Modern Methods for the Determination of Non-Metals in Non-Ferrous Metals: Applications to Particular Systems of Metallurgical Importance [Reprint 2011 ed.] 9783110862447, 9783110103427

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Modern Methods for the Determination of Non-Metals in Non-Ferrous Metals

C. Engelmann · G. Kraft J. Pauwels · C. Vandecasteele

Modern Methods for the Determination of Non-Metals in Non-Ferrous Metals Applications to Particular Systems of Metallurgical Importance

W G DE

Walter de Gruyter · Berlin · New York 1985 on behalf of the Commission of the European Communities

Authors Charles Engelmann CEN Saclay F-91191 Gif-sur-Yvette, Cedex France Günther Kraft Metallgesellschaft AG, Frankfurt, and Universität Frankfurt, Fachbereich Chemie D-6000 Frankfurt Federal Republic of Germany Jean Pauwels Commission of the European Communities Joint Research Centre Central Bureau for Nuclear Measurements Steenweg naar Retie B-2440 Geel Belgium

Carlo Vandecasteele Research Associate of the National Fund for Scientific Research Rijksuniversiteit Gent Instituut voor Nucleaire Wetenschappen Proeftuinstraat 86 B-9000 Gent Belgium Document No. EUR 8612 EN of the Commission of the European Communities Directorate-General Information Market and Innovation Luxembourg

Library of Congress Cataloging in Publication Data Modem methods for the determination of non-metals in non-ferrous metals. „Document no. EUR 8612 EN"—Τ p. verso. Includes bibliographies and index. 1. Nonmetals—Analysis. 2. Nonferrous metals—Analysis 3. Chemistry, Metallurgie. I. Engelmann, Ch. II. Commission of the European Communities. QD131.M63 1985 669'.92 84-28816 ISBN 0-89925-010-6 (U.S.) CIP-Kurztitelaufnahme der Deutschen Bibliothek Modem methods for the determination of non-metals in non-ferrous metals.: applications to particular systems of metalurg. importance / C. Engelmann . . . On behalf of the Comm. of the Europ. Communities. - Berlin ; New York : de Gruyter, 1985. ISBN 3-11-010342-7 (Berlin . . . ) ISBN 0-89925-010-6 (New York) NE: Engelmann, C. [Mitverf.]

ISBN 3-11-010342-7 Walter de Gruyter · Berlin · New York ISBN 0-89925-010-6 Walter de Gruyter, Inc., New York Copyright © 1985 by ECSC, EEC, EAEC, Brussels/Luxembourg. All rights reserved, including those of translation into foreign languages. No part of this book may be reproduced in any form - by photoprint, microfilm or any other means nor transmitted nor translated into a machine language without written permission from the copyright hölder. Printing: Gerike GmbH, Berlin. Binding: Dieter Mikolai, Berlin. - Fhrinted in Germany. Legal Notice Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information.

PREFACE For more than 10 years, a few highly qualified industrial and university laboratories in Europe have

availed

themselves of

the opportunities

offered by the Commission of the European Communities to collaborate on measurements and analytical problems,

with the double aim

of improving

the accuracy of results and of furthering European harmony. of

In the case

the analysis of non-metallic impurities in non-ferrous metals the

Commission has provided financial assistance. The reports of several of the participants in scientific journals show that considerable progress has

been achieved through this collaboration : at times differences of

more than one order of magnitude had to be explained and overcome. The present volume is

a

collection of

the results of this collaborative

work. It is

not frequent that fruitful collaboration has been

such a long period of time.

continued over

It is a pleasure, therefore, to acknowledge

the work of those who participated. Dr. H. MARCHANDISE Head of the Community Bureau of Reference (BCR)

The preparation of certified reference materials has

always been a

difficult task, as accuracy and not only reproducibility is intended. Systematic errors must therefore be eliminated as far as possible. These can

usually only be detected by applying

techniques. If problem

is

nitrogen,

the component to be certified is

even more

sensitivity, high

blank

severe

due

at trace level, the

to contamination,

insufficient

values, ... . Elements such as boron, carbon,

oxygen, sulphur

non-ferrous metals in few ng/g.

widely differing analytical

and phosphorus are usually present in

concentrations between several hundred pg/g and a

It is well known that these elements influence the mechanical

and physical properties

of many non-ferrous metals

to a considerable

extent. The users' requirements therefore become increasingly strict and

VI

often

the

producer

is

faced

analytical difficulties.

with

considerable

It is therefore to be

reference

materials

will fulfil a real

certified

an large number of reference

technological

expected that certified

need in

this

area.

materials based on

numerous interested laboratories, both from industry and institutions.

The

participation of

sometimes

causes long

different

analytical

delays,

several

has two

techniques

usually

institution and it allows the participants intercompared, but sources of

eliminated.

Participation

in

has

the work of

from

research

: it offers available

more

in

one

to become aware of their own

systematic errors : during successive round-robins the only

BCR

laboratories, although it

advantages

than

and

results

are not

systematic errors are detected

these

round-robins

thus

and

allows

the

participants to evaluate their analytical technique and to correct it if necessary. The

role

particle

that

nuclear

analytical

and

photon

activation

intercomparisons which

is

must

usually

that

blank

meant as routine methods, improving

the

accuracy

the

charged

in

these

from the sensitivity,

chemical

they

are

the analyte element and they

are

values. These

but they of

of

played

Apart

that of

independent of the chemical state

e.g. neutron,

analysis,

also be stressed.

higher

usually also free from

techniques

methods

methods,

are of course not

contributed in a significant way to chemical

methods and to

eliminating

systematic errors. The

present work summarises the experiences gained during several years

of collaboration and of critical investigation of the methods applied. Prof. Dr. J. Hoste Institute of Nuclear Chemistry Rijksuniversit eit Gent Belgium The

authors

thank

all those

who contributed

non-metals in non-ferrous metals. to N. Verboomen for typing the Geeraerts for the drawings.

to

the

BCR program on

Grateful acknowledgement is also made

manuscript

and

to

J. Van Saene and R.

C O N T E N T S

Chapter I : THE INFLUENCE OF NON-METALS ON THE TECHNOLOGICAL PROPERTIES OF NON-FERROUS METALS

1.

Introduction

1

2.

Oxygen

1

2.1 2.2 2.3 2.4 2.5 2.6 2.7 3.

6.

copper aluminium lead titanium zirconium niobium and tantalum molybdenum and tungsten

Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon

1 3 5 5 7 7 9 10

in in in in in

titanium and zirconium niobium tantalum molybdenum tungsten

Carbon 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9

5.

in in in in in in in

Nitrogen 3.1 3.2 3.3 3.4 3.5

4.

Oxygen Oxygen Oxygen Oxygen Oxygen Oxygen Oxygen

10 11 12 13 14 14

in in in in in in in in in

aluminium copper nickel titanium zirconium niobium tantalum molybdenum tungsten

Boron

14 15 15 15 15 16 16 16 17 17

5.1

Boron in zircaloy and aluminium for nuclear purposes

17

5.2

Boron as grain refiner in Al-Mg a l l o y s

17

Sulphur

18

6.1

Sulphur in copper

18

6.2

Sulphur in nickel

18

7.

Phosphorus

18

8.

References

19

VIII Chapter I I : GENERALITIES ON NUCLEAR METHODS

1.

2.

3.

4.

Neutron activation analysis

20

1.1 1.2 1.3

20 21 22

Introduction Activation analysis with reactor neutrons Activation analysis with 14 MeV neutrons

Charged particle activation analysis

25

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

25 28 31 48 49 55 65 65 67

Nuclear reactions Stopping power and range Standardization Samples and standards Irradiation Influence of radionuclides formed at the surface Chemical separation of the radionuclides produced Determination of the induced a c t i v i t y Calculations

Photon activation analysis

68

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

68 70 72 74 74 77 79 83 93

Introduction Gamma photon generators Standardization Nuclear reactions Nuclear interferences Theoretical p o s s i b i l i t i e s Irradiation f a c i l i t i e s Radiochemical separation Specific advantages of the method

References

94

Chapter I I I : SAMPLE PREPARATION AND SURFACE ANALYSIS

1.

Introduction

101

2.

Measurement of surface concentrations

105

2.1 2.2 2.3 2.4

105 108 109 110

3.

Principle of the method Nuclear reactions Standardization Comment

Mechanical shaping of analysis samples

110

3.1 3.2

110 111

Rough preparation of the analytical sample Final shaping of the sample

4.

Shaping of large series of samples

113

5.

Chemical etching

117

IX 6.

7.

Recommended procedures

118

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13

118 119 119 120 120 120 122 123 123 124 124 125 125

Primary ingot aluminium Aluminium-silicon alloys Aluminium-magnesium alloys Copper Brass Lead and lead alloys Nickel Titanium and TiA16V4-alloy Zirconium Molybdenum Tungsten Niobium Tantalum

References

126 Chapter IV : THE DETERMINATION OF BORON

1.

2.

Chemical methods

129

1.1 1.2

129 137

Charged particle activation analysis

143

2.1 2.2 2.3

143 147

2.4 3.

4.

The determination of boron in aluminium The determination of boron in zirconium and zircaloy Nuclear reactions Standards The determination of boron in aluminium and aluminium-magnesium alloys The determination of boron in zirconium and zircaloy

148 149

Evaluation of methods

158

3.1 3.2 3.3

158 160 161

The determination of boron in aluminium The determination of boron in aluminium-magnesium alloys The determination of boron in zirconium and zircaloy

References

164 Chapter V : THE DETERMINATION OF CARBON

1.

Chemical methods 1.1 1.2 1.3 1.4 1.5 1.6

Introduction The determination of carbon The determination of carbon and zircaloy The determination of carbon molybdenum and tungsten The determination of carbon The determination of carbon

167 in aluminium in titanium, zirconium

167 168 170

in niobium, tantalum, in copper in nickel

175 179 180

χ 2.

Charged particle activation analysis 2.1 2.2 2.3 2.4 2.5 2.6

3.

4.

5.

Nuclear reactions The determination The determination The determination The determination The determination

of of of of of

carbon carbon carbon carbon carbon

in in in in in

181 aluminium nickel zirconium and zircaloy niobium and tantalum molybdenum and tungsten

181 181 183 183 185 186

Photon activation analysis

187

3.1 3.2 3.3 3.4 3.5

187 189 190 190

The determination of carbon in sodium The determination of carbon in aluminium The determination of carbon in nickel The determination of carbon in refractory metals Other examples of carbon determinations in non-ferrous metals

194

Evaluation of methods

195

4.1 4.2 4.3

195 197 198

The determination of carbon in aluminium The determination of carbon in zirconium and zircaloy The determination of carbon in molybdenum and tungsten

References

200 Chapter VI : THE DETERMINATION OF NITROGEN

1.

Chemical methods 1.1 1.2 1.3 1.4 1.5

2.

3.

The The The The The

determination determination determination determination determination

206 of of of of of

nitrogen nitrogen nitrogen nitrogen nitrogen

in in in in in

zirconium and i t s alloys titanium and i t s a l l o y s niobium and tantalum molybdenum and tungsten nickel

206 213 216 229 231

Charged particle activation analysis

232

2.1 2.2 2.3 2.4 2.5 2.6 2.7

232 232 234 236 237 241 244

Introduction Nuclear reactions The determination The determination The determination The determination The determination

of of of of of

nitrogen nitrogen nitrogen nitrogen nitrogen

in in in in in

titanium and i t s a l l o y s nickel zirconium and zircaloy niobium and tantalum molybdenum and tungsten

Proton activation a n a l y s i s

245

3.1 3.2 3.3 3.4 3.5

245 246 246 248

The determination of nitrogen in sodium The determination of nitrogen in aluminium The determination of nitrogen in nickel The determination of nitrogen in refractory metals Other examples of nitrogen determinations in non-ferrous metals

249

XI 4.

Evaluation of methods 4.1 4.2 4.3 4.4

5.

The determination The determination TiA16V4-alloy The determination The determination

249 of nitrogen in zirconium and zircaloy of nitrogen in titanium and of nitrogen in molybdenum and tungsten of nitrogen in nickel

References

249 251 252 253 254

Chapter V I I : THE DETERMINATION OF OXYGEN

1.

Chemical methods 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

The The The The The The The The and 1.9 The 1.10 The 1.11 The

2.

3.

determination determination determination determination determination determination determination determination their a l l o y s determination determination determination

259 of of of of of of of of

oxygen oxygen oxygen oxygen oxygen oxygen oxygen oxygen

in in in in in in in in

aluminium aluminium a l l o y s copper copper alloys lead lead alloys nickel zirconium, titanium

of oxygen in niobium and tantalum of oxygen in molybdenum of oxygen in tungsten

259 266 267 280 287 290 290 291 299 303 306

14 MeV neutron activation analysis

307

2.1 2.2 2.3 2.4 2.5 2.6 2.7

307 307 308 309 310 315 318

Nuclear reactions Apparatus I r r a d i a t i o n and measuring conditions Samples, standards and flux monitors Sources of errors Precision and s e n s i t i v i t y Results

Charged particle activation analysis

318

3.1 3.2 3.3 3.4

318 320 324

3.5 3.6 3.7 3.8 3.9

Nuclear reactions The chemical separation of The determination of oxygen The determination of oxygen and their a l l o y s The determination of oxygen The determination of oxygen The determination of oxygen The determination of oxygen The determination of oxygen

F in aluminium in titanium, zirconium in in in in in

nickel copper molybdenum and tungsten tantalum lead

327 328 329 331 333 333

XII

4.

5.

Photon activation analysis

334

4.1 4.2 4.3 4.4 4.5 4.6

334 335 336 336 341

Evaluation of methods 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

6.

The determination of oxygen in sodium The determination of oxygen in aluminium The determination of oxygen in nickel and copper The determination of oxygen in refractory metals The determination of oxygen in lead and i t s alloys Other examples of oxygen determinations in non-ferrous metals The The The The The and The The The

determination determination determination determination determination their alloys determination determination determination

342 343

of of of of of

oxygen oxygen oxygen oxygen oxygen

in in in in in

aluminium copper lead and i t s alloys nickel titanium, zirconium

of oxygen in niobium and tantalum of oxygen in molybdenum of oxygen in tungsten

References

343 345 346 347 348 348 350 351 351

Chapter V I I I : THE DETERMINATION OF PHOSPHORUS

1.

Chemical methods

366

2.

Charged p a r t i c l e activation analysis

372

2.1

Nuclear reactions

372

2.2

Experimental procedure for the determination of phosphorus in aluminium a l l o y s

372

2.3

Results and discussion

375

Neutron activation analysis

376

3.1 3.2 3.3

376 376

4.

Nuclear reactions Applications The determination of phosphorus in aluminiums i 1 icon a l l o y 3.4 The determination of phosphorus (and sulphur) in copper and nickel Evaluation of methods

5.

References

380

3.

377 378 380

XIII

Chapter IX : THE DETERMINATION OF SULPHUR 1.

2.

3.

Chemical methods

382

1.1 1.2

382 392

The determination of sulphur in copper The determination of sulphur in nickel

Charged particle activation analysis

394

2.1 2.2

394

Nuclear reactions Experimental procedure for the determination of sulphur in copper and nickel 2.3 Results and discussion Neutron activation analysis

395 397 398

3.1 3.2

398 399

Nuclear reactions Applications

4.

Evaluation of methods

399

5.

References

399 A p p e n d i x

Subject index

401

CHAPTER I

THE INFLUENCE OF NON-METALS ON THE TECHNOLOGICAL PROPERTIES OF NON-FERROUS METALS

1.

INTRODUCTION

The presence of non-metallic elements in metals may affect their propert i e s in a variety of ways.

Non-metals may influence mechanical and

physical properties such as corrosion resistance, hardness, hot and cold d u c t i l i t y , mechanical strength, conductivity, c a s t a b i l i t y , nucleus forming properties and s i n t e r a b i l i t y . properties of metals.

Boron influences the nuclear

These effects may be f e l t at very low concentra-

tions even below 0.1 Mg/g. Depending on the non-metal element, i t s concentration and the matrix, the determination of the concentration of these elements requires more or less sophisticated analytical equipment.

Classical

analytical

methods are only of limited p o s s i b i l i t i e s ; techniques such as reducing fusion allow the determination of oxygen and nitrogen at concentration levels of a few Mg/g.

To date, an accurate determination of concentra-

tions below 1 Mg/g i s only possible using nuclear activation methods such as e.g. photon- or charged particle activation. This chapter gives a brief survey of some important effects of nonmetallic impurities in non-ferrous metals.

Emphasis i s put on oxygen

and nitrogen, which were the main subject of analytical work within the Community Bureau of Reference (BCR). 2.

OXYGEN

2.1. OXYGEN IN COPPER -4 The s o l u b i l i t y of oxygen in copper i s of the order of 2.10 (0.5 /ig/g) at 500°C and 3.10"

2

at.% (75 pg/g) at 1000°C.

at.% The conduc-

t i v i t y , which decreases with increasing oxygen content, i s an important

2 property for electrotechnical applications: for an oxygen concentration of 4000 Mg/g the conductivity is 50 S, for 1000 jug/g oxygen it is 56 S. Oxygen contents below 200 μς/g do not seem to influence the electrical conductivity of copper which for oxygen free copper is at least 57 S. When the oxygen content is higher than 400 Mg/g, precipitates of ( ^ 0 become harmful.

They decrease the formability of the metal, and there-

fore drawing to fine wires becomes difficult. Copper containing oxygen may not be used in the presence of hydrogen which would reduce the oxygen and create porosity and blisters (hydrogen sickness).

For such applications the copper melts are deoxidized with

phosphorus. Unalloyed copper is mainly used in the electrical industry. It is produced either by electrolysis or by fire-refining.

In the latter oxygen

is of particular importance, because the copper is purified by an oxidizing treatment.

Electrolytically refined copper is cast after its

recovery into forms suitable for subsequent treatment, whereby it is once more refined to eliminate contained gases. copper may contain varying oxygen contents.

After refining, the

So the phenomenon known

as "raising", which occurs in casting copper into wire bars, is closely connected to the oxygen content and results from the interaction with hydrogen present.

It causes porosity in the cast metal.

The lower the oxygen content, the higher is the softening point of copper. This means that, e.g. in annealing rolled wires, the annealing time is lengthened, thus increasing the cost of the operation.

When copper is

used as trolley wire a low oxygen content may have a positive effect on wear losses, a higher oxygen content definitely has a negative effect. The influence of oxygen on the strength of copper is believed to be restricted to small hardening effects only. Unalloyed copper of commercial quality can be subdivided in three types: 2.1.1.

Electrolytical tough pitch copper (ETP)

The oxygen content of this type of copper normally ranges from 200 ßg/g to 500 ^g/g.

Although the oxygen concentration influences the mechanical

qualities to a certain extent, it also suppresses the effect of impurities less noble than copper by converting them into oxides.

The result is on

3 one hand a high conductivity and on the other a low softening temperature. The copper oxide present in the structure slows down grain growth by recristallization, and contributes to stabilizing a favourable structure. 2.1.2.

Oxygen free copper (OFHC)

Oxygen contents in this quality of copper are far below 10 Mg/g, usually around 1 μς/g.

It may be used in an atmosphere containing hydrogen, in

particular as tubes.

In oxidizing atmosphere a compact and very adhesive

oxide film forms at the tube surface, which does not crack and peel off whilst water quenching. 2.1.3.

Oxygen free copper is also used for plating.

Phosphorus deoxidized copper (DLP)

If absence of oxygen has to be guaranteed the copper is deoxidized with phosphorus.

For safety, the phosphorus additions are slightly in excess

over what is needed. vity of the copper.

However too much phosphorus decreases the conductiThe exact knowledge of the oxygen content before

the phosphorus treatment is thus of essential

importance (2).

2.2. OXYGEN IN ALUMINIUM The affinity of aluminium for oxygen is high, which means that all phases of its processing may be strongly influenced by oxygen. for both unalloyed and alloyed aluminium. aluminium is virtually zero (1).

This is the case

The solubility of oxygen in

Aluminium is produced by electrolysis

using alumina and cryolite, and is obtained as liquid metal under a salt cover.

At that stage it always contains finely dispersed non-metallic

particles, and sufficient time should be allowed for decantation before casting.

In the holding furnaces and casting channels oxides can form

at the surface of the metal, if sufficient precautions are not taken to minimize oxidation.

Oxide particles can be introduced into the metal

during casting and are therefore detrimental to the properties of the metal.

Oxygen is therefore usually kept to very low levels.

The metal thus purified is cast into bars of different sizes.

Thereby,

the liquid metal has to pass through channels from the holding furnace into the casting implements.

The casting channels cover with a thin

oxide skin and it is not excluded that parts of this skin enter the liquid metal.

4 According to the pouring techniques used, they may remain totally or partially in the cast metal as oxide inclusions. Both oxide types have a negative effect on the further processing of the metal.

When bars are rolled into plates, the oxide inclusions cause the

formation of dull and porous spots, which make the product useless.

In

foil production even the presence of oxide plancton has a negative effect as it causes the formation of holes and, in greater accumulation, of cracks. In sheet production especially the oxide plancton acts as recombination centers (nuclei) for hydrogen which is always present in atomic form in the molten metal, in which it is 20 times more soluble than in the solid metal.

If recombination takes place near the surface of plates, it may

cause the formation of holes: this phenomenon is especially observed with sheets of about 1 mm thickness.

The oxide inclusions resulting from the

skins of the casting channels are not only present in the form of aluminium oxides, but can also contain hydroxyl groups, which during the transformation of the aluminium may yield water and hydrogen.

The cor-

relation between the presence of oxides and the formation of blow-holes on the sheets is therefore very complex. Oxygen also plays an essential role in the production of aluminium casting products.

This is valid as well for the oxides which are already present

in the pigs, as for those which are newly formed in remelting and casting. When the latter enter the castings, which happens preferably in the form of dross or skins, it causes macroscopic faults similar to those reported above for the production of rollings.

They cause failures in the cast

material, which influence the mechanical properties negatively.

Any

oxide plancton results in an effect on the flowing behaviour, and on the mold-filling properties during casting. Finally, oxides are important in shaping of aluminium cast products by chips removal: when oxides are present, the hardness of the products influences significantly the lifetime of the working tools thus causing additional costs, not only because the tools have to be renewed, but also because the processing yields become worse the more the tools are damaged (2).

5 2.3. OXYGEN IN LEAD The solubility of oxygen in lead is very low.

According to Fromn and

Gebhardt (1) it is about 0.08 Mg/g at 350°C and about 8 Mg/g at 540°C. Lead is worked and used in both unalloyed and alloyed form.

Hard lead

e.g. contains antimony and is used in batteries. The oxidizing treatment of molten lead plays an important role in the refining processes, especially to remove traces of antimony, arsenic and tin.

The oxides resulting from this process separate slowly because of

the small difference in density with liquid lead.

When the resting

periods are too short, there is a risk that the cast lead contains oxide inclusions. Another difficulty results from the tendency of liquid lead to oxidize. The importance of this tendency depends essentially on the kind and quantity of metallic impurities.

Lead varieties exist which, in the liquid

state keep a shiny surface for a relatively long time in contact with air, but others show gray till blue annealing after a very short time of air exposure.

The oxides formed in this way (casting oxides) enter into the

product during the casting, are fixed between the growing crystals during the solidification and are often present in the form of thin skins.

Both

these oxides and the refining oxides may be the origin of different types of faults, e.g. insufficient corrosion resistance of lead e.g. for sanitary purposes or cable sheaths, battery plates, etc.

An optimal control

of quality depends of the availability of accurate techniques for the determination of oxygen independent of the chemical form and at concentrations below l O p g / g . Another need for the determination of oxygen in lead is deoxidizing treatments which are carried out e.g. for hard lead.

If accurate analyses are

carried out the addition of deoxidizing agents can be adjusted accurately (2). 2.4. OXYGEN IN TITANIUM Titanium is known for its high mechanical strength, good resistance against corrosion and low density.

It is particularly suited for aeronautical and

astronautical applications, chemical equipment and in car industry.

6 Titanium is used both in unalloyed and alloyed form: at present the best known alloys are the TiAlV ones. Titanium is obtained by the reduction of its tetrachloride by magnesium, in the form of a metal sponge.

This sponge is then melted under vacuum,

either in a consumable electrode arc furnace or in an electron beam furnace.

The titanium sponge has an oxygen content of 500 to 1500

The remelting process virtually does not change these values.

ug/g. Titanium

with a significantly lower oxygen content can be obtained by the van Arkel-process (iodide-titanium). This type of titanium generally contains oxygen concentrations of 10 to 100 vg/g

only.

At room temperature, oxygen is soluble in titanium up to about 8 at.% (27 mg/g).

The oxygen present is then really dissolved in the metal.

It is present in the hexagonal structure of α-titanium like an alloying element and increases the strength of the metal: with increasing oxygen content the hardness, yield point and tensile strength increase. In the range of 500 to 1500 μg/g, oxygen only decreases slightly the ductility of titanium and its alloys, the corrosion resistance is not significantly influenced by the oxygen content. It is thus possible to adjust the strength of unalloyed titanium by an appropriate oxygen content as shown in Table 1-1. Table 1-1:

Mechanical strength of unalloyed titanium as a function of the oxygen content.

Oxygen content

Yield point 2

Tensile strength

Hardness (HV 10)

2

(^g/g)

(N/mm )

(N/mm )

(N/rnri2)

500

0.015

0.028

0.11

1500

0.028

0.045

0.16

The effect of oxygen on the strength decreases with rising temperature and nearly disappears at 400°C.

Between the titanium alloys, the palla-

dium alloy behaves in a similar way as the unalloyed metal, as far as the influence of oxygen on the physical properties is concerned.

On the con-

trary, for the high tensile alloys of TiAl6V4 type, the strength mainly results from the martensitic transformation which takes place (2).

7 2.5. OXYGEN IN ZIRCONIUM The behaviour of zirconium is very similar to that of titanium.

It is

obtained in the same way, mainly as sponge, and is melted into ingots under vacuum. In the unalloyed form, zirconium is used for the construction of chemical equipment.

Of much higher importance are however the zirconium alloys,

from which especially the types zircaloy-2 (1.5 % Sn, 0.1 % Fe, 0.1 % Cr, 0.05 % Ni) and zircaloy-4 (1.5 % Sn, 0.1 % Cr, 0.2 % Fe) are of interest. They are used as fuel cladding materials in pressure and boiling water nuclear reactors and for structural elements in the reactor core. As it is the case for titanium, oxygen acts as an alloying element with very high solubility and influences mechanical strength (Table 1-2). Table 1-2:

Mechanical properties of unalloyed zirconium as a function of the oxygen content.

Oxygen content

Yield point

Tensile strength

Elongation

Hardness (HV 30)

(N/nm 2 )

(N/mm 2 )

(%)

500

0.015

0.025

22

0.10

1500

0.023

0.035

9

0.14

M/g)

(N/mm 2 )

Semiproducts of pure zirconium, intended for the construction of chemical equipment, contain approx. 1000 Mg/g oxygen.

Normal oxygen contents in

remelted zirconium sponge amount to 600-1400 μς/g, and to 200-600 ng/g or even lower for iodide zirconium. For recristallized zircaloy-4 semi-products, the correlation between the oxygen content and the physical properties is given in Table 1-3. Just as for titanium and its alloys, the influence of oxygen on the physical properties decreases considerably with increasing temperature (2). 2.6. OXYGEN IN NIOBIUM AND TANTALUM Both metals show a high affinity for oxygen.

In solid solution, niobium

can contain 1000 ßg/g of oxygen, and tantalum about 200 ßg/g.

8 Table 1-3:

Physical properties of zircaloy-4 as a function of the oxygen content

Oxygen content

Yield point

Tensile strength

2

Elongation

(%)

2

(N/mm )

(N/rrm )

700

0.031

0.047

25

1000

0.036

0.049

25

1250

0.040

0.057

24

(Mg/g)

The solubilities of oxygen in both metals are given in Table 1-4. Table 1-4:

Solubility of oxygen in tantalum and niobium

Metal

Temperature (°C)

Solubility (^g/g)

Ta

500

2000

1800

5000

500

2000

1800

8000

Nb

In practice the oxygen contents of both metals are however below the saturation values. Just as for titanium and zirconium, oxygen strongly influences the mechanical properties of both metals (Table 1-5). Table 1-5:

Metal Ta

Nb

Vickers-hardness as a function of oxygen content

Oxygen content (Mg/g)

2 Vickers-hardness (N/mm )

200

0.10

800

0.25

2000

0.40

400

0.10

1000

0.15

4000

0.30

9 In the very pure electron beam melted and zone-refined quality, the hardness of niobium is 0.04-0.05 N/mm 2 , that of tantalum 0.07-0.09 N/mm 2 . Commercial grade metals which are vacuum melted or vacuum sintered have hardnesses of 0.10 to 0.16 N/mm 2 for niobium of 99.9 to 99.7 % purity and 0.09 to 0.15 N/mm 2 for tantalum of 99.95 to 99.9 % purity. Niobium is known for its high ductility; deformation up to 75 % by cold rolling is possible without problem, even at oxygen contents of 1 mg/g. At a content of 2 mg/g however, cracks already appear at a deformation of 25 %.

Tantalum on the contrary already becomes brittle at oxygen contents

of 200 μg/g.

The transition temperature brittle/ductile, which depends on

the oxygen content, is of a special practical importance for pure metals. Determined by the transverse bending test the transition temperature of niobium is approx. - 200°C when the oxygen content is 200 Mg/g; for an oxygen content of 1000 Mg/g it is - 30°C and for 2000 Mg/g + 20°C. The tensile strength of niobium increases with increasing oxygen content: 2 it reaches 7 N/mm

at 400°C, 300°C and room temperature for oxygen concen-

trations of resp. 600 Mg/g, 700 Mg/g and 1200 Mg/g.

