. Elementary biophysics: selected topics. Biophysics. 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 t
. Elementary biophysics: selected topics. Biophysics. 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, sodium, phosphorus, sulfur, potas- sium, chlorine, magnesium, and 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 X 1012 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 1018 ionizations per cc. If these are distributed so as to average one ionization per volume equal to the virus volume, the virus volume must
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