186 



closely that of fluorescence. The corresponding optical pro- 

 blem, however, gives rise to no such lack of symmetry ; we 

 find in its case no corresponding asymmetry of distribution 

 or of quality of the secondary effect. 



If, however, we attemjDt an explanation upon the ''mate- 

 rial" 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 la 

 to change the direction of motion of the incident primary 

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

 tered 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 y pair, or it may be 

 both. The distribution of the scattered radiation will pro- 

 bably depend upon the nature of collision, and may be in- 

 fluenced largely by the atomic structure of the matter 

 with which the y particle has collided. We have seen, 

 for instance, that in the case of Pb, where the secondary 

 radiation is produced mainly from the hard bundle 

 of y rays proceeding from the Ra, that much more of the 

 scattered y 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 I ^ be the number 

 of y 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 7 — Z^^-a-^- In a distance, dx, the number 

 which have suffered serious collision is I ^Xe-^^dx. Of these 

 let a fraction, q, be merely scattered, at the same time soft- 

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

 in which their coefficient of absorption (so called) is \'. 



The remainder represented by the fraction (1 — q) are 

 turned into ^ rays at once. Let i^ = qT^\e—\'^dx. 



As a result of scattering in a direction inclined to the 

 original at an angle 6, let i ^FB represent the number of par- 

 ticles per sec. which cross the unit area of a spherical sur- 

 face of unit radius described with centre at the point where 

 scattering occurs ; then the number of scattered rays which 

 emerge per sec. from the flat plate of thickness L 



L 



2 -\x -\'{L-x) Sec. e 



\27r Sin. O.FB.q J ^Xe e dO dx 



Li 



