128 RADIATION BIOLOGY 



5-2c. Effective Volumes and Effective Areas. We have been considering 

 the rate of action, a, in Eq. (49) as the probabiUty of producing a certain 

 "event" in an organism "per unit dose of radiation." As mentioned in 

 Sect. 5-lb, the "dose" of radiation indicates in general the amount of 

 energy dissipated per unit volume of the material. 



Energy dissipation is a comprehensive macroscopic concept which 

 applies equally to all radiations but is unrelated to the microscopic nature 

 of each radiation or to the mechanism of its physical action. Therefore 

 it is also of interest to relate the number of the ultimate biological events 

 caused by radiation to the number of primitive elements of the radiation 

 action: particles or photons, activations or ionizations. To this end the 

 dose is expressed in terms of the flux of particles or photons or in terms of 

 the number of activations of various kinds produced in the material under 

 consideration. 



A dose of monochromatic light is most simply expressed in atomistic 

 terms. The total light energy incident on the surface of a material 

 divided by the energy of one photon may be regarded as a measure of the 

 "flux of photons." However, the number of photons absorbed in a mate- 

 rial is more pertinent "to the radiation action than the mere flux of pho- 

 tons. The number of light photons absorbed coincides with the number 

 of activations produced and equals the amount of radiation energy 

 absorbed divided by the energy of one photon. 



When the dose of light is expressed in terms of the number of photons 

 absorbed, the rate of action a represents the probability of occurrence of 

 the "event" under consideration "per photon absorbed." The absorp- 

 tion of photons is subject to statistical fluctuations, as we know. Thus 

 the proportionality of effect to dose establishes a definite correlation 

 between the statistically variable frequency of biological events and the 

 statistically variable number of photons absorbed in each portion of 

 material. 



This statistical relation may be specified by indicating the size of a por- 

 tion of material in which photon absorptions occur as frequently as the 

 biological events occur in an organism. Consider a hypothetical example 

 in which the observed probability of bacterial "killing" by ultraviolet 

 light equals lO^^'^ for every photon absorbed in 1 cm^ of bacterial matter. 

 Since photons are absorbed here and there at random, the death of a 

 particular bacterium has the same probability as the absorption of one 

 photon within a volume of 10^^° cm* of bacterial matter. The indication 

 of this volume serves to represent the frequency of the corresponding 

 biological event. Owing to the analogy with the corresponding formula- 

 tion of the frequency of atomic phenomena (see footnote 5, Sect. 2-2a) 

 this volume is called the "effective volume" for the killing of the specified 

 kind of bacteria by ultraviolet light of specified wave length. 



The rate of action of ionizing radiations may be represented in a 