Also the elongation

depends on the oxygen content: at room temperature it amounts for niobium to 30 % for an oxygen content of 200 Mg/g and to 17 % for an oxygen content of 1500 Mg/g. The most interesting fields of application of niobium, tantalum and their alloys are heat exchangers, heat protection shields in furnaces, chemical /

equipment and aerospace materials.

Due to the relatively low neutron cap-

ture cross section, niobium may have some interest in nuclear applications. 2.7. OXYGEN IN MOLYBDENUM AND TUNGSTEN The dependence of the properties of molybdenum and tungsten on their oxygen content differs significantly from that of niobium, tantalum, titanium and zirconium.

Even very low oxygen contents cause such an embrittle-

ment that considerable difficulties occur during the fabrication and use. When molybdenum and tungsten are allowed to cool slowly, the oxygen content in solid solution is only about 1 Mg/g.

Even in the range of 1500 to

1800°C, the solubility of oxygen in both metals is only of the order of 50 Mg/g, the value for molybdenum being somewhat higher than the one for tungsten.

As a consequence of this low oxygen solubility, both molybdenum

10 and tungsten are normally supersaturated with oxygen, and consequently are brittle.

The transition temperature from ductile to brittle depends

very much on the oxygen content.

For molybdenum it is - 60°C at 1 Mg/g,

and + 200°C at both 2 Mg/g and 6 Mg/g (2). The hardness is also influenced by oxygen.

Monocrystals of tungsten 2 rolled at 1000°C, have a Vickers-hardness of 0.015 N/nrn at an oxygen ?

content of 8 Mg/g after recristallization (approx. 10 grains per mm ) and of 0.02 N/mm2 at 40 Mg/g. Technical processing of molybdenum and tungsten is preferentially done by reducing the oxides with hydrogen; the powders thus obtained are subsequently sintered. Tungsten is mainly used for heating conductors, electric bulbs, thermionic valves and electrodes.

Because of its suitable electrical and

thermal properties, molybdenum is used in high temperature furnaces. 3.

NITROGEN

3.1. NITROGEN IN TITANIUM AND ZIRCONIUM Nitrogen is highly soluble at all temperatures in both titanium and zirconium (3).

In titanium the solubility amounts to 25 at.% (74 mg/g)

at 1985°C, to 40 mg/g at 700°C and to 35 mg/g at room temperature.

In

zirconium it is soluble up to 21 at.% (37 mg/g) at 2350°C and up to 10 at.% (17 mg/g) at 600°C.

The reaction of nitrogen with titanium

and zirconium starts perceptibly at 600-700°C and becomes violent above 1000°C. For all qualities the nitrogen content is below 200 Mg/g for titanium and below 60 Mg/g for zirconium. The following properties of both metals are highly influenced by the nitrogen content: -

resistivity: for titanium, it increases from 47 to 50, 53 and 56 μΩ.αιι for nitrogen contents of 0.75, 1.5 and 3 mg/g respectively.

-

2 hardness: the Vickers-hardness (VH 10) increases from 0.11 N/im for 2 pure titanium (van Arkel) to 0.26-0.28 N/mm for titanium containing 3 mg/g of nitrogen.

11

-

y i e l d point and elongation: these parameters are more influenced bv nitrogen than by oxygen.

The y i e l d point of van Arkel titanium i s

increased to the threefold value when the nitrogen content i s

increased

to 1 at.%, and the elongation diminishes by a factor of 2.5 for a nitrogen content of 0.25 at.% and by a factor of 3 for 1 at.% (3). 3.2. NITROGEN IN NIOBIUM Literature contains a v a r i e t y of data on the s o l u b i l i t y of nitrogen in niobium.

They do not always agree; Table 1-6 summarizes some of them.

Table 1-6:

S o l u b i l i t y of nitrogen in niobium

T°(C)

300

κ g/g

0.125

700-800

1100

1200

0.25-0.4

~ 0.5

0.5-2

1800

2200

2400

3.5-7.5

7.5-15

~ 25

The s o l u b i l i t y of nitrogen in niobium varies with temperature according to S i e v e r t ' s square root law.

Nitrogen can therefore be removed by

vacuum f u s i o n . The t r a n s i t i o n temperature depends on the - e s p e c i a l l y i n t e r s t i t i a l impurities in the metal.

-

T r a n s i t i o n temperatures measured under compa-

rable t e s t - c o n d i t i o n s (creep rate, grain s i z e ) , depending on kind and concentration of the impurity elements, are compiled in Table 1-7. Table 1-7:

T r a n s i t i o n temperature in °C of niobium measured in the t e n s i l e t e s t at p l a i n rods

Nitrogen

Hydrogen

°C

. ^g/g

°C

" g/g

100

- 200

200

-

30

500

+

80

600

+ 100

20

Oxygen

Carbon °c

f^g/g

f g/g

- 200

150

- 200

200

+

70

500

- 100

400

+ 100

1300

-

10

2500

+

30

1500

°c - 200

12 The values mentioned show that the influence of nitrogen exceeds that of oxygen.

For a recristallized metal of average purity (150 Mg/g 0 2 ,

100 Mg/g N 2 , 10 ^g/g H 2 , 800 Mg/g Fe and 2000 Mg/g Ta) the transition temperature is of the order of - 150 to - 195°C. The hardness also depends to a large extent on the interstitial impurities, but up to now the influence of nitrogen has not been differentiated from that of other impurities. 3.3. NITROGEN IN TANTALUM Also for the solubility of nitrogen in tantalum different data are given in the literature. Table 1-8: T(°C) ^g/g

Table 1-8 shows the data of Albert (3).

Solubility of nitrogen in tantalum 20

~ 0.2

350

500

700

1000

1500

2300

4.5

10.20

50

27.60

37.80

~ 125

As for niobium, the solubility of nitrogen in tantalum follows Si evert's square root law.

The absorption and desorption of nitrogen depend on the

temperature and on the N2-partial pressure.

So, the nitrogen content can

be reduced to below 0.1 at.% (77 Mg/g) at 2000°C and 1.3 10"4 Pa, whereas at 3000°C the same nitrogen content can already be obtained at 0.67 Pa. The nitrogen content of tantalum ranges from 20 to 500 Mg/g depending on the applied fusion technique.

Samples, zone refined in an electron beam,

contain only 2.5 Mg/g of oxygen and nitrogen. The hardness (Brinell) of tantalum increases from 0.1 N/mm 2 1.2 N/mm

2

to 0.75 and

for nitrogen contents of 4 at.% (3.1 mg/g) and 7 at.% (5.4 mg/g)

respectively. The diffusion of nitrogen is slower than that of oxygen.

Variations of

hardness by heat treatment in oxygen and in nitrogen at 1200°C, show that nitrogen has a less decisive influence both at the surface and in the bulk of the metal (3).

13 The yield point of tantalum is influenced by nitrogen and oxygen. -3 tion [1] describes their influence on the elastic limit a (10 a

=

Equa2

N/mm ):

12.6 + 69 C N + 40 Cq

[1]

C N and C Q are the concentrations of nitrogen and oxygen in at.%. The resistivity is described by equation [2]: PHe

=

5.1 C N

In this equation

[2]

is the residual resistivity (μΩ.αη) at the tempera-

ture of liquid helium, whereas C^ is the nitrogen concentration in at.%. Equation [2] is valid up to 6 at.%, which means 5.3 mg/g oxygen and 4.6 mg/g nitrogen respectively (3). 3.4. NITROGEN IN MOLYBDENUM Nitrogen reacts with molybdenum at high temperature.

Its solubility is

given in Table 1-9. Table 1-9:

Solubility of nitrogen in molybdenum

T(°C) Mg/g

1300

1700

2000

2400

87

330

650

1300

3 At about 1600°C and under nitrogen partial pressure of 2.10 Pa to 4 5.33 10 Pa, the solubility follows Si evert s square root law: CN

=

0.216

. exp (- 20400/RT)

[3]

in which C N is the nitrogen concentration in at.% (1 at.% = 1.46 mg/g). Hence, the nitrogen concentration can be lowered to less than 1.5 ^g/g, at 6.7 10" 2 Pa and 1550°C or at 4.0 Pa and 2400°C.

Due to the low solu-

bility of nitrogen in molybdenum, nitrides are precipitated during cooling to room temperature. The nitrogen remaining in solution by quenching increases the resistivity of molybdenum with 2.4 μΩ.αιι per at.% of nitrogen.

14

Depending on the fusion technique and the conditions of production, the nitrogen content of molybdenum ranges from below 10 to 20 pg/g. For a metal of medium purity, the b r i t t l e f a i l u r e i s very problematic as the t r a n s i t i o n occurs at room temperature or even at higher temperatures. The t r a n s i t i o n temperature i s influenced by the following factors: -

content of i n t e r s t i t i a l l y dissolved impurity elements

-

processing technique

-

final state

-

kind of stressing (tension, bending, push) velocity of s t r e s s i n g , state of sample (grooved or ungrooved).

3.5. NITROGEN IN TUNGSTEN The s o l u b i l i t y of nitrogen in tungsten follows S i e v e r t ' s square root law (3): CN

=

0.92

. exp ( - 46700/RT)

[4]

one at.% of nitrogen corresponding to 770 Mg/g. In a nitrogen atmosphere the s o l u b i l i t y consequently i s : 15 jug/g at 3000°C, 3 Mg/g at 2400°C, 0.013 ßg/g at 1200°C, and becomes v i r t u a l l y zero at 25°C. 4.

CARBON

Carbon interacts in many ways with non-ferrous metals due to e.g. the different s o l u b i l i t y in these metals. 4.1. CARBON IN ALUMINIUM Although the s o l u b i l i t y of carbon in aluminium i s as small as 0.1-0.01 at.% (400 - 40 ßg/g)

(1), carbon i s of some importance for the

production of aluminium, as an indicator for possible faults in the operation of e l e c t r o l y s i s c e l l s .

When the metal contains carbon even

at the level of a few ßg/g only, defects may appear in the sheets obtained by r o l l i n g .

4.2. CARBON IN COPPER In copper carbon residues are an indicator for the co-precipitation of organic additives during the electrowinning of the cathodes. sions have to be kept as low as technically possible.

These inclu

Carbon contents

near the surface of copper plates or sheets allow to draw conclusions on the r o l l i n g process and the a u x i l i a r y materials used for i t . 4.3. CARBON IN NICKEL Nickel i s a non-ferrous metal with a rather high carbon s o l u b i l i t y : at the melting point i t i s about 10 at.% (20 mg/g) and at 700°C about 0.5 at.% (1 mg/g).

Carbon contents even below 1 at.% (2 mg/g) increase

the electrical r e s i s t i v i t y of the metal in a s i g n i f i c a n t way.

The same

applies for the tensile strength, the y i e l d point and the torsion-modulus Elongation however i s decreased (1). 4.4. CARBON IN TITANIUM The s o l u b i l i t y of carbon in titanium i s s l i g h t l y below 2 at.% (5 mg/g) at 1000°C for the α phase and at 1400°C for the β phase, but decreases very rapidly with decreasing temperature. titanium carbide layers.

With graphite titanium forms

After annealing at 850°C no precipitates can

be detected by l i g h t microscopy, even i f 1 at.% (2.5 mg/g) of carbon i s present.

At carbon concentrations of 1 at.% (2.5 mg/g) the l a t t i c e

parameters increase already in a s i g n i f i c a n t way.

The hardness of the

metal, the t e n s i l e strength, the y i e l d point and the e l a s t i c i t y modulus are s i g n i f i c a n t l y increased by carbon.

The elongation and the notch

impact strength are lowered (1). 4.5. CARBON IN ZIRCONIUM The interaction between carbon and zirconium i s in some way similar to that between carbon and titanium.

The s o l u b i l i t y i s however lower in

zirconium than i t i s in titanium: in α zirconium i t i s very low, in β zirconium, even at 1000°C, i t i s only 0.1 at.% (130 Mg/g) (1).

16 4.6. CARBON IN NIOBIUM The solubility of carbon in niobium is very low: at 1200°C it amounts to 13 Mg/g and at 2000°C to 0.1 at.% (130 Mg/g).

During cooling of carbon

containing systems, precipitates of Nb 2 C and/or NbgC 2 are formed. At temperatures above 1500°C, the resistivity increases linearly with the carbon concentration.

2

Up to 200 Mg/g of rarbon the hardness increases

from 0.04 to 0.06 N/mm ; at higher carbon contents it decreases due to the precipitation of Nb 2 C.

When 0.42 at.% (540 Mg/g) of carbon is added, the

yield point increases from 0.004 to 0.012 N/mm 2 at 720°C.

The transition

temperature ductile/brittle decreases to below -100°C when the carbon content is raised from 0.3 at.% (400 Mg/g) to 1.3 at.% (1.7 mg/g) (1). 4.7. CARBON IN TANTALUM The influence of carbon on tantalum is similar to that of carbon on niobium, the solubility being even lower: 0.1 at.% (66 Mg/g) at 1400°C. The resistivity at higher temperature increases linearly with the carbon content.

As the solubility of carbon is very low, the hardness is only

influenced to a limited extent by carbon at room temperature (1). 4.8. CARBON IN MOLYBDENUM The solubility of carbon in molybdenum is 0.04 at.% (50 Mg/g) at 1500°C and 1 at.% (1.25 mg/g) at 2200°C.

When a metal containing more carbon

is cooled off, precipitates of Mo 2 C and MoC are formed.

The resistivity

increases with 3 μΩ.cm/at.% in the range below 0.1 at.% (125 Mg/g).

The

hardness is only slightly influenced by carbon contents up to 1 at.% (1.25 mg/g).

The ductility seems to be improved by carbon. In sintering

molybdenum, additions of carbon cause grain refinement.

The dependence

of the mechanical properties of molybdenum on the carbon content has not yet been clearly demonstrated, as mostly oxygen and nitrogen are present at the same time.

Precipitates of carbides, nitrides and oxides within

the grain and especially at the grain boundaries, however influence the mechanical properties considerably.

Carbon contents of 0.05 at.%

(60 Mg/g) increase the transition temperature ductile/brittle (bending test) to close to 0°C, contents of 0.1 at.% (125 Mg/g) even up to 30-50°C (1).

17

4.9. CARBON IN TUNGSTEN The s o l u b i l i t y of carbon in tungsten i s 0.05 at.% (32 pg/g) at 1600°C and 1 at.% (650 Mg/g) at 3000°C. The mechanical properties are i n f l u enced in a similar way as for molybdenum. to 0.05 at.% (32 Mg/g) of carbon.

The y i e l d point increases up

The t r a n s i t i o n temperature ductile/

b r i t t l e of tungsten polycrystals containing carbon increases much more than the one of tungsten monocrystals.

For polycrystals, about 0.1 at.%

(65 Mg/g) of carbon causes an increase to approx. 400°C, for monoc r y s t a l s only to nearly 100°C (1). When producing tungsten wires for e l e c t r i c bulbs, carbon has an essential effect due to an increasing tendency f o r ruptures. 5.

BORON

Due to the very high neutron cross section of ^ B , boron i s of particul a r interest in materials used in the core of nuclear reactors. 5.1. BORON IN ZIRCALOY AND ALUMINIUM FOR NUCLEAR PURPOSES Materials used in nuclear watercooled reactors are f i r s t of a l l , zircaloy which i s preferentially used as cladding material for nuclear fuel elements and pure aluminium which may be used for certain constructional units near the reactor core.

For both metals, at present, a boron

content of maximum 0.2 Mg/g i s tolerated, but one i s inclined to lower t h i s l i m i t to 0.1 or even to 0.05 Mg/g.

The reason why t h i s step has

not yet been taken i s that no generally accepted and reliable a n a l y t i cal techniques are available to determine these low concentrations. 5.2. BORON AS GRAIN REFINER IN AlMg-ALLOYS Aluminium-magnesium a l l o y s are grain refined by small boron additions together with titanium and seldomly with zirconium. considerably improved physical properties.

They obtain thereby

Moreover, the casting pro-

perties and surface f i n i s h i n g are also improved to a great extent. The concentrations of interest are s i g n i f i c a n t l y higher (10 to 20 Mg/g) than in nuclear reactor materials.

Accurate analysis for boron are

required to control the grain refining treatment.

18 6.

SULPHUR

6.1. SULPHUR IN COPPER Sulphur has a negative effect on the conductivity of unalloyed copper and - j u s t l i k e oxygen - causes at higher temperatures in an hydrogen atmosphere the formation of pores resulting in a decrease in strength. The latter i s also v a l i d for some copper a l l o y s .

Moreover, sulphur has

a negative effect on the corrosion resistance of copper. 6.2. SULPHUR IN NICKEL For nickel and i t s a l l o y s the dependence of the corrosion resistance on the sulphur content i s even more characteristic than i t i s for copper, especially at higher temperatures. 7.

PHOSPHORUS

Phosphorus i s technologically and economically important in aluminiumsilicon alloys.

On one hand i t regulates the mechanism of s o l i d i f i c a t i o n

of eutectic (12.5 % S i ) and nearly eutectic a l l o y s , on the other hand i t grain refines the primary s i l i c o n in the hypereutectic system (15-25 % S i ) When the eutectic or nearly eutectic aluminium-silicon alloys contain less than 5 Mg/g of phosphorus, the alloy s o l i d i f i e s into a lamellar structure.

When the phosphorus concentration i s above 9 Mg/g a globular

structure i s obtained.

In hypoeutectic a l l o y s with about 7 % of s i l i c o n ,

the s o l i d i f i c a t i o n i s only fine lamellarly at phosphorus contents between 2 ;ig/g and 4 μg/g.

When magnesium i s present, even below 2 Mg/g a globu-

lar structure i s obtained. Most of the aluminium-silicon a l l o y s are cast in a modified state whereby strength and elongation are improved.

Phosphorus consumes however modi-

fying agents (sodium or strontium): an eutectic alloy containing 10 to 15 jjg/g of phosphorus needs about 45 to 50 Mg/g of sodium.

I t i s there-

fore a matter of economical necessity to dispose of accurate methods for the determination of phosphorus in aluminium-silicon alloys (4).

19

8.

REFERENCES

(1)

Ε. Fromm and Ε. Gebhardt Gase und Kohlenstoff in Metallen, Springer-Verlag, B e r l i n (1976)

(2)

G. Kraft Eurisotop Information Booklet N° 59 (1971)

(3)

Ph. Albert Eurisotop Information Booklet N° 90 (1974)

(4)

S. Bercovoco Giesserei 67, 522 (1980)

CHAPTER II

GENERALITIES ON NUCLEAR METHODS

1.

NEUTRON ACTIVATION ANALYSIS

1.1. INTRODUCTION Upon irradiation with neutrons some of the nuclides present in the sample are transformed by nuclear reactions into radionuclides.

These decay with

a characteristic half-life under emission of radiation (α, β, γ) that can be detected.

The radioactivity produced from a given element is propor-

tional with the amount of this element. It can be shown that the activity A (desintegrations/s) at a moment t^ after the irradiation is given by: A = with:

m Ν. Φ ο θ 5 (1 Μ

-Xt e

irr

) e

-Xt c

m

mass of the element of interest (g);

Ν A Φ

Avogadro's constant;

ο

[1]

1 neutron flux (neutrons/cm .s);

2 cross-section of the considered reaction (cm );

θ

isotopic abundance of the nuclide that gives the reaction of interest;

Μ

mass of one mole of the element of interest;

λ

desintegration constant (s~*);

t. irr

irradiation time (s); decay time (s).

Eq. [1] allows, in principle, to deduce the mass of the analyte element from the induced activity.

This assumes that the neutron flux Φ is deter-

mined absolutely and that a is known with good accuracy. knowledge of the decay characteristics is required.

In addition,

In general a relative

method is applied, whereby a standard is used containing a known quantity

21 of the analyte element.

When the sample and the standard are irradiated

and measured under exactly the same conditions, m^ can be deduced from: a

mx

=

X nig — a

[ 2]

S

with: a = count rate (counts/s).

The subscripts X and S refer to the

sample and the standard. Eq. [2] assumes that the neutron flux and the detection efficiency with which the induced a c t i v i t y i s measured are the same for the sample and the standard. As a neutron source a nuclear reactor, a 14 MeV neutron generator or an isotopic neutron source can be used.

For the applications considered in

t h i s book, the neutron flux obtained with an isotopic neutron source i s in general too low. 1.2. ACTIVATION ANALYSIS WITH REACTOR NEUTRONS Activation with reactor neutrons allows the determination of a wide range of metallic and non-metallic elements.

I t can in general not be used for

the determination of traces of carbon, nitrogen and oxygen.

Among the

elements considered in t h i s book, boron can be determined by prompt methods based on the ^ Β ( η , α γ ) \ ι reaction (2), whereby the α - p a r t i c l e s or γ-rays emitted are measured during the i r r a d i a t i o n .

Phosphorus and sulphur can

be determined by reactor neutron activation analysis as described in chapters V I I I and IX. The nuclear reactor as a neutron source i s described in some detail by De Soete et a l . (3).

In a nuclear reactor, f i s s i o n neutrons slow down by

e l a s t i c c o l l i s i o n s with the moderator atoms until equilibrium i s reached with the thermal motion of the moderator atoms.

The neutron spectrum

consists of f a s t neutrons, epithermal neutrons and thermal neutrons. neutrons have an energy above 1 MeV.

Fast

For epithermal neutrons the energy

i s above a certain value depending on the convention used.

In the Hfigdahl

convention, for instance, under cadmium i r r a d i a t i o n i s applied with an energy cut-off value of 0.5 eV.

The energy of thermal neutrons i s d i s t r i -

buted according to a Maxwell-Boltzmann d i s t r i b u t i o n .

The most probable

velocity at 20°C i s 2200 m/s, corresponding to an energy of 0.025 eV. f a s t to thermal flux r a t i o s may vary between wide l i m i t s .

A reasonably

The

22 well thermalised neutron flux i s obtained in the reflector and in the socalled "thermal column", found in some reactors.

To avoid activation by

thermal neutrons the sample may be covered during the i r r a d i a t i o n by a cadmium f o i l of 0.7 - 1 mm thickness.

Cadmium has indeed a very high

absorption cross-section for thermal neutrons. neutrons induce mainly (η,τ) reactions.

Thermal and epithermal

With f a s t neutrons (η,ρ), (η,α),

1

(n,2n) and (η,η ) reactions are observed. 1.3. ACTIVATION ANALYSIS WITH 14 MeV NEUTRONS The principles of and the apparatus used for activation analysis with 14 MeV neutrons (14 MeV NAA) have been described in detail e.g. by Nargolwalla and Przybylowicz (4), by Adams et a l . (5), by De Soete et a l . (3) and by Wood (6).

An exhaustive survey of the literature up to the end

of 1972 i s given in the annotated bibliography of Van Grieken and Hoste (7). In a neutron generator, deuterons accelerated in a high vacuum to an energy of 100-600 keV bombard a tritium target made e.g. of t r i t i a t e d titanium on a copper backing. 3

Η +

takes place.

2

The fusion reaction

Η

>

4

He +

1

η +Q

This reaction i s exoergic with a Q-value of 17.6 MeV. Accord-

ing to the laws of conservation of energy and momentum, the neutron energy i s 14.9, 14.1 and 13.3 MeV for a neutron emitted under an angle of resp. 0, 90 and 180° with respect to a 150 keV deuteron beam.

In t h i s

way a nearly isotropic and monoenergetic source of neutrons i s obtained. Around the tritium target of a 14 MeV neutron generator strong flux gradients e x i s t , even for a homogeneous deuteron beam that impinges on a homogeneous tritium target.

The importance of the

flux gradients depends on the

Fig. 11—1:

Neutron flux d i s t r i b u t i o n

round a 30 rim target (10)

23 diameter of the target and of the deuteron beam (8)(9).

This was studied

in detail by Van Grieken et al. (10). Fig. 11—1 gives the results obtained by these authors for a 30 mm target. Because of the flux gradients, it is practically impossible to irradiate a stationary sample and a standard simultaneously with the same neutron flux. Therefore, e.g. the following solutions can be adopted: (a)

the sample and the standard, which have identical dimensions are irradiated simultaneously while rotating e.g. around an axis parallel to the deuteron beam, in a plane parallel to the target.

The sample and

the standard are thus exposed to the same average external neutron flux.

To assure homogeneous irradiation the samples and the standard

are also rotated around their longitudinal axis (11)(12)(13).

The

complexity of required set-up results in a larger target to sample distance and thus in a lower sensitivity; (b)

The sample and the standard are irradiated successively in the same position and the neutron flux is monitored during the irradiation in order to allow corrections for the differences in neutron output. Indeed, the neutron output for 2 successive irradiations may differ significantly as a consequence of variations of the deuteron beam intensity.

Two methods are in practice most suitable for flux

monitoring: -

measurement of the activity induced in a flux monitor irradiated simultaneously with the sample or the standard and placed close to it.

To take also variations of the neutron output during the

irradiation into account, the activity induced in the flux monitor must have the same half-life as the one of the sample and the standard.

In the case of the determination of oxygen via the reaction, the ^ N activity in an oxygen containing

flux monitor can be measured. -

direct neutron counting e.g. with a BF^ counter surrounded by some paraffin in order to thermalize the neutrons before counting.

Because of target depletion and of limited life of sealed neutron tubes, neutron generators are most suitable for short irradiations, hence for the formation of short-lives isotopes. usually pneumatic.

This implies a fast transfer system,

24

Hoste et a l . (14) described a system that allows the i r r a d i a t i o n and the subsequent measurement of a sample and a standard under s t r i c t l y constant geometrical conditions. A flux monitor i s irradiated and measured simultaneously with the sample and the standard.

A double rectangular section (internal dimensions

26.5 χ 9.5 mm) aluminium transport system i s used.

At the i r r a d i a t i o n

s i t e , the tubes are placed one after another, the sample being nearest to the target. This i s shown schematically in F i g . 11-2.

The external dimensions of the

sample, standard and flux monitor are 9 mm thickness and 26 mm diameter. Van Grieken et a l . (63) studied the effect of inaccurate sample thickness, inaccurate sample diameter, inaccurate axial positioning and inaccurate lateral positioning for different target diameters for this system.

Al-TRANSPORT

TUBE

TRITIUM TARGET

\

DEUTERON BEAM

MASSIVE SAMPLE

0XY6EIM FLUX MONITOR

/

Fig. 11-2:

Geometry at the i r r a d i a t i o n s i t e

F e , 0 , • GRAPHITE

25 2.

CHARGED PARTICLE ACTIVATION ANALYSIS

2.1. NUCLEAR REACTIONS When matter is irradiated with charged particles (protons, deuterons, helium-3, helium-4), some of the nuclides are transformed by nuclear reactions into radionuclides.

The possibility of a nuclear reaction A(a,b)B

is determined by the energy Q liberated per reaction.

Q can be deduced

from the mass difference between starting and end products by: Q = (m A + m a - m ß - m b ) c

2

[ 3]

with : m^, m a , mg and m^ = masses of the atoms A, a, Β and b. If m A , m a > mg, m^ are expressed in atomic mass units, Q (expressed in MeV) is given by: Q = 931.5 (m A +

- mß -

rab)

[4]

The compilation of Keller et al. (15) gives Q-values for a large number of nuclear reactions.

If Q is positive (exoergic reaction) the reaction

can in principle occur with particles of any energy.

If Q is negative

(endoergic reaction), the incident particle a must have a minimum kinetic energy.

This energy, the threshold energy, Ey, is given by:

Ey with:

m 'fl + m' a - — m'A

=

Q

[ 5]

m'A

=

mass of the target nucleus A;

m'

=

mass of the incident particle a.

α If the reaction is energetically possible (Q > 0 or, if Q < 0, Ε > Ε γ ) , it can take place with a high probability if the energy is sufficient to overcome the coulomb barrier between the target nucleus and the incident particle.

The Coulomb barrier energy (MeV) is approximately given by: Aa + Α Ζ Ζ E c = 1.02 — SL ii-2 [6] %

with:

ZA, Za

=

V

/ 3

* « a

1 / 3

atomic numbers of the target nucleus and of the incident particle;

A a , Afl

=

mass numbers of the target nucleus and of the incident particle.

26 It increases with the atomic numbers of the target nucleus and of the incident particle. Fig. 11-3 gives the Coulomb barrier energy for incident protons, deuterons and helium-4 as a function of the atomic number of the target nucleus. Charged particles with an energy below the Coulomb barrier energy can give a nuclear reaction via "tunneling" through the barrier. The cross-section decreases rapidly with decreasing energy. In the case that b is a charged particle, it must also have, according to classical theory, a sufficient energy to overcome the Coulomb barrier.

Again,

according to quantum mechanics, ATOMIC NUMBER

there exists a finite probability for a reaction to occur,

Fig. 11-3:

even if the particle does not

protons (p), deuterons (d) and helium-4

have sufficient energy to over-

(a) as a function of the atomic number

come the barrier.

of the target nucleus

Coulomb barrier energy for

Because of the Coulomb barrier an instrumental determination of a light element in a heavy element matrix is sometimes possible. An incident energy above the Coulomb barrier of the light element and well below the one of the heavy element matrix must be chosen. The cross-section σ for a given nuclear reaction depends on the energy of the incident particle.

The cross-section is equal to zero at the threshold

energy, increases with increasing energy, in general goes through a maximum about 10 MeV above the threshold and then decreases again as reactions with higher thresholds start to compete.

The excitation function for a nuclear

reaction gives the cross-section a as function of the energy.

Experimen-

tally determined excitation functions for a number of nuclear reactions

27

were compiled by Keller et a l . (18) and McGowan et a l . (19). More recent data are available in Nuclear Data Tables (20) and Atomic Data and Nuclear Data Tables (21). F i g . 11-4 gives, as an example, the excitation function for the 14

N (ρ,®)

C reaction, deter-

.—,

Ν [ρ.α} C

240-

X.1

Ε 200ζ ο t— 160 υ 1 in11 in 120in ο α υ eo-j < ΙΟ 40-

fl 10

mined experimentally by Jacobs et a l . (16).

