312 



Supplement to ''Nature,'' Auj^st 25, 1923 



Adijenijum.* 



It may he of interest to give here a brief review of 

 some additional facts in connexion with the a-particle, 

 brought to light in recent years. It has long been 

 known tliat a-particles, although projected from the 

 source at the same speed, travel unequal distances 

 through a gas. For example, the maximum distance 

 travelled by the a-particles from radium C in air is 

 7*04 ( m. at 760 mm. and 15° C., the minimum distance 

 is about 6'4 cm., and the mean distance about 6*8 cm. 

 Some " straggling " of the a-particles is to be anti- 

 cifiated on general grounds, since the a-particle loses 

 its energy mainly in liberating electrons from the atoms 

 of matter in its path. On the laws of probability, one 

 u-particle may meet more atoms and liljerate more 

 electrons than another, and thus lose energy at a faster 

 rate. The amount of straggling observed is, however, 

 much greater than can be accounted for in this way, 

 and the occasional large deflexions of the a-particles 

 due to nuclear collisions are so rare, except near the 

 end of the range, that they do not seriously influence 

 the final distribution. 



Henderson has suggested tiiat the property of an 

 u-[)article of capturing and losing electrons will introduce 

 a new factor in causing straggling. No doubt this is 

 the case, but the rates of capture and loss observed 

 appear to be too rapid to account entirely for the 

 discrepancy between theory and experiment. Another 

 interesting suggestion has been made by Kapitza to 

 account for the magnitude of this straggling. From 

 the experiments of Chadwick and Bieler on the collision 

 between a-particles and hydrogen nuclei, it has been 

 deduced that the a-particle or helium nucleus has an 

 asymmetrical field of force around it. This asymmetry 

 of the electric field must become small at the distance 

 of the orbits of the electrons in the neutral helium atom, 

 but may be sufficient to fix the plane of the orbit of 

 an electron relative to the axis of the helium nucleus. 



Suppose that the a-particles liberated from a radio- 

 active source have their axis orientated at random, 

 and that the direction of the axis of each individual 

 particle remains unchanged during its motion. In 

 some cases, for example, the captured electron will 

 describe an orbit of which the plane is nearly in the 

 direction of motion of the a-particle ; in other cases 

 nearly perpendicular to it. It is to be expected, 

 however, that the chance of losing the captured 

 electron by collision will be greater in one case than 

 the other ; or, in other words, the mean free path of 

 the singly charged a-particle before loss of its electron 

 will be different in the two cases. 



On this view, it is to be anticipated that one group 

 of a-particles will lose energy faster than, the other, 

 and the ranges will be different. In order to test 

 whether a-particles show the individual differences to 

 be expected on this theor)', Kapitza has photographed 

 in the Cavendish Laboratory the tracks of a number 

 of a-particles by the Wilson expansion method, using 

 a strong magnetic field of about 70,000 Gauss, produced 



• This did not 'form part of the Royal Institution discourse, but it may 

 usefully supplenaent one or two of the points surveyed in that lecture. 



by a momentary' « urrent of great intensity. Tlu 

 magnetic field was sufficiently strong to cause a marked 

 bending of the track of the a-particlc. It was found 

 that the curvature of the tracks at equal distance*- 

 from the ends showed marked variations. iJefore an . 

 definite decision can l>e reached, a large numtwr t,\ 

 tracks obtained in this way must Ix' carefully measured 

 up and allowance made for the sudden liends wliich 

 occur due to a nuclear collision with the atoms f*f 

 nitrogen or oxygen. The frequency of these !■ " 

 near the end of the range complicates the interpret, 

 of the apparent tnirvature which is measured. Tii' 

 experiments, which are still in progress, are difliicult 

 and require great technical skill, and it will be a matter 

 of much interest if any definite asymmetry in th< 

 orbits of the singly charged a-particles can be established 

 by this or other methods. If such an asymmetry exist>. 

 it must influence to a small extent the arrangement o! 

 the two electrons round the helium tt'- "' •• ■•• ' ■ <ibly 

 their spectrum. 



During the past two years, Blacken, in iin- ( avtndish 

 Laboratory, has made a careful examination of the 

 frequency of occurrence of sharp bends or forks in the 

 tracks of a-particles near the end of their range in air 

 and other gases. For this purpose, a simple form of 

 Wilson expansion chamber, of the type designed by 

 Shimizu, has been used, and each track has l>een 

 photographed in two directions at right angles to each 

 other to fix the angle of the forks in space. A large 

 number of photographs have been taken, and the 

 frequency of the forks has been examined in different 

 gases, particularly in the last centimetre of the range 

 of the a-particle. Assuming that these forks arise 

 from nuclear collisions, it is possible to deduce from 

 the experimental data the variation of velocity of the 

 a-particle near the end of its range. It is known from 

 the work of Geiger and Marsden that the maximum 

 velocity v of the a-particlcs of emergent range R is 

 given hyi^<xR, when R is not less than one centimetre. 

 Blackett finds that this relation between velocity and 

 range no longer holds near the end of the track but is 

 replaced by a relation of the form v^'^ cc R. 



In the course of these experiments a number of 

 well-defined forks have been photographed in hydrogen, 

 helium, air, and argon by Blackett, and also by Auger 

 and Perrin in Paris. By measuring the angles between 

 the original direction of the a-particle and the direction 

 of the colliding particles after collision, the accuracy 

 of the laws of impact can be directly tested. The 

 results are found, within experimental error, to be in 

 agreement with the view that the impacts are perfectly 

 elastic and that the conservation of energy and of 

 momentum hold in these nuclear collisions. Conversely, 

 by assuming that the impacts are perfectly elastic, it 

 is possible to deduce the mass of the recoil atom in 

 terms of the a-particle of mass 4-00. For example, 

 a fork in helium gave the mass of the recoil atom 4-03, 

 and a fork in hydrogen gave the mass of the recoil 

 atom 1-024. Iri a collision between the a-particle and 

 a helium nucleus the angle between the forks should 

 be exactly a right angle ; the value measured was 

 89° 45'- 



Printed in Great liritain /^ R. & R. Clark, Limited, JiiiiHcutgk. 



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