Radiation Dose in Autoradiography 125 



If Aq has been applied, combination of equations (1), (2) 

 and (3) gives the general expression for the dose rate at 

 time t: — 



d.T. = 6l^^^^^^^ e^t f(T)e-^^T reps per day (4) 



The total dose for the time T the tracer remains in the 

 organism will be: — 



^^ei^Co ^^, J k(^)f(^)e-Ar dr reps (5) 



o 



For practical use, knowledge of the minimum concentra- 

 tion Cq is essential, and solutions for the integral have to be 

 found. The factor k itself can be a function of time and in 

 many cases the function k(r)f (r) will be simpler than f(T) itself. 

 In most cases two possibilities are of interest: (a) when the 

 radioactivity present in the organism after removal of the 

 specimen at time t is not important, in which case the integral 

 has to be evaluated for T=t; (b) when the dose after 

 removal of the specimen is important for a longer or shorter 

 period, i.e. when T>t. This applies especially to humans, 

 where T=oo. 



Minimum Concentration of Radioactive Material to 

 Obtain Autoradiographs 



Using short-lived isotopes the autoradiographs (ARG) can 

 be exposed for times of up to two half-lives. After this time 

 75 per cent of the possible radiation has been utilized and 

 longer exposures would not increase the strength of the ARG 

 to any appreciable extent. Only one half of the particles 

 emitted will hit the film and thus the overall efficiency will be 

 37-5 per cent, or less if developed grains in the penumbra 

 are not counted. 



The grain yield in terms of the number of photographic 

 grains rendered developable per incident electron has been 

 found by Berriman et al. (1950) to be of the order one for 



