PRINCIPLES OF RADIOLOGICAL PHYSICS 129 



variety of ways which seem equally plausible a priori. The dose of an 

 ionizing radiation may be represented by the total number of activations 

 which it produces per unit volume or per unit mass of the treated material. 

 This total number equals the energy dissipated per unit volume or mass 

 divided by the average energy of each activation. The rate of action is 

 then expressed as an "effective volume" for the production of the bio- 

 logical "event" under consideration by an "average activation." 



The estimation of the average energy of activations is not quite 

 straightforward. It depends on the production of activations discussed 

 from the qualitative viewpoint in Sect. 3-1 and on data of this kind pre- 

 sented in Sect. 2-4c. 



Instead of expressing the dose in terms of the estimated total number 

 of activations, we may express it in terms of the number of activations of 

 some special type. Ionizing activations are often considered in this con- 

 nection. Their number is estimated from the total energy dissipation 

 per unit volume or mass and from experimental data on the average 

 energy dissipation per ionization in a gas having a chemical composition 

 similar to that of the treated material (see Sect. 3-lb). 



Alternately, the dose may be expressed in terms of the flux of charged 

 particles within the treated material. This expression is quite straight- 

 forward in some special instances, for example, when bacteria laid on an 

 agar surface are exposed to a beam of heavy particles. Most often the 

 calculation of the flux of particles is subject to uncertainties, as when fast 

 electrons of various energies are produced by X rays. 



Once the flux of particles is known, we may calculate the size of an area 

 which is crossed by a particle with a probability equal to the probability 

 of occurrence of the biological event under consideration. For example, 

 the frequency of "death" of bacterium coh caused by polonium a par- 

 ticles was found to be 1 per 2 X 10^ particles per square centimeter. 

 Therefore the chance of a kilUng effect equals the chance of crossing an 

 area of 1/(2 X 10*) = 5 X 10"^ cm' laid in front of the a particle beam. 

 An area of this size is called the "effective area" or the "effective cross 

 section" for the specified killing effect by the specified kind of particles. 



The alternate ways of indicating the biological effectiveness of radia- 

 tion by means of effective volumes or effective areas appear equally 

 legitimate in principle. Further considerations, to be presented in the 

 next section, and especially in Sect. 5-5, indicate that one representation 

 or another may be more significant depending on the circumstances. 



5-2d. Target Theory. The statistical link between the frequency of the 

 activations produced by radiation and the frequency of the ultimate 

 biological effects has certainly something to do with the accidental dis- 

 tribution of activations throughout an organism. It is becoming pro- 

 gressively clear that macroscopic biological phenomena are steered to a 

 great extent by submicroscopic structures, such as the genes, and also by 