12

1 1 16 18

U

Γ

20

PROTON ENERGY (MeV)

The determination of an absolu-

Fig. 11-4: Total cross-section for the 14 11 Ν(ρ,α) C reaction as a function of

te cross-section requires the

the proton energy (16).

i r r a d i a t i o n of a thin target. The incident beam i n t e n s i t i e s and the absolute number of reactions in the target must be measured.

I t i s usually simpler and s t i l l of considerable

interest to determine the relative cross-sections of a reaction at d i f f e r ent energies.

Frequent use i s made of the method of "stacked f o i l s " ,

whereby several target f o i l s containing the element of interest (e.g. mica or mylar for oxygen), with or without energy degrading f o i l s are irradiated in the same beam.

interposed,

After the i r r a d i a t i o n the a c t i v i t y in

each f o i l i s measured. Fig. 11-5 and 11-6 give the r e l a t i v e excitation functions for the 0( 3 He,nx) 1 8 F and 4

18

0( He,nx)

F reactions deter-

mined experimentally by Vandecasteele et a l . (17).

The

experimental data were obtained using stacks of mica and mylar foils.

The range-energy rela-

tions for mica, mylar and

15

calculate the energy scale and to obtain the "activation curve" in s i l i c o n .

13

II

9

ENERGY (MeV)

s i l i c o n (see 2.2) were used to Fig. 11-5:

Relative excitation function 3

for the 0( He,nx) 1 8 F reactions (17)

28

Curves that give the thick target y i e l d , i . e . the a c t i v i t y produced by a given nuclear reaction in a given thick target, as a function of the energy are sometimes also useful.

When i t

i s d i f f i c u l t to obtain thin targets of the elements of interest, these curves are more e a s i l y determined experimentally than the excitation functions. ENERGY (MeV)

2.2. STOPPING POWER AND RANGE

Fig. 11-6:

Relative excitation function 4

for the 0( He,nx) 1 8 F reactions (17)

Charged p a r t i c l e s interact strongly with the electrons of

the irradiated material, unlike neutrons of gamma photons.

The particles

lose rapidly their energy, so that they do not deeply penetrate the sample. This has several important consequences: -

the analysed part of the sample i s a thin layer beneath the surface;

-

whereas for neutrons the cross-section for a given reaction may be considered constant over the entire sample, for charged particles

it

varies with the depth in the sample. The greatest part of the energy l o s s of medium energy particles occurs by c o l l i s i o n with electrons, energy loss to nuclei being very small for all energies of interest here.

For hydrogen projectiles i t amounts to only

1-2 % at 10 keV and increases in relative importance with decreasing energy.

The stopping power i s defined by: S(E) = - — (E) dx

[7]

dx

[8]

= ρ .dl

with: Ε = particle energy; ρ = density; dl = thickness corresponding to the energy variation dE. 2

2

The stopping power may be expressed in J.m /kg. However MeV.cm /g i s most often used as a practical unit.

29 Fig. 11-7 gives the stopping power for protons, deuterons and

1

2000

'

1 ' ' Ί

helium-4 of carbon and tungsten 1000

as a function of energy (22)(23).

a/c^v

:

The range (pathlength) at the energy Ej i s defined by:

5 Σ

R(Ej) = / 0

E, Ί

:

500

dE

[9]

- dE/dx(E)

I t corresponds to the average depth of penetration of charged particles of energy Ej.

Because

IT Ol

200

\ ^s.

Ϊ ε ιοο ο

—

-

α.

ο

SO

ν

-

10

0,5

. . ..1

1

distributed around the average value (range s t r a g g l i n g ) .

A

sample thicker than the range i s called a "thick sample". Fig. 11-8 gives the range for

, , 1 , . ,,

2

5

10

20

ENERGY (MeV)

tion of the particles of a monoenergetic group i s s t a t i s t i c a l l y

-

20

of the discrete nature of the energy l o s s , the depth of penetra-

a/w*-\

F i g . I1-7:

Stopping power for protons,

deuterons and helium-4 of carbon and tungsten as a function of the energy (22)(23) 200

protons, deuterons and helium-4 in carbon and tungsten as a function of the energy. Williamson et a l . (24) compiled the ranges and stopping powers for protons, deuterons, t r i tons, helium-3 and helium-4 in 37 elements for energies from 0.05 to 500 MeV.

Only a crude indication

of the accuracy i s given.

It is

stated that, for energies above 1 2

1 MeV per nucleon and for targets

5

10

E N E R G Y (MeV)

with an atomic number below 50, the values appear to agree with

Fig. 11-8:

published values to within 5 %

and helium-4 in carbon and tungsten as

and often much better.

a function of the energy (22)(23)

For lower

Range of protons, deuterons

30

energies and/or for atomic numbers above 50 deviations as high as 10 % can occur.

Andersen and Ziegler (22) and Ziegler (23) give formulae that allow

the calculation of the stopping power of hydrogen and helium ions in all elements using tabulated coefficients.

In the high energy region the Bethe

stopping-power formula was used as the theoretical 4 2 4ir e ΖΓΝΖ. S(E) = (E) = 1—£ dx me ν ρ ,F

with:

2 mν In — J

basis

2

+ In

1 1 - β

^ - Γ

9

Cc - -zA

[10]

S(E)

=

stopping power,

e

=

electron charge,

Z fl ,Z^

=

atomic number of projectile and target,

Ν

=

number of atoms per unit volume,

m e ν

=

electron mass,

=

projectile v e l o c i t y ,

Ρ

=

density,

J

=

mean excitation potential,

β

=

projectile velocity divided by the velocity of l i g h t ,

C^/Z^

=

shell correction.

For hydrogen, stopping power data for 27 elements over a s u f f i c i e n t l y broad range of the high energy region were considered to construct the f i n a l fits.

For these elements, appropriate power series were f i t t e d to the

shel1-corrections deduced from the experimental data.

For other elements

shell corrections were obtained by linear interpolation in Z^ and similar power series were f i t t e d to these values.

The low and medium energy region

(energy below 0.6 MeV) i s less important for activation a n a l y s i s .

In t h i s

region appropriate semi-empirical interpolation formulae were used to f i t the experimental data. The data of Andersen and Ziegler (22) and of Ziegler (23) may be considered as the best data available based on the most recent experimental and theoretical work.

For hydrogen p a r t i c l e s , the accuracy can be estimated better

than 1 % at high energies, about 5 % for energies near 0.5 MeV and 10-20 % for energies around 0.01 MeV. For mixtures and compounds, the stopping power can be calculated by the Bragg-Kleeman formula: S(E) =

Σ ω . S. (Ε) 1 i =l 1

[11]

31 where: S(E)

=

stopping power of a mixture or a compound with η components 2

(MeV.cm /g), Si(E) = ω.

stopping power of the i

th

component, L weight fraction of the i component.

=

j.

When the stopping power as a function of the energy i s known for a pure element, a mixture or a compound, Eq. [9] can be used to calculate the range (pathlength) by numerical

integration.

2 The range (g/cm ) for a compound or a mixture can be deduced also from the ranges R^ in the η components with weight fractions ω.. by: 1 — R

η Σ i=l

=

ω. —R.

[12]

2.3. STANDARDIZATION 2.3.1.

Introductory considerations

Standardization for charged particle activation analysis i s more complicated than for neutron and photon activation a n a l y s i s .

This i s a r e s u l t

of the strong interaction of charged particles with matter, resulting in limited ranges of these p a r t i c l e s . When a thin sample i s irradiated with a beam of charged particles of energy Ej, perpendicular to the surface, the increase per unit of time of the number of radioactive product nuclides Β of the reaction A(a,b)B at time t during the i r r a d i a t i o n i s given by: dN* dt with:

=

n A i σ dx - λ Ν*

[13]

Ν*

=

number of radioactive product nuclides Β at t,

n^

=

number of nuclides A per g,

i

=

beam i n t e n s i t y , number of particles a that impinge per s on the target,

a

=

dx

=

2

cross-section of the reaction A(a,b)B (cm ), 3 p.dl with ρ = density (g/cm ); dl = thickness of the target (cm),

λ

=

decay constant of the radionuclide produced ( s _ 1 ) = ln2/tl/2.

32

I t follows that the a c t i v i t y A (desintegrations/s) after an i r r a d i a t i o n time t. i s 3given by: J ι rr A

=

n A i S a dx

-xt. irr n A 1 (1 - e ) adx

=

[14]

-Xt. with:

S

=

saturation factor = 1 - e

, on the condition that the

beam intensity i s a constant. The cross section i s a function of the energy and changes with the depth χ in the sample. A

For a "thick" sample, the a c t i v i t y i s given by:

=

n.iS

Ι 0

A

R

ο (x)dx

[15]

Since c Ν. -

nA with:

c

=

10" 6

β

[ 16]

Μ

concentration (μς/ς),

N^ =

Avogadro's constant,

θ

=

natural isotopic abundance of the nuclide of interest A,

Μ

=

mass of one mole of the element to be determined,

the concentration of the analyte element i s given by: A = — iS

c

Μ 106

1 / 0

a(x)dx

[17]

Ν. θ A

When a sample (X) and a standard (S) are irradiated separately with particles of the same incident energy and the induced a c t i v i t i e s are measured with the same efficiency, R x

s

"

ax

i s Ss

J 0

a

/ 0

^ with:

s

ζ

S

°(x)dx 0(x)dx

a = count rate (counts/s).

a/S corresponds to the count rate at the end of the i r r a d i a t i o n for an i n f i n i t e i r r a d i a t i o n time and i s given by a/S

=

C X (1 - e

—λ t

m

1 —Xt· -1 H i irr ) " i (1 - e )"1 e d

[19]

33 with:

C

=

measured number of counts,

t = m t.jrr =

measuring3 time,

tj

waiting time between the end of the irradiation and the

=

irradiation time, start of the measurement.

To calculate the concentration of the element of interest, -

the concentration in the standard (c^);

-

a/S for the standard and the sample;

-

the ratio of the beam intensities (i) for the standard and the sample;

-

the correction factor for the different stopping powers;

must be known. The correction factor for the different stopping powers of the sample and the standard can also be written as:

F

/RS„(x)dx

f°

0 -u

E T [ dE/dx(E)] ς Ε, -- = -4

=

R

/ X„(x)dx 0

since

°(E)dE

E

dE

°< ) Ej [dE/dx(E)] x

/

E

T __o(jQdE_

/ Ej

[dE/dx(E)U -E

[ 20]

dE

°( ) [dE/dx(E)] x

σ = 0 for Ε < E-j-.

The factor F can be calculated in a straightforward way on condition that (a)

the cross section as a function of the energy (the excitation function) is known, at least in a relative way;

(b)

the stopping power as a function of the energy for the standard and the sample are known.

The second condition is fulfilled in practice as discussed sub 2.2. first condition is not fulfilled for all useful reactions.

The

Therefore

various approximate methods (Table 11-1) have been proposed for the calculation of F without any knowledge of the excitation function. 2.3.2.

Methods not requiring knowledge of the excitation function

Vandecasteele and Strijckmans (30) show the validity of Eqs. [ 21]-[ 24] as follows.

34

Table 11—1:

Methods for standardization in charged particle activation

Methods not requiring knowledge of σ(Ε) Ricci 1 (25)

F = ^

.

Ricci 2 (26)

F =

R

S

ίro

Cr

Ο

ΣΙ Ί-

αι C Φ

>

ν

+J φ c ε: Ο )— -σ •ιΟ C ι ι

Ο 1—I

ΣΙ

L T )

Γ0

•

*—.

C O

X3

C M

Ε

ι — I

σι

II

II

•

II

-—»

-σ > r —φ Οy χ: — ' Ι Λ αι SJZ ι-

C O « — 1

C M

£=

αι

ΙΧΙ

co • IX)

Ο

,—,

1—1 Έ:

ο •Ι— -1-1 υ to Φ

CC

>1 CN sΦ C φ iΦ •rSSro JD

>> en ίαι

Χ < ο Ε:

αι Ι -Ο ro I —

ι/) ro 3

LLJ -—^

>>

τ — 1

Γ Ο

C O

Ο

•

*

C _ >

Ο

ο

Λ

Λ

Λ

ο·

Cr

Cr

LLJ • *

13 Ο

•

ο L u l •« ι ο

•

o

J—

ο UJ

> «1

•

Ο CM

c R· C L

'

•Z.

«DT—1

ο

ι—Ι τ—i

_—^ D

*

Q.

—-Ζ

R-H •

Ι—1

ζ Γ Ο ι — I

— . .

c Λ "Ό •—.»

Ο

CM

-

,—.

ι — I

C Λ TJ —

ζ:

1—I

Τ—1

CM

ο

LT)

•

co

ο ι — I ι — I

, . Ö Φ

C O (_)

CM

F-H •

•

LN

'

L i _ 00 Γ—Ι ,—S Ο. Η Φ 3: co Ν * ο νο Γ-Η •

U3

φ -C +-> 10

ε U

I Ι I •—Η

-σ Φ ιο

- —

_Ε

C•M·

u r — r0 υ φ -C +J

φ C φ η —

S< 0 ΑΙ ο 3 C

c ο

•Γ-

Ε

(Λ c ο • Γ—

IT)

Ll_ 00 » — 1

.— C CL d

ο to Ι—1 •

r·^

c ο • r — +J υ rö >1 Φ cn i- iΦ o tz •r- φ C D 1- T3 Φ ίΟ ο X -c φ Ι Λ φ ί. ίΟ -C L I - +->

,—^

38 the maximum possible error is larger, so that more complex standardization methods are required. Eq. [28] can also be used to deduce Ricci1 s method (Eq. [21] and [22]). Indeed R(E T ) - R(E T ) = 1

1

/ 1 ET

^ - dE/dx(E)

[33]

Assuming that K(E) is a constant Κ for Ε γ < Ε < Ej, it follows that E, j "I

dE

R s (Ej) - R S (E T )

ET

[dE/dx(E)]

R x (Ej) - R X (E T )

/i

dE

ET

[dE/dx(E)]

Κ

[ 34]

X

Using Eq. [29], one obtains: F

=

Rs

(Ei) -

Rs

( Ε τ>

"

Rx

(Ei) -

Rx

(M

[35]

Eq. [22] can be obtained in a similar way, K(E) must however be assumed constant in the 0 to Ej energy interval, instead of the Εγ to Ej energy interval.

As appears from Figs. 11-9 to 11-12 the former assumption is

less justified than the latter, so that Eq. [21] can be expected, a priori, to yield more accurate results than Eq. [22]. 2.3.3.

Comparison with the numerical integration method

Vandecasteele and Strijckmans (30) calculated F according to Eq. [27] by numerical integration using excitation function data from the literature and stopping power data from Ref. (24). The results were compared to those obtained by Eqs. [21], [22] and [23].

Table 11-3 summarizes some data on

the nuclear reactions studied and on the standards used. Figs. 11-13 to 11-16 summarize this intercomparison.

The percent difference between the

F-value from Eq. [27] and that obtained via Eqs. [21], [22] and [23] is plotted versus Ζ of the sample for different energies and a given standard. From the intercomparison several conclusions can be drawn: -

Eqs. [21] and [23] yield practically the same errors.

These are smaller

than for Eq. [22], especially for high threshold reactions;

39 Table 11-3:

Data on the nuclear reactions studied

Threshold (MeV)

Reaction

Standard

Reference for excitation ^ function

1.

14

N(p,n) 1 4 0

6.3

Nylon 6

(38)

(45)

2.

I VΝ(ρ,α) x " Cr

3.1

Nylon 6

(16)

(57)

3.

12

C(d,n) 1 3 N

0.3

Graphite

(58)

(59)

4.

14

N(d,n) 1 5 0

Q> 0

Nylon 6

(60)

(61)

5.

12

C(3He,a)UC

Q> 0

Polyethylene

(62)

6.

16

0( 3 He,p) 1 8 F

Q> 0

Quartz

(17)

7.

16

0(a,pn)18F

20.4

Quartz

(17)

*

The f i r s t reference was used for the comparison of the numerical integration method with the results obtained using Eqs. [ 2 1 ] , [22] and [ 23]

Al

V

Zr

Pb

Al

V

Zr

Pb

ATOMIC NUMBER

Fig. 11-13:

Percent difference between the F-val ue from Eq. [ 27] and from

Eq. [21]: (o), Eq. [22]: (·) and Eq. [23]: (χ) for different incident 14 14 /"Λ energies. The standard i s nylon 6 for the N(p,n) 0 reaction and the

14

N ( p , a ) U C reaction (z)

40

Al

V

Zr

Pb

Al

ATOMIC

Fig. 11-14:

V

Zr

Pb

NUMBER

Percent difference between the F-value from Eq. [27] and from

Eq. [21]: (ο), Eq. [22]: (·) and Eq. [23]: (x) for different incident 12 13 energies. Standards are graphite for the C(d,n) Ν reaction and nylon 6 for the Al

V

14

N(d,n) 1 5 0 reaction ( ? )

Zr

Pb

ATOMIC

Fig. 11-15:

Al

V

Zr

Pb

NUMBER

Percent difference between the F-value from Eq. [27] and from

Eq. [21]: (o), Eq. [22]: (·) and Eq. [ 2 3 ] : (χ) for different incident 12 3 11 energies. Standards are polyethylene for the C( He,a) C reaction (5J 16 3 18 and quartz for the 0( He,p) F reaction (6y

-

when Eq. [23] i s used, the error i s s i g n i f i c a n t l y smaller

Al V

than the maximum possible

Zr

Pb

error given in Table 11-2; -

when Eq. [21] or [23] i s used, the error increases with decreasing threshold energy;

-

the error increases with Ζ for the sample, at least when a low Ζ standard i s used;

-

the error decreases as the incident energy increases, as long as the cross-section at the incident energy remains significant.

This can be

Fig. 11-16: Percent difference between

expected since the assumption

the F-val ue from Eq. [ 27] and from

K(E) = constant i s a better

Eq. [21] : (o), Eq. [22] : (·) and

approximation at higher energy; -

in a l l cases a negative error 3 i s made.

2.3.4.

Eq. [23]: (x) for different incident Ienergies. The standard i s quartz for the

16

0(a,pn) 1 8 F reaction

0

The average stopping power

The standardization methods that do not require knowledge of excitation function do not always y i e l d accurate results

(2.3.3.).

I s h i i et a l . (29) therefore developed the "average stopping power method", the average stopping power < — > being defined by dx J° Εj

dE/dx(E)

=

- i —

f°

Ej

o(

E)dE

dx A so-called average energy Ε

may be defined by

[36]

42

so that , dE/dx(E )

=

;0

σ(E)dE

ET -Π /U

dE/dx(E)

E

[38]

σ(E)dE

I

Using Bethe's formula - Eq. [10] - in the n o n - r e l a t i v i s t i c form and series expansion of the logaritmic term, one can show that to a good approximation Ε i s 3given by: J m f E

m

Zl1

Ε σ(E)dE

= τ : / 1 0

[39]

a(E)dE

Calculation of Em with t h i s equation y i e l d s Em values very close to those obtained with equation [38] . The factor F in equation [ 20] i s given by: F

=

[dE/dx (E )] X — — [dE/dx ( E m ) ] s

[ 40]

I s h i i et a l . (29) give also formulae allowing to estimate the errors in t h i s approximation. act One can also make use of Em (31) which may be calculated by:

Eact =

-

m

1 + with:

Ε, -

L

-

Y(Ej)

/ 0

[41] Ε, 1

Y (E)dE 7

Ε

Ej

=

incident energy;

Y(E)

=

thick target y i e l d at energy E.

act The value of Ε i s very close to that of Ε111 , so that i t can be used m act instead of Ε . An advantage i s that Ε can be deduced from data of the m m thick target y i e l d as a function of the energy. These data are often more easy to determine with good accuracy than the excitation function.

43 2.3.5.

Influence of stopping power data and e x c i t a t i o n f u n c t i o n data on the accuracy

2.3.5.1.

§togging_gower_data

I t i s c l e a r from Eq. [20] that the accuracy of the stopping power data has d i r e c t r e p e r c u s s i o n s on the f i n a l

result.

For pure element samples and

standards o n l y the accuracy of the stopping power data f o r pure elements, as d i s c u s s e d under 2 . 2 . , must be considered. For chemical compounds, e . g . some of the standards used in charged p a r t i c l e a c t i v a t i o n a n a l y s i s , p o s s i b l e d e v i a t i o n s from B r a g g ' s Rule - Eq. [11]

-

due to the i n f l u e n c e of chemical binding on the stopping power must a l s o be considered.

Several authors have reported experimental t e s t s of the

v a l i d i t y of t h i s r u l e .

The experimental values can be expected to f a l l

below those predicted by B r a g g ' s Rule since the outer e l e c t r o n s would be more t i g h t l y bound i n the compound and therefore play a l e s s e r r o l e in the stopping of i n c i d e n t ions (32). Moreover the d i f f e r e n c e between c a l c u l a t e d and experimental values expected to decrease with i n c r e a s i n g energy.

is

Langley and Biewer (32)

reported experimental stopping powers that were resp. 0-9 % and 4-13 % lower than those predicted by B r a g g ' s Rule f o r 2.5-0.5 MeV protons and 2 . 5 - 0 . 5 MeV a l p h a - p a r t i c l e s . Blondiaux et a l . ( 3 3 ) ( 3 4 ) ( 3 5 ) ( 3 6 ) studied the influence of p o s s i b l e chemical e f f e c t s on the stopping power o f A ^ O ^ , TiO^, ZnO, f ^ O ^ and Ta^Og f o r 2 . 3 - 2 . 7 MeV protons and of BeO f o r 1.9-2.2 MeV α - p a r t i c l e s . tal technique was as f o l l o w s .

The experimen-

The prompt γ - r a y s produced from the metal of

i n t e r e s t upon i r r a d i a t i o n of a t h i c k t a r g e t of a metal oxide (e.g.

A l )

and of the corresponding metal ( A l ) with protons or α - p a r t i c l e s are detect27 27 ed. In the case of aluminium the 842 keV γ - r a y s produced by Al(p,p'7) Al are used.

The r a t i o o f the 2 t h i c k target y i e l d s i s c a l c u l a t e d .

In the

"average stopping power" formalism . Sjij

. a2

fl c2

according to Eqs. [18] and [38] .

.

[dE/dx(E

m)]2

[dE/dx(Em)]1

[42]

44 The subscripts 1 and 2 indicate the pure metal (e.g. aluminium) and the oxide (e.g. alumina).

The ratio of the stopping powers for a metal/metal-

oxide pair were compared to those predicted by Bragg's Rule combined with the data of Andersen and Ziegler (22) and of Ziegler (23). The experimental and calculated values for 2.3-2.7 MeV protons agree a l l within 1.1 %, i n d i cating that for these energies chemical effects on the stopping power are negligible. 2.1 to 8.8 %.

For 2.0-2.8 MeV α particles on BeO the deviations range from I t i s however not sure that the deviations are due to a

chemical effect. Baglin and Ziegler (37) have tested Bragg's Rule for 2 MeV helium ions in a number of compounds.

Within an experimental uncertainty of c i r c . 2 %

agreement i s reported for RhSi, HfSi2, SiC^, A ^ O g , SigN^, A1N, l^Ng, SiC, melamine and adenine. Vandecasteele and Strijckmans (30) showed that Bragg's Rule can be used to calculate the stopping power of nylon for 2.9 MeV deuterons. As a general conclusion Bragg's Rule can be used for accurate activation analysis. 2.3.5.2.

Excitation_function_data

Excitation function data can be subjected to several kinds of errors.

In

addition to random errors e.g. due to counting s t a t i s t i c s , systematic errors may occur due to: -

inaccurate detector efficiency or beam intensity c a l i b r a t i o n ;

-

interfering nuclear reactions;

-

errors on the incident energy;

-

inaccurate target thickness measurement.

When the "stacked f o i l " technique i s used, inaccurate target thickness measurements can r e s u l t in errors on the incident energy for each f o i l . The influence of inaccurate excitation function data on the f i n a l

result

can be calculated i f the excitation function i s known. Vandecasteele and 1 fi 3 ιQ Strijckmans (30) deduced from the excitation function for the 0( He,p) F reaction (17) the curves that would be obtained i f some of these sources of error would occur. Curve 1 in Fig. 11-17 i s the excitation function from Ref. (17). To simulate the influence of random errors the experimental points were alternately shifted over resp. 10, 20 or 30 % to higher and

45 lower cross section values (curves 2, 3 and 4 not shown in Fig. 11-17). Curves 5 and 6 give excitation functions determined by the stacked f o i l technique. Curve 5

ζ ο

corresponds to the experimental situation where i t i s assumed that the incident energy i s

ΙΛ Ο OL 0 on σ (Ε)

3

0.00

+ 0.01

+ 0.02

+ 0.02

+ 20 ί on σ (Ε)

4

0.00

+ 0.01

+ 0.03

+ 0.05

+ 30 1 on α (Ε)

5

+ 0.27

+ 1.24

+ 2.50

+ 4.30

+ 10 ί on Ε j

6

+ 0.14

+ 0.65

+ 1.31

+ 2.20

-

Table 11-5:

Zr

Pb

Error (see text)

10 ί on thickness

Comparison of F-values obtained from two different literature data (standard and literature references are mentioned in Table 11-3)(30)

Reaction

Difference

%

ET (MeV)

I (MeV)

Al

V

Zr

Pb

E

1.

14

Ν(ρ,η) 1 4 0

6.3

10

0.06

0.10

0.16

0.27

2.

14

N(p,a)UC

3.1

9

0.08

0.12

0.20

0.33

3.

12

C(d,n) 1 3 N

0.3

3

0.52

0.91

1.29

1.59

4.

14

N(d,n) 1 5 0

Q > 0

3

2.10

3.35

4.61

5.20

The fact that the exact shape of the excitation function has only a small influence on F is a consequence of K(E) being approximately constant and could be expected since, as shown in 2.3.3., reasonably accurate results can be obtained without any knowledge of the excitation function. Using the "average stopping power" formalism, Ishii et al. (29) arrived at the same conclusion.

They showed that errors on the excitation function

have a small influence on the average energy Ε . error on E„ m

In addition,for a small

the relative error on F is even smaller,

47 2.3.6.

Conclusion

When data on the excitation function of the reaction of interest are available, it is in general recommended to use the "numerical integration method" - Eq. [27] - or the "average stopping power method" - Eq. [ 26].

When these

data are not available, the approximate methods that do not require knowledge of the excitation function may be used.

As appears from 2.3.3. these

methods yield a negligible systematic error for high threshold reactions and for low Ζ matrices.

For low threshold reactions and for high Ζ matri-

ces the systematic errors may be high, so that it may be necessary to determine the excitation function experimentally. The basic equation of charged particle activation analysis - Eq. [18] assumes that the activity of the sample and the standard correspond to the same incident energy.

Because

the sample is in general chemically etched after the irradiation and this etch is not completely reproducible, this is difficult to realize in practice.

Fig. 11-18 illustrates

the procedure used for the determination of nitrogen in nickel via the reaction.

14

N(p,a)UC

Three series of

standards (nylon discs) are irradiated, covered with a copper beam intensity monitor foil and resp. 3, 4 and 5 aluminium foils. The incident energy is resp. 14.05, 13.93 and 13.80 MeV.

This yields

a/iS as a function of the inci-

r^NQAPc, Immmmmm mmmmmmm mmmmw//mm

r ™ » » »

dent energy in the 14.05-13.80 energy range.

The thickness to

be removed from a nickel sample must range ?from 16.8 to 28.1 mg/cm to yield an inci-

Ι

ϋ

ϋ

ETCH

Fig. 11-18:

R ^ i

Al

Cu

Calculation of the acti-

vity of the standard for a given thickness removed from the sample

48 dent energy between 14.05 and 13.80 MeV.

For an analysis, the actual

incident energy is calculated and the corresponding a/iS of the standard is deduced by interpolation.

In practice, one can also plot a/iS of the

standard as a function of the etched thickness for the sample. used in Eq. [15] can be obtained by direct interpolation.

a/iS to be

Of course, a

curve such as the one in Fig. 11-18 can also be obtained by varying the incident energy from the accelerator.

The latter approach can be applied

when the beam intensity is determined by current measurement, but not when a beam intensity monitor foil is used. 2.4

SAMPLES AND STANDARDS

Samples for charged particle activation analysis are usually thicker than the range (thick samples).

Cylindrical metal discs of 15-20 mm diameter

and 1-2 mm thick are most often used.

The preparation of such samples is

described in Chapter III. As standards materials with: -

sufficient purity;

-

known concentration of the element of interest;

-

good stability under irradiation;

are required. standards.

Table 11-6 gives some materials that proved suitable as

These materials are preferentially used in the form of discs

or pellets of similar dimensions as the samples.

Powders are however

also sometimes used. Table 11-6:

Some standards for charged particle activation analysis

Element of interest Boron

Standard Pure boron (disc) Boric acid (pellet)

Carbon

Graphite (pellet or disc) Polyethylene (disc)

Nitrogen

Nylon (disc)

Oxygen

Quartz (disc)

Phosphorus

Di sodi umhydrogenphosphate (powder)

Sulphur

Sodium Sulphate (pellet)

49 2.5. IRRADIATION 2.5.1.

Particle accelerator

Charged particles of the energies required for activation analysis can be obtained with a cyclotron or a Van de Graaff accelerator.

Most of the

applications described in this book can be carried out with a so-called compact cyclotron.

An example is the CGR-MeV 520 machine, which allows

to accelerate 2.5-24 MeV protons, 3-14.5 MeV deuterons, 6-32 MeV helium-3 and 6-29 MeV helium-4.

Maximum beam intensities range from 10 to 100 μΑ,

depending on the energy. A small or medium-sized Van de Graaff allows also a large number of the applications described.

Such a machine is the preferred-accelerator, when

low energy (1-5 MeV) particles are needed. energies are known with good accuracy.

It has the advantage that the

Higher energies can be obtained

with a tandem Van de Graaff accelerator. 2.5.2.

Irradiation facility

The samples and the standards are in general irradiated in vacuum, placed in an appropriate sample holder, to be mounted on the accelerator beam transport system.

