Reflexion of the ^-Particles by Matter. 



855 



weight of the reflector. When a differential curve was 

 plotted, which would show the absorption of the reflected 

 rays alone, the coefficient of absorption for the reflected rays 

 was considerably greater than for the incident rays. For 

 example, in the case of the j3 rays from Ra E, when air was 

 underneath, the coefficient of absorption //, = 42'4 (cms ) _1 ; 

 when lead was underneath fi = 44*8, while the differential curve 

 gave for the reflected rays yLt = 51'0 cms. -1 for aluminium. 

 Schmidt* had already drawn attention to this fact. 



Table II. gives the values of the coefficients of absorption 

 in terms of cms. of Al for the /3-particles from RaE, ActC, 

 Th D, and Ra C, when different substances are placed under 

 the active material. 



Table II. 



Substance 

 underneath. 



Coefficients of Absorption. 



RaE. 



ActC. 



ThD. 



EaC. 



Aii* 



42-4 - 



27-6 

 28-3 

 28-5 

 28-9 

 292 

 29-3 

 293 

 29-5 

 30-1 

 30-2 

 30-4 

 30-4 

 30-4 

 30-4 



15-7 

 161 

 16-3 

 16-4 

 164 



16'4 



16-7 

 16-7 



130 

 13-2 

 134 

 13-6 

 136 

 139 

 13-8 

 140 

 14-2 

 14-3 

 14-7 

 14-7 

 14-7 

 147 







Al 



S 



430 

 433 

 43-8 

 44-3 

 44-3 

 444 

 444 

 446 

 44-7 

 448 

 44-8 

 44-8 

 44-8 



Fe i 



Ni 



Cu 



Zn 



Ag 



Sn 



Pt 



Au 



Pb 



Bi 



It will be observed that in all cases the absorption coefficients 

 are greater when a substance of greater atomic weight is 

 placed underneath. The value for air underneath would 

 apparently be the value of /j, for the incident rays. It follows 

 from this that the average velocity of the " /3-particles is 

 decreased by reflexion. 



Since the ^-particles are reflected by matter, it is obvious 

 that the absorbing matter above the active material will 

 reflect downwards a certain fraction of the incident radiation, 

 the amount of which will depend on the velocity of the rays 

 themselves (as will be shown later) and on the thickness and 



* H. W. Schmidt, loc. cit. 



