Secondary 7 Radiation. 441 



furnish a partial explanation of the facts ; but there are many 

 difficulties in the way of a pulse theory, even modified to 

 this extent ; some of these have been pointed out in a recent 

 paper by Professor Bragg and myself (Trans. Pov. Soc. 

 S. Aus. vol. xxxii. 1908). 



When one attempts to further explain by the pulse theory 

 the lack of symmetry which exists in the case of the secon- 

 dary 7 rays, the difficulties become much greater. 



If we are to suppose that the incidence and emergence 

 7 rays are true secondary effects produced by vibrations of 

 electrons in the absorbing material, the effect resembles very 

 closely that of fluorescence. The corresponding optical 

 problem, however, gives rise to no such lack of symmetry ; 

 we find in its case no corresponding asymmetry or distribu- 

 tion or of quality of the secondary etfect. 



If, however, we attempt an explanation upon the " material " 

 theory, these difficulties at once disappear. 



Let us consider in the first place the effects produced by 

 a homogeneous bundle of hard y rays. These in passing 

 through matter suffer collision ; the effect of such collision 

 is to change the direction of motion of the incident primary 

 ray — in other words, to scatter it ; at the same time the 

 scattered ray loses a certain amount of energy — it has become 

 softened ; this softening may be due either to a change in 

 its speed or to a change in moment of the 7 pair, or it may 

 be both. The distribution of the scattered radiation will 

 probably depend upon the nature of collision, and may be 

 influenced largely by the atomic structure of the matter 

 with which the 7 particle has collided. We have seen for 

 instance that in the case of Pb, where the secondary radia- 

 tion is produced mainly from the hard bundle o( 7 rays 

 proceeding from the Ra, that much more of the scattered 

 7 radiation moves on in the direction of the original rays 

 than returns in the opposite direction. 



Consider the case of a fine stream of homogeneous rays 

 moving in the direction .r, a plate of absorbing material of 

 thickness L being placed in their path, at right angles to the 

 direction of motion of the particles. Let l (l be the number 

 of 7 particles which are sent out by the Ra per unit time. 

 The number I which are able to proceed a distance x through 

 the absorbing plate without suffering appreciable effect is 

 such that I = I e _/V . In a distance dx, the number which 

 have suffered serious collision is I X<?~ Aa ' dx. Of these let a 

 traction, q, be merely scattered, at the same time softened 

 or reduced to what we may call an intermediate stage, in 

 which their coefficient of absorption (so called) is V. 



