M. Levin — Absorption of the a-Bays from Polonium. 11 



.The shape of the curve is very similar to that obtained by 

 Bragg and Kleeman for the a-rays from a thin film of radium 

 and by McClung for a thin film of radium C. The ionizing 

 power of the a-particle steadily increases with the distance, 

 passes through a well-marked maximum and then rapidly falls 

 off. In the experiments the ionization chamber had a depth 

 of 5 mm as stated above, and the cone of rays was not very nar- 

 row. The ionization per centimeter of path consequently 

 appears to fall off more slowly than would be observed with a 

 shallower ionization chamber and a very narrow cone of rays. 



1 





























fc 





&_ 







K-^. 





-~4—^. 











k. H 





___*/__ 















--^-— . 





) 







—a. 



/ 



y 



/ 





• 















Ionization. 



Figure 2 shows the curves obtained when the rod was cov- 

 ered with different layers of aluminium foil, the numbers of 

 the curves giving in each case the number of layers of alumi- 

 nium foil employed. 



It is seen from the curves that the interposition of a sheet of 

 aluminium foil does not at all alter the general shape of the 

 curve, but that the effect of the screen is to lower the ioniza- 

 tion curve by a definite distance corresponding to the equiva- 

 lent in air of the stopping power of the aluminium screen. 

 The amount of the maximum ionization is not affected by the 

 absorption of the a-rays in the aluminium foil, thus agreeing 

 with the view of Bragg, that the a-rays are not absorbed 

 according to an exponential law, but that the whole number 

 of a-particles- passes the aluminium sheet, their velocity all 

 being diminished by a definite amount. 



In Tables II and III, the distances between the rod and the 

 lower plate of the ionization chamber are given, for which the 

 a-particle begins ionizing. 



