Section III., 1914 hsil Trans. R.S.C. 



The Penetrating Power of the y Rays excited in lead by the ft Pays 

 of Radium E. 



By J. A. Gray, D.Sc. 

 (Presented by Dr. A. S. Eve, F.R.S.G.). 



Barkla 1 and others have shown that under suitable circumstances, 

 most elements can emit at least two types of X radiation, character- 

 istic of the element?: They are called the X rays of the K series and 

 of the L series, the former being much more penetrating, e.g. for 

 silver, the mass absorption coefficient in aluminium (fi/p Al) = 

 2-5 for the rays of the K series and 700 for the rays of the L series 

 Moseley 2 has examined the X ray spectra of many elements, and has 

 shown for elements ranging from aluminium to silver, that in the 

 rays of the K series there are at least two lines of different wave 

 lengths, which he calls the a and the /3 lines. Of these lines, the 

 a line is the more intense, and has the longer wave length. Its 

 frequency ^a is connected with the atomic number N by an approxi- 

 mate relation of the form — 



^a = C(N — l) 2 where C is constant. 



Rutherford and Richardson 3 had examined the rays of most of 

 the 7 ray products and showed that it was probable that these y rays 

 were characteristic of the elements emitting them. 



Radium B emits three types of y rays for which n/p Al = 87, 14-7, 

 0-188, RaC one type for which /x/p Al = • 0424, and Rutherford and 

 Andrade 4 have since examined the spectra of the 7 rays of these 

 elements. In their first paper they showed by direct experiment, 

 that the rays for which ////? Al=14-7 were identical with the lead X 

 rays of series L. As both RaB and lead have the same atomic 

 number N = 82, this shows that these rays form the L series for radium 

 B. 



In the second paper they give the wave lengths of the more pene- 

 trating 7 rays of Ra B, and of the 7 rays of Ra C. 

 g§ The wave length of the most intense line of the 7 rays of radium 

 B (ju/P Al = 0-188) is 1-65 XlO -9 cms. and the frequency ^=1-83 

 X10 19 



'Barkla and Sadler, Phil Mag. 16, p. 550, 1908. 

 Barkla and Nicol, Proc. Phys. Soc. 24, p. 9, 1911. 

 2 Moseley, Phil Mag. vol. 27, p. 703, 1914. 



3 Rutherford and Richardson, Phil Mag. 25, p. 722 (1913), 26, p. 324, 1913. 

 'Rutherford and Andrade, Phil Mag. 27, p 854, 1914, 28, p. 263, 1914. 



