ABSORPTION OF PHOTO-ELECTRONS 



63 



absorption curve between the limits of thickness investigated. 

 A secondary phenomenon occurring- for thicknesses of less 

 less than 10" ^cm does not make its appearance in the region 

 investigated here. The absorption coefficients were then com- 

 puted from the half value relations by means of the equation 

 a = ^-^^ Sample results are given in the accompanying table 

 and show samples of the values which the limitations of the 

 method allow. 



VALUES OF a IN uu-'cm-^ FOR X 2536 



Lenard- investigating the properties of cathode rays showed 

 that their coefficient of absorption was very nearly proportional 

 to the density " p " of the material. The maximum variation in 

 his results for - were 2070 and 5610, although the densities 

 of the absorbing material varied between 3.6X10"'^ and 19.3. 

 Rutherford^ found that a similar relation held approximately 

 for the faster B rays from Uranium X. Though the elements 

 of large atomic weight were found to possess a nialss absorption 

 coefficient (-) nearly twice as large as the lighter elements, 

 Crowther* who extended these latter observations for thirty-two 

 elements, found that the mass absorption coefficient exhibited 

 a progressive increase from the lighter to the heavier elements. 

 The following values of — for B rays of platinum and silver 

 quoted from his paper will give some idea of the magnitude of 

 his results. 



For comparison the values of — for photoelectrons (^2536) 

 are given and it is seen that the ratio of the mass absorption 



^Lenard. Ann. d. Phys. 56 p. 275, 1895. 

 ^Rutherford, Phil. Mag. 47, p. 109, 1899. 

 ^Crowther, Phil. Mag. 12, p. 379, 1906. 



