.204 Dr. Tycho E:son Auren on 



V. Absorption and Scattering ofy-rays. 



Formula (18) is valid merely on condition that the 

 term p c q is so small relatively to 100 that it is negligible. 



Already, at a wave-length of X = 0*166, on the assumption 

 that in Cu there are 5 outer electrons, absorption produced 

 by scattering amounts to 3*4 per cent, of the whole absorp- 

 tion ; and in case of a still shorter wave-length this part 

 is very rapidly augmented. According to what was 

 pointed out by Barkla and Grlocker, absorption for the 

 heavier elements will therefore in 7-radiation in a great 

 measure, and for the lighter elements necessarily mostly, 



depend on scattering. When calculating - of 7-radiation, 



the term p Cn g is therefore not negligible, and in this case 

 formula (17) must be used. 



Without too great a mistake we may assume that the ratio 



q-Qu = 0*678 is correct also for shorter wave-lengths than 



P m 



those given in Table XII. ; then, in consequence, the value of q 



can be found when - r „ is known. In Table XV. we 



again find the last-mentioned values for 7-rays emitted from 

 some radioactive elements in accordance with determinations 

 performed by Soddy and Russell. Indeed, \ m may not be 

 thought the same for the different substances, but the 



values ol - showing that the difference cannot be very 

 great, mean values for - 0u may be used for approximate 



calculation. By aid of this mean value, 0*0407, we find 

 q to be 16*7, which, put in formula (17), for Al gives 

 the value 0*043. By direct observations, Ishino * has for 

 the same metal found the value 0*045, which very nearly 

 agrees with the value calculated here. 



When inserting the just-named value of q, also *" a / Cu for 



Fe, S and Al may be calculated if the values found for 

 K aICn are assume d to be valid for 7-rays. Using the values 

 given in Table VIII., and assuming that p-values for Al, S, 

 Cu according to the above are 5, 4, 5 respectively, and 

 for Fe 5, we in the same way a3 in the case of formula (14) 

 obtain the values given in the table below. The assumption 

 that there are 5 outer electrons in Fe is quite arbitrary, but 



* Ishino, Phil. Mag. xxxiii. p. 129 (1917). 



