[ 393. } .- 



XLI. A Note on Beta Rays and Atomic Number. 

 By J. L. Glasson, M.A., D.Sc* 



1 . A LL workers on the passage of fast ft rays through 

 XjL matter hare noted that the uncorrected absorption 

 coefficient appears to be a periodic property of the weight of 

 the atom. A resume of the present state of our knowledge 

 of this subject is to be found on pages 227 to 234 of Ruther- 

 ford's ' Radioactive Substances,' where the results of Crowther 

 and of Schmidt are fully stated and discussed. Crowther's 

 curve connecting y,/D with atomic weight seems to show that 

 there are two effects at work, one of which increases steadily 

 throughout the series of the elements while the other is a 

 periodic function of the atomic weight. The investigation of 

 Schmidt (Ann. d. Pliys. xxiii. p. 671, 1907 ; Pliys. Zeit. x. 

 p. 929, 1909; Pliys. Zeit. xi. p. 262, 1910) bas disentangled 

 two effects. An outline of Schmidt's theory is given by Ruther- 

 ford together with a complete table of his results. Schmidt 

 finds that the passage of ft rays through matter can be use- 

 fully considered in terms of two coefficients, the absorption 

 coefficient a, and the scattering coefficient ft ; and he reaches 

 the conclusion that the absorption coefficient a is directly 

 proportional to the density (D), and inversely proportional 

 to the cube root of the atomic weight (A), whereas the scat- 

 tering coefficient ft is directly proportional to the product of 

 A and D. Schmidt finds that a=OJ)h.-* and /3 = C 2 AD, 

 and considers that d and C 2 are universal constants of matter 

 for a given radiation. Professor Rutherford interprets these 

 results to mean that a is proportional to the cross section 

 of the atom and ft to the volume of the atom. Schmidt 

 believed that his results proved that there was no periodic 

 effect either in a or in ft, and that his results were at 

 variance with those of Crowther. 



2. In view of recent tendencies it appears desirable to 

 express Schmidt's results in terms of atomic number, and 

 this is done in the following table. It is found that several 

 obvious discrepancies disappear and the regularities become 

 more obvious. Gruided by analogy with the « ray case, it 

 seems likely that the results will be easier to interpret if 

 instead of measuring the coefficients in reciprocal centimetres 

 as Schmidt does, we use the atomic absorption and scattering 

 coefficients defined by the relations 



a = ^- and o=^-. 

 * Communicated by Prof. Sir E. Rutherford, F.R.S. 



