442 Mr Growther, On the Transmission of ^-rays. 



On the Transmission of ^-rays. By J. A. Crowther, M.A., 

 St John's College. 



[Read 14 March 1910.] 



I 1. The problems connected with the transmission of /8-rays 

 through matter are of considerable theoretical importance. Un- 

 fortunately they are involved at present in considerable obscurity, 

 and a fairly extensive study by various experimenters has so far 

 only served to demonstrate the complexity of the phenomena. 

 The present paper deals only with a few of the many questions 

 involved. 



§ 2. The first question to be considered is the nature of the 

 absorption itself. The absorption of the /3-rays is almost invariably 

 measured by finding the diminution in the ionization produced 

 by them in some suitable ionization chamber, when various 

 absorbing media are interposed in their path. This absorption 

 may be due to two causes : — 



(1) to a loss of velocity by the ;8-particles, such as is found 

 to take place during the absorption of the a-rays ; 



(2) to an actual decrease in the number of the /8 -particles 

 themselves, due either to stoppage or deflection. 



To decide this question it is necessary in the first place to 

 determine the velocity of a homogeneous beam of ^S-particles 

 before and after transmission through a suitable thickness of 

 absorbing material. 



A certain amount of indirect evidence already exists on the 

 subject. In 1907 H. W. Schmidt* performed an experiment 

 which at the time seemed to prove conclusively that the yS-rays 

 suffered no change of velocity even when transmitted through 

 very considerable thicknesses of absorbing material. The essential 

 features of his method are simple, and may be described here as 

 they form the basis of the methods employed in the present 

 experiments. 



Three circular holes (fig. 1) are made in three lead screens, so 

 as to define an arc of a circle of suitable radius. The system is 

 placed between the poles of an electromagnet so that the magnetic 

 field is perpendicular to the plane of the circle. By a suitable 

 adjustment of the strength of the field a beam of /3-rays entering 

 the magnetic field at A can be made to describe the circular path 

 AGB, and will produce ionization in a chamber placed at B. 

 The velocity of the rays transmitted through this system is known 

 from the value of the magnetic field, and the radius of curvature 

 of the path, 



* Phys. Zeit. Vol. viii. p. 371. 



