The existance of radioactive recoil ions of high mobility

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VOL. 15, 1929

PHYSICS: LOEB AND LOEB

305

THE EXISTENCE OF RADIOACTIVE RECOIL IONS OF HIGH MOBILITY BY LoRA LAN" LOJB AND LZONARD B. LOUB

DZPARTMENT or PHYsIcs, UNImRSInY OF CALFwoRNiA,

BuRRXILuY

Communicated March 4, 1929

Introduction.-Using an air blast method of mobility measurement,

Erikson"2 in 1924 determined the mobility of the positive ions formed about recoil atoms from radium, thorium and actinium emanations. He observed positive ions having closely the mobility of 1.56 cm./sec. per volt/cm. originally found by Franck,3 and Franck and Meitner,4 for the same type of ions. In addition he observed a second group of positive ions having a mobility of some 4.35 cm./sec. per volt/cm. The peaks in the mobility curves characterizing both classes of ions were, however, poorly defined in comparison to the peaks which Erikson usually5 obtained by this method, and the ions failed to show the aging effect usually observed. Erikson first attributed the high mobilities to unclustered ions,' but later work convinced him that the explanation must be sought for elsewhere. He tentatively assumed the faster ion to be one having a double charge.2 In a theoretical discussion of these results one of the writers6'7 showed that to give such a mobility as 4.35 cm./sec. at least a triple charge or possibly even a quadruple charge was required on the ion. It was further shown that although the recoil atoms studied by various workers had always appeared singly charged7 one might account for such multiply charged carriers of high mobility by the Auger effect together with certain theoretical deductions of Franck.7 This explanation, however, appeared to be somewhat too artificial and it was deemed essential to verify if possible the existence of the high mobility radioactive recoil ions by a different method before further speculation was made. Experimental Method.-In making such a test it was decided to use the Rutherford8 alternating current method with high fields and a high frequency square-wave potential from a commutator. A commutator set capable of giving from 2000 alternations per second to 10 alternations per second was used. The method was exceedingly simple. Two circular brass plates of 10 cm. diameter, forming a parallel plate condenser were separated 0.5 cm. by means of an ebonite ring. In their centers were two circular holes 3 cm. in diameter with shoulders into which could be placed discs of 3 cm. diameter which were to serve for the source and detection of the ions. The discs were provided with insulating handles and were secured in place by spring clips. One of these plates N was faced by a nickel plate 3 mm. thick on the face which went into the chamber. The other plate B was merely brass. The one condenser plate B was grounded,

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PHYSICS: LOEB AND LOEB

PRoc. N. A S..

the other N was connected to the commutator brush, through a two-way switch which permitted grounding the plate or placing the potential from the commutator on it. The potential was taken from a bank of small lead accumulators whose center was grounded and whose two end terminals were connected to the slip-ring leads of the commutator. The radioactive source for these measurements consisted of a deposit of Th C obtained by rotating the nickel disc in a hot HCl solution of Radio Thorium, in equilibrium with 3 mg. of Mesothorium II, for 3 minutes. Fifteen minutes after removal from the solution this disc is coated with a deposit consisting of largely Th C. The Thorium C has a half life of 60 minutes and decays through a particle emission to Th C'. The recoiling Th C' atoms that left the nickel disc N, could be collected on the brass disc B of the condenser under the appropriate conditions of field strength and frequency. Th C' has a half-life period of 3.5 minutes, emitting an intense A radiation which is easily measured. These substances are therefore ideal for such measurements in that they decay sufficiently rapidly so as to make serious contamination practically negligible. The short life period of Ra C', however, required that the electroscopic measurements be made by an observation of the distance fallen through by the gold leaf in a measured interval of time and taken a measured interval of time after the test plate B with Th C' had been removed from the condenser. The microscopic eye piece on the electroscope was provided with a transparent scale divided into 0.1-mm. intervals, there being 50 such divisions in the field of view. The rate of fall of the gold leaf was tested and found to be sensibly uniform over the working range. The procedure of making a measurement was as follows: Fifteen minutes after the removal of the nickel source N from the solution the intensity was measured, using the (3 radiation from the source. Then the source was placed in the lower plate of the condenser, and the collector plate B, carefully polished with the finest grade of emery paper, was inserted into the top plate. The alternating potential was then applied to the lower plate N, the upper plate B being grounded. The accelerating potential on the positive ions was always 10 or 20 volts below the negative or retarding potential in order to prevent the ions "working" across the plates over several cycles due to diffusion. The alternating potential was applied for 3 minutes and the upper plate B was removed. Sixty seconds after the removal of the plate B the measurement of its activity was begun. In this minute the electroscope was charged and brought to the zero of the scale. The scale reading on the electroscope after one minute gave the activity of the plate. The plate B was then cleaned with emery, wiped with filter paper and the process repeated using other values of the alternating potential. In this way some 18 or 20 points for various values of the potential could be made before the source N had decayed to values

vow. 15, 1929

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PHYSICS: LOEB AND LOEB

which precluded further work. At the end of the 20 readings the T-ray activity of the source N was again taken and the two values were checked

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