[gray] rays of radium 147 



rays from carbon is reduced from 5-40 to 1'91 or to 35 % while the 

 intensity of the primary radiation falls to 42%. Similar results are 

 to be obtained from Table 2. In the case of the scattered radiation 

 from lead the much greater absorption of the softer rays, both primary 

 and scattred, vitiates this comparison. 



The fact that softer rays are scattered to a greater extent is, of 

 course to be expected. In a previous paper the writer introduced a 

 coefficient S j/p , S j dx representing the fraction of the radiation scattered 

 from a parallel beam in a layer of thickness dx, between the angles 

 90° and 270°. 



In the case of X rays S^/p is of the order 0* 100. 



In the case of 7 rays of radium E S^/p is of the order 0'050. 

 In the present case a special experiment showed that a carbon radiator 

 reflects about 12% of the 7 rays of radium and it is easy to prove 

 that if M represents the absorption coefficient of the primary rays in 

 carbon, y the absorption coefficient of the ''reflected" rays and P the 

 the fraction of rays reflected. Si/p = P (/* + X)/p 



For carbon ixj p = 0-044 X/p =0-080 P=0-12 

 Si/p = 0-015 



showing that scattering decreases as the 7 rays become harder. 



The numbers, although approximate, are sufficiently accurate to 

 show that the scattering of 7 rays is quite marked in comparison with 

 direct absorption. For this reason it practically impossible to separate 

 the primary and scattered rays, and these scattered rays play a great 

 part in absorption phenomena. 



Absorption of 7 rays. 



When examined by most materials, the 7 rays of radium become 

 more and more penetrating, this being particularly marked in the case 

 of absorption by lead and other substances of high atomic weight. 

 After passing through a certain amount of any material the absorption 

 becomes exponential, Russell* finding that the absorption in mercury 

 is exponential for a thickness of 1 cm. to 22-5 cms., the intensity dimin- 

 ishing in the ratio 360.000 to 1. 



The tables show us that even though the absorption may be 

 exponential, the rays are not homogenous owing to scattered 7 rays 

 being softer than the primary. Ordinarily we would expect the rays 

 to become more and more penetrating and ultimately homogenous. 

 That this is not the case is of course explained by the fact that although 

 the softer rays are more quickly absorbed their place is taken by other 



*Russell Proc. Ray. Sec. A. 87, 1912. 