A simple sample holder is shown in Fig. 11-19.

It is

watercooled and an aluminium tube is placed on the sample holder to minimize the escape of secondary electrons, which may result in inaccurate beam intensity measurements.

Before the sample holder a collimator is

placed as shown in Fig. 11-21b. Powdered samples or standards are irradiated in appropriate sample holders such as the one described by Mortier et al. (95). When short-lived radionuclides are used, it is important that the sample is available as soon as possible after the irradiation.

Moreover the radia-

tion level at the target station directly after the irradiation may be very high, so that entering the shielded target area and manual removal of the sample holder from the beam transport system must be avoided. Therefore a pneumatic transfer system such as the one described by Strijckmans (39) can be used.

This system allows samples mounted on a copper rabbit to be send

from the sending station to the irradiation station situated in a room shielded with thick concrete walls.

50

Fig. 11-19:

Sample holder

Fig. 11-20 illustrates the operation of the system and shows the rabbit. Initially all electro-pneumatic valves (V) are closed and the turbine and the pump are off.

The following sequence is then carried out under control

of a microprocessor: I

1.

the turbine is on and valves V^ and V^ are opened;

2.

the rabbit leaves the sending station and is detected by the optical sensors 2 and 1;I

3.

valves V^ and V^ are closed and the turbine off;

4.

the vacuumpump is on and valve V 3 is opened;

5.

the vacuum is measured with a thermo-couple probe.

51

TURBINE

1

2 *

ι ι ι

ιιι

Ο

5

ι

ιι

3 i 1

5 Η

ι ι 10 ( c m )

OPTICAL SENSOR 2

VACUUM SENDING AND ARRIVAL STATION

IRRADIATION STATION

1. rings for transport and thermal contact with the cooling mantle; 2. 0-ring; 3. sample; 4. b e a m intensity monitor foil; 5. ring with screw thread also serving as a diaphragm. For the other symbols: see text.

Fig. 11-20: Pneumatic transportsystem and section of the rabbit (brass) As soon as the pressure is below 5 Pa(5.10

_5

bar), V, is closed, the pump -2

is off and V^ is opened.

As soon as the vacuum is better than 10

Pa,

the irradiation can be started by lifting the Faraday cup that stopped the beam.

The rabbit is in contact via 2 brass rings with a brass cooling

mantle cooled with deionised water.

The cooling mantle is electrically

insulated and connected to a μΑ-meter situated in the control room.

In

front of the sample an insulated ring-shaped electrode and an interchangeable collimator are placed.

The electrode is at a negative potential

(- 300 V) to suppress emission of secondary electrons.

At the end of the

preset irradiation time, the irradiation is stopped by lowering the Faraday cup.

The following sequence is then carried out under microprocessor

control:

52 1.

valve V^ is closed, the turbine is on and valves V^ and

2.

the rabbit is sent back and passes the sensors 1 and 2;

3.

valves V 2 and

are opened;

are closed and the turbine is off.

The sample is available about 10 s after the end of the irradiation and is removed from the rabbit behind a lead wall. 2.5.3.

Determination of the beam intensity

As appears from Eq. [ 18], the beam intensity for the sample and the standard or at least their ratio is required.

In general the beam inten-

sity is kept constant during the irradiation. The beam intensity can be determined directly by measuring the current incident on the sample and the standard.

The following relation exists

between the incident current I(MA) and the beam intensity i(particles/s): i where:

ζ

=

=

6.242 10 12 I/z

[43]

charge of the incident particle expressed as the number of elementary charges.

The sample holder must be insulated with respect to the experimental setup at ground potential.

For the cooling of the sample holder a cooling

liquid with a sufficiently high specific resistance must be used e.g. deionised water.

In front of the sample holder a collimator is placed

to avoid beam falling in on the sample holder instead of on the sample. Emission of secondary electrons can also result in an inaccurate measurement of the beam intensity.

Therefore, an insulated ring-shaped elec-

trode at a negative potential (- 300 V) is placed between the sample holder and the collimator, or a metal tube is placed on the sample holder, so that escape of secondary electrons is only possible via a small solid angle.

This is illustrated in Fig. 11-21.

The current can be measured

with a digital μΑ-meter and recorded with a x,t-recorder or integrated with a digital current integrator such as e.g. ORTEC model 439. Direct measurement of the current has the following disadvantages: -

Inaccurate beam intensity determinations may result from the emission of secondary electrons or from an insufficient vacuum;

-

At low beam intensities (< 10 nA as used for some standards) the determination of the current is difficult and may easily be disturbed.

53

F i g . 11-21:

Determination of the beam intensity by current measurement

An indirect method for the determination of the beam intensity makes use of a beam intensity monitor f o i l .

During the i r r a d i a t i o n a thin metal f o i l

i s placed before the sample and the standard. f o i l i s a measure for the beam intensity. (X) are both covered with a monitor f o i l

The induced a c t i v i t y in the

When a standard (S) and a sample (M) with thickness D(D 0

Εγ

l/2

(keV)

d

1115

min

670;962

3.41 h

283;656

5.5 Q > 0

43.0 min

390

Cu

65

Cu(3He,2n)66Ga

5.0

9.4 h

1039

Cu

63

Cu(a5n)66Ga

8.0

9.4 h

1039

15.0

78.1 h

65

Cu(a,2n)

67

Ga

300

55 2.6. INFLUENCE OF RADIONUCLIDES FORMED AT THE SURFACE-REMOVAL OF A SURFACE LAYER AFTER THE IRRADIATION The possibility of eliminating surface contamination by etching after irradiation is one of the major advantages of activation analysis.

In charged

particle activation analysis, a surface layer is in general removed from the sample after the irradiation to remove radionuclides formed from the element of interest at the sample surface that recoil into the sample. At the same time, radionuclides formed in the beam intensity monitor foil that recoil into the sample are removed and the influence of contamination before the irradiation is overcome. 2.6.1.

Nuclear recoil

The thickness of the surface layer that must be removed is determined by the maximum range of the recoil nuclei. The energy of the recoil nucleus for a given nuclear reaction can be calculated as a function of the angle of emission by means of the formulae given by Marion and Young (40).

For the nuclear reaction illustrated in

Fig. 11-22, the energy in the laboratory system for the heavy product is given by: Eg with:

= (E a + Q)a [cos f + (y/a m

B[

m

+

? Γ

f)

+

(m; + m^)

(m;

ι/?

?

- sin

ÄH

[46]

[47]

+ m

bD

ηΐβ) (η·

Κ

(1 + +

m ^

Q

-

) +

[ 48]

Q)

Only the plus sign is used, unless a > γ, in which case f

max = s i n - V / a ) 1 / 2

As an example, Fig. 11-23 gives the recoil energy for 1 ft ιß 0( He,p)

F reaction.

[49]

18

F from the

For an incident energy of 18 MeV the maximum ener-

gy in the forward direction is 6.7 MeV. The stopping power for the heavy ion of interest of different target elements can be obtained from the compilation of Ziegler (41). The ranges can be deduced using Eq. [9] . Ranges of heavy ions can also be18 obtained directly from the tables of Northcliffe and Schilling (42). For

F ions of 6.7 MeV, the range is circa 4.5 Mm in

aluminium, 2.3 μπι in nickel and 2.8 μπι in tantalum.

Since the tables of

56

Fig. 11-22: Kinematics of nuclear reactions

Fig. 11-23: 18

Recoil energy for

F formed by the

16

0(3He,p)18F

reaction as a function of the 3 angle of emission f, for He particles of different energies (64). In this case a > y, so that for each angle up to •ι Q

ΠΊαΧ

F nuclei of 2 different energies are observed, corresponding to the plus and minus signs in Eq. [46]

C ( LAB)

Northcliffe and Schilling are not very accurate below 1 MeV per nucleon (96), it is recommended to remove always a surface layer several times thicker than calculated. 2.6.2.

Removal of a surface layer after the irradiation

A surface layer can be removed after the irradiation by: -

chemical etching;

-

mechanical

-

a combination of both.

grinding;

57 Mechanical grinding alone is in general not applied, since it is difficult to avoid contamination of the deeper layers of the sample. Chemical etching is applied by several authors (43)(44)(45).

In choosing

an appropriate etching solution, usually a mixture of acids, several factors must be considered: -

speed;

-

the etch must yield a smooth surface.

This point may be controlled by

microscopic methods or by using a surface-texture measuring instrument; -

reproducibility;

-

it is preferable to carry out the etching at room temperature.

Recommended etching solutions for the applications considered will be given in chapters IV to IX. It is recommended to etch at least twice in two etching solutions with the same composition.

Afterwards the sample is rinsed with water and

alcohol or acetone and dried. Several authors (46)(47)(48)(49)(50) recommend a combination of mechanical grinding and chemical etching.

Blondiaux et al. (48)(49) showed that

during the irradiation a "carbon layer" is deposited on the sample surface at the beam spot.

A mechanism for the formation of this layer is given

in (50) and it is shown that the formation of the "carbon layer" takes place only in the irradiated zone.

The velocity of formation is deter-

mined by: -

the current density and the temperature of the target.

The curve that

gives the velocity of growth of the "carbon layer" as a function of the 2 current density shows a maximum around 150 to 200 nA/mm . For temperatures above 100°C the velocity is very low. At still higher temperatures desorption of the carbon-containing products takes place until a given surface concentration depending on the temperature is reached. -

the energy and nature of the incident ion. influence on the velocity of growth.

The energy has only a small

The nature of the incident ion

has however a very important influence; -

the nature of the target;

-

the quality of the vacuum.

The "carbon layer" delays chemical etching of the sample after the irradiation.

This is shown in Fig. 11-24 giving the aspect of the sample surface

after irradiation and chemical etching.

The profiles were obtained with

58

the help of a surface-texture measuring instrument.

It is

Profile No 2

clear that at the beam spot the sample i s etched much less than

Beam area Profile N o 1

at the other parts. I f a s l i g h t mechanical polishing i s carried out after i r r a d i a t i o n and before chemical etching, normal etching takes place.

1 mm

Number 1

Blondiaux et a l . (48)(49) carried out the mechanical polishing with a f e l t impregnated with very fine diamond powder (0-0.2 Mm).

In the case of

germanium less than 40 nm of

Number 2

germanium i s removed. Blondiaux et a l . (48)(49) showed also

Fig. 11-24:

that incomplete etching after

germanium sample irradiated with 3 MeV

the i r r a d i a t i o n , when only a

t r i tons and chemically etched.

chemical etch i s carried out,

beam density was 6 μ A/cm (49)

Surface p r o f i l e for a

may cause s i g n i f i c a n t errors in the determination of oxygen in germanium via the

A

The

Ifl " 0 ( t , n ) ' l u F reaction.

The apparent oxygen content was high and increased with the amount of oxygen present at the surface of the sample.

When the sample was mechani18

c a l l y polished before the chemical etching, as described, no detected in germanium, even when irradiated behind a mica f o i l .

F was An upper

l i m i t for the oxygen concentration was 6 ng/g. 2.6.3.

Measurement of the thickness removed

In the case of neutron or photon activation, measurement of the removed thickness i s not very c r i t i c a l , because the samples are almost uniformly activated.

Therefore, i t i s s u f f i c i e n t to weigh the sample after etching.

For charged particle activation, however, the etching i s very c r i t i c a l because activation of the sample i s not uniform and i s limited in depth. The lower the energy of the incident p a r t i c l e , the smaller the range and the larger the error for a given error on the measurement of the thickness removed.

59 Several methods can be used to determine the removed thickness: -

measure the thickness before and after etching with -

a mechanical micrometer [a] ,

-

an electronic micrometer [ b];

-

weigh the sample before and a f t e r the etch [ c] ;

-

cover the non-irradiated side of the sample before the etch and measure the thickness before and after etching with an electronic micrometer in the i r r a d i a t e d area [ d] ;

-

measure the a c t i v i t y formed from the matrix, before and after the etching [ e] .

For methods [a] to [ c] the sample i s in general measured before the i r r a d i a t i o n instead of immediately before the chemical etching in order to avoid contamination of the measuring equipment and f o r reasons of radioprotection. Method [a] i s very simple but the accuracy i s seldom better than a few Mm. In addition the surface of the measuring instrument i s large, so that, for rough surfaces as sometimes obtained after etching, the maximal thickness instead of the average thickness i s determined. Method [ b] makes use of an e l e c t r o n i c micrometer (e.g. Tesatronic d i g i t a l , Tesa, Renens Switserland). The set-up i s shown in F i g . 11-25.

3

CM

ΙΟ
range) so that all the 0 + -radiation emitted is attenuated and yields 511 keV radiation.

The i?+-emitter is identified by the determination of

its half-life. Since the energy of the 511 keV radiation does not yield any qualitative information, Nal(Tl) detectors can be used.

These detectors are available

in large sizes, e.g. 7.5 cm diameter by 7.5 cm height and are less expensive than germanium semi-conductor detectors.

The superior resolution of

the germanium detector is in this case not a significant advantage.

For

a more selective detection of annihilation radiation, 2 Nal(Tl) detectors at 180° connected to 2 single channel analysers in coincidence can be applied.

Practical experimental set-ups are described in references (17)

and (39). Radionuclides that are ß+, y (or ß~, y) emitters are usually identified from the γ-ray energy.

In general, germanium detectors are used for this

purpose,as the resolution is more than one order of magnitude better than for a Nal(Tl) detector.

A typical figure is 2.0 keV at 1332 keV.

The

principles of y-spectrometry are described in references (54) and (97). 2.8.2.

Analysis of decay curves

In cases where a sample contains several ß + -emitters, the activity of the radionuclide of interest can be determined after chemical separation or by measurement of the annihilation radiation as a function of time followed by analysis of the decay curve.

At the same time a best estimate for the

half-life of the considered radionuclide may be obtained, which may serve as a qualitative criterion. graphically.

Analysis of a decay curve may be carried out

This method is however subjective and time-consuming, so

that appropriate computer programmes are preferable.

67 Several computing methods have been developed with varying degrees of complexity.

According to their field of application they can be divided into

3 groups, applicable to: (a)

a system with 1 component: the initial count-rate and the half-life are calculated with the method of least squares;

(b)

a system with a known number of components with known half-lives: the method of least squares is used to obtain the initial count-rate of each component.

Some programmes allow also, starting with an

estimate, to calculate by iterative methods a best estimate for the half-life of one or several components (55); (c)

a system with an unknown number of components with an unknown halflife: the number of components, the half-lives and the initial countrates are obtained.

This method requires a high precision for the

measured count-rates,which can only be realised for high count-rates (56). In activation analysis one applies usually a method from group (b). 2.9. CALCULATIONS To obtain the concentration of the element of interest in the sample Eq. [18] is used. (a)

The following calculations must thus be made:

Calculation of the incident energy on the sample, taking into account the beam intensity monitor foil and the surface layer removed by chemical etching. is available.

This calculation is simple if a range-energy table

The compilation of Williamson et al. (24) directly

gives range-energy tables for a number of elements.

The compilations

of Andersen and Ziegler (22) and Ziegler (23) give formulae that allow the calculation of the stopping power of hydrogen and helium ions in all elements as a function of energy.

The range can be

obtained by numerical integration based on Eq. [9] .

For compounds,

Bragg's Rule - Eq. [11] - is used to calculate the stopping power. For a beam of charged particles with energy Ej traversing a "thin" target with thickness d and density Ρ the energy of the outcoming beam E n is calculated as follows:

68

-

R(Εj) i s calculated with the range-energy table using

linear

interpolation -

R(E q ) i s given by R(E0)

-

=

R(EJ)

-

Φ

[51]

EQ i s deduced from the range-energy table using l i n e a r

inter-

polation. (b)

a / i S f o r the standard and f o r the energy corresponding to the e f f e c t i v e i n c i d e n t energy of the sample i s calculated as described in 2.3.6.

(c)

The c o r r e c t i o n factor F - Eq. [20] - can be calculated by numerical integration.

The r e l a t i v e c r o s s - s e c t i o n as a f u n c t i o n of the energy

i s obtained from the l i t e r a t u r e or determined experimentally.

Some-

times (as discussed in 2 . 3 . 3 . ) approximate methods that do not require knowledge of the e x c i t a t i o n f u n c t i o n may be used to c a l c u l a t e F. 3.

PHOTON ACTIVATION ANALYSIS

3.1

INTRODUCTION

Photon a c t i v a t i o n a n a l y s i s ( 6 5 ) ( 6 6 ) ( 6 7 ) ( 6 8 ) ( 6 9 )

i s based on the use of a

nuclear r e a c t i o n A ( r , x ) B between the atomic nuclei of the element A, which i s to be determined, and the gamma photons used to a c t i v a t e

it.

For an appropriate choice of the i n c i d e n t gamma photon energy, t h i s

inter-

a c t i o n g i v e s r i s e to the production of a radioisotope Β characterized by i t s h a l f l i f e and i t s type of r a d i o a c t i v i t y (nature and energy of p a r t i c l e s or photons emitted). The gamma photons must have an energy higher than the threshold of the reaction considered, below which the r e a c t i o n can a b s o l u t e l y not take place. The value of t h i s threshold energy depends e x c l u s i v e l y on the components of the r e a c t i o n .

More s p e c i f i c a l l y , i f the mass of the target atom A i s

denoted by m^, that of the atom Β produced by nig, and the sum of the masses of the p a r t i c l e s χ ( η , ρ , α , . . . ) produced by the r e a c t i o n by sm , the threshold energy i s defined by the f o l l o w i n g formula:

69 Ετ

=

mA - (mß + Σ>ηχ)

c


28

>

34

>

32

[ x: \ i , atn, ... ] 20., < .11. Ne(r,y) C [y: 9 Be, 2an, ... ] 23

Na(7,z)UC 12

[z: 24

Mg(7,u)nC 13

[u: 27

C , 3an, ... ]

Al(7,v)nC 16

[v: 28

N , 3atn, ... ]

cSl(7,w) it ,,\Hr C 17

[w: Nitrogen

B , 2atn, ... ]

0 , 4an, ... ]

16

0(7,t) 1 3 N

19

F(7,X') 1 3 N

[x 1 :

6

He, a2n, ... ]

25 >

25

76 Table 11-11 (Continued)

20

[ y·: 23

[ z': 24

[ u': 27

[ν': 28

[w1:

Oxygen

Ne(r,y')13N 7

U ,

at,

B e , 2a2n,

N,

2at,

C , 3a2n,

N , 3at,

F(r,tn)150

20

Ne(7,x")150

23

[y": 24

[z": 27

[u": 28

[v": 3

[w":

5

He, an,

...

26

>

27

L i , atn,

>

21

>

34

>

29

>

34

>

28

>

34

]

... ]

Mg(T,z")150 9

>

28

Na(7,y")150 8

28

... ]

19

[x":

>

...]

Si(7,w')13N 15

28

... ]

A1(γ,ν')13Ν 14

> ... ]

Mg(T,u')13N U

28

... ]

Na(7,z')13N 10

>

B e , 2aη, . . . ]

A1(7,U")150 12

B , 2atn,

...]

Si(T,v")150 13

C , 3an,

W ' ) 16

1 5

... ]

o

N, 3atn,

...]

77 Furthermore, by comparing the threshold values given in Tables 11-10 and 11-11, it is clear that it is in all cases possible to eliminate totally the effect of competing nuclear reactions, by the appropriate choice of the maximum energy of the gamma photon beam. The curves in Figures 11-33, 11-34 and 11-35 provide indications concerning the apparent carbon, nitrogen and oxygen

iO Errax

45

50

(MeV)

contents which competing nuclear reactions could produce.

Fig. 11-33: Apparent carbon and nitrogen 14 11 contents produced by (1) N(?,t) C,

3.6. THEORETICAL POSSIBILITIES

(2)

16

0(r,t) 1 3 N, (3)

16

0 ( 7 , a n ) U C reac-

tions in a material containing 1 μg/g of Figures 11-36 and 11-37 show

nitrogen (1) or of oxygen (2)(3), as a

schematically the characteris-

function of the maximal energy of the

tic forms of the excitation

gamme photon beam

function of the photoneutron reactions and of the Bremsstrahlung spectrum behind a target bombarded by high energy electrons.

The appearance of the curves indicates that

the activation of an element by a (γ,η) reaction increases with increasing maximum photon energy.

However, as shown above, the relative importance

of the competing nuclear reactions also increases with the energy of the incident photons: hence the need to select an optimal value for the latter. As a rule, to determine carbon, nitrogen or oxygen in pure metals, a maximum energy (equal to the kinetic energy of the electrons used to generate bremsstrahlung photons) is chosen between 25 and 35 MeV. tions, the ^ C , insignificant.

Furthermore, the carbon, nitrogen and oxygen concentrations

N and

15

In these condi-

13

0 yields from elements heavier than fluorine are

in metals are often of the same order of magnitude, and those of fluorine and especially neon are considerably lower, so that the influence of

78

Fig. 11-34:

Apparent carbon,

nitrogen and oxygen contents in sodium produced by the 23 11 nuclear reactions: Na(r,z) C, 23

Na(7,z') 1 3 N,

23

Na(7,y") 1 5 0 as

a function of the maximal energy of the gamma photon beam

Emax (MeV)

Fig. 11-35:

Apparent carbon,

nitrogen and oxygen contents in aluminium produced by the 27 11 nuclear reactions: ΑΙ(γ,ν) C, 27

Α1(γ,ν') 1 3 N,

27

A1(t,u") 1 5 0 as

a function of the maximal energy of the gamma photon beam

< an t—

ζ Η 1.0o (J ζ e 10s), after i r r a d i a t i o n the sample may be scoured mechanically (by abrasion), chemic a l l y or by both methods successively.

In this way a surface layer of

controllable thickness i s removed (as determined by weighing or by means of a micrometer), thus eliminating the influence of surface effects.

The

new film produced after the removal of the f i r s t , will exert no influence whatsoever, as i t no longer contains any radioactive atoms. the oxygen determined i s only that i n the sample.

As a result

The latter point consti-

tutes one of the big advantages of activation analysis compared with any other analytical method". To-date i t i s known that this statement i s only v a l i d i f a l l are taken and "clean" well prepared samples are used.

precautions

I t i s however

d i f f i c u l t to quantify potential errors and i n fact only l i t t l e i s known about the various phenomena that may possibly affect the sample during i r r a d i a t i o n and cause contaminants to migrate at varying speeds towards deeper layers of the material.

The phenomenon may however not a priori be

neglected, as was demonstrated at various occasions in round robins organized by the Community Bureau of Reference (BCR). A typical example of the importance of suitable sample preparation was encountered during the c e r t i f i c a t i o n - provisional in 1974 (5) and f i n a l i n 1978 (6) - of oxygen i n lead.

In the f i r s t round, sample preparation

was carried out by the participating laboratories which are mainly physic a l l y oriented.

In the final c e r t i f i c a t i o n a l l samples were prepared in

the same specialized laboratory after extensive study of the problem. Although in both campaigns samples have been etched between i r r a d i a t i o n and measurement, the results d i f f e r by a factor of 3 in absolute value and a s i g n i f i c a n t improvement i n precision was obtained i n the second campaign (Table I I I - 2 ) .

104 Table 111—1:

Influence of surface contamination on bulk content (Oxygen in non-ferrous metals) (2) Specimen

Actual

(%)

Permissible 'contamination (Mg/cm 2 )

20

< 0.1

20

< 0.1

0.5

> 250 > 250 > 100

20

< 0.2

2.00

2.2

55

10

0.2

2

2.00

2.3

58

10

0.2

Pb

< 1

3.00

2.5

>

83

20

< 0.2

Pb-Sb

< 1

3.00

2.5

>

83

20

< 0.2

content

Error if

("9/9)

(9)

Surface area (cm 2 )

Al

< 1

2.00

4.9

Al-Si

< 1

2.00

4.9

W

< 1

0.50

Cu

2

Cu-Zn

Metal

Weight

C s =lMg/cm 2

{%)

Permi ssible error

Ti

800

0.25

0.9

0.5

0.5

1.0

Zr

600

0.25

0.7

0.5

0.5

1.0

Nb

25

0.50

0.9

7

5

0.7

Ta

25

0.50

0.6

5

5

1.0

Mo

15

0.50

0.8

11

5

0.5

Table 111-2:

Results for oxygen in lead obtained by photon and charged particle activation analysis (6) Provisional certification results

Final certification results

METHOD x("9/9)

s(Mg/g)

Labs

x^g/g)

s(Mg/g)

Labs

16

0(7,n) 1 5 0

2.0

0.9

1

0.6

0.1

1

16

0(a,pn) 1 8 F

2.2

0.8

1

0.7

0.3

2

16

0(3He,p)18F

-

0.8

0.3

2

-

-

105 2.

MEASUREMENT OF SURFACE CONCENTRATIONS

As shown in a literature survey and c r i t i c a l evaluation of surface analysis methods by Quaglia and Weber (7), the most appropriate method to evaluate residual contamination of oxygen, nitrogen and carbon (for boron and phosphorus s i g n i f i c a n t surface contaminations were never observed) on the surface of metallic samples i s microanalysis by nuclear reactions (8-11). This method consists in bombarding the surface of a sample ("target") with accelerated low energy charged particles with simultaneous detection of the p a r t i c l e s emitted when a nuclear reaction takes place, using a s i l i c o n surface-barrier detector.

As a source of charged particles generally a

Van de Graaff accelerator i s used. as incident p a r t i c l e s .

Mostly low energy deuterons are used

Since to our knowledge the only systematic study

of surface preparation of non-ferrous metal samples and characterization of residual oxygen, nitrogen and carbon contamination was carried out within the BCR programme, we will r e s t r i c t ourselves here to describe the method as used in t h i s context by L. Quaglia and G. Weber at the I n s t i t u t de Physique Nucleaire of the Universite de 1'Etat ä Liege (7). 2.1

PRINCIPLE OF THE METHOD

The principle of the method i s i l l u s t r a t e d in Fig. 111—1: a beam of deuterons, the energy and incident direction of which are fixed or defined with a maximum of precision, strikes the sample (target) which i s located in a vacuum chamber.

The energy i s of the order of one MeV.

On the l i g h t

nuclei of the target these particles give r i s e to reactions such as (d,p) or (d,a).

According to the energy levels of the product nuclide, one or

more groups of charged particles of different energies are emitted as i s i l l u s t r a t e d in Fig. 111-2 for the

16

0 ( d , p ) 1 7 0 reaction with 1 MeV deuterons.

A detector i s placed at a backward angle 0 with respect to the direction of the incident particles and corresponding to a s o l i d angle Ω.

Through

measurement electronics and a multichannel analyzer (MCA) this detector gives the energy d i s t r i b u t i o n or the spectrum of the particles emitted in the s o l i d angle ii by the reactions induced at the surface of the sample. To a specific reaction corresponds, in these spectra, a well defined peak. In general the detector i s a s i l i c o n surface barrier detector with a 100 % efficiency for the charged particles crossing the dead layer.

The surface

106

VACUUM CHAMBER

INSULATOR COLLIMATORS BEAM LINE

///////,

PREAMPLIFIER

INTEGRATOR LINEAR

AMPLIFIER

//w//.

MCA

F i g . 111-1:

Experimental set-up (23)

η of a peak then indicates the number of reactions of a given type which led to emission of a p a r t i c l e in the s o l i d angle Ω when a number nQ of incident p a r t i c l e s reached the target.

The value of nQ i s obtained by

measuring the incident charge Q collected on the target during the i r r a diation.

I f charges are expressed in Coulombs, t h i s means:

1.6 · 10

-19 13

· ζ

with: ζ = charge of the incident p a r t i c l e expressed as the number of elementary charges. For a number of detected p a r t i c l e s n, the surface concentration can be calculated using the equation:

107

COMPOUND NUCLEUS MeV

MeV

m 16 r /

Ü

m

'l6o* m d 8.677

8.2/46 7.358

r 16

6.49

0+d

5.619

Q = 1.919 MeV

170 , +p

0.0

!8p

Fig. III-2:

η

16

Energetics of the r e a c t i o n

=

η

C

da(E.,9) 5

F

dn

0(d,p)

17

0 f o r 1 MeV deuterons

4]

Ω

with η

= number of charged p a r t i c l e s detected i n Ω;

η ο

= number o f i n c i d e n t 2

Cs

= number o f g/cm

N^

= Avogadro's c o n s t a n t ;

Μ

= mass of one mole of the element to be determined;

θ

= natural i s o t o p i c abundance o f the n u c l i d e of

do/dn

= d i f f e r e n t i a l cross2 s e c t i o n o f the r e a c t i o n at an angle θ and energy E^ (cm / s r ) ;

Ω

= s o l i d angle of d e t e c t i o n

particles;

f o r the element of

interest;

interest;

(sr).

Although the p r i n c i p l e of the method i s simple, the experimental

determina-

t i o n o f the v a r i o u s parameters used i n equation [4] c a l l s f o r a number of precautions:

108

a)

The number of deuterons that actually reach the target can be deduced from the integrated charge Q received by the target, provided that measures are taken to avoid the escape of secondary electrons.

If

these electrons are not collected, the integrated charge i s overestimated. To minimize t h i s effect, the currents collected on the target and on the walls of the reaction chamber are placed in p a r a l l e l . A study by Quaglia (12) using electrostatic and magnetic suppressors has shown that the contribution of secondary electrons amounts to only a few tenths of a percent in normal collimation conditions. b)

The s o l i d angle of detection can be determined by geometric evaluation of the distance from the target to the detector and of the diameter of the collimator, or by using a calibrated α source substituted for the target.

c)

Results of both methods agree within 2 % (7).

The accurate evaluation of the absolute value of the differential cross section of a nuclear reaction i s a more delicate problem. Numerous examples can be found in the literature where cross sections of the same reaction - at identical energies and angles - scatter over a wide range.

