6 RUSS, Note on Radio-active Recoil. 



the particles breaking up per second actually leave the 

 surface. 



Since the factor due to the nrst cause will be the 

 same whether Radium A or Radium B is projected, and 

 probably also due to the second, we may for simplicity 

 take it unity. 



After lo minutes exposure the number of Radium A 

 particles on the disc will be "9//, and that of Radium C 

 •I3«. 



Hence at the instant of removal of the disc the initial 

 alpha ray activity will be these numbers multiplied by 

 the radio-active constants Aj, A„ of Radium A and Radium 

 C respectively. 



It has previously been shown that the activity at the 

 instant of removal of the disc, due to the Radium C on it 

 which has been produced from the Radium B directly 

 fired on to the disc is given by R, where 



R = 



—-n{ \ -ae - + />e - ). 



The evaluation of this expression shows that the 

 number of Radium C particles on the disc at the moment 

 of removal after an exposure of 10 minutes is ■34^, and 

 consequently its alpha ray activity will be '24.n\„. 



We have then at the instant of removal of the disc 

 after 10 minutes exposure : — 



(i) Due to the radiation of radium A an alpha ray 

 activity given by 'g/i\ Radium A particles and 'i^nX.^ 

 Radium C particles, breaking up per second. 



The variation of this activity with time may be read 

 from the graph referred to. 



(2) Due to the radiation of Radium B an alpha ray 

 activity given by •34«A, Radium C particles breaking up 

 per second. 



