358 ENERGY LOSS AND BIOLOGICAL EFFECTS 



phenomena, that it may lead us to proving invariance of certain quanti- 

 ties and thus to more or less general biological properties. The model 

 proposed in this paper is presented because some of the constants in the 

 model are supposed to be invariant and they are accessible to experi- 

 mental determination. This should stimulate experimenters to check 

 the validity for the prediction of the model in a wide. range of environ- 

 mental conditions. In order to stimulate progress, it is helpful if 

 deviations from the theoretical predictions of the model are experi- 

 mentally detected. If this is the case and such deviations are quanti- 

 tatively measured, one is usually able to evaluate a better model with 

 greater range of validity and more detailed explanations. In the present 

 paper the phenomenon of inhibition of cell division is examined theoreti- 

 cally and experimentally in a set of varied conditions. Some of the 

 statements below are in agreement with the general scheme presented 

 by Zirkle in the previous paper. In effect the simple assumptions under- 

 lying the theory correspond to some of the "relevant" processes in 

 Zirkle's scheme. Some processes classified "irrelevant" are omitted. 

 One should bear in mind, however, that for different types of biological 

 effects different processes may prove to be relevant. Further, the 

 irrelevant processes will also modify quantitatively the predictions of 

 the theory; usually there is a necessity to add small corrections to 

 account for these. 



For the past twenty years those interested in the mechanism of 

 biological effects of radiation have realized that any satisfactory theory 

 of such effects must allow for the variation in the magnitude of radiation 

 effects depending on the rate of energy loss (REL) * of the ionizing 

 particles in the medium. By and large it was found that the quality of 

 the effects did not vary appreciably in a wide range of REL, but that the 

 sensitivity of most organisms for various types of effects did vary. The 

 effectiveness of radiations with different REL is often called the relative 

 biological effect (RBE) . Because of difficulties in producing homogeneous 

 radiations and uncertainties of dose measurement in the past, data were 

 obtained only after long and tedious effort. An excellent review of the 

 work up to 1943 was written by Zirkle (1). During the past few years 

 experimental facilities have vastly improved, and it is now possible to 

 obtain reliable data in a fraction of the time it would have previously 

 taken. The author's own interest in this field was originally in the 

 physical properties and measurement of radiations. Some of the work 



* The term "rate of energy loss" refers to the quantity of energy expended in an 

 absorbing medium as ionization or excitation by a single particle during its passage 

 through unit thickness of an absorber. 



