128 S. R. Pelc 



in days. The concentration necessary for obtaining an auto- 

 radiograph after exposure for two half-hves will therefore be: — 



^ = 0.3758x4"6xlO»xH =mX^-^X^""VC/°^'- (^> 



For work with special stripping film, taking n = 10^ grains 

 per sq. cm. and 8 = 5 xlO-* cm. we get: — 



Cs=iij^^ (7) 



Similarly for X-ray film, where n— 2 xlO^ 8=2 x 10"^. 



Cx-0-58g (8) 



For elements with long half-life, exposure for two half-lives 

 will be out of the question and the minimum concentration 

 will then depend on the exposure time chosen. For inter- 

 mediate half-lives, such as ^^S, where the full exposure time 

 might be practicable, but tedious, allowance for the shorter 

 exposure time can be made, taking the decay into account. 

 For elements with very long half-life, the decay during 

 exposure can be neglected. 



We need a total of nf/0 • 5 electrons emitted per ml. of 

 tissue. 1 fiC gives 3-2 xlO® disintegrations per day and the 

 necessary concentration, w^here d is the exposure time in 

 days, is: — 



For stripping film and X-ray film we obtain: — 

 C's=i^V/ml. (10) 



0-625f 

 Cx= — T — /xC/ml. (11) 



