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THE ROYAL SOCIETY OF CANADA 



(2) To determine the ranges of the /3-rays in different substances 

 and the relation between range and atomic number. 



Part I 



When electrons strike a metal anticathode, X-rays are produced. 

 In the same way, when /5-rays impinge on matter, a metal plate for 

 example, secondary 7-rays are produced, some of the /3-rays are 

 absorbed, some are scattered, and some are transmitted if the plate 

 be not too thick. The question arises as to what relation exists 

 between these various factors. If the 7-rays are due to the scattering 

 of the /3-rays then the scattered jS-rays should show a loss of energy 

 comparable to the energy of the 7-rays produced. If no such loss is 

 detectable we are justified in assuming that 7-rays are not produced 

 when jS-rays are scattered, but when they are stepped by some parti- 

 cular type of collision. 



The experimental procedure was as follows: The preparation of 

 Ra.E was enclosed in a small lead case {A) (see .Fig. 3), with one open 

 face and was mounted centrally in front of, but turned away from, 

 the foil face of the electroscope. The latter was a 14 cm. cube. Be- 

 tween it and the active material was placed the absorbing material 

 {B), and in front of the active material stood the radiator {R). Thus 

 only rays scattered through approximately 160° to 180° could enter 

 the electroscope. 



The intensity of the direct 

 radiation was obtained by re- 

 placing the radiator by the 

 active material with its open 

 face towards the electroscope. 

 Corrections had to be made 

 in both cases for 7-rays. 

 This is possible to a high 

 degree of accuracy if the 

 mass-absorption coefficient for 7-rays be known. Those coefficients 

 have been determined for various substances, including carbon and 

 aluminium, by Dr. Gray, who has shown that whereas the mass- 

 absorption coefficient of |8-rays in carbon is approximately 16, that of 

 7-rays in carbon is . 100. In the case of the scattered radiation a 

 further correction was necessary to eliminate the effect of air-scatter- 

 ing. This presented no greater difficulty than the careful repetition 

 of every reading with the radiator completely removed. 



