THE TARGET THEORY 



111 



20 40 60 80 



X-ray dose in units of 1000 roentgens 



Fig. 52. The x-ray inactivation of bacteriophage alfa. The dose for produc- 

 ing 37% survival is found by the dashed lines to be about 23,000 roentgens. 



properties of all the radiations we have mentioned earlier, and the num- 

 ber of ionizations produced per cc of substance is a number which can be 

 found by looking in the proper books on radiation biology. It depends 

 on the nature of the substance, for it depends on how many electrons are 

 contained per unit volume. For typical biological material (wet tissue) 

 there will be something like 10% hydrogen, 12% carbon, 4% nitrogen, 

 73% oxygen, 0.1% sodium, 0.2% phosphorus, 0.2% sulfur, 0.35% potas- 

 sium, 0.1% chlorine, 0.04% magnesium, and 0.01% calcium. Knowing 

 this, it is possible to compute the number of ionizations produced per cc 

 by each kind of ionizing particle. The unit of dose was chosen by the 

 physicists, who called the amount of radiation that will produce 1.8 X 10 12 

 ionization per cc in air one roentgen. Note that the unit is expressed as 

 ionizations per cc. For these sparsely ionizing radiations, such a unit is 

 reasonable, since we have indicated that the probability of producing an 

 ionization is proportional to the volume traversed. For substances such 

 as water or normally occurring organisms, the density of the substances 

 is about 1000 times greater than that of air; therefore this density 

 factor must be taken into account in computing the number of roentgens 

 delivered to the substance. 



The upshot of the physicists' calculations is that the number of ioniza- 

 tions per cc of substance can be computed from the nature of the radia- 

 tion and of the substance. Thus, suppose that the dose given to produce 

 37% survival of our viruses corresponds to 3 X 10 18 ionizations per cc. 

 If these are distributed so as to average one ionization per volume 

 equal to the virus volume, the virus volume must be 1/(3 X 10 18 ) cc. 

 Thus, from the biological measurement of the 37% survival dose (D 37 in 



