418 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



received by the tissue in terms of conventional units is easily found as 

 follows: 



a. Dosage rate in rep per sec or in gm r per gm per sec 



, E d 3.7 X 10 4 p rep/sec 



" 52.5 X 10 12 == 52.5 X 10 12 3 or gm r/gm/sec 



where 52.5 X 10 12 is the factor for conversion of ev of energy absorbed to rep. 



b. Accumulated and total dose in rep 



n Ed 3.7 X 10 4 n p rep 



" 52.5 X 10 12 == 52.5 X 10 12 " or gm r/gm 



c. Integral dosage rate in gm r per sec 



Ld - = 52.5 X 10' 2 gm r/sec 



where m is the weight of the tissue containing active material. 



d. Accumulated and total integral dose 



t n mE ° 



LD - = 52.5 X 10> 2 gm r 



e. Dosage rate in ionization units 



1.58 X 10 12 W J/ 



/. Accumulated and total dose in ionization units 



Ed 



D = 



1.58 X WW 



the factor 1.58 X 10 12 is the number of ion pairs per J, and W is the average 

 energy required to form one ion pair in the gas of a cavity ionization chamber 

 placed in the medium. For air, W — 32.5 ev per ion pair formed by an 

 electron. 



In principle, d, D, Id., and I.D. should be multiplied by a fraction / 

 (< 1) to take into account the available energy that is not absorbed in the 

 tissue under consideration. If, judging from the linear dimensions of the 

 tissue mass relative to the beta-particle range, it can be assumed that the 

 available decay energy is absorbed almost entirely within the tissue, then 

 / ~ 1. If the dimensions of the tissue mass are relatively small, then/ < 1 

 and can be evaluated only by graphical methods [16] or, if the beta-particle 

 absorption coefficient for the substance is known, the beta particles may be 

 treated by the same method used for computing gamma-ray dose (see 

 Sec. 16.5). 