2.2

NUCLEAR REACTIONS

For the determination of surface oxygen Quaglia and Weber (7) use the reactions 16

0(d,po)170

and

16

0(d,Pl)170

which y i e l d protons of energies corresponding to the formation of * 7 0 in i t s fundamental and in i t s f i r s t excited state as shown in F i g . 111-2. The second reaction has a larger maximum cross section, and therefore one has advantage in trying to use i t . The

850 keV i s a suitable energy.

reaction i s interfered by ^N(d,pg)^ 5 N, which means that

i f important amounts of nitrogen are present i t cannot be used to determine oxygen.

At 850 keV the cross section of the interfering reaction i s

20 times smaller than the one of the

reaction, so that for

equal amounts of oxygen and nitrogen the interference i s 5 %.

I f , anyhow,

109

16 the interference i s too high, the use of the recommended.

17 0(d,p Q )

0 reaction i s

At 850 keV t h i s reaction i s interfered by d(d,p)t so that

i t i s advisable to carry out the analysis at 650 keV. The comparison of the height of the different peaks produced by reactions on nitrogen shows that for t h i s element i t i s interesting to use the reaction 14 N(d,p,-) 15 N, or to sum the 1 4 N(d,p,) 1 5 N and 1 4 N(d,p d ) 1 5 N reaction 14 11 14 11 peaks. One can also use the reactions N(d,a^) C and N(d,a Q ) C which are only seldomly disturbed by interferences. 12 The

13 C(d,p Q )

C reaction peak i s free from interference and allows the

determination of carbon with a high precision and s e n s i t i v i t y , even i f large quantities of oxygen and nitrogen are present. 2.3

STANDARDIZATION

As already mentioned absolute differential cross section values given in the literature often show for the same reaction larger differences than expected from the quoted experimental errors.

I t i s therefore indicated

to give some information on the measurements used to determine the surface contamination values given further on. For oxygen, a literature survey led to the use of the data of Seiler et a l . (13) and Kim et a l . (14), obtained with a gaseous target. ' The d i f f e r ential cross sections of the ^ ( d . p ^ ^ O and determined from 800 keV up to 1700 keV.

16

0 ( d , p o ) 1 7 0 reactions were

To extend the energy range down

to 300 keV a series of relative measurements was carried out by Dumont et a l . (15).

The obtained curve agrees within 10 % with that measured by

Amsel and David (16) on a p a r t i c u l a r l y reliable Ta20^ target prepared by anodic oxydation. For nitrogen, the differential cross sections determined by Amsel and David (17) were used. known at 10 % r e l a t i v e .

They are based on tantalum nitride reference layers The differential cross section used i s believed

to be accurate within 15 % relative. In view of the extremely scattered r e s u l t s published for the reaction 12 13 C(d,p o )

C (18-21), the differential cross section of this reaction was

redetermined by Huez et a l . (22) using the accurately known cross section of the

16

0 ( d , p 1 ) 1 7 0 reaction as a reference.

at + 20 %.

This provided a value known

110

2.4

COMMENT

It has been observed frequently that the surface concentration of a sample varies during the irradiation (23-25), as illustrated in Figure 111-3.

To

allow extrapolation of the measurement to the start of the irradiation, it is essential to carry out a prior study for each type of target in order to understand its behaviour under irradiation.

TOTAL CHARGE (\xC )

This allows the

determination of optimal expe-

Fig. 111-3:

rimental conditions, in parti-

content of a copper sample with the

cular with respect to the total

incident charge (23). Current density

charge received by the target.

on the target = 12.5 nA/mm

3.

MECHANICAL SHAPING OF ANALYSIS SAMPLES

3.1

ROUGH PREPARATION OF THE ANALYTICAL SAMPLE

Evolution of the oxygen

2

Normally the sample to be analysed has to be taken from a larger piece of material.

This operation must be carried out with all necessary care in

order to avoid heating of the sample.

This can not only result in oxida-

tion of the surface, but, depending on the temperature and the metal, also in fairly deep penetration of surface contaminants into the sample. In general, a sharp metal saw is a suitable tool to carry out this preliminary operation.

Abrasive wheels should only be used for very hard metals,

whereas soft metals may also be sampled by e.g. careful punching.

cold rolling and

In every case, care must be taken that the sample surface is

not fissured or smeared, but is smooth and has a bright appearance.

If

this is not the case, there is always a risk that contaminants penetrate deeply during the first stage of sample preparation, and that further finishing will not completely eliminate these contaminants.

111

3.2

FINAL SHAPING OF THE SAMPLE

The primary aim of final shaping i s of course to transform the sample to i t s final form as required for analysis. surface contamination to a minimum.

At the same time i t reduces the

As far as the form i s concerned,

spherical samples would be ideal since their surface to volume ratio i s minimal.

This i s however unpractical, and in general the accepted compro-

mise between small surface to volume ratio and difficulty of preparation consists in making cylinders or cubes. Cylindrical samples are machined from the crude samples by turning on a lathe, whereby also the base surfaces have to be rectified. purpose steel, hard metal or diamond tools can be used.

For this

The operating

parameters may differ from one metal or alloy to another, but must always end up in f l a t and clean surfaces.

As a result of

several years of international cooperation within BCR, the Central Bureau for Nuclear Measurements (C.B.N.M.), Geel, Belgium, proposed a standardized tool form, which i s schematically represented in Fig. 111-4 (3).

The specific tool mate-

r i a l s and turning parameters to be used to remove the last 0.1 to 0.2 mm are summarized in Table I I I - 3 . Cubic samples are generally prepared using thin cutting wheels coated with hard metal, but such techniques are only allowed i f the metal i s s u f f i ciently hard.

TOOL P A R A M E T E R S FOR

TURNING

A N G L E OF ATTACK CLEARANCE ANGLE RAKE ANGLE RAKE ANGLE INCLINATION CUTTING E D G E TOP R A D I U S 0.2-0.3mm DEPENDING

In some cases, surface contami-

Fig. 111-4:

nation i s sufficiently low after

ters for turning (3)

: X : 45" :y : 20-30° : α : 6-10° : a,: 3 - 6 ° : X : 0° : · : Z E R O OR ON THE METAL

Standardized tool parame-

112

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^

ι—< - -

CSJ

—-

υΊ •

•

00

Ε:

•

•

n:

ζπ

•

s: •

m

113 shaping to enable direct a n a l y s i s of the sample after simple degreasing. I f t h i s i s the case,the time between preparation and a n a l y s i s should however be kept to a minimum and the sample should be manipulated using clean tweezers. 4.

SHAPING OF LARGE SERIES OF SAMPLES

For the production of reference materials i t may be required to shape several thousands of identical small samples.

Machining by turning on a

lathe or by conventional diamond wheel cutting techniques may then become c o s t l y because of poor material u t i l i z a t i o n , low production rate and high tool consumption. I t therefore seems interesting to describe in some detail a procedure developed a t C.B.N.M. to produce large series of small cubic samples of molybdenum, titanium and titanium aluminium vanadium a l l o y (TiAl6V4) s t a r t i n g from round bars (26-27).

The procedure consists of the combina-

tion of two machining operations: s l i c i n g of the bar into disks and subsequent dicing into cubes using adapted high speed a i r bearing sawing machines. For the s l i c i n g operation a high speed a i r bearing sawing attachment, called ROTO (Semitron Ltd.,Cricklade, Swindon W i l t s . , U.K.), i s f i t t e d on the saddle of a lathe as shown i n Figure I I I - 5 .

An appropriate thin

abrasive wheel i s f i t t e d to the arbor of the ROTO and the metal bar i s held in the chuck of the lathe. The rotating water cooled abrasive wheel i s advancing into the bar at 90° to the axis of the chuck which i s a l s o r o t a t i n g , as schematically shown in Figure 111-6. The r e s u l t i n g e f f e c t i s that material i s removed in a modified creep-feed technique. The dicing operation involves the use of multiple wheel arbors f i t t e d to an automatic Semitron machine as shown in Figures I I I - 7 and 111-8. In t h i s instance the s l i c e i s clamped i n a f i x t u r e and i s sawn twice, in order to obtain cubes of correct s i z e .

To obtain optimum s l i c i n g and

dicing conditions a number of parameters were investigated. important were:

The most

114

Fig. 111-5:

S l i c i n g attachment mounted on a lathe (26)

F i g . 111-6:

Principle of

s l i c i n g (26)

115

F i g . 111-7:

Principle of

dicing (26)

Fig. 111-8:

Dicing machine with multiple cutting wheels (26)

116 -

choice of cutting blade material and thickness;

-

spindle speed, relative rotational direction, lathe chuck speed and feed for dicing;

-

coolant composition and effectiveness of coolant distribution.

The most useful conditions for molybdenum, titanium and TiA16V4 are reported below: a)

Slicing "

Cutting_blade -

Mo

Phenolic resin bonded A1 2 0 3 wheel (A 240 NB 60) of 150 mm diameter and 0.4 mm thick

-

Ti

Phenolic resin bonded green SiC wheel (GC 150 NB 34) of 150 mm diameter and 0.5 mm thick

-

TiAl6V4

Phenolic resin bonded green SiC wheel (GC 80 NB 34) of 150 mm diameter and 0.5 mm thick

b)

"

ROTOsgindlespeed

"

L?ihe_chuck_sgeed

-

Lathe_feed

"

!?§i!ial_wear/cut

-

Coolant

8750 rpm 80 rpm 0.025 mm/revolution 3 mm current water

Dicing "

Cutting_bl ade -

Mo

:

Rubber bonded A1 2 0 3 (A 150 QR 12 DT) wheels of 101.6 mm diameter and 0.5 mm thick

-

Ti and TiAl6V4 : Phenolic resin bonded mixture of diamond and SiC wheel (GSD - GIA 801 SD 100R 13B01) of 101.6 mm diameter and 0.5 mm thick

-

Multicut on Semitron "Auto 2000" -

Number of blades

2, 3 or 5

-

Spindle speed

8000 rpm

-

Feed speed

20 mm/mi η

-

Radial wear/100 cuts

Coolant

0.1 mm

80 volumes of water and 1 volume of Miracol 80 (Castrol) at 3 1/min

117

Compared to more conventional techniques, this sawing technique enables 10 times higher production rates and close dimensional tolerances (maximum mass variation for 18 samples: 2 %).

As a result of the dicing

operation the cubes thus produced however present burrs, which can be eliminated either by etching (Ti and TiA16V4) or by barrel finishing (Mo). For molybdenum, 50 % random shaped alumina pieces of 2 to 4 mm are mixed with 50 % alumina powder of 240 grit in two times its volume of water to produce an abrasive medium, and a Turbula shaking mixer with PVC container is used as barrel finishing machine.

Batches of 150 molybdenum

cubes are correctly deburred after a 4 hours time. After appropriate etching, the above described mass production techniques lead to equivalent residual surface contaminations as piece by piece preparation by turning on a lathe. 5.

CHEMICAL ETCHING

In general it is necessary to subject the samples to a chemical treatment of the surface after their final preparation by mechanical means in order to achieve the minimum possible surface contamination. It is extremely important that this chemical treatment is reproducible. An initial survey of appropriate etching solutions for different nonferrous metals was provided by Kraft (28).

It formed the basis for the

thorough and far-reaching investigations of a BCR specialists group, the results of which were published by Quaglia et al. (3). For all etching solutions used and for all etching conditions specified, the etching process is discontinued in the same way.

The sample is

removed from the etching bath by means of a pair of platinum coated tweezers, and put immediately into a glass vessel containing distilled water.

Dilution of the etching bath to stop the etching is unacceptable.

The initial rinse is followed by two further rinsing stages, each with fresh quantities of distilled water.

The sample is then dipped successi-

vely in three vessels containing alcohol and is subsequently dried in a stream of warm air at about 60°C (a hair drier) or, in special cases, in a stream of inert gas.

118

6.

RECOMMENDED PROCEDURES

Surface treatment has been studied systematically for a number of nonferrous metals and alloys and recommended procedures have been proposed (3).

These recommendations take into account that the proposed treatment

must: -

reduce surface contamination to a minimum;

-

be reproducible;

-

be easy to perform with standard laboratory equipment.

Therefore the proposed solutions may be compromises between some of these requirements.

Furthermore, other techniques not mentioned here may be

well suited too (e.g. cold r o l l i n g of copper), but their absence i s explained by the fact that they were not examined and evaluated. In the recommended procedures turning parameters refer to Table I I I - 3 , s l i c i n g and dicing parameters to section 4, and etching i s carried out as indicated in section 5. 6.1

PRIMARY INGOT ALUMINIUM /

Two shaping techniques were found to be acceptable: turning on a lathe and cold r o l l i n g . turned pieces.

In the l a t t e r thin plates are prepared from freshly

The reduction in thickness must be smaller than 5 % per

r o l l i n g pass. In the case of oxygen and carbon shaping has to be followed by etching. Etching parameters are: Bath

1 volume HF (40 %) + 1 volume HN03 (65 %; d = 1.4)

Temperature

20°C

Duration

60 s

Arrest

3 χ H20

Drying

3 χ CH30H + warm a i r (60°C)

Tvpical residual surface concentrations are 0.3 - 0.4 Mg/cm and 0.1 - 0.2 ^g/cm 2 for carbon.

2

for oxygen

Experiments conducted in parallel in two laboratories have shown that the mechanical surface preparation of the specimen i s e s s e n t i a l .

As

appears from Table 111-4 s l i g h t differences may produce substantial deviations, even after identical chemical etching (2).

119

Table I I 1 - 4 :

Surface oxygen concentration measured on aluminium and A l - S i alloys 2 Measured surface oxygen (Mg/cm )

Specimen Turning

Analysis

Al 99.5

AS 3

AS 7

AS 13

Lab I

Lab I

0.22

0.27

0.24

0.33

Lab I

Lab I I

0.28

0.42

0.43

0.39

Lab I I

Lab I I

0.80

1.10

1.10

0.70

6.2

ALUMINIUM-SILICON ALLOYS

Two shaping techniques were found to be acceptable for alloys containing 3 (AS3), 7 (AS7) and 13 % (AS13) s i l i c o n , provided the samples are etched subsequently: turning on a lathe and cold r o l l i n g . Using the latter technique 1 mm thick plates may be prepared provided freshly turned pieces are rolled as follows: -

AS3

:

- starting thickness: 10 mm - maximum reduction : 5 % per pass

-

AS7 and AS13 :

- starting thickness: 3 mm - maximum reduction : 2 % per pass.

Etching i s carried out as for unalloyed aluminium, but the duration i s increased to 2 minutes.

The typical residual surface oxygen concentration

2

i s 0.2 - 0.4 pg/cm . 6.3

ALUMINIUM-MAGNESIUM ALLOYS

Turning on a lathe was found to be an acceptable shaping technique for aluminium-magnesium a l l o y s , provided the samples are etched subsequently. The etching procedure used i s the same as for aluminium, but the duration i s increased to 5 minutes. 2

t i o n i s 0.2 - 0.4 Mg/cm .

The typical residual surface oxygen concentra-

120

6.4

COPPER

For unalloyed copper, turning on a lathe i s s u f f i c i e n t in sample preparation.

I f needed, a chemical etching i s possible as well.

Chemical etching i s carried out in 2 successive steps: a)

b)

Bath 1

1 volume HCl (30 %·, d = 1.15)

Temperature

20°C

Duration

2-3 min

Arrest

3 χ H20

Bath 2

1 volume HN03 (65 %\ d = 1.4) + 1 volume CH3C00H

-

-

(96 %; d = 1.06) + 1 volume H 3 P0 4 (85 %·, d = 1.71) Temperature

20°C

Duration

1 min

Arrest

3 χ H20

Dryi ng

3 χ CH30H + warm a i r (60°C)

The typical residual surface oxygen concentration i s i n both cases 2 (turning or etching) 0.2 - 0.4 Mg/cm . 6.5

BRASS

Brass samples may be prepared by turning on a lathe, provided a subsequent etching i s carried out. Etching parameters are -

Bath

4 volumes HN03 (fum.) + 1 volume H 2 0

-

Temperature

40°C

-

Duration

5 s

-

Arrest

3 χ H20

-

Dryi ng

3 χ CH30H + warm a i r (60°C)

The typical residual surface oxygen concentration i s 0.3 - 0.5 Mg/cm . 6.6

LEAD AND LEAD ALLOYS

Three shaping techniques were found to be acceptable for lead and a l l o y s containing respectively 0.2 % Sn + 0.07 % Cd and 0.4 % Cu + 0.04 % Te, provided the samples are subsequently etched.

121

a)

turning on a lathe

b)

rolling : one mm thick plates may be prepared by this technique, provided freshly turned pieces are cold rolled with a maximum reduction in thickness of 10 % per rolling pass

c)

punching: cutting with a punching die, using kerosene as cutting fluid, of round or rectangular pieces is possible up to a thickness of 10 mm, provided one starts from freshly turned samples.

As chemical etching of lead and lead alloys is particularly delicate and i t s success depends on a lot of details, i t seems worthwhile to explain the procedure in more detail than for the other metals.

The method

described here i s based on a similar one proposed by the European Lead Development Committee (29).

Its principle is illustrated in Figure 111-9

(30).

I Vol Η,Ο] • I V o l CHjCOOH

H j O DIST

HjO DIST.

20*C

60 - 70*C

20 *C

CHjOH 20-C

100 ml

200 m l

500 ml

200 m l

t5s

5s

THERMOMETER

MAGNETIC

ARGON

Fig. 111—9:

Principle of the etching of Pb, PbSbCd and PbCuTe (30)

A sample of 1 to 5 g, kept between chromium or platinum coated tweezers, i s dipped for 45 s in a 100 ml etching bath composed of 1 volume of hydrogen peroxide (30 %) and 3 volumes of acetic acid (97 %) at room temperature and rinsed in 200 ml of water at 60-70°C. magnetically stirred.

Both liquids are

For each sample fresh solutions are used.

Then the sample i s dipped for 5 s in two further vessels containing respectively 500 ml of water (20°C) and 200 ml of methanol (20°C), placed in an ultrasonic device.

These solutions are replaced after six specimens.

122 Drying i s c a r r i e d out f o r 2 minutes i n a pre-heated argon f l o w (80°C). The g l a s s vessel i n which the sample i s d r i e d i s pre-heated using a h a i r d r i e r . The a n a l y s i s can be c a r r i e d out a f t e r 1 to 15 minutes, without change i n the surface oxygen c o n t e n t . ones.

significant

The chemicals used are p . a . grade

Hydrogen peroxide i s s t a b i l i z e d with maximum 0.005 % phosphate.

The e t c h i n g time may vary between 40 and 50 s .

When e i t h e r too s h o r t or

too long e t c h i n g times are a p p l i e d , the sample shows grey spots or p i n holes.

The argon used f o r d r y i n g the samples i s 99.999 % pure, and con-

t a i n s l e s s than 1 μ 1/1 o f oxygen and 3 μ 1/1 of water. not to use almost empty gas b o t t l e s .

Care must be taken

Furthermore the above quoted storage

time i s o n l y v a l i d on the c o n d i t i o n t h a t the samples are preserved from e t c h i n g vapours. T y p i c a l r e s i d u a l surface c o n c e n t r a t i o n s are 0.2 - 0.4 μg/cm

2

2

and 0.15 - 0.2 μg/cm 6.7

f o r oxygen

f o r carbon.

NICKEL

Two shaping techniques were found to be acceptable f o r n i c k e l ,

provided

a subsequent e t c h i n g i s a p p l i e d : t u r n i n g on a l a t h e and sawing. The l a t t e r was c a r r i e d out as f o l l o w s : -

tool

:

B a k e l i t e bonded SiC wheel (Feldmühle 80N/99001)

-

speed

:

4000 rpm

-

cutting f l u i d

:

water

Chemical e t c h i n g parameters are: -

Bath

:

75 volumes CH,C00H ( g l a c i a l ) + 25 volumes HNO,, (65%; d = 1.4) + 1.5 volume HF (40 %)

Temperature

60°C

Duration

60 s

Arrest

H 2 0 (under u l t r a s o n i c

Rinsing

2 χ H20

Dryi ng

3 χ CH,OH + warm a i r (60°C)

vibration)

T y p i c a l r e s i d u a l surface c o n c e n t r a t i o n s are 0.1 - 0.2 μg/cm 0.15 - 0.2 μg/cm

2

f o r carbon and < 0.03 Mg/cm

2

2

for nitrogen.

f o r oxygen,

123 6.8

TITANIUM AND TiA16V4-ALL0Y

Beside the mass production technique described in section 4, two other shaping techniques, turning on a lathe and cutting with a diamond saw, were found to be acceptable, provided samples are subsequently etched. Cutting with a diamond saw is carried out as follows: -

cutting blade

:

diamond bronze bonded wheel, 150 mm diameter and 0.65 mm thick

-

speed

:

2300 rpm

-

cutting fluid

:

water

Chemical etching parameters are: -

Bath

:

4 volumes HN0 3 (65 %; d = 1.4) + 1 volume HF (40 %)

-

Temperature

:

20°C

-

Duration

:

60 s

-

Arrest

:

3 xH^O

-

Drying

:

3 χ CH 3 0H + warm air (60°C)

Typical residual surface concentrations are 0.4 - 0.6 Mg/cm 0.1 - 0.2 Mg/cm 6.9

2

for carbon and < 0 . 1 Mg/cm

2

2

for oxygen,

for nitrogen.

ZIRCONIUM

Two shaping techniques, turning on a lathe and cutting with a silicon carbide saw, were found to be acceptable provided samples are subsequently etched. Cutting is carried out as follows: -

cutting blade

:

bakelite bonded SiC wheel (Feldmuhle 80N/99001), 150 mm diameter and 1 mm thick

speed

2300 rpm

cutting fluid

water

Chemical etching parameters are: Bath

5 volumes HN0 3 (65 %\ d = 1.4) + 0 . 5 volume HF (40 %) + 5 volumes H^O

Temperature

20°C

Duration

50 s

Arrest

3 χ H20

Dryi ng

3 χ CH-.0H + warm air (60°C)

124

Typical residual surface concentrations are 0.4 - 0.6 ^g/cm 2

0.1 - 0.2 jjg/cm

for carbon and < 0.05 ^g/cm

2

2

for oxygen,

for nitrogen.

6.10 MOLYBDENUM Besides the mass production technique described in section 4, one shaping technique - cutting with a diamond saw - was found to be acceptable, provided samples are subsequently etched. Cutting i s carried out as follows: -

cutting blade

:

diamond bronze bonded wheel (Norton PD 180-N1006-1 11044 Z), 150 mm diameter and 0.65 mm thick

-

speed

:

4000 rpm

-

cutting f l u i d

:

water

Chemical etching parameters are: -

Bath

:

4 volumes HF (40 %) + 1 volume HN03 (65 %; d = 1.4)

-

Temperature

:

20°C

-

Duration

:

10 s

-

Arrest

:

HCl (36 %)

-

Rinsing

:

3 χ h^O

-

Drying

:

3 χ CH30H + warm a i r (60°C) 2

Typical residual surface concentrations are 0.2 - 0.4 A0)

' L i P.n) Be

1.8)

(11.2)

9

14.9)

7

Be p , t ) B e

12~

14, 10 U

B(d,n)nc n

B(d,2n) c

(Q > 0) (5.9)

7

p,pan) Be 7

Sensitivity

(37)

0.92 ( 5 MeV) 0.24 (10 MeV)

28.5)

p,2a) Be

11.3)

C d,t)UC 14. d,an)UC

14.5)

3.5

( 5 MeV)

5.9)

2.1

(10 MeV)

12

0.75 (15 MeV) 10 U

B(d,an) 7 Be 7

B(d,a2n) Be

(1.3) (14.8)

7 J

Li d,n) Be

Q > 0)

7 Li d,2n) Be

5.0)

7 Be d,tn) Be

12-

33.3)

d, 2an ) 7 Be

14.5)

J

Be 12.

3

He,n)UC

Q > 0)

He,a)

14

3

14 (Q>0) n

B(3He,t)nC

(2.5)

r 16, 10 n

Β(α5η)13Ν

B(a,2n)13N

(Q > 0)

(14.2)

21.5)

d,ap2n) 7 Be

12

3

44

( 5 MeV)

C

Q > 0)

8.2

(10 MeV)

n

10.2)

3

(15 MeV)

U

6.3)

1.3

(20 MeV)

He,ad) C He,2a) C

( a,t)13N

23.8)

20

(26 MeV)

a,an)

Ν 13

13.6)

10

(30 MeV)

16 ( a , a t )

Ν

16c

19

F

19r

a,7Li)13N a,2a2n) 1 3 N a,10Be)13N

26.7)

3.5

(38 MeV)

28.2) 34.5) 19.5)

Boron concentration (ng/g) i n an aluminium sample that y i e l d s at the end of an i r r a d i a t i o n (1 μΑ i n t e n s i t y , i r r a d i a t i o n time = one h a l f l i f e ) a t the i n d i c a t e d energy an a c t i v i t y of 100 desintegrations/min

145 i n i t s p r i n c i p l e s i m i l a r to the one f o r the simultaneous determination of l i t h i u m and boron (see below). use the

I t i s however i n general recommended to

r e a c t i o n (24), which f o r energies below 5.9 MeV allows

an i n t e r f e r e n c e - f r e e determination o f boron, a l b e i t with a somewhat lower sensitivity. The ^ B ( p , a ) 7 B e and

10

7

the long h a l f - l i f e o f 7

B(d,an) 7 Be r e a c t i o n s have the f o l l o w i n g advantages: Be allows complex radiochemical s e p a r a t i o n s ; because

Be emits γ - r a y s o f 478 keV, i n some cases an instrumental

is feasible. criterion.

determination

Moreover t h i s γ - r a y energy can be used as a q u a l i t a t i v e The disadvantages are: i n t e r f e r e n c e o f l i t h i u m cannot e a s i l y

be avoided (28); long i r r a d i a t i o n and counting times are r e q u i r e d f o r good sensitivity.

S a s t r i e t a l . (29) give the t h i c k t a r g e t y i e l d s as a f u n c t i o n

of the energy f o r the ^ B ( p , a ) 7 B e r e a c t i o n and the i n t e r f e r i n g as w e l l as f o r the

10

7

B(d,an) Be r e a c t i o n and the i n t e r f e r i n g

reactions,

reactions.

For protons ( F i g . IV-3) w i t h energies i n the 4-14 MeV range, the

7

Be a c t i -

v i t y from l i t h i u m i s about 10 times t h a t produced from boron. Above 14 MeV 1 i n t e r f e r e n c e by the tant. 10

4

7

9

N(p,2a) Be and

7 B e ( p , t ) Be r e a c t i o n s becomes impor-

Since the curves f o r the

B ( p , a ) 7 B e and the

7

Li(p,n)7Be

r e a c t i o n s show a p a r a l l e l

course

BORON

between 4 and 14 MeV, i t i s imp o s s i b l e to determine l i t h i u m and boron by i r r a d i a t i o n w i t h protons o f 2 d i f f e r e n t energies i n t h i s energy i n t e r v a l . When deuterons are used ( F i g . the s i t u a t i o n i s

IV-4)

different.

In the 4-7 MeV energy range, the y i e l d f o r l i t h i u m i s somewhat lower than f o r boron, but

2x10»

both are e s s e n t i a l l y o f the same o r d e r .

Above 7 MeV, the

y i e l d f o r l i t h i u m i n c r e a s e s more r a p i d l y than f o r boron.

10

The

i n f l u e n c e of n i t r o g e n and

F i g . IV-3:

b e r y l l i u m i s o n l y important

10

above 15 and 20 MeV r e s p e c t i v e l y .

15

20

E N E R G Y (MeV)

7

Thick t a r g e t y i e l d s f o r the

B(p,a) Be, 14N(p,2a)7Be, 9 7 and B e ( p , t ) Be r e a c t i o n s

7

Li(p,n)7Be

146

I f two samples are irradiated, 6x10'

one at energy Ej^ and one at

LITHIUM-

energy E 2 (E 2 > E ^ , the following set of equations can

5x10'-

be written: A

r

B

+ A

B

A

B

Li +

~ r

A

Li \ i

0

58. 5

d

Table IV-4:

I r r a d i a t i o n conditions

12 MeV

Proton energy Energy corresponding to etching depth

9.8 - 10.0 MeV

Beam intensity

4 μΑ

I r r a d i a t i o n time

3 h

Beam intensity monitor

Zr f o i l (100 μπι)

Nuclear reaction

92

t

l/2

Ε

7

Zr(p,n) 9 2 m Nb

10.16 d 935 keV

155

2.4.2.2.

Chemical_segaration_of_^Be

Zirconium ^Be must be separated from niobium, zirconium and yttrium radionuclides 48 56 produced from zirconium and from V and Co produced from titanium and iron by the nuclear reactions given in Table IV-3.

Anion exchange on

Dowex 1 X 8 in 6 Μ hydrofluoric acid followed by precipitation of yttrium fluoride and barium beryllium fluoride was used. The optimum conditions for the chemical separation by anion exchange were studied experimentally.

An irradiated zirconium sample was dissolved in

10 ml of 6 Μ hydrofluoric acid, brought on a column f i l l e d with Dowex

1X8

resin and eluted with 6 Μ hydrofluoric acid.

Successive 10 ml fractions 92m 95 of eluate were collected and the a c t i v i t y measured. No Nb and Zr a c t i v i t y was observed in the eluate.

These elements are thus quantitati-

vely 56 retained 48 on the column. Fig. IV-6 shows the elution curves for ^Be, 87 7 Y, Co and V. The elution curve for Be was determined by treating irradiated boric acid dissolved in 6 Μ hydrofluoric acid together with an inactive zirconium sample in a similar way. Discarding the f i r s t 80 ml of eluate and collecting the next 200 ml allows to separate ''ße quantitatively from Nb and Zr a c t i v i t i e s and also from the 48 56 87 48 56 main part of the V, Co and Y a c t i v i t i e s . The remaining V and Co 7 87 do not affect the detection of Be. A further separation of Y which emits 485 keV γ-rays i s necessary.

