148 THE ROYAL SOCIETY OF CANADA 



rays produced by scattering. Consider a liomogenous beam of -y rays. 

 As a certain fraction of these rays is scattered per unit mass, the rays 

 coming through any absorption plate would become less and less 

 penetrating as the thickness of the plate increased. Ultimately, 

 however, the absorption would be come exponential as only a definite 

 fraction of the unscattered primary rays, which would be unchanged 

 in quality, could be scattered per unit mass and we must reach a stage 

 where the production of softer 7 rays is balanced by their absorption. 

 A certain similarity to the problem of the absorption of /S rays will 

 be noted. In the case of jS rays the slower j8 rays are produced by the 

 faster /3 raj^s losing velocity as they pass through matter. One dis- 

 tinction must be drawn however. In the case of j8 rays, true absorption 

 only takes place to any great extent when the j8 rays become very slow, 

 whereas in the case of 7 rays absorption can take place at any point 

 of their path. 



The writer has shown that an exponential law for /3 rays can only 

 be approximate and the question arises as to whether the same thing 

 may not be true for /3 rays. This depends on the question: Can 7 

 rays be directly scattered? Crowther has shown that of a pencil of 

 j9 rays every /3 ray is scattered through a small angle after passing 

 through very small thicknesses of matter and Geiger has shown the 

 same^ thing for a rays. If something similar took place in the case of 

 ;' rays, we would have the result that 7 rays, as a whole, must become 

 less and less penetrating so that like /3 rays, an exponential law could 

 only be approximate, the absorption ultimately becaming greater and 

 greater. If on the other hand we start with a beam of 7 rays and a 

 definite percentage of these rays is scattered per unit mass the remaining 

 keeping their direction unchanged, the exponential law can be an 

 accurate one. The fact that Russell found the absorption in mercury 

 exponential over a range of intensity of 360,000 to 1, shows that this 

 is very nearly the case and this has been tacitly assumed above. Al- 

 though it must be admitted that Russell obtained evidence that Y rays 

 could not penetrate more than 26 cms. of mercury. 



The tables show that much less scattered radiation escapes from 

 a lead radiator than from a carbon radiator, consequently when the* 

 rays are exponentially absorbed by the two materials, the rays coming 

 through the carbon will be more heterogeneous than those coming 

 through the lead. Consequently if 7 rays are being exponentially 

 absorljed by carbon or aluminium, the issuing rays will not be expo- 

 nentially absorbed by lead as they contain too great a proportion of soft 

 rays. When examined by lead these softer rays will be cut out until 

 the 7 rays are exponentially absorbed by lead, i.e. lead will "harden" 

 the rays. Evidence for this is found in recent experiments of Rutherford 



