[douglas] 



/3-RAYS FROM RADIUM 



123 



In Fig. 7 are shown the absorption curves terminating the ranges 

 for carbon, aluminium and copper. 



'34 '36 -JB -40 -42 -^A -46 'AS 



RANGE in GMS./CM^ 



Fig. 8 



In Fig. 8 the range has been plotted against the atomic number, 

 and a smooth curve is found to result. It would be necessary, how- 

 ever, to examine the range in many more substances before the relation 

 between effective range and atomic number could be definitely 

 established. By analogy to Crowther's and McClelland's curve of 

 mass-absorption coefficients against atomic number, and Bragg's 

 curve of molecular diameters against atomic number, it seems a 

 plausible forecast that a broken curve of that nature might be found, 

 the breaks occurring at the atomic numbers of the inert gases. 



The range of a-particles in different substances has been found by 

 Bragg and Kleeman (Phil. Mag., 1905) to vary very nearly as the 

 square root of the atomic weight. At fiist sight it appears strange 

 that the range of the ;8-particle should follow an entirely opposite 

 law and decrease with increase of atomic weight. 



This leads to the distinction already referred to between effective 

 and actual range. It will be seen from Table II that the effective 

 range decreases very slightly for large increases in the atomic number 

 of the absorbers. On the other hand, it has been shown by Schmidt 

 and others that the coefficient of scattering increases very rapidly 

 with atomic number. This means that the amount of scattering 

 from plates of equal mass per unit area increases the higher the atomic 