This i s achieved by precipitation of

yttrium fluoride (YF ? ). Eventually ^Be i s precipitated as barium beryllium 7 fluoride (BaBeF^) to allow the detection of Be in the same geometrical conditions as the standard and with high efficiency. Procedure for the separation of ^Be from zirconium metal: Dissolve the sample and 10 mg of beryllium metal in 10 ml 6 Μ hydrofluoric acid in a p l a s t i c bottle.

Add 2 ml concentrated hydrofluoric acid.

Add

the solution to the top of a teflon column ( i . d . 1.8 cm; height 40 cm) f i l l e d with 30 g of Dowex 1 X 8 resin kept at room temperature.

The resin

was previously converted to the fluoride form by washing with 250 ml 6 Μ hydrofluoric acid. rate.

Elute with 6 Μ hydrofluoric acid at a 2.5 ml/min flow

After discarding the f i r s t 80 ml of eluate, collect the next 200 ml.

Add dropwise 50 mg of yttrium nitrate hexahydrate dissolved in 10 ml of water while s t i r r i n g with a magnetic s t i r r e r . Continue s t i r r i n g for 15 min.

156

ml

Fig. IV-6:

ml

Elution curves

F i l t e r the yttrium fluoride precipitate on a membrane f i l t e r . s t i r r i n g 6 ml of 0.5 Μ barium nitrate.

Add while

After 15 min, f i l t e r off the

barium beryllium fluoride precipitate, wash with ethanol and dry at 80°C. The effectiveness of the chemical separation i s i l l u s t r a t e d by the Ge(Li) spectra, given i n Fig. IV-7. Zi rcaloy After chemical separation by anion exchange as described above, the eluate of an irradiated zircaloy sample contains important a c t i v i t i e s mainly of 120m 52

Sb,

Mn,

48

122

Sb and

V and

56

124

Sb (from Sn), of

87

Y,

Co (from Cr, Ti and Fe). 5+

oxidized to Sb ed by the resin.

88

Y and

91

Y (from Zr) and of

When before the elution Sb? + i s

with hydrogen peroxide, the antimony i s completely retainThe fraction of the eluate between 80 and 280 ml s t i l l

157

Fig. IV-7:

Spectrum of a dissolved sample measured for 5 min at low

detection efficiency.

Spectrum for a BaBeF^ precipitate measured for

60 h at high detection efficiency.

BG means background a c t i v i t y

48 contains considerable amounts of

56 V,

52 Co and

Mn, because the zircaloy

contains more chromium, iron and titanium than zirconium.

To avoid s i g n i -

ficant coprecipitation of these radionuclides with the barium beryllium fluoride precipitate some vanadium, cobalt and manganese carriers are added before precipitation. Procedure for the separation of 7Be from zircaloy: Dissolve the sample as for zirconium. and 2 ml hydrogen fluoride (50 %).

Add 2 ml hydrogen peroxide (30 %)

Add the solution onto the column and

elute as for zirconium, collecting the fraction of the eluate between 80 and 280 ml. Precipitate yttrium fluoride at 70-80°C and s t i r for one hour. F i l t e r o f f on a membrane f i l t e r . Dissolve 50 mg vanadium(III) oxide, 50 mg manganese(II) chloride dihydrate and 50 mg c o b a l t ( I I ) chloride hexahydrate i n the f i l t r a t e and add dropwise 6 ml 0.5 Μ barium nitrate while s t i r r i n g . F i l t e r o f f the precipitate, wash with ethanol and dry at 80°C.

158 2.4.2.3.

De te tTQi^Qa tjon

he_c hemi ca X

i el_d

Calculation of the chemical y i e l d from the weight of the precipitate i s inaccurate, because some barium fluoride coprecipitates. I t was attempted to avoid t h i s coprecipitation but t h i s resulted in low y i e l d s of barium beryl 1ium fluoride. Therefore, the chemical y i e l d was checked by different methods. acid pellet was irradiated and the ^Be a c t i v i t y measured.

A boric

The pellet was

dissolved together with 1 g of zirconium and 10 mg of beryllium metal and the chemical separation was carried out.

The barium beryllium fluoride

precipitate contained 93-97 % of the original

7

Be a c t i v i t y .

Atomic absorption and ICP emission spectrometry were also applied. A known quantity of beryllium (10 mg) was added during d i s s o l u t i o n of the sample and the amount of beryllium in the precipitate was determined. cases the standard addition method gave accurate r e s u l t s . line of beryllium was measured.

In both

The 234.86 nm

The y i e l d s obtained ranged also from 93

to 97 %. 2.4.2.4.

Measurements

The samples and the standards were measured with a Ge(Li) γ-spectrometer to detect the 478 keV 7 - r a y s of ^Be. 1 d after the i r r a d i a t i o n .

The samples were measured for 60 h,

The zirconium beam intensity monitors were

also measured with the Ge(Li) detector. 2.4.2.5.

Discussion

Mortier et al. (38) obtained precisions ranging from 3.5 to 10.7 % for concentrations between 1.02 and 0.16 ^g/g.

The detection l i m i t i s

0.033 Mg/g, but can be lowered by carrying out the measurements with an anti-compton spectrometer.

In t h i s way an upper l i m i t of 0.015 /jg/g was

obtained for undoped zirconium. 3.

EVALUATION OF METHODS

3.1. THE DETERMINATION OF BORON IN ALUMINIUM The boron content of aluminium metal i s in general rather low and can only be determined with a limited number of routine chemical methods.

159 In addition to emission spectrometry with plasma excitation as described by Grallath et al. (8), which i s claimed to be very precise even at the sub-Mg/g level, activation analytical methods are well suited for t h i s type of a n a l y s i s .

In the frame of a BCR round robin (Table IV-5) good

agreement was obtained between instrumental proton activation analysis and deuteron activation analysis with radiochemical separation of 7Be or n

C

(31).

These results indicate that interference from lithium in proton

activation analysis i s not s i g n i f i c a n t in normal primary ingot aluminium. Table IV-5:

Results of a BCR round robin for boron in primary ingot aluminium

Method

Results (fg/g)

Photometry (methylene blue)

1.27 + 0.08

Emission spectrometry (plasma excitation)

1.20 + 0.03

10

1.17 + 0.18

B(p,a) 7 Be

1.16 + 0.10 10

B(d,an) 7 Be

1.11 + 0.17

10

B(d,n)nc

1.18 + 0.09

Isotope d i l u t i o n mass spectrometry

1.25 + 0.05

Spark-source mass spectrometry

1.0

+ 0.2

Accurate results were also obtained using the methylene blue method as described by Meier et a l . (20).

However, as i t i s the case for most of

the photometric methods c r i t i c a l examination i s required to avoid errors due to small alterations in the operational conditions (8).

Also the

correction for blanks can be a problem, especially when the method i s used near i t s detection l i m i t .

This was demonstrated within the above

BCR round robin: a f i r s t result of (0.30 +_ 0.12) ^g/g was obtained for the same aluminium as above.

When a purer aluminium sample, accurately

characterized with charged particle activation a n a l y s i s , was used to determine the blank, a value of (1.27 + 0.08) ^g/g was obtained.

160

The results were confirmed by isotope d i l u t i o n mass spectrometry which i s often considered as a " d e f i n i t i v e method" in the c e r t i f i c a t i o n of reference materials.

In t h i s method the chemical concentration i s obtained

from the determination of three isotope ratios (sample, spike and blend) which means that no quantitative separations are required. F i n a l l y good results were also obtained by spark source mass spectrometry as described by Beske et a l . (21).

I t should be noted that in the method

used no reference materials are required for c a l i b r a t i o n . 3.2. THE DETERMINATION OF BORON IN ALUMINIUM-MAGNESIUM ALLOYS The boron concentration of aluminium-magnesium alloy i s

significantly

higher than the one of unalloyed aluminium, so that the determination of boron i s less problematic.

All methods described can probably be used

with acceptable precision and accuracy.

Table VI-6 summarises the results

of three round robins organized by BCR on an AlMg3 a l l o y s containing approximately 65 and 30 pg/g boron. Both the methylene blue and the curcumine method allow precisions as good as + 2-3 %, which i s generally better than those obtained with charged particle activation analysis (5 to 10 %). The latter method seems however to be less influenced by uncontrolled systematic deviations than the photometric methods. As for pure aluminium (see 3.1.) the results of charged particle activation are in good agreement with those obtained by emission spectrometry with plasma excitation (8) and by isotope d i l u t i o n mass spectrometry (45).

An etched 0.5 g aluminium sample i s spiked with a standard solution of boron enriched in I 0 B (1.202- 101 7 at.B/g; 1 0 B = 88.06 at.%), and d i s solved in 2 ml concentrated n i t r i c acid (suprapur) at 70°C under addition of 1 ml hydrochloric acid (suprapur) every 10 to 15 minutes in order to prevent excessive foaming. After addition of 6 ml hydrochloric acid a clear solution i s obtained. To t h i s solution 10 g methanol (purified by d i s t i l l a t i o n over a mixture of sodium hydroxide and mannite) i s added, and boron i s d i s t i l l e d at 95°C in the form of trimethylborate. After d i l u t i o n to 20 ml with water and addition of 2 Mg mannite, the solution i s evaporated down to 0.5-1 ml at 60°C. 25 μΐ of t h i s solution are transformed with lanthanum into lanthanum nitrate and loaded on the rhenium filament of a thermal ionization mass spectrometer. The filament i s heated stepwise to 1050°C, and 10 BOT and 1 1 BOj ions are measured alternatively (10 times each) using a Faraday cup. The obtained r a t i o ' s are corrected for the isotopic composition of oxygen (45).

161 Table IV-6:

Results of BCR round robins on boron in AlMg3 alloys

Method

Results (Mg/g) Batch 1 (40)

Batch 2 (46)

Batch 3 (46)

Photometry -

methylene blue

73.0 + 1.0

30.5 + 1.1

- - -

61.9 + 1.3 68.3 + 3.2 70.9 + 1.6 -

curcumi ne

Emission spectrometry (plasma excitation) 10

B(p,a) 7 Be

70.2 + 2.5 —

—

- - -

31.1 + 1 .8

64.1 + 4.1

33.2 + 3 .4

67.9 + 6.9

32.8 + 2 .3

31.8 + 1 . 5 —

65.6 + 2.3 10

B(a,n)13N

67.4 + 4.5

10

B(d,an) 7 Be

...

10

B(d,n) n c

Isotope dilution mass spectrometry Spark source mass spectrometry Ion-selective electrode

3.3

—

- - -

32.3 + 2 .8

—

34.6 + 1 .9

—

—

31.3 + 3 .5

—

30.0 + 5.6

—

29.0 + 0.3 —

29.7 + 1.0

—

THE DETERMINATION OF BORON IN ZIRCONIUM AND ZIRCALOY

In the frame of BCR a number of analysis methods for boron in zirconium have been evaluated using pure zirconium samples doped quantitatively with boron using high frequency levitation melting.

The essential feature

of this quantitative alloying method is the homogeneous mixing of compounds of known mass into a single sample with no loss or contamination.

During

the preparation the sample floats in a magnetic field without any direct contact with its surroundings.

From the masses and purities of the com-

ponents, the concentration of each element in the sample may thus be calculated, possible small loses of material being considered as the uncertainty (41).

162 When applied to boron i n unalloyed zirconium, the technique proved to y i e l d homogeneous samples and to be q u a n t i t a t i v e and r e p e t i t i v e w i t h i n the quoted uncertainties.

Only using secondary ion mass spectrometry heterogeneities

could o c c a s i o n a l l y be detected i n the samples with the highest boron contents (42). These were due to expected zirconium boride m i c r o p r e c i p i t a t i o n s . At high concentration l e v e l s (100 and 20 vg/g)

several a n a l y s i s methods

y i e l d e d accurate r e s u l t s as shown i n Table IV-7 ( 4 3 ) . spectrometry was however found to be f u l l y Table IV-7:

Classical

emission

unappropriate.

Results of BCR round robins on pure zirconium doped with 100 and 20 Mg/g boron

Method

Results

(pg/g)

Photometry (methylene blue)

97.5 +

Spark source mass spectrometry

98

Results

4.2

+ 11

Emission spectrometry (plasma e x c i t a t i o n )

(Mg/g)

19.4 + 1.0 21.5 + 1.9 22.7 + 2.7

Neutron induced a measurement

98.0+

4.0

18.3 + 2.2

Neutron induced 7 measurement

99.1+

2.6

19.6 + 1.2

102.5+

3.6

20.7 + 1.0

102.2+

1.0

19.7 + 0.4

100

2

20.0 + 0.4

10

7

B(p,a) Be

Quantitative a l l o y i n g

+

At lower concentration l e v e l s (1.0 and 0.5 Mg/g), which are of p r a c t i c a l i n t e r e s t i n nuclear technology, only few methods remain precise and accurate (Table I V - 8 ) .

Spark source mass spectrometry - as well as secondary

i o n mass spectrometry at the higher concentrations - provides

interesting

information on the degree of homogeneity, e s p e c i a l l y on a microscale. Although the obtained r e s u l t s are g e n e r a l l y good, real q u a n t i t a t i v e

results

w i l l only be obtained using the method of Beske et a l . (8) i n spark source mass spectrometry or when reference samples are a v a i l a b l e . The l a t t e r remark a l s o applies to emission spectrometry on s o l i d samples, on the metal

itself

or a f t e r o x i d a t i o n , which provided only semi-quantitative r e s u l t s . In the l a s t case, standard a d d i t i o n s may be c a r r i e d out, but the difference i n physical s t a t e between samples and standards may even then lead to e r r o r s which are d i f f i c u l t to evaluate a p r i o r i .

Neutron induced prompt γ and

e s p e c i a l l y a , measurements do not give good r e s u l t s for low concentrations.

163

Table IV-8:

Results of BCR round robins on pure zirconium doped with 1.0 and 0.5 μς/g boron (40)

Method

Results

Photometry: - methylene blue

- curcumine Spark source mass spectrometry

Emission spectrometry (plasma excitation)

10

B(p,a) 7 Be

Quantitative alloying

Mg/g)

Results (Aig/g)

0.97 + 0.06

0.62 + 0.03

1.02 + 0.06

0.51 + 0.04

1.03 + 0.13

0.53 + 0.06

1.03 + 0.17

0.36 + 0.04

1.44 + 0.12

0.47 + 0.06

1.10 + 0.12

0.52 + 0.12

1.19 + 0.10

0.55 + 0.16

1.02 + 0.18

0.68 + 0.11

1.11 + 0.14

0.58 + 0.06

1.04 + 0.08

0.48 + 0.02

1.00 + 0.04

0.50 + 0.03

Emission spectrometry with plasma excitation i s based on chemical separat i o n of boron by wet chemistry and i s matrix and form independent.

The

s e n s i t i v i t y i s good (0.01 to 0.02 pg/g) but the method requires several d i f f i c u l t manipulations, thus highly s k i l l e d operators.

The latter also

applies to photometry which was used under different forms: the methylene blue method, with varying BF^ formation times and with and without reextraction of f l u o r i n e , and the curcumine method.

Under optimal condi-

tions, these methods allow precisions and accuracies of + 10 % at the 0.5 to 1 Mg/g level. Proton activation analysis via the reaction

10

B(p,a) 7 Be i s one of the most

interesting methods as i t i s precise and accurate at + 5 %, provided 7Be i s separated radiochemically before counting and the lithium concentration in the sample i s very low, which i s generally the case. The use of protons of lower energy (e.g. 2.75 MeV) theoretically allows the analysis to be carried out in an instrumental way, but experience proved that i t i s d i f f i c u l t to obtain a good precision. 1

Deuteron activation analysis via the

^ ( d . n ) ^ reaction i s an alternative.

This method i s free from nuclear

interferences but requires a radiochemical separation of ^ C .

164 As shown in Table IV-9 this method was applied to pure zirconium and to industrial zircaloy (32)(36) with results which compare well with those obtained by the previous method (38)(43). Table IV-9:

Results of boron analysis in pure zirconium and zircaloy Results (pg/g)

Method

Pure zirconium

Zircaloy

B(d,n) n c

0.024 + 0.002

0.163 + 0.032

B(p,a) 7 Be

< 0.03

0.158 + 0.017

10 10

Emission spectrometry (plasma excitation)

< 0.04

Photometry (methylene blue)

< 0.05

4.

REFERENCES

( 1)

A. Ichiryu and S. Hashimoto Jap. Analyst 10 (1961) 1137

( 2)

D. Kerin Mikrochim. Acta (1964) 670

( 3)

Analyse der Metalle, Vol. 1, Schiedsanalysen, 3. ed., Springer-Verlag, Berlin, 1966, p. 26

( 4)

A.A. Nemodruk, P.N. Palej and Che Chunj-I Zavodskaja Laborat. 28 (1962) 406, ref. Z. Anal. Chem. JL99 (T964) 151

( 5)

L.M. Budanova and A.N. Gurevich Zavodskaja Laborat. 32 (1966) 1208

( 6)

P. Pakalns Met. Metal Form. 39 (1972) 308

(7)

M. Patricot Analusis 5 (1977) 236

( 8)

E. Grallath, P. Tschöpel, G. Kölblin, U. Stix and G. Tölg Ζ. Anal. Chem. 302 (1980) 40

( 9)

M. Codell, G. Norwitz Anal. Chem. 25 (1953) 1446

165

10)

P.V. Mar^enko Zavodskaja Laborat. 27 (1961) 801

11)

E.N. Pollock and L.P. Zopatti Analyst 10 (1963) 118

12)

R.C. Calkins and V.A. Stenger Anal. Chem. 28 (1956) 399

13)

M. Freegarde and J. Cartwright Analyst 87 (1962) 214

14)

W.T. El well and D.F. Wood Analysis of the new metals T i , Zr, Hf, Nb, Ta, W and their a l l o y s , Pergamon Press, Oxford, 1966

15)

W.T. Elwell and D.F. Wood Analyst 88 (1963) 475

16)

E. Sudö and S. Ijeda Japan Analyst 17 (1968) 1197

17)

H. Ginsberg Leichtmetallanalyse, W. de Gruyter, Berlin, 1955, p. 77

18)

H. Ginsberg Leichtmetallanalyse, W. de Gruyter, B e r l i n , 1965, p. 67

19)

Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, Part 32, 1971, p. 135

20)

K. Meier, K. Wandelburg and G. Wegner Analysis of Non-Metals in Metals, G. Kraft, Editor, W. de Gruyter, B e r l i n , 1981, p. 509

21)

H.E. Beske, G. Frerichs, M.D. Jahn and F.G. Melchers Mikrochim. Acta 32 (1979) 493

22)

Ch. Engelmann J. Radioanal. Chem., 7 (1971) 281

23)

N. Krasnov, P. Dmitriyev, S. Dmitriyeva, I . Konstantinov and G. Mol in Applications of Cyclotrons in Chemistry, Metallurgy and Biology, C. Amphlett, Editor. (1970), 341, Butterworths, London

24)

Ch. Engelmann Int. J. Appl. Radiat. Isotopes, 18 (1967) 569

25)

K. Strijckmans, C. Vandecasteele and M. Esprit Fresenius Ζ. Anal. Chem., 303 (1980) 106

26)

P.N. Kuin EUR 3896 d-f-e (1968)

27)

H. Rommel Anal. Chim. Acta, 34 (1966) 427

(28)

Β. Vialatte J. Radioanal. Chem., 8 (1971) 269

(29)

C.S. S a s t r i , R. Caletka and V. Krivan Anal. Chem., 53 (1981) 765

(30)

S. Shibata, S. Tanaka, T. Suzuki, H. Umezama, J.G. Lo and S.J. Yeh Int. J. Appl. Radiat. Isotopes, 30 (1979) 563

(31)

R. Mortier, C. Vandecasteele and J. Hoste Unpublished results

(32)

R. Mortier, C. Vandecasteele and J. Hoste Unpublished results

(33)

P. Goethals, C. Vandecasteele and J. Hoste Analysis of Non-Metals in Metals, G. Kraft, Editor, W. de Gruyter, Berlin, 1981, p. 259

(34)

R. Mortier, C. Vandecasteele and J. Hoste Analysis of Non-Metals in Metals, G. Kraft, Editor, W. de Gruyter, Berlin, 1981, p. 267

(35)

M. Fedoroff and C. Loos-Neskovic Radiochem. Radioanal. Lett., 39 (1979) 121

(36)

R. Mortier, K. Strijckmans and C. Vandecasteele B u l l e t i n Soc. Chim. Belg. 90 (1981) 297

(37)

J. Petit, J. Gösset and Ch. Engelmann J. Radioanal. Chem., 55 (1980) 69

(38)

R. Mortier, C. Vandecasteele and J. Hoste Anal. Chim. Acta, 121 (1980) 147

(39)

Ch. Engelmann J. Radioanal. Chem., 7 (1971) 89

(40)

Internal report BCR/54/79 (1979)

(41)

J. Van Audenhove and J. Joyeux J. Nucl. Mat. _19 (1966) 97

(42)

P. Block and F. Adams Personal communication

(43)

Internal report BCR/86/79 (1979)

(44)

J. Van Audenhove Personal communication

(45)

K.G. Heumann Personal communication

(46)

Internal report BCR/85/82 (1982)

CHAPTER V

THE DETERMINATION OF CARBON

1.

CHEMICAL METHODS

1.1

INTRODUCTION

The determination of carbon in metals i s generally carried out by combustion in a stream of oxygen and subsequent determination of carbon dioxide by gravimetry (e.g. after absorption on soda lime), manometry, titrimetry after absorption in an alkaline solution or by gas chromatography.

Which

of these techniques should be preferred depends on the carbon concentration, the sample weight and the accuracy required.

As non-ferrous metals

mostly contain low carbon concentrations, only coulometry, conductometry and gas chromatography are of practical interest.

A new relative conduc-

tometric method, which enables accurate simultaneous determinations of micro amounts of carbon and sulphur and can work without fluxing agents, may become of special interest (1). In the analysis procedure the combustion of the sample i s the crucial step.

Only in exceptional cases and when the sample i s in a f i n e l y devided

form, can the combustion be carried out without addition of fluxing agents. Appropriate fluxing agents are discussed for each metal separately. Blank values produced by crucibles, boats, tubes and combustion gas are of high importance, especially when low carbon concentrations have to be determined.

Their importance depends on the type of heating used.

In

general, lower and more constant blank values are obtained when tube furnaces are used.

In this case i t i s generally s u f f i c i e n t to preheat

the combustion boats and tubes for some time to a few hundred °C above the analysis temperature.

This procedure i s more d i f f i c u l t to apply and

not so r e l i a b l e when induction furnaces are used, since the combustion boat containing the sample couples much better with the induction f i e l d than the empty boat, so that during the analysis a higher temperature i s reached than during the blank determination.

Furthermore, the oxide

168 melt formed during combustion may attack the combustion boat so that parts which remain inaccessible in the blank determination are attacked. Only high quality ceramics can be used for tubes, crucibles or boats.

If

tube furnaces are used, i t may be advantageous to use quartz tubes or packets to protect the ceramic tube from splashing combustion products, thus avoiding their destruction. Furthermore, the blank value of the oxygen used as combustion gas may not be overlooked, as i t may contain small amounts of carbon dioxide or organic impurities (e.g. o i l vapours). solution i s not s u f f i c i e n t .

Washing the combustion gas in an alkaline

I t i s necessary to combust the organic com-

ponents by leading the gas over copper oxide at 300°C and subsequently trap the carbon dioxide in a l k a l i . 1.2

THE DETERMINATION OF CARBON IN ALUMINIUM

Aluminium generally contains less than a few Mg/g carbon.

In r o l l e d sheet

the concentration may be higher, especially in zones near the surface (8). Cast products also have higher carbon concentrations, especially when the melt was treated with carbon separating mixtures for refining purposes. 1.2.1.

Wet chemical methods

In e a r l i e r times, carbon was determined in aluminium using wet chemical methods.

This was mainly due to the d i f f i c u l t i e s encountered in the

combustion of the metal.

The sample may be dissolved in chromic-sulphuric

acid (2), or a combined wet-dry method may be used to dissolve i t in non-oxidizing mediums (3), or i t i s v o l a t i l i z e d in a hydrochloric acid or chlorine gas stream (4,5,6,7), the remaining insoluble parts or rests being burnt. -

The problems with these procedures are obvious:

carbides which are transformed into methane by acid reaction are not quantitatively determined,

-

most of these methods have important blanks.

1.2.2.

Combustion

Fundamental work was carried out by Fischer and Schmidt (8) using a tube furnace and combustion boats of pythagoras-mass.

Carbon dioxide i s absorb-

ed in barium hydroxide solution (pH = 9.9) and determined by back t i t r a t i o n

169 using Potentiometrie indication.

The combustion is carried out at 1050 to

1100°C in a stream of moistened oxygen (250 ml/min). An amount of copper wire equal to the sample weight and 1/5 of this amount of a lead-antimony alloy (50/50) are used as fluxing agents. The aluminium sample is melted together with the fluxing agents before the combustion is started.

Samples up to 2 g can be used, compact material being preferred.

The analysis time is 15 min.

Before analysis all materials are cleaned

by acid etching and the combustion boats and tube are annealed. blank values are low and surprisingly constant.

Remaining

In pure aluminium, carbon

concentrations smaller than 1 ^g/g have been determined using this method. The above procedure does however not guarantee complete combustion and aluminium pearls are always observed in the combustion slags. Satisfactory conditions are however reached when metallic bismuth is used as fluxing agent in a 1:1 ratio to aluminium, the further procedure being similar to the above one. Sulzberger (9) analyzed this recommendation thoroughly and applied it to aluminium alloys as well.

The operational conditions are summarized in

Table V-l. Table V-l:

Operational conditions proposed by Sulzberger (9) Temperature (°C)

Al : Bi

Al

1200

2 : 1

AlCu & AlZn

1150

2 : 1

AISi (5-14 % Si)

1350

3 : 2

AlMg (< 3 % Mg)

1350

3 : 2

AlMg (> 3 % Mg)

1350

1 : 1

Material

The analyses are carried out on cylindrical samples of 4 mm diameter and 20 mm length, or on strips of 5 χ 5 χ 1 mm.

Sample weights are 1 to 2 g,

except when the combustion is carried out at 1350°C, in which case they are limited to 0.5 g, because larger amounts may react very violently.

170

1.3

THE DETERMINATION OF CARBON IN TITANIUM, ZIRCONIUM and ZIRCALOY

As the carbon concentrations of titanium, zirconium and zircaloy range from 10 to 100 μς/g their determination seems easier than for aluminium. I t i s however complicated by the fact that these materials can mostly not be burnt in compact form and that f i n e l y divided samples may react in an explosive way. 1.3.1.

Wet chemical methods

The f i r s t carbon determinations in titanium were based on a known method of steel analysis (11), namely the photometric measurement of the yellow colour resulting from the decomposition of carbides with n i t r i c acid. The procedure i s as follows: a 1 g sample i s dissolved in 20 ml sulphuric (1+3) and 10 ml fluoroboric acid (prepared by addition of 130 g boric acid to 280 ml hydrofluoric acid (48 %) under ice cooling) under gentle heating. After d i s s o l u t i o n , 5 ml n i t r i c acid (1+1) i s added and the solution i s f i l t e r e d i ηto a 50 ml calibrated f l a s k and analyzed by photometry at 450 nm.

The colour i s stable for 3 hrs.

Impurities present in technical

titanium do not interfer. Although t h i s method seems easy to apply, i t found no use in practice, but i s interesting from a theoretical point of view. 1.3.2.

Combustion

A combustion method with

g r a v i m e t r i c _ d e t e r m i n a t i o n

of carbon dioxide after

absorption on ascarite i s described by Nunemaker and Shrader (12). 0.5 g f i n e l y divided sample i s mixed with 0.5 g iron chips and 0.3 to 0.5 g brass, and i s burnt at 1200°C in an oxygen stream in a sintercorundum boat f i t t i n g in a quartz tube.

Before absorption, the combustion gases are

purified in a tube f i l l e d with copper oxide, platinum asbestos and s i l v e r wool at 600°C.

The method allows the determination of 50 to 10000 ßg/g

carbon with an accuracy of 50 Mg/g. A similar method i s described in the "Handbook on Titanium Metal" The combustion i s carried out using a high-frequency furnace.

(13).

The combus-

tion boat i s coated with aluminium oxide and no fluxing agents are used. The method can be used for powder, sponge and compact material containing more than 100 ßg/g carbon.

171

A kind of " c o n t r o l l e d combustion" i s described by Payne (14).

In t h i s

method, a boat of sintercorundum l i n e d with aluminium oxide, containing a 2 g sample, i s put i n the cold furnace.

An oxygen stream of 100 ml/miη

i s led through and the temperature i s slowly r a i s e d to 900°C. s t a r t s suddenly.

Combustion

When i t i s completed, the device i s flushed with oxygen

f o r several minutes.

The produced carbon dioxide i s absorbed on a s c a r i t e

and determined g r a v i m e t r i c a l l y .

The method i s only applicable to f i n e l y

divided material and the detection l i m i t i s of the order of 100 ßg/g. Reference (15) describes a gravimetric method in which the combustion i s c a r r i e d out by high-frequency heating at 1550°C.

A t i n f l u x (1 g Sn f o r

1.4 g T i ) i s used and the oxygen flow i s 400 ml/min. Codel1 et a l . (20) use lead as f l u x i n g agent. 2 g granulated lead i s used.

For a 1 g titanium sample,

The furnace temperature i s 900°C, the flow

of p u r i f i e d oxygen 500 ml/min.

For concentrations in the 2000 to 3000 ßg/g

range, p r e c i s i o n s of 2 to 3 % r e l a t i v e are quoted. Elwell and Wood (21) obtain more c o n t r o l l a b l e combustion conditions by f i l l i n g the combustion tube f i r s t with argon at the used temperature and furtheron f o r c i n g i t g r a d u a l l y out with oxygen. l a r g e r samples can be burnt.

An advantage i s that

Titanium and i t s a l l o y s need a lead f l u x

(3 g Pb f o r samples of 3 to 4 g ) , whereas zirconium and i t s a l l o y s need no f l u x at a l l .

