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PRIMARY AND SECONDARY /S-RAYS 



127 



Subtracting these figures from those in column 4 of Table I we 

 get finally the figures in column 5. 



The results of the initial experiments on the primary rays are 

 given below in Table III, intensities being given in arbitrary units, 

 and for the sake of comparison, corresponding intensities of the 

 secondary /3-rays have been placed in column 4. 



TABLE III 



The secondary /3-rays have their origin in the lead foil and there- 

 fore lose energy before they escape from it. We can, however, 

 neglect this loss of energy when making a comparison between the 

 velocities of primary and secondary rays, because, according to the 

 measurements of Miss Douglas, the primary /S-rays are only just 

 stopped by lead of mass 0.354 grammes/cm.", or lead 20 times a 

 thick as that in which the secondary j8-rays originate. 



We see at once from the tables that a very large percentage of 

 the secondary iS-rays have slower velocities than the primary /3-rays, 

 since 80 per cent, of the primary rays pass through one sheet of paper 

 as compared with 30 per cent, of the secondary, and 40 per cent, of 

 the primary rays pass through 6 sheets of paper as compared with 

 1.2 per cent, of the secondary rays. It is impossible to compare 

 accurately the average energy of a secondar}^ /3-ray with that of a 

 primary, but a rough comparison may be made as follows. The 

 primary rays are reduced to half value by 5 sheets of paper, the 

 secondary rays to the same extent by one sheet of paper. A table is 

 given on page 243 of. Rutherford's "Radioactive Substances and their 

 Radiations" which indicates that when absorption coefficients of 

 /3-rays vary as 5 to 1, the energies of such /3-rays vary as 3 to 10. If 



