16 



THE a PARTICLES OF URANIUM AND THORIUM. 



By W. H. Bragg, M.A., Elder Professor of Mathematics and 

 Physics in the University of Adelaide. 



[Read April 3, 1906.] 



This paper is divided into two parts. Tiie first contains 

 a discussion of the magnitude of the ionisation current due to 

 a layer of radio-active material scattered on the floor of an 

 ionisation chamber, and covered by a uniform sheet of metal 

 foil. The result is expressed in a formula which is somewhat 

 complicated in its general form, but is capable of simplifica- 

 tion under suitable conditions. Account is taken of the varia- 

 tion of ionisation with velocity. The second contains an 

 account of experiments which show : — 



(a) That the values of the current in various cases, cal- 

 culated from the formula, agree very well with the 

 results of observation. 



(h) That the ranges of the a particles of uranium and 

 thorium are very nearly, perhaps exactly, equal 

 to the range of the a particle of radium. 



(c) That the rate at which thorium atoms break down is 

 '19 of the similar rate for uranium. 



Part I. 



The method which was used by Mr. Kleeman and myself 

 in the determination of the ranges of the a particles emitted 

 by radium and its products does not lend itself to the corres- 

 ponding determinations in the cases of uranium and thorium. 

 It is a necessary feature of the method that all a particles 

 except those moving normally to the horizontal layer of radio- 

 active material should be prevented from reaching the ioni- 

 sation chamber, below which the radium is placed. This is 

 done by the use of a bundle of vertical tubes, which stop 

 all a particles other than those moving in the desired direc- 

 tion. But this limitation diminishes very greatly the num- 

 ber of effective a particles, and in the cases of uranium and 

 thorium the effect is reduced below the limits of convenient 

 measurement. This is the case even when a large surface of 

 radio-active material is employed. In order to determine the 

 ranges of uranium and thorium another method must be 

 devised. 



I have, therefore, calculated the ionisation due to a 

 radio-active layer over which a screen has been placed. The 