The r e p r o d u c i b i l i t y of the method i s 100 ßg/g for carbon

concentrations of 1000 ßg/g

in titanium (39).

The method i s useful

for

concentrations above 500 ^g/g. The sample must be f i n e l y divided, d i s t r i b u t e d evenly in a thin layer over the bottom o f the combustion boat and covered with the f l u x i n g agent.

For

the a n a l y s i s of zirconium and i t s a l l o y s , i t i s advised to use 2 g bismuth chips and 1 to 2 g granulated crude i r o n (< 50 μg/g carbon) as f l u x . Wood and Williams (22) a l s o prefer to use a mixture of bismuth and i r o n f o r the a n a l y s i s of zirconium.

Following e s s e n t i a l l y the same procedure,

they determine carbon concentrations between 200 and 2000 ßg/g with an accuracy of 50 ßg/g using a gravimetric determination method. weight i s 4 g. i s used.

For concentrations below 50 ßg/g,

The sample

a conductometric method

For one gram samples an accuracy of 10 μg/g i s claimed.

According to reference (15) a 2asvolumetric_method should be appropriate as w e l l .

Combustion i s c a r r i e d out at 1370°C, using t i n ( r a t i o = 1:1) as

172 fluxing agent.

The gas volume i s measured before and after absorption of

carbon dioxide in sodium hydroxide s o l u t i o n . The determination of low carbon concentrations i n titanium using a titrimeΐΓΪ9_0!§£[}9ί!_

is

described by Fischer and Schmidt (19).

The method i s iden-

t i c a l to the one described in 1.2.2, combustion being carried out using a tube furnace.

Even for f i n e l y divided titanium there i s no violent reac-

tion up to ca. 900°C, but only an oxidation of the surface. and 1100°C the combustion starts after 20 to 60 s .

Between 900

Above 1100°C the reac-

tion s t a r t s v i o l e n t l y after a few seconds whilst brightly f l a s h i n g , whereby the combustion boat (pythagoras-mass) i s heavily corroded.

This corrosion

can be avoided by laying a magnesia groove into the boat, but even then at 1250°C the combustion i s not always complete. Combustion becomes however f u l l y s a t i s f a c t o r y using the fluxing agents l i s t e d in Table V-2.

The combustion temperature i s 1150°C.

When a

moistened oxygen stream of 400 ml/min i s used, the reaction i s not too violent.

The samples to be used are compact pieces.

Both samples and

fluxing agents are etched before analysis (HNO.,, conc. = 4:1). Table V-2:

Fluxing agents used for the analysis of carbon in titanium

Carbon concentration

Sample weight

Flux (g)

(g)

(fg/g) 5000 -

10000

0.1

0 . 5 Cu + 0 . 1 PbSn (1:1)

1000 -

5000

0.2

0 . 5 Cu + 0 . 1

500 -

1000

0.5

1

Cu + 0 . 2

1

2

Cu + 0 . 4

< 500

Copper i s used as wire, the lead-tin alloy i s rolled down to a 0.2 mm f o i l . When the fluxing agents i n the boat are distributed in such a way that the titanium i s on top, the copper wire at the bottom and the lead-tin f o i l between

in

and they do not touch the wall, the violent reaction with the

boat i s avoided and the combustion proceeds without interferences.

Both

unalloyed titanium and titanium alloys have been analyzed in a r e l i a b l e way using this method.

In titanium sponge, concentrations between 30 and

80 Mg/g were found, i n pure titanium 40 to 60 Mg/g and in alloys up to 300 μς/g.

173 Ogneva et al. (28) use essentially the same method: sample weight of 0.5 g, oxygen stream, 1250°C, coulometric production of reagents. described by Arsenijevic and Tomasevic (29) is similar too.

The procedure For the com-

bustion of 0.1 to 1 g titanium an iron and copper flux is used, for the analysis of zirconium 1 g iron and 0.5 g tin are used.

The combustion

temperature is 1000°C.

Carbon dioxide is determined using a differential

conductometric method.

The sensitivity limit of the method is 5 Aig/g,

the repeatability + 10 % relative. Elwell and Wood (39) use the conductometric_determination of carbon dioxide to determine carbon in titanium and zirconium. out similarly as in their gravimetric method.

The combustion is carried The reproducibility is

+ 10 μς/g at the 200 Mg/g level. ASTM (46) also recommends this procedure to determine carbon in zirconium. Chips or similar forms with a diagonal length of 7 mm maximum are used. The range of application of the method is 200 to 600 μg/g-

When the

combustion is carried out in a high-frequency furnace at 1480°C, iron and tin chips - 1 gram of each - are used as accelerator. by 0.8 g copper(II)-oxide.

Tin can be replaced

When the combustion is carried out in a tube

furnace at (1315 + 14)°C, 2 g tin can be used.

The 0.5 g zirconium sample

is put in the combustion boat onto a bed of iron chips, covered by the rest of the iron and tin.

Carbon dioxide is absorbed in barium - or

sodium hydroxide solution, sulphur dioxide being oxidized and trapped on manganese dioxide.

The amount of carbon in the sample is determined by

measuring the change in conductance of the absorbing solution and comparing it with a calibration curve based on the carbon contents of steel reference samples. For the determination of low carbon concentrations in zirconium and zircaloy, Kraft and Kahles (24) carry out the combustion in a tube furnace and determine carbon dioxide by titrimetry, the reagents being produced coulometrically.

They found that fine zircaloy drillings burn uniformly

and quantitatively when they are previously heated under argon and the argon is subsequently forced out with oxygen. 0.8 to 1.2 g.

The sample weight used is

Compact pieces of 40 to 60 mg (total sample weight: 0.2 g)

do not burn under the same conditions at 1200°C in an oxygen stream of 400 ml/min.

Tin and copper fluxes are without effect.

(2:1 to 3:1) reproducible but low results are obtained.

Using iron as flux

174

Quantitative r e s u l t s are obtained for compact pieces, when these are previously heated (together with a 3:1 granulated iron f l u x ) at 1400°C in an argon stream of 400 ml/min for 5 minutes, and then gradually burnt by forcing out the argon by oxygen (400 ml/min). tion i s 15 min.

The duration of the combus-

Thus obtained results are identical with those found by

others at temperatures far above 1550°C (induction heating) using a copper flux (1.4:1).

Grallath (25) confirmed that the above method i s also s u i t a -

ble for the analysis of carbon in titanium, titanium alloys and zirconium alloys. 1.3.3.

Oxidizing fusion

Already a long time ago several authors (16){17)(18) tried to analyse carbon by oxidizing fusion.

The metal was fused in vacuum in an alumina

or in a beryllium oxide crucible, under addition of oxidizing materials such as copper- or iron oxide, whereby carbon i s transformed to carbon monoxide.

After oxidation to carbon dioxide, this i s measured by absorp-

tion or manometrically. Improvements in this procedure have been reported.

The procedure proposed

by Karpov and Natansov (26) i s as follows: a nickel-iron alloy containing 85 to 90 % nickel (e.g. 10 g) i s fused in a sintered alumina crucible and saturated with oxygen. into the melt.

Then the apparatus i s evacuated and the sample put

Thereby the carbon present i s oxidized to carbon dioxide,

which, in the case of titanium, leaves the melt for 95 % in 2 min, and i s measured by gas chromatography. For a 1 gram sample 10 g n i c k e l - i r o n alloy i s used and an uncertainty of 0.03 to 0.25 Mg/g i s claimed.

The standard deviation of the method i s 2

to 35 % for carbon concentrations between 300 and 0.8 vg/q. Similar results were obtained by Vasilevski et a l . (27) for titanium and zirconium. 1.3.4.

Conclusion

The conditions for accurate carbon determination in titanium and zirconium depend e s s e n t i a l l y on the fact whether the material i s f i n e l y divided or not.

Finely divided material can normally be burnt without fluxing agents

in an oxygen stream at temperatures between 1000 and 1200°C.

Compact

175

material needs suitable fluxes in any case.

The most r e l i a b l e method uses

the combined argon-oxygen combustion technique and subsequent coulometric titration.

Fischer and Schmidt (19) described optimal conditions for the

analysis of titanium, Kraft and Kahles (24) for the analysis of zirconium. Both use tube furnaces.

Induction furnaces can be used as well, but no

convincing experimental parameters have until now been given in l i t e r a t u r e . I t i s furthermore evident that correct carbon analyses are only possible i f the samples and the fluxing agents are clean. generally obtained by etching. contamination.

Clean surfaces are

Furthermore, samples must be stored without

As indicated by Lassner (30), p l a s t i c containers are not

suited for t h i s purpose since f i n e l y divided samples, especially powders, stored in p l a s t i c can absorb considerable amounts of carbon. 1.4

THE DETERMINATION OF CARBON IN NIOBIUM, TANTALUM, MOLYBDENUM AND TUNGSTEN

1.4.1.

Combustion

Mallett (31) recommends combustion in a high-frequency furnace in pure oxygen, followed by conductometric determination of carbon dioxide after absorption in barium hydroxide solution. carried out when fluxing agents are used. and 1 g t i n are generally needed.

The combustion can only be For a 1 g sample, 1 g iron

For materials that are d i f f i c u l t to

combust the amount of sample i s reduced to 0.5 g.

I f the sample combusts

e a s i l y , the amount of iron can be reduced to 0.5 g, the amounts of sample and t i n being 1 g. El well and Wood (39) burn niobium, tantalum and tungsten in a tube furnace at 1200°C.

The sample weight i s 2 g, 3 g lead being used as f l u x .

For

zirconium and hafnium containing a l l o y s , 1 g iron and 2 g bismuth are used as f l u x . AGARD (40) discusses the problem of the analysis of carbon in refractory metals in d e t a i l .

As far as the choice of the combustion furnace i s con-

cerned, the following i s stated: " In making a choice between these methods i t should be noted that the principal merit of induction heating l i e s in i t s simplicity and r a p i d i t y , but until accelerators of lower blank contents are available i t cannot

176 " match the accuracy of resistance heating for the determination of carbon at very low l e v e l s , when detection systems of equivalent s e n s i t i v i t y and reproducibility are used." Thereby a range of application of 10 to 120 Aig/g carbon i s taken as a basis, or even lower when reaching low enough blank values.

Millings,

turnings or fragmented pieces, degreased and etched in hydrochloric acid (1+1), are used. The question of combustion fluxing agents i s commented on as follows: " In the case of niobium, i t i s recommended that a flux be used to ensure complete i g n i t i o n of the carbon contained in the sample.

The most

suitable flux for t h i s purpose i s copper but i t i s necessary to pretreat before use to reduce the carbon content to a minimum.

The method of

treatment i s to heat turnings of wire-bar grade copper in quartz boats in a quartz tube at 500° to 600°C in a slow stream of a i r to induce oxidation of the copper and any carbon i t may contain.

After oxidation,

the tube i s allowed to cool and hydrogen used to purge the a i r .

The

copper oxide i s then heated to 850° to 900°C in a stream of hydrogen for two hours.

After reduction, the heat i s switched off and the resultant

copper i s allowed to cool in hydrogen.

Copper pretreated in t h i s manner

should contain less than 1 Mg/g of carbon '(calculated on a 1 g sample basis)." Copper of a similar grade can be produced by reducing copper oxide "wire" supplied by chemical reagent producers for elemental organic a n a l y s i s . For tantalum the method of analysis i s identical to the one used for the determination of carbon in niobium except that no flux i s required. The method of analysis of molybdenum i s identical to that used for the determination of carbon in niobium except that a temperature of 1200°C instead of 1350 to 1400°C i s used.

The use of the copper flux i s recom-

mended to prevent v o l a t i l i z a t i o n of molybdenum t r i o x i d e . For tungsten the analysis method i s identical to that for niobium. combustion temperature i s 1350 to 1400°C. oxygen.

The

Burning i s carried out in pure

Carbon dioxide i s determined by conductometry after absorption

in a 0.003 Μ sodium hydroxide solution. adsorbed on manganese dioxide.

Any sulphur dioxide formed i s

Carbon monoxide i s oxidized to carbon

dioxide on copper oxide at 325°C.

177

A reproducibility of + 5 μς/g is claimed at the 15 μς/g level. According to Kraft and Kahles (45) the determination of carbon in refractory metals can be carried out as follows: 0.5 to 1 g of low carbon iron is weighed into the prepared boat as lowest layer.

Thereon, depending on

the expected carbon concentration, 0.2 to 1 g sample (degree of fineness: 0.2 mm or chips) is stratified and either 0.5 g tin millings or 0.25 g tin millings mixed with 0.25 g granulated bismuth are put.

The combustion is

carried out at 1350 to 1400°C, and carbon dioxide is determined by coulometry or conductometry. In a comprehensive study, Lassner (32) comes to the following conclusions: the combustion of niobium, tantalum, molybdenum and tungsten in a stream of oxygen is easy, but to analyse carbon accurately the use of a flux is necessary.

In the case of molybdenum it suppresses the sublimation of

molybdenum trioxide, in the case of niobium and tantalum it lowers the reaction velocity.

Larger pieces of these metals burn so quickly that

the boats fuse and can thus not be used even with a flux.

Tungsten,

finally, burns slowly at a temperature below 1300°C, but so quickly above this temperature that for a short time very high temperatures are reached which cause melting on of the formed oxide. As it solidifies very quickly and encloses the rest of the metal, it causes low carbon results. In all cases a copper oxide flux in wire form, which is glowed carbon free at 800°C, is used.

The combustion is carried out in a tube furnace and

carbon dioxide is determined by coulometry or conductometry.

Operational

conditions are summarized in Table V-3. Table V-3:

Operational conditions for the analysis of carbon in refractory metals

Nb

Ta

Mo

W

Sample weight (g)

1

2

1

4

CuO-flux (g)

2

2

2

2

Analysis parameters

Temperature (°C) Duration of combustion (min)

1250

1250

1250

1250

4

4

4

5

178 To calibrate the apparatus, calcium carbonate, tungsten carbide and potassium hydrogenphtalate are suitable.

A standard deviation of 2 μς/g was

obtained for molybdenum powder containing about 40 Mg/g. Antonova et a l . (33) discuss different fluxing agents in combination with combustion in a tube furnace at 1250 to 1300°C in an oxygen stream of 200 ml/min.

Carbon dioxide i s determined by titrimetry.

For niobium

a l l o y s they propose copper as fluxing agent, for molybdenum alloys a mixture of zinc oxide and copper, and for tungsten a l l o y s copper oxide. The standard deviation i s claimed to be better than 10 % for carbon concentrations between 30 and 1000 A0)

13

C(P,n)13N

(3.2)

12

12

C(d,n)13N

C(3He,a)UC

(0.3)

(5.5)

110

14

N(p,d)13N

(8.9)

40

( 7 . 5 MeV)

14

N(p,pn)13N

(11.3)

18

(15

MeV)

14

N(d,t)13N

(4.9)

7

( 5

MeV)

14

N(d,dn)13N

(12)

0.7

(10

MeV)

16

0(d,an)13N

(8.4)

0.4

(20

MeV)

( 5

MeV)

(10

MeV)

0.85 (20

MeV)

9

(Q > 0)

a)

(25)

)

0(ρ,α)13Ν

n

C(a,an)UC

a

16

Be(3He,n)UC

(Q > 0)

B(3He,d)UC

(Q > 0)

B(3He,t)UC

(2.5)

10

12

Sensitivity (48)

12 3.6

14

N(3He,ad)nc

(10.2)

16

0(3He,2a)UC

(6.3)

Be(a,2n)UC

(18.8)

10

B(a,t)UC

(15.6)

4.4

11

B(a,tn)11C

(30.8)

14

N(a,at)nC

(29.2)

16

0(a,2an)UC

(32.5)

9

16

( 5

MeV)

(34

MeV)

(38

MeV)

1.75 (42

MeV)

Carbon concentration (ng/g) in an aluminium sample that yields at the end of an irradiation (1 μΑ intensity, irradiation time = one half-life) at the indicated energy an activity of 100 desintegrations/min.

183 samples are chemically etched and dissolved in a sodium hydroxide solution. Ammonium chloride carrier i s added, ammonia i s d i s t i l l e d and trapped in diluted sulphuric acid.

The detection limit i s 1 Mg/g and the precision

about 25 %. A similar method was applied by Goethals et a l . (51). The samples are i r r a diated for 10 min with a 4 μΑ beam of 7 MeV deuterons, which i s degraded to 4.7 - 4.9 MeV.

Graphite and polyethylene disks are used as standards.

A 20-30 Mm surface layer i s removed by etching in a 7/2/1 (v/v/v) mixture of concentrated phosphoric, sulphuric and n i t r i c acids for 5 min at 80°C. 13 The sample i s dissolved in 10 Μ sodium hydroxide. NH3 i s steam d i s t i l led and trapped in diluted hydrochloric acid.

The annihilation a c t i v i t y

i s measured with 2 Nal(Tl) detectors in coincidence.13Analysis of the decay curve confirmed that the d i s t i l l a t e contained only N. For industrial aluminium (99.5 % purity) the results obtained were 0.25 + 0.05 pg/g (n=10). 2.3

THE DETERMINATION OF CARBON IN NICKEL

Strijckmans et a l . (52) describe the determination of carbon in nickel. After i r r a d i a t i o n with 7 MeV deuterons (degraded to 4.1 - 5.0 MeV) the sample i s chemically etched in 3 volumes 40 % hydrofluoric acid and 2 volumes 14 Μ n i t r i c acid and partly dissolved in 25 ml of a solution of 140 mg/1 ammonium hexachloroplatinate in 6 Μ hydrochloric acid.

The ammo-

nium hexachloroplatinate i s added to speed up the d i s s o l u t i o n . 30 min. i s 2 s u f f i c i e n t to dissolve 0.14 - 0.30 g/cm . The volume i s adjusted to 50 ml by adding water.

The solution i s tranferred to a steam d i s t i l l a t i o n appa13 ratus, 40 ml 7.5 Μ sodium hydroxide i s added and NH^ i s steam d i s t i l l e d . The d i s t i l l a t e i s collected in 25 ml 6 Μ hydrochloric acid until the total volume i s 100 ml.

The d i s t i l l a t e i s measured with 2 Nal(Tl) detectors in

coincidence. To check the y i e l d of the chemical separation the procedure was applied to an inactive nickel sample to which after the partial d i s s o l u t i o n some 13

NH 3 was added.

The average y i e l d was (99.6 + 3.8) %.

The results obtained for commercial nickel were (86.4 + 8.3) ρg/g (n=5). 2.4

THE DETERMINATION OF CARBON IN ZIRCONIUM AND ZIRCALOY 12

For zirconium and zircaloy the

13 C(d,n)

Ν reaction allows an instrumental

determination of carbon. The method described was developed by Vandecasteele

184 et a l . (53) for zirconium, by Mortier et a l . (54) and Vandecasteele et a l . (92) for zircaloy.

The detailed experimental procedure i s based on the

work of the l a t t e r authors (92). 2.4.1.

Experimental procedure

The samples are c y l i n d r i c a l disks of 13 mm diameter and 1 mm thick, the standards are graphite pellets of the same dimensions. The i r r a d i a t i o n conditions are summarized in Table V-5.

After the irradia-

tion, the samples are etched in a 1.2 % hydrofluoric acid solution to ? remove a 6-8 mg/cm surface layer. The annihilation photons of the sample and the standard are measured with a y-y coincidence system with 2 Nal(Tl) detectors.

After a 20 to 40 min waiting time, the sample is repeatedly

measured during a 3 to 4 h period.

The only 0 + -emitters present are

13

N from 12 C(d,n) 13 N, 18 F ( t 1 / 9 = 109.8 min) from 1 7 0(d,n) 1 8 F, 55Co ' RR Q?m ( t 1 / 9 = 17.5 h) from Te(d,n) DO Co and 3 mNb ( t 1 / 9 = 10.1 d) from 91 ' 92m ' Zr(d,n) Nb. The decay curve i s therefore analyzed starting with halflives of 9.96 min, 109.8 min, 17.5 and 10.1 d.

The contribution of the

long-lived components at the start of the measurements is 2-4 % of the 13

N activity.

Table V-5:

Irradiation conditions

Deuteron energy (MeV) Energy corresponding to etching depth (MeV)

Sample

Standard

5

5

3.90 - 4.06

—

Beam intensity (μΑ)

1

0.1

Irradiation time (min)

3

2

Beam intensity monitor

Mo-foil (9.18 mg/cm2)

Nuclear reaction

92m , , >93mT Mo(d,n) Tc

t

43.5 min

l/2 Ε y

390 keV

185 2.4.2.

Interferences

No nuclear interferences occur for deuteron energies below 4.9 MeV.

From

+

the Ge(Li) spectrum of an irradiated sample i t follows that no β , γ-emit13 62 ters that can interfere with the measurement of N, are formed. Cu ( t 1 / 9 = 9.73 min) i s the only pure 0 + -emitter l i a b l e to interfere with the 13 61 62 measurement of N. This radionuclide can be produced by the Ni(d,n) Cu and the ^ Z n ( d , a ) ^ C u reactions.

From nickel, ^ C u ( t 1 / 9 = 3.41 h, 60

Ε γ = 283 and 656 keV) i s formed by the 65

61

Ni(d,n)

Cu reaction.

Ga ( t 1 / 2 = 15.2 min, E y = 115 keV) i s formed by the

64

From zinc,

Zn(d,n) 6 5 Ga

reaction.

By means of pure nickel samples, the ratio of the 283 and 62 656 keV peaks in the Ge(Li) spectrum to the Cu contribution to the annih i l a t i o n peak was deduced for a given waiting time after i r r a d i a t i o n . With 65 pure zinc samples, the ratio of the 115 keV peak of Ga to the annihilation peak was determined.

From the Ge(Li) 6? spectra of the analysed mater i a l s i t followed that the interference of Cu formed respectively from nickel and zinc with the determination of carbon i s at most 0.6 and 0.4 %. 2.4.3.

Results

The following results were obtained: Zirconium: (65.1 Zircaloy:

+ 2.6) Mg/g

(n=14)

(119.6 + 7.6) pg/g

(n= 8)

A precision of 4-6 % i s thus possible. Because of the low deuteron energy used, i t i s necessary to give particular attention to the standardization method. 2.5

THE DETERMINATION OF CARBON IN NIOBIUM AND TANTALUM

The conditions for the determination of carbon in niobium and tantalum are similar to those for zirconium and zircaloy. given in reference (53). 13

Decay curve analysis i s required to separate the

N-contribution from the one of

and of

15

Experimental details are

17

F ( t 1 / 2 = 66 s) formed by

0 ( t J / 2 = 2.05 min) formed by

14

N(d,n)150.

16

0(d,n)17F

The results

obtained were (24.8 + 1.5) pg/g (n=5) for niobium and (1.04 + 0.15) μg/g (n=6) for tantalum.

186 2.6

THE DETERMINATION OF CARBON IN MOLYBDENUM AND TUNGSTEN

Vandecasteele et a l . (53)(55)(56) describe the determination of carbon in molybdenum and tungsten.

The i r r a d i a t i o n conditions are given in

Table V-6. Table V-6:

I r r a d i a t i o n conditions

Deuteron energy, MeV

5

Energy corresponding to the etched dept, MeV

2.3 - 2.7

Beam i n t e n s i t y , μA

2

I r r a d i a t i o n time, min

20

Beam intensity monitor f o i l

Mo, 9.34 mg/cm2

Beam intensity normalization

92

Mo(d,n) 9 3 m Tc

43.0

^1/2' Ε , keV

390

7

The molybdenum samples are etched in a mixture of 4 volumes hydrofluoric acid (40 %) and 1 volume n i t r i c acid (14 M) at room temperature, dipped for 20 s in 12 Μ hydrochloric acid, rinsed successively with water and acetone and dried in a stream of hot a i r .

This sequence i s repeated twice.

For tungsten the samples are etched twice in a mixture of 1 volume hydrof l u o r i c acid (50 %) and 1 volume n i t r i c acid (14 M) at room temperature, rinsed and dried. The annihilation a c t i v i t y i s repeatedly measured with a γ-γ coincidence set-up with 2 Nal(Tl) detectors. For carbon i n molybdenum the decay curve 13 93 contained 2 components: Ν and Tc ( t , / 9 = 2.73 h). For carbon in 13 tungsten i t corresponded to pure N. The choice of the incident energy i s not c r i t i c a l for the determination of carbon in tungsten.

I t i s however very important for the instrumental

determination of carbon in molybdenum. target y i e l d s for the

92

Mo(d,n)

function of the energy (55).

93m

Tc and

Fig. V-l gives relative thick 100

Mo(d,p) 1 0 1 Mo reactions as a

These were determined by i r r a d i a t i o n of a

stack of molybdenum f o i l s with 5 MeV deuterons.

For comparison, the thick

187

, 13,

12,

reaction i s also given.

io5

The

Ν. 92 93m Ν. Moid,η) To Ν. EY= 390 KeV 43 min

a c t i v i t y produced from molybdenum decreases rapidly with energy.

104

For an incident energy

below 3 MeV, the matrix a c t i v i t y i s less than 1/500 of that obtained at 5 MeV.

An incident

energy between 2.3 and 2.7 MeV

Vi

Ζ 3 > U L 3 < 103 ω < >-

was chosen.

12 13 Hd.n) Ν Ev= 511 kev^* ti/2=10.0 min .

2

I< ίο α Ui ο α ζ ίο'

The results were (72 + 32)ng/g (n=7) and (97 + 44)ng/g (n=4) for carbon in molybdenum and

4) Σ rs >

\

100 101 Mold.p) Mo EY = 1013keV tv:-14.6min

\

\

\ N.

< 15 and (8.4 + 2.4)ng/g (n=4) for 2 different types of tungsten. 3.

1

1

PHOTON ACTIVATION ANALYSIS Fig. V-1:

3.1

THE DETERMINATION OF CARBON IN SODIUM

for the

92

1 ENERGY (MeV)

Relative thick-target y i e l d s Mo(d,n) 9 3 m Tc (E

23

100

t 1 / 2 = 43.5 min) and (Ε γ = 1013 keV,

Two photonuclear reactions can 23 22 activate sodium: Na(T,n) Na

I

511 keV (ß ),

Mo(d,p) 1 0 1 Mo

= 14.6 min) reac12

tions and for the +

= 390 keV,

=

C(d,n)13N

10

·

0

min

)

(E

=

reaction.

18

12.5 MeV) and Na(T,an) F 13., l l r , 14.., ,11„ 18 r , 18 n , A8C . 93nv. C(p,n) N, C from Ν(ρ,α) C, F from 0(p,n) F and Mo 93 93m ( t 1 / 9 = 6.95 h) from Nb(p,n) Mo; the one for tantalum was only composed of 11C and 18F. Because of the long waiting time after i r r a d i a t i o n , 13 the contribution of Ν was negligible in the case of tantalum. The decay curves were analysed with the CLSQ programme (see Chapter I I I - 2.8.2). Nylon-6 was used as a standard. 14 As discussed in section 2.2, the by

Ν(ρ,α)

11 C reaction i s interfered with

An upper limit for the boron concentration was determined

by instrumental deuteron activation.

Upper limits for boron of 0.50 a'g/g

and 0.018 Mg/g for niobium and tantalum resp. were obtained corresponding to an upper l i m i t for the interferences of boron of resp. 0.68 and 0.025 Mg/g of nitrogen. 2.6.2.

Using the

14

N(d,n) 1 5 0 reaction

Strijckmans et al. (42)(46) irradiate a sample, placed behind a nickel beam intensity monitor f o i l , for 1 to 2 min with a 2 μΑ beam of 7 MeV deuterons.

The beam intensity was normalized by measurement of

(Ε γ = 282.9 keV, t 1 / 2 = 3.41 h) formed by the

60

61

Cu

61

N i ( d , n ) C u reaction.

After chemical etching the samples were repeatedly measured with a Ge(Li) γ-spectrometer for 1 min during a 20 to 30 min period starting 1.5 to 2.5 min after the i r r a d i a t i o n .

Nylon-6 was used as a nitrogen standard.

Fig. VI-5 shows the decay curves for a niobium and tantalum sample.

The

decay curves for the samples and the standards were composed of ^ F from 16

0 ( d , n ) 1 7 F , of

2.6.3.

15

0 from

Using the

14

14

N(d,n) 1 5 0 and of

13

N from

12

C(d,n) 1 3 N.

N(p,n) 1 4 0 reaction

The conditions for the determination of nitrogen in niobium and tantalum 14 14 ' (42)(45)(46) using the

N(p,n)

0 reaction are similar to those described

under 2.3 except for the i r r a d i a t i o n (12 MeV protons, 0.5 μΑ for Nb and 3 to 4 aA for Ta) and measuring conditions (No lead absorber needed, detector with 5 % relative detection e f f i c i e n c y ) .

243

Fig. VI-5:

Decay curve of a Nb and Ta sample, irradiated with 5 MeV deuterons

Fig. VI-6 shows a Ge(Li) spectrum for a niobium sample. and 1477.2 keV 7-rays of 14 peak of

93m

Mo formed by

93

Nb(p,n)

93m

The 263.2, 684.6

Mo and the 2313 keV

0 appear in the spectrum, along with the pulser peak used for

the correction for counting losses.

CHANNEL NUMBER

Fig. VI-6:

Ge(Li) γ-ray spectrum of a Nb sample, irradiated with 11 MeV protons

244 2.6.4.

Conclusion

Table V1-7 (46) compares results obtained by the 3 instrumental methods discussed.

The results agree within the experimental errors.

Table V1-7:

Results for nitrogen in niobium and tantalum (Mg/g) Mean + Standard deviation

Reaction

Niobium

Tantalum

14

N(p,n) 1 4 0

218 + 14

10.0 + 1.1

14

N(p,a)nc

242 + 35

11.1 + 0.7

14

N(d,n) 1 5 0

224 + 38

11.1 + 1.0

The r e s u l t s for niobium obtained via the (ρ,α) and (d,n) reactions are less precise than those for tantalum.

