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XLIII. On the Coefficient of Absorption of the /3 Rays from 

 Uranium, By J. Arnold Crowther, B.A., Hutchinson 

 Student of St. Johns College, Cambridge, and Open Re- 

 search Student of Emmanuel College, Cambridge*. 



Introduction. 



THE ft rays of radioactive substances consist of small 

 negatively charged corpuscles travelling with a high 

 velocity. On entering any material substance they are more 

 or less rapidly absorbed, the intensity of the radiation de- 

 creasing, as the thickness of matter traversed is increased, 

 very nearly in accordance with an exponential law. Thus 

 if I is the initial intensity of the radiation before entering 

 the substance, the intensity I after passing through a thick- 

 ness d of the material is given by the equation 



i=i. e - w , 



where A, is the coefficient of absorption of the given substance 

 for the particular /3 radiation employed. 



We may regard the absorption of the /3 rays by matter as 

 being due to collisions between the swarm of corpuscles, con- 

 stituting the rays, and the particles composing the substance. 

 From the minute size of the corpuscles, however, and their 

 high velocity we must consider them as colliding, not with the 

 atom or molecule of the absorbing substance as a whole, but 

 rather as penetrating the atom, and coming into collision 

 with the individual corpuscles of which the atom is con- 

 structed t- The absorption of the /3 rays is thus due to 

 collision between the j3 corpuscles and similar corpuscles in 

 the atom of the absorbing medium. 



On this view, therefore, the fundamental quantity in the 

 absorption of the /3 radiation is the stopping-power of the 

 individual corpuscles in the atom, and this quantity can be 

 immediately deduced from the coefficient of absorption.. For 

 if p is the density of the absorbing medium, M its atomic 

 weight, and n the number of corpuscles in the atom, then the 

 number of corpuscles N in unit volume is given by the equation 



But w/M is a constant, since the number of corpuscles in an 

 atom is simply proportional to the atomic weight, and thus 



Noc p. 

 Hence the absorption per corpuscle is simply proportional to 

 X/p. 



* Communicated by Prof. J. J. Thomson, F.R.S. 



f J. J. Thomson, • Conduction of Electricity throush Gases,' p. 312 

 <1903j. 



