Particles from Thorium Active Deposit. 579 1 



active source, should' be about 1*5 times the number 

 observed in position (2) when the mica is placed near the 

 zinc-sulphide screen and further away from the active 



source. 



Fig. 2. 



=g 



ZZ2 



-&&?* 





■CZZ22S 



4-7,™. 



^ 



mm 



Foissr/OAf. (2.) 

 Pos/ rr o /v. Q) 



Acr/ve Sukfa C£ . 



(tig A h/*S TO <S C A ce. / 



In a particular experiment of this nature the total 

 distance from the active deposit to the zinc-sulphide screen 

 was 5 mm., the distance from the mica to the active source 

 in (1) being 1 mm., and in (2) 4*2 mm. Within the 

 limits of possible error in experiment, the number of 

 scintillations observed was the same in each case. Repeti- 

 tions of this experiment always gave the same result. 

 Whilst these observations suggest that the majority of the 

 long-range particles come from the active source, they do 

 not entirely preclude the possibility of a, small fraction 

 originating in the mica. Before discussing this result, 

 therefore, it appears desirable to give the data obtained by 

 the second method of long-range particle analysis. 

 (/>) In the second method a direct comparison is made 

 between the number of long-range scintillations observed 

 when screens of (1) mica and (2) aluminium are used to 

 stop the 8 6 cm. a-particles. A comparison here is made 

 between the effects observed when the absorbing screen 

 contains oxygen, as in mica, and when it is free from 

 oxygen (except as a slight impurity perhaps), as in 

 aluminium. 



If the long-range particles consist entirely of oxygen 

 atoms from the mica, it is evident that the number of 

 scintillations observed through mica will be far greater 