This i s probably due to the more

complex decay curve for niobium. 14 The

14 N(p,n)

0 reaction i s most suitable for the determination of n i t r o -

gen in niobium and tantalum, because: -

no nuclear or spectral interferences occur;

-

the method i s purely instrumental;

-

no decay curve analysis i s required;

-

the method i s very f a s t ;

-

the precision i s similar (tantalum) or better (niobium) than for the other reactions discussed.

2.7

THE DETERMINATION OF NITROGEN IN MOLYBDENUM AND TUNGSTEN

An instrumental determination of nitrogen in molybdenum and tungsten via the (ρ,η), (ρ,α) or (d,n) reactions i s not possible (42). Therefore Strijckmans et a l . (42)(46) and Vandecasteele et a l . (54) used the * 4 N ( p , a ) ^ C reaction with chemical separation of ^ C as ^CC^. The samples are irradiated for 20 min with a 1 to 2 μΑ beam of 15 MeV protons.

245 A f t e r i r r a d i a t i o n the samples are chemically etched and placed in an alumina combustion tray together with some 0 . 1 mm copper (Mo:Cu r a t i o = 1) or 1 mm lead f o i l vely.

(W:Pb r a t i o = 1) f o r molybdenum and tungsten r e s p e c t i -

The same apparatus and procedure was used f o r the combustion as f o r

nitrogen i n nickel

(see 2 . 4 ) .

The gas flow from the furnace passes through

a condenser, through Schütze reagent, through 2 absorption v e s s e l s each containing 100 ml 6 Μ n i t r i c acid to trap matrix a c t i v i t y and f i n a l l y through 2 absorption v e s s e l s each containing 100 ml 0.5 Μ sodium hydroxide and 1 ml butanol.

The carbon dioxide i s q u a n t i t a t i v e l y trapped in the

f i r s t sodium hydroxide s o l u t i o n . The absorption v e s s e l s are measured with a y-y coincidence set-up with 2 N a l ( T l ) detectors in a 180° geometry. To obtain the decay curve repeated 5 min measurements are made during 60 min s t a r t i n g 30 min a f t e r the i r r a diation.

The decay curves corresponded to pure

^C.

Nylon 6 was used as a standard. The r e s u l t s obtained were 505 + 35 ng/g (n = 4) and 74.0 + 1.2 ng/g (n = 4) f o r molybdenum and tungsten r e s p e c t i v e l y .

By deuteron a c t i v a t i o n using

the ^ B ( d , n ) ^ C reaction and chemical separation of

as described,

upper l i m i t s f o r boron of 3.7 and 2.2 ng/g were found f o r molybdenum and tungsten r e s p e c t i v e l y .

This corresponds to an interference of l e s s than

5 . 1 and 3.0 ng/g f o r nitrogen in molybdenum and tungsten, which i s

negli-

g i b l e at the actual nitrogen concentrations. 3.

PHOTON ACTIVATION ANALYSIS

3.1

THE DETERMINATION OF NITROGEN IN SODIUM 38

Because of the important a c t i v a t i o n of potassium (formation of

Κ with a

h a l f - l i f e of 7.71 minutes), which i s always present i n sodium, the determination of nitrogen cannot be c a r r i e d out without chemical separation of 13 Ν a f t e r

irradiation.

Chapyzhnikov et a l . (55)(56)(57) developed a procedure based on the Kjeldahl method. water.

A 1 g sample i s d i s s o l v e d in a mixture of alcohol and 13 Ammonia containing NH^ i s d i s t i l l e d , trapped in a s u l p h u r i c acid

s o l u t i o n , and the a c t i v i t y measured. 10 μg/g.

The concentrations ranged from 1 to

The method can a l s o be used f o r l i t h i u m .

246

I t should be noted that oxidizing fusion i s also applicable, and can be 11 13 used to separate

C and

Ν simultaneously (see also chapter V).

This

method was applied to determine nitrogen concentrations between 0.3 and 0.5 Mg/g in sodium from a breeder reactor. Oxidizing fusion i s more r e l i a b l e and of more general use than the Kjeldahl method and can be used to analyze larger amounts of sodium (5 to 7 g).

An

additional advantage i s that i t allows the determination of nitrogen and carbon simultaneously. 3.2

THE DETERMINATION OF NITROGEN IN ALUMINIUM

Oxidizing fusion in helium discussed in chapter I I , section 3.8.2, has been used to determine nitrogen in primary ingot aluminium. The procedure i s identical to that described in chapter V, section 3.2. (same sample size and treatment before and after i r r a d i a t i o n ) . The graphite discs are replaced by nitrogen standards (A1N).

Furthermore, the

samples were irradiated for 20 min in a beam of 30 MeV maximum energy. 13 This ensures that the production of 27

Ν by the competing nuclear reactions:

Al(7,v')13N

mentioned in Table 11-11 i s v i r t u a l l y impossible as shown in Fig. 11-35. The nitrogen content observed was 0.8 μg/g with a standard deviation of 0.1 i 0.1 mm), they are not t o t a l l y dissolved by the oxidizing bath, so that extraction of nitrogen i s incomplete. These two methods have been used to determine nitrogen in nickel discs (diameter 15 mm, thickness 1 mm, weight = 1.6 g).

The samples and a l u -

minium nitride standards were irradiated for 20 min each in a beam of 35 MeV maximum energy (average electron beam intensity = 75 μΑ).

They

247

were then thoroughly etched (thickness removed per face > 50 ;im) at 90°C in three successive baths of 3 volumes n i t r i c , 1 volume sulphuric, 1 volume phosphoric and 5 volumes acetic acid.

After each etch they were

rinsed with d i s t i l l e d water, and dried in hot a i r , before being placed in the respective crucibles. 3.3.1.

Chemical separation of

13

Ν by reducing fusion

Reducing fusion was carried out using the equipment described in chapter I I , section 3.8.3. HF generator (15 kW). a bath i s unnecessary. 3.3.1.1.

Heating at 2200°C was carried out using a

As nickel readily dissolves carbon, the use of Each sample was treated for 2 min.

Effect _of_ tempera ture _2n_the_ext]2action

iel^d

The effect of the temperature on the extraction y i e l d was investigated using a Leco EF 10 furnace, far easier to use than an HF generator for operation at predetermined temperatures. In addition, i t can be used up to 3000°C. The results obtained, given in

1500

Ν i s constant

2500

3000

°C

Fig. VI-7, show that the extrac13 tion y i e l d of

2000

13 Fig. VI-7:

Ν extraction y i e l d from

nickel as a function of reducing fusion

above 2000°C.

temperature 3.3.2.

Chemical separation separation of

13

Ν by oxidizing fusion

After i r r a d i a t i o n and etching, the samples are rolled to reduce their thickness to about 0.1 mm.

They are then treated by oxidizing fusion at

a temperature of 1200°C for 3 min each by means of the equipment described in chapter I I , section 3.8.2.

248

3.3.3.

Results

The c o n c e n t r a t i o n s observed by the two methods are r e s p e c t i v e l y (1.04 + 0.19) μ ς / g and (1.10 + 0.07) Mg/g.

O x i d i z i n g f u s i o n has thus a

b e t t e r r e p e a t a b i l i t y than reducing f u s i o n . 3.4

THE DETERMINATION OF NITROGEN IN REFRACTORY METALS

Schmitt and Fusban (58) simultaneously determined n i t r o g e n and oxygen i n 13 15 r e f r a c t o r y m e t a l s . To separate Ν and 0, i n e r t gas f u s i o n i n a g r a p h i t e c r u c i b l e was used.

The samples were t r e a t e d i n the presence of a bath

( P t , Pt + Fe or Ni + Sn depending on the type of m e t a l ) .

Heating (2200 to

2500°C) was provided by an HF generator. Low n i t r o g e n c o n c e n t r a t i o n s (0.01 to 1 ng/g) were observed i n zirconium, niobium, molybdenum and tungsten.

The samples analyzed were taken from

monocrystals or batches subjected to s p e c i a l p u r i f i c a t i o n treatments.

In

the case o f tungsten, an i n d u s t r i a l product was used. 3.4.1.

Nitrogen i n t i t a n i u m 13

The chemical s e p a r a t i o n of

Ν by o x i d i z i n g f u s i o n (chapter I I ,

3 . 8 . 2 ) was used to determine n i t r o g e n i n t i t a n i u m .

section

However, to prevent

e x c e s s i v e l y v i o l e n t i n i t i a t i o n o f the o x i d a t i o n r e a c t i o n , i t i s

important

f o r the sample to be placed i n the c r u c i b l e beneath the f l u x . The samples ( 7 x 7 x 1

mm, weight — 0.15 g) were i r r a d i a t e d f o r 10 min by

gamma photons w i t h a maximum energy of 27 MeV (average e l e c t r o n beam i n t e n s i t y = 80 μ Α ) .

Since i n d u s t r i a l t i t a n i u m o f t e n contains l a r g e amounts

of oxygen > 500 ^g/g), t h i s energy was chosen to prevent i n t e r f e r e n c e by 16 13 the competing nuclear r e a c t i o n 0(r,t) N. A f t e r a c t i v a t i o n , the samples were etched three times a t 20°C i n a bath c o n t a i n i n g 4 volumes h y d r o f l u o r i c and 1 volume n i t r i c a c i d .

The concentra-

t i o n observed i n an i n d u s t r i a l t i t a n i u m was (128 + 11) ng/g (n = 13). 3.4.2.

Nitrogen i n zirconium 13

The s e p a r a t i o n o f

Ν by reducing f u s i o n i n a g r a p h i t e c r u c i b l e i n the

presence of platinum (weight r a t i o P t / Z r = 20) proved r e l i a b l e .

249

This method was used to analyze nitrogen in zirconium batches containing 500 to 1200 μg/g oxygen (59).

The samples (diameter 8 mm, thickness 1 mm,

weight 0.30 g approximately) were irradiated simultaneously with aluminium nitride standards using a rotary device (10 min).

The maximum beam energy

was limited to 25 MeV (average electron beam intensity = 80 μΑ) for the reasons already discussed for titanium.

After deep etching in several

baths containing 50 volumes n i t r i c acid, 5 volumes hydrofluoric acid and 50 volumes water (etching temperature: 20°C) followed by r i n s i n g s with d i s t i l l e d water and drying with hot a i r , the samples, placed in a sandwich between two platinum d i s c s , were treated for 2 min by the method described i n chapter I I , section 3.8.3.

However, to reach the high temperatures

( > 2500°C) needed for the complete extraction of nitrogen, the HF generator was replaced by a Leco EF 10 furnace. 3.5

OTHER EXAMPLES OF NITROGEN DETERMINATION IN NON-FERROUS METALS

Gorenko et a l . (60) determined nitrogen in several metals (Be, V, Y and Nb).

The samples were activated by a photon beam with a maximum energy 13

of 25 MeV.

Ν was separated by an oxidizing fusion method in helium.

The observed concentrations range between 0.1 and 10 Μ g/g. Nitrogen was also determined in lithium (55)(56) and can be determined non-destructively, at least under certain conditions, in beryllium (61)(62). 4.

EVALUATION OF METHODS

4.1

THE DETERMINATION OF NITROGEN IN ZIRCONIUM AND ZIRCALOY

Zirconium was the f i r s t metal for which a thorough study of analytical methods for nitrogen was carried out under the a c t i v i t i e s of BCR (63). I t was shown that at the 30 jug/g level the following methods were useful: -

reducing fusion under inert gas or i n vacuum on samples of 0.3 to 0.5 g, analyzed together with platinum or palladium according to the sandwich technique, i n a bath-to-metal ratio of 20:1 at extraction temperatures between 2000 and 2200°C;

-

the micro-Kjeldahl method according to Werner and Tblg (11); 13 photon activation a n a l y s i s , with separation of Ν according to the above reducing fusion conditions;

250 -

proton activation analysis, with separation of ^ C as described in chapter V;

-

15

deuteron activation analysis, with separation of

0 by reducing fusion

as described in section 2.5.1. The results of this study are summarized in Table VI-8. excellent.

The agreement is

At the 30 Mg/g level a precision and accuracy of the order of

5 % can be obtained.

In 1980 these methods were used to certify nitrogen

in three existing reference materials for oxygen in zirconium (41). Table VI-8:

Results of a BCR study on the determination of nitrogen in zirconium

Method

Results (^g/g)

Reducing fusion

31 + 2 32 + 2 30 + 2

Micro-Kjeldahl

32 + 1

14.,, ,ll r Ν(ρ,α) C

34 + 1

14

N(d,n) 1 5 0

31 + 4

14

N(7,n)13N

32 + 2

The certified values are listed in Table VI-9.

The values for nitrogen

represent the unweighted mean values of 52 accepted individual measurements obtained using five independent methods in 12 laboratories.

The quoted

uncertainties are based in the precision of the methods used, and on differences between the results which may be due to systematic errors or inhomogeneity of the metal. Table VI-9:

BCR certified reference materials for nitrogen in zirconium

BCR CRM-No.

Certified values (Mg/g)

21

26.6 + 2.7

56

11.7 + 1.8

57

12.0 + 1.7

251

Table VI-10 summarizes the results of a f i r s t intercomparison for nitrogen in zircaloy.

The conclusions are similar as for zirconium.

Table VI-10:

Results of a BCR-study on the determination of nitrogen in zircaloy

Method

Results (A 2100

below 0.1 atmospheres and at 2000°C it is nearly 1 atmosphere.

So in

theory the requirements for the reaction to take place are fulfilled for all the reaction temperatures listed in Table VII-2, since the necessary precondition namely that oxygen is present in the form of aluminium oxide and is not dissolved in the metal, can be regarded as fulfilled.

263 The only process that has been accepted in the investigations carried out by BCR, is the one that makes use of a copper bath at 1750°C, uses argon as the carrier gas, and determines carbon monoxide by coulometric titration after oxidation to carbon dioxide with Schütze reagent (12®5 ened with concentrated h^SO^ and absorbed on silica gel)(33).

m0is

^"

Only this

process reached the sensitivity and reproducibility required (43). This procedure (33) gives the same results as the use of platinum or palladium baths, and is thus clearly superior to the latter for economic reasons.

Working without an auxiliary metal bath did not produce usable

results.

All the aluminium-oxygen compounds that might be expected in

aluminium are reduced in a copper bath; even compact sintered corundum is determined quantitatively.

The joint investigations have shown that

the same reaction conditions also yield success if the reaction is carried out in vacuo. 1.1.3.2.

Pr99§dure

Carrier gas:

Argon, purity 99.998 %\ 15-25 1/hour

Reaction crucible:

A new crucible for every analysis; it does not need to be closed, but can be.

Before analysis, the cruci-

ble should be outgassed at 2000-2100°C until it gives a constant experimental value.

Several analyses per

crucible are possible, but can lead to complex conditions.

The crucible should be cylindrical

(and not

conical), otherwise there is a risk that the melt will creep up and destroy the crucible. Bath metal:

Copper The quantity of copper required is fused in the crucible and is reduced to completion at 1750°C.

Cu:Al ratio = 5 : 1

The aluminium sample is introduced into the molten copper in the form of a compact piece.

Weight of Al sample:at least 2 g; 5 g if the apparatus permits. Reaction time:

For the oxygen contents to be expected in normal industrial aluminium, about 6-10 minutes.

The reaction is

complete as soon as the indicator reverts to the blank value for the apparatus determined before the analysis.

264 Naturally, this value depends on the apparatus available.

With the apparatus used by Kraft and Kahles (33),

at the determination intervals of 3 minutes maintained in that case, it amounts to about 30 to 40 pulses, corresponding to 6 to 8 Mg oxygen.

The standard devia-

tion of the blank value determination is + 1.5 pulses, which means 0.3 Mg oxygen.

The graphite components in

the reaction furnace constitute the essential of the blank value.

source

According to Kaiser and Specker

(41), the limit of detection is 3.\/"7. s ß ~ 1.3 ßg oxygen, corresponding, for an aluminium sample weighing 2 g, to 0.65 jug/g, or, at the recommended sample weight of 5 g, to about 0.25 pg/g. Lower limit of detection: Reproducibility:

+3

0.5 to 1 Mg/g.

% for oxygen amounts of approximately 100 jug.

Supplementary investigations at higher temperatures (up to 2050°C) and at higher copper to aluminium ratios (up to 10:1) did not give oxygen yields which differed from those obtained under the standard conditions. Calibrating the apparatus: -

to test for leaks and to check the reading known quantities of carbon monoxide or carbon dioxide are put through the apparatus with an empty crucible.

Carbon dioxide is preferable, since at the working tempera-

ture it is reduced to give twice its volume of carbon monoxide: the signals obtained when it is passed through the reaction furnace under cold and hot conditions must, therefore, be in the ratio of 1:2.

If

they are not, there is probably a leak in the apparatus. -

to check whether the reaction is taking place quantitatively 100 to 200 ^g aluminium oxide of known composition, wrapped up in a little aluminium foil, are introduced into the copper bath and analysed. A blank value should be determined separately for the foil and allowed for in the calculation. The loss on calcination at 1100°C must be known for the aluminium oxide used; if this is appreciable, allowance must be made for it.

265 1.1.3.3. -

Sgecial_factors

Increasing the weight of the sample: Owing to the great difference in density between the bath metal and aluminium (P C u = 8.96; Ρ ^ = 2.7, both figures relating to the s o l i d state), when the aluminium sample i s introduced into the l i q u i d bath metal, complete mixing does not occur instantaneously, so that a uniform l i q u i d phase i s not formed; on the contrary, the mixture arranges i t s e l f in layers, the l i q u i d aluminium or an aluminium melt of r e l a t i vely low copper content resting on top of an aluminium melt rich in copper.

I f t h i s aluminium-rich layer has only a low copper content,

a vigorous reaction with the graphite crucible occurs.

As would also

be the case with the aluminium alone, t h i s leads to the formation of aluminium carbide, so that the graphite i s wetted by the melt to a considerable extent.

The result of this i s that the melt creeps over

the edge of the crucible and even reaches the outside of the crucible. A thin film of aluminium of t h i s type has a pronounced getter action on carbon monoxide, so that low values are obtained for oxygen; in addition, i f the melt does creep over, there i s great danger of the furnace being destroyed. This phenomenon i s especially observed i f , for example, aluminium samples weighing more than 2 g are used, at a 5:1 bath metal to aluminium r a t i o , irrespective of whether the bath metal used i s copper, platinum or palladium.

As would be expected, the latter two metals have exhib-

ited t h i s effect p a r t i c u l a r l y readily ( p p t = 21.45 and p p d = 12).

The

larger the aluminium samples used, the more pronounced t h i s problem i s . I t cannot be eliminated by taking larger melts for the bath, corresponding, say, to a bath/sample ratio of 9:1. I t has been possible to overcome these d i f f i c u l t i e s in a very simple manner, by interposing a glass tube so as to r a i s e , by about 30 cm, the device for introducing the samples, which i s usually located immediately above the upper edge of the reaction furnace in the commercially a v a i l a ble apparatus.

This has the effect of causing the aluminium sample, as

i t f a l l s into the copper bath, to penetrate more deeply into the l a t t e r , so that the layer formation described occurs either nor at a l l , or no longer to a harmful extent.

This method made i t possible to analyse

5 g samples (8 mm in diameter and 50 mm long) without any problems, in

266 virtually the same time as required for 2 g samples (25 g of Cu forming the bath, arid one analysis per crucible).

There was no gettering

action here either, nor was any ejection of bath metal from the crucible noticeable. 1.1.3.4. If the aluminium under examination contains appreciable quantities of magnesium (> 1000 jug/g) or manganese (> 3000 jug/g), these elements will volatilize during analysis and condense on the colder parts of the apparatus.

These thin films have a particularly strong getter action on

carbon monoxide.

If zinc and/or cadmium are present in the sample, they

will also volatilize, but their getter effect is appreciably less than that of magnesium or manganese.

1.2

THE DETERMINATION OF OXYGEN IN ALUMINIUM ALLOYS

In recent times, only occasional studies have been carried out to determine the oxygen content of aluminium alloys by means of chemical processes. The results of initial attempts (46) to solve the problem by the use of chemical methods of separation, such as gaseous chlorine or bromine/ methanol, for example, do not stand up to critical evaluation any more than those relating to the determination of oxygen in unalloyed aluminium. The bromine-methanol process in particular is subject to a serious systematic error, since it ultimately responds to aluminium amounts, present in the undissolved silicon component of the alloy, as if they were Al^O^, and thus reports them as oxygen. As has been shown by initial comparison tests with activation analysis techniques under the auspices of Eurisotop and BCR Study Groups, good results are obtained with aluminium-silicon alloys - free from magnesium or only containing less than 3000 pg/g of it - if reducing fusion in a stream of carrier gas is employed in the manner suggested by Kraft and Kahles (47) for the analysis of unalloyed aluminium, with the sole difference that the reaction temperature is increased to 1950°C.

Like for

unalloyed aluminium, the oxygen contents reported are near the detection limit, and only increase to values of a few pg/g at silicon contents of 7 % or more.

267 For a number of reasons, it has still not been possible to analyse successfully alloys with a fairly high magnesium content.

It may be possible to

overcome the difficulties due in this case to the volatilization of magnesium during the analysis by deliberately volatilizing it before the actual reducing fusion.

High frequency levitation melting may offer interesting

possibilities here. Volatilization of magnesium from a crucible has hitherto been unsuccessful, because either magnesium reacted with the crucible material or the oxygen content of the sample already underwent a change in this preliminary phase. There is no record of attempts to solve the problem by means of vacuum fusion.

1.3

THE DETERMINATION OF OXYGEN IN COPPER

1.3.1.

General

The solubility of oxygen in copper is extremely high and it is markedly dependent on pressure and temperature.

The following equations for this

relationship are quoted by Fromm and Gebhardt (48): in solid Cu: log CQ = 1/2 log p Q

-1.53 + 1860/T ( 750-1030°C)

in 1iquid Cu: log Cg = 1/2 log p Q

-1.46 + 3953/T

(1065-1400°C)

The excess of oxygen absorbed in the molten state, compared with the solid state, which separates out as Cu 2 0 on solidification, can be detected by metallographic means under polarized light in the form of very fine reddish precipitates.

This formed the basis of one of the first analytical

pro-

cesses for determining oxygen in copper: counting the precipitates of Cu^O within a specified area in the roughly polished surface of the sample under investigation (49).

Even today, this process is still used and recommended

as a rapid means of obtaining information in reaching metallurgical sions.

deci-

According to current opinion, it is considered to be appropriate

for this purpose - but only for this purpose - in the range between 120 and 6000 jug/g (50). The hydrogen reduction process meets a much higher analytical standard, and even today is still regarded as a recognized analytical method

(51)(52)(53).

268 A few other chemical methods will merely be mentioned and not dealt with in detail, because in practice they were never used on a widespread scale: reaction with sulphur and sulphur chlorides (24)(54)(55) and spectrochemical analysis (56)(84). The outlook is different for the direct electrochemical determination of oxygen in the copper melt using a zirconium oxide sensor stabilized with calcium oxide (57)(80)(81)(85). This method is used to an ever-increasing extent. Reducing fusion has become the preferred method for the analysis of solid samples; it was used initially in vacuo (58-63), but later on it was increasingly used under carrier gas (64).

Both variants are used in a

very great diversity of methods of determining 1.3.2.

the reaction gases.

Reducing fusion

1.3.2.1.

Carrier_gas_methods

A detailed study of these methods has been published by Kraft and Kahles (64).

The apparatus employed uses argon (purity 99.998 %) as the carrier

gas (flow: 5 to 40 1/hour), the carbon monoxide formed as the product of the reaction with oxygen is oxidized to carbon dioxide with Schütze reagent ( ^ s

moistened with concentrated H^SO^ and adsorbed on silica

gel), and the latter is determined by coulometric titration after being adsorbed in a slightly alkaline solution (pH about 10) containing B a + + . The reaction furnace is heated by direct passage of a current through graphite heating elements.

The reaction crucibles are made of graphite,

have a cylindrical shape, with a slightly conical base, and are put into the furnace without a cover.

The crucibles can be very simply changed

after each analysis. A new crucible is used for each determination. -

Preferred pa£ametersj_ Duration of determination: 3 minutes Blank value for the apparatus: depending on the size of the crucible and the temperature blank values between 4 and 10 μς oxygen (preferably about 6 Mg) are obtained. Reproducibility of the blank value: usually about 0.2 ßg oxygen; under adverse circumstances 0.4 ^g oxygen.

269 Optimum temperature for unalloyed copper: 1450°C. Preferred size of crucible: external diameter 15 mm; height 15-25 mm; heat up to about 1800°C before the analysis, until the blank value for the apparatus becomes constant.

This

takes about 6 to 9 minutes. Optimum speed of carrier gas for this size of crucible: 15 1/hour. Check on correct functioning: this is carried out using known quantities of very pure carbon dioxide, which is reduced quantitatively at the used temperature to give a double amount of carbon monoxide.

The titration thus indica-

tes twice the quantity fed in. Calibrating the apparatus: by means of a weighed sample of pure CuO, containing a known quantity of oxygen, and introduced into a borehole in the central axis of a cylinder of very pure copper, which has been reduced with hydrogen. Weight of sample: 1 to 5 g, depending on the concentration of oxygen expected. Yield:

better than 99 %.

Standard deviation: 1 /jg/g for oxygen concentrations up to 10 A'g/g 2 ^g/g for oxygen concentrations of the order of 100 Mg/g. Reaction time:

not more than 9 minutes for oxygen concentrations of the order of 1000 jug/g; about 3 minutes at less than 20 pg/g.

0thj;r_pro£osed parameters^ Inert gas fusion is of course not limited to the above described procedure.

The variants listed in Table V11-3 were also used in the joint

programmes of Eurisotop and BCR, even though they yielded not always reliable results particularly for very low oxygen contents (69)(70). It is not possible to quote details here, since these are not available in easily accessible literature.

It should merely be pointed out that

the calibration recommended by one instrument manufacturer, of analysing known quantities of Ag~0, cannot be regarded as suitable.

270 Table V11-3:

Variants in the determination of oxygen in copper by inert gas fusion

Carrier gas

Temperature (°C)

Method of determination

N2

2500

C0 2 : thermal

He

1450

C0 2 : coulometric titration

Ar

1850

C0 2 : coulometric titration

conductivity

For the determination of oxygen in copper, Dahlmann and Fasse! (87) use an auxiliary bath of 80 % platinum and 20 % tin in a 3 to 1 bath to sample ratio; the reaction temperature is 1725 to 1775°C.

The

final concentration of copper in the bath should not exceed 10 %. However this procedure must be regarded as completely obsolete; it has, in any case, only been used for the determination of fairly high oxygen concentrations. 1.3.2.2.

Vacuum_methods

Several variants of vacuum fusion are also available for the determination of oxygen in copper.

The parameters used in european joint research work

(61)(70) are given in Table VII-4. Table VII-4:

Variants in the determination of oxygen by vacuum fusion

Temperature (°C)

Method of determination

2500

CO :

IR photometry

1250 and 1350

C0 2 :

pressure measurement after freezing out

CO :

gas chromatography

1450

The principle of calibrating the vacuum apparatus is the same as that described in detail for the carrier gas technique.

271

When carrying out the process in vacuo, i t must be borne in mind that the v o l a t i l i z a t i o n of copper i s much more pronounced at a given temperature than i t i s under a carrier gas.

This can become a limiting factor.

Temperatures of 2500°C can only be used with graphite capsules.

In this

case the duration of the analysis i s extremely short (about 30 seconds). Although the formation of primary carbon dioxide i s thus suppressed, a very appreciable blank value i s produced, which appears in general to make the process only suitable for the determination of f a i r l y high oxygen concentrations. F i n a l l y , i t must be borne in mind that the phenomenon of primary carbon dioxide formation becomes p a r t i c u l a r l y pronounced when high pumping rates are used. 1.3.2.3.

Problems

I t i s known (62) that, in the determination of oxygen in copper by both inert gas and vacuum fusion, carbon monoxide i s not the only oxygen containing reaction product formed; carbon dioxide i s also formed to some extent. This occurs p a r t i c u l a r l y in vacuo and for high oxygen concentrations (77). Since the latest analytical apparatus in general no longer measure carbon monoxide and dioxide, but only monoxide which in every case i s the p r i n cipal reaction product, the formation of primary carbon dioxide always leads to low values. In order to counteract t h i s loss factor, Frohberg and Leygraf (62) used, in connection with a vacuum apparatus, a procedure in which the reaction gas was impeded from escaping rapidly from the hot reaction zone. Attempts were made to effect t h i s obstruction by enclosing the sample to be analysed in a graphite capsule with a t i g h t l y f i t t i n g screw stopper as spec i f i e d by Paesold (65).

A considerable reduction of primary carbon

dioxide i s achieved by t h i s method, but as confirmed later by the same author (37) i t does not eliminate the problem completely. Frohberg and Leygraf (62) suppressed the formation of primary carbon dioxide by the reduction of the high oxygen potential in copper by carrying out the reaction in a melt of a metal with a f a i r l y high a f f i n i ty for oxygen. Nickel was used for this purpose. However, the experiments

272 did not y i e l d satisfactory r e s u l t s , in particular because foam graphite was deposited very rapidly on the surface of the nickel melt, which considerably retarded, or even prevented, the penetration of the copper sample into the melt and i t s d i s s o l u t i o n in i t .

F i n a l l y , i t was attempted

to overcome the formation of carbon dioxide very simply by using a prereduced copper bath to obtain a considerable d i l u t i o n of the copper to be analysed. However, Paesold (66) shows that, although the quantity of carbon dioxide formed i s reduced by d i l u t i o n , i t cannot be reduced to zero with certainty, and that the likelihood of success in t h i s direction decreases as the oxygen content in the sample increases. He suggests, as the best solution for suppressing the formation of carbon dioxide, a premelt consisting of a copper-nickel-chromium alloy containing approx. 15 % nickel and 0.5 % chromium.

McLauchlan (67) takes up t h i s suggestion.

He uses a sample to

bath metal ratio of 1:0.5 for oxygen contents up to 150 A