RADIO-ACTIVE RECOIL. 



I'.v WALTER MAK()\M:k. M.A.. D.Sc 



It is well known that when a shot is fired from a 

 cannon, the latter attempts to move backwards and 

 is said to recoil. Furthermore, if the cannon is free 

 to move, then its velocity is such as to make the 

 momentum of the cannon equal to that of the 

 shot which causes it to recoil. The momentum 

 of a body is a quantit}' which juay be taken as 

 giving a measure of the (]uantit\' of motion 

 in a s\'Stem and is measured hv the product of 

 the mass and velocity of the body considered. 



The question whether the ordinary laws 

 of dynamics apply to the case of atoms 

 as well as to that of molecular masses has 

 frequently been discussed, but till recently 

 it has been be\ond the possibility of direct 

 experiment to test the point. By the 

 recent discoveries in radio-activit\' the 

 motion of streams of atoms, and in some 

 cases of single atoms, can be observed 



^ 



FiGU 



RE 1. 



and studied 

 afforded of 

 in motion, 

 radio-active 

 changes in 

 cerned, and 



and 



the possibility has thus been 

 the dynamics of atoms 

 It is now generallv accepted that 

 processes are the manifestations of 

 the constitution of the atoms con- 

 are of a more subtle nature than 



chemical changes, in which atoms of various kinds 

 react on each other without affecting the internal 

 structure of the atoms themselves. Thus to take as 

 an example the case of the best known 

 radio-active substance — radium, discovered 

 by Mme Curie. It is now certain that 

 this element, which is similar to barium 

 in its chemical behaviour, is slowly but 

 constantly ejecting positiveh- charged 

 atoms known as n particles which have 

 been shown by Rutherford and others 

 to be nothing other than charged atoms 



helium. This can be proved by collecting the 

 « particles, which can be detected in many ways and 

 then subjecting them to spectroscopic analysis. The 

 remainder of the atom from which the helium has 

 been expelled constitutes a new element also dis- 

 playing radio-active properties, which happens to be a 

 gas, and is called the emanation — a name which has 

 persistedsince the time of its discovery, when its nature 

 was not yet fully understood. The emanation, w hose 

 chemical and ph\-sical projierties are entirely distinct 

 from those of the radium which produced it, in turn 

 gives off a particles, and disintegrates into a new- 

 product which has been called radium A. The only 

 essential difference between this change and that 

 undergone by the radium exists in the difterence in 

 the speed of the disintegration. Whereas radium 

 takes tlK)usnnds of \ears to change into the 



emanation and helium, the emanation is much more 



unstable, and disappears in the course of a few weeks. 



A similar process occurs with radium .\, which in 



turn gives rise to a series of successive radio-active 



products, known as radium B, radium C, and 



so on. It should here be mentioned that all 



radio-acti\e {processes are not exactly of the nature 



just described: for in some cases the disintegration 



is associated merel\' with the expulsion of an 



electron — that minute negatively charged particle 



- first detected by J. J. Thomson. It will. 



however, be sufficient for present jnu-poses 



to confine attention to those transforma- 



t| tions in which the radio-active change is 



— ' associated with the expulsion of an a 



particle, which we have seen is a charged 



atom of helium. 



The change of radium .\ into radium B, 



mentioned above, presents a particularly 



convenient case for stud\ing the phenomena of 



atomic recoil: for radium A has a very short " life." 



and quickly disappears with the production of radium 



I), an (1 particle being ejected during the process. The 



details of the method of obtaining pure radium .\ 



from the emanation need not be considered here : 



suffice it to sa\ tliat if a plate is exposed to 



the emanation for a short time, as the kathode 



in an electric field, it becomes coated with a 



strongly-acti\e film of radium A. If the 



plate is removed from the emanation and 



left for some minutes, most of the radium 



.A w ill have turned into radium B in position 



and after about a quarter of an hour prac- 



ticalh' no atoms of radium A will be left. 



It is to be noticed tliat the radium B formed 



remains on the plate. The thing is different 



of when the experiment is carried out //; vacuo, for 



according to the laws of probabilit}' half of the 



number of a particles ejected will be shot out from 



the radium A atoms awa\' from the plate and half 



towards it. Thus in half the disintegrations of the 



radium A when an « particle is shot into the plate, 



the residual atoms of radium B possess momentum in 



directions away from the plate and will be detached 



unless there is some force preventing them from 



leaving. When the experiment is performed under 



ordinar\- conditions at atmospheric pressure, the atoms 



radium 15 are prevented from tra\'elling more than of 



a fraction of a millimeter before colliding with 



molecules of air h\ which the\' are stopped : they 



then mostly diffiise liack to the plate from which 



they came. On the other hand, //; vacuo there is 



nothing to stoj) the motion oi the' atoms of radium B, 



and the\- continue on their course until tln'\- 



262 



