RADIOACTIVITY 37 



the atom ; whereas observation shows that the emission is 

 concentrated largely round the original line of movement of the 

 stream of fi particles which it more or less continues. Most of 

 those which move off the original line are only slightly deflected 

 and a much smaller number are turned through a large angle. 

 Clearly this is characteristic rather of the simpler hypothesis 

 we have considered first : it is just what we should expect if the 

 original /8 particles were scattered or deflected and there is no 

 need to suppose a new emission. In order to see this effect we 

 must use a thin plate, say a tenth of a millimetre of aluminium, 

 allowing the ft rays to fall perpendicularly upon it. In that 

 case the /3 particles have usually suffered only one large de- 

 flection, if any; we see the effects of single collisions, as shown 

 in fig. 14. But even if we put aside the hypothesis that the 

 internal energy of the atom enters into the question, there still 

 remains the possibility of a sort of secondary /3 radiation arising 

 in another way. A /3 particle might strike another electron in 

 the atom which is crossing, with a blow so direct that the latter 

 electron is driven out with great speed ; so great that we may 

 be said to have two /3 rays where we had one before. It would 

 be a very great advance on our knowledge of atomic and sub- 

 atomic mechanics could we give a definite decision on this point. 

 If it could happen, then it would be still easier to understand 

 the seeming disappearance of the /3 particle while going at a 

 high speed, for with its full energy it often crosses a metre of 

 air and it has but to divide its energy equally with two other 

 electrons in succession and it can only cross three or four 

 centimetres and so on. But there is no clear evidence that 

 such a sharing ever takes place. When an a particle goes 

 through an atom it or some part of it must sometimes nit 

 an electron as squarely as a /3 particle could do. Yet we 

 find no /3 particles springing out from the atoms traversed by 

 the a particle : it is true, of course, that they might exist in too 

 small numbers to be observed. We do indeed find ionisation 

 and we suppose this to mean that electrons are ejected from the 

 atoms which the ionising particles, a or /3 rays, traverse. But 

 these electrons have little more than the " critical " speed already 

 mentioned. In the case of the a particle there has been some 

 measurement of their speed and it is clear that it never exceeds 

 this critical value by more than a small margin, no matter what 

 the ionising agent may be. This process, the real process of 



