138 S. E. PIERCE. 



THE IONISATION CAUSED BY PENETRATING 

 7 RAYS IN A CLOSED THICK- WALLED VESSEL. 



By S. E. Peircb, b.Sc 



Deas-Thomson Scholar in Physics in the University of Sydney. 



(Communicated by Professor Pollock ) 



[Bead before the Royal Society of N. S. Wales, September 3, 1913.~\ 



This research is an extension of some experiments described 

 by Professor Bragg, 1 in a paper on "The Consequences of 

 the Corpuscular Hypothesis of the y and X Rays, and the 

 Range of the P Rays," in connection with the discussion 

 of the ionisation caused by penetrating y rays in a closed 

 thick-walled vessel in terms of his corpuscular theory. 



According to this theory when a stream of y rays crosses 

 a block of any material some of the y particles are lost to 

 the stream by conversion into p. rays through collision with 

 atoms. Each y ray of given quality is converted into a /3 

 ray of a definite initial velocity, irrespective of the sub- 

 stance in which the change takes place. The number of 

 P rays formed in unit volume of the material will thus be 

 proportional to the coefficient of absorption (k) of the y 

 rays per unit mass, and the average intensity (I) of the 

 y rays in that volume. 



The P rays thus formed, to continue Professor Bragg's 

 argument, are deflected by collision with atoms and lose 

 energy from the same cause, the average loss of energy 

 at a collision varying with the nature of the material only. 

 This loss of energy finally brings the electrons to rest. 



The total distance which a P particle travels in any 

 material will not vary much, for p rays of given initial 



1 Bragg, Phil. Mag., S. 6, Vol. xx, p. 385, 1910, also " Studies in Eadio- 

 activity," p. 94. 



