Section III., 1913. [139] Trans. R.S.G. 



The Scattering and Absorption of the y Rays of Radium 

 By J. A. Gray, D.Sc. 

 Presented by Prof. H. T. Barnes, F.R.S.C. 



(Read May 28, 1913) 



It was discovered by Eve* that secondary y rays were emitted by 

 any body or radiator through which the y rays of radium passed, 

 the secondary rays being less penetrating than the primary rays.f 

 Investigations on the nature of these secondary rays have since been 

 carried out by Kleeman,^ Madsen* and Florance,^ the experiments of 

 Florance giving us perhaps the most definite information. Florance 

 ascribed the secondary y rays to scattering of the primary rays, as 

 the quality of the secondary rays appeared to be independent of the 

 nature of the radiator. He obtained the important result that the 

 secondary or scattered rays became less penetrating as the angle of 

 scattering increased. It appeared as if more y radiation was scattered 

 in the emergent direction than in the incident direction, the scattering 

 being apparently similar to that suffered by a pencil of a or /9 rays. 

 As the primary rays were heterogeneous, Florance explained his. results 

 by assuming that the softer rays were scattered through larger angles 

 than the primary and this is the explanation usually given.^ 



The writer^ came to the conclusion that y rays could be directly scat- 

 tered but further consideration of the experiments of Madsen and Florance 

 showed that the interpretation of the scattering of 7 rays given above 

 was probably not sufficient, as it appeared that when the intensity 

 of the primary rays was diminished by lead, the softer scattered rays 

 were not cut down so quickly as one would expect (see tables given 

 below). In consequence, experiments were carried out to test the 

 interpretation. The results obtained show that 7 rays are not scattered 



*Eve, Phil. Mag. 8., p. 669, 1904. 



tNoTE. — In this paper y rays striking a body or radiator are referred to as 

 primary rays, rays coming off from the radiator in a different direction as secondary 

 rays. 



IKleeman, Phil. Mag., 15, p. 638, 1908. 



* Madsen, Phil. Mag., 17, p. 423, 1909. 

 » Florance, Phil. Mag., 20, p. 921, 1910. 



* Bragg, "Problems in Radio-activity"; Rutherford, "Radio-active Substances 

 and their Radiations." 



' Gray. Prcc. Ray. Sec. A. 87 p. 489, 1912. 



