124 S. R. Pelc 



to the cells will be due to the total concentration of ^^P before 

 fixation whilst only ^^P insoluble in hydrochloric acid will be 

 of value in an investigation concerning deoxyribonucleic acid 

 in this material.* The fraction of the tracer utilized in 

 producing the autoradiograph depends on (a) tracer lost 

 involuntarily during preparation of the specimen, {b) tracer 

 extracted during the preparation in order to retain only the 

 radioactive element in those compounds which are of interest. 

 Let C be the concentration in /xC per ml. in the latter com- 

 pound or compounds at time t, a, the total concentration of 

 tracer and a/C=k(T) their ratio. If an amount, A. ixC per 

 ml. is applied at time the concentration C at time r 

 will be: — 



C=Af(T)e-^- /xC/ml. ^ (1) 



If a minimum concentration Cq is needed at the time (t) 

 of removal of a specimen to obtain an autoradiograph, either 

 after an exposure of two half-lives for short-lived isotopes or 

 after d days for long lived ones, the minimum concentration 

 Aq to be applied will be: — 



Ao=-^ (2) 



Electronic equilibrium in the tissue in question has been 

 assumed for the following calculations. This will be so when 

 the energy of the emitted electrons is dissipated in the 

 material. It is realized that this condition will not always be 

 fulfilled, but the mathematical difficulties of the more general 

 case would make the final equations unwieldy. The dose 

 rate will be: — 



d.r. = 61aEy3==61 Ck(T)E^ reps per day (3) 



where E^ is the average electron energy in ]\IeV. 



*More detailed information on this work can be found in a paper by A. 

 Howard and S. R. Pelc in this volume. 



