[gray] rays of radium 145 



in quality as the angle of scattering increased. Further after passing 

 through a few millimeters of lead the scattered rays would be of about 

 the same penetrating power as the primary. 



We are therefore forced to the second explanation and this is quite 

 sufficient. The change in quality is probably very small when the angle 

 of scattering is small as experiment shows that in this case the scattered 

 rays do not differ much in penetrating power from the primary. A 

 certain similarity to the scattering of a rays may here be noted. When 

 an a ray is scattered through a large angle there is little doubt that it 

 loses velocity, the loss being greater the greater the angle of scattering. 



It was stated above that Sadler and Mesham found that X rays were 

 softened by scattering. The change in quality appeared to be greater 

 the greater the penetrating power of the X rays and this point is sup- 

 ported by the present experiments, the y rays scattered through an 

 angle of 110° being not much more penetrating than the 7 rays excited 

 in lead by the j8 rays of radium E. It seems quite probable that the 

 change in quality is small for very soft X rays. It will be seen that the 

 rays scattered through 50° are still much softer than the primary rays 

 even after passing through a centimetre of lead so the softening must 

 happen to every type of 7 ray scattered. 



We can deduce the following results from the tables. 



1. The quantity of radiation scattered per unit mass is approxi- 

 mately independent of the nature of the radiator. 



At first sight it would appear that lead 7 scattered much less 

 radiation per unit mass than either carbon or iron. This, however, 

 is mainly due to the fact that lead absorbs the scattered radiation so 

 much, that less of it escapes from the radiator. 



Consider Table 1 (b). The primary rays pass through the radiator 

 approximately normally, the scattered rays at an angle of 50° with 

 the normal. If [i is the absorption coefficient of the primary in the 

 radiator, that of the scattered rays, 7 the density, A the area, d the 

 depth of the radiator, the amount of scattered radiation entering the 

 electroscope will be approximately proportional to 



/. 



