the X Badiation from a Coolidge Tube. 



159 



penetrating gamma rays from radium C are those given in a 

 recent paper by Ishino *, where the coefficients of absorption 

 and scattering were separately determined. The values of the 

 mass-scattering coefficients, cr/p, for the gamma rays were 

 found by him to be '045 for aluminium and *034 for lead — 

 values much smaller than those previously found for ordinary 

 X rays. 



Voltages 

 (volts). 



Wave- length in | 

 A.U. 



X. 



Mass Absorption Coefficient fx/p 



in Aluminium. 



in Lead. 



*84,000 ; 0-147 



92,000 0135 



*102,000 0-122 



144,000 -086 



183,000 -068 



196,000 -063 



Gamma rays 1 ? 



from radium J 



0-154 

 014 



Oil' 



•085 



•026 



1-50 

 300 



105 



0-75 



•042 



It will be observed from the table that the value of pujp 

 in aluminium decreases very slowly between 84,000 and 

 196,000 volts, even at a slower rate than the first power of 

 the wave-length; while for longer waves it is well known 

 that the value of p, varies approximately as the cube of the 

 wave-length. As we should expect, the variation in pu/p 

 with wave-length is much more rapid for lead than for 

 aluminium over the same range. It will be noted that, while 

 the value of p\p for aluminium for X rays generated at 

 183,000 volts is only 3 times the value for the gamma rays, 

 the corresponding ratio in the case of lead is more than 20. 

 The general results suggest that when the value of p/p 

 becomes of the same order of magnitude as that of o\p, the 

 former coefficient varies slowly with the wave-length, the 

 latter probably remaining constant. In addition, it appears 

 not unlikely that there is a definite connexion between 

 absorption and scattering, and that, for very short waves, 

 the absorption like the scattering may ultimately reach a 

 minimum value independent of wave-length. From some 

 points of view such a connexion between these two quantities 

 is not improbable, but unfortunately no waves of sufficiently 

 short wave-length are available to test the relation expe- 

 rimentally. 



The two shortest wave-lengths of the gamma rays observed 



* Ishino, Phil. Mag-, xxxiii. p. 129, January 1917. 



