862 Dr. A. F. Kovarik on the Absorption and 



Multiple Reflexion. 



Since the /3-particles are reflected from a substance into 

 which they penetrate, the reflected rays should in turn be 

 reflected it* a substance is placed in their path. Suppose we 

 have two parallel plates of lead and /3-particles of the type 

 pair Ra E, say, to be emitted from the lower plate. Let us 

 also suppose the velocity after reflexion to remain constant, 

 and consequently the percentage of reflexion and the ioniza- 

 tion of the particles involved in multiple reflexion to be 

 constant also. The following calculation then shows what 

 the ionization due to multiple reflexion should be. 



Let I represent the ionization due to the incident rays and 

 p represent the percentage of reflexion. Then for the rays 

 passing upwards the ionization is equal to 



I+pl + pn+p*I + ... &c 



and for the rays initially passing downwards, after reflexion 

 into the ionization chamber 



pI+p 2 I + /0 3 I+ ... &C. 



giving a total ionization of 



I + 2pI(l+/> + /> 2 + /D 3 + ... fto.) 



->['♦£] 



= 587 per cent, of the ionization due to the incident rays; 

 or that due to multiple reflexion alone is 487 per cent. 

 The numbers substituted are obtained from Table V. 



Since the reflected particles have a smaller velocity than 

 the incident, and the amount of reflexion decreases with 

 decreasing velocity, it follows that the above ratio must 

 be too high, unless the variation of the ionization with the 

 velocity is very marked. In order to test this point and 

 to see how the multiple reflexion varies with different 

 reflectors and with rays of different velocities, a special 

 apparatus was constructed. It was then found experi- 

 mentally for the Ea E radiation that the multiple reflexion 

 was 236 per cent, of the incident radiation, measured by 

 the ionization. This seems to justify the conclusion that 

 the rays after the first reflexion are not as readily reflected. 

 To see what the average value of the reflexion percentage 



