DIFFUSION MODEL 



365 



over the whole range of ionization. For typical survival curves see Figs. 

 4 and 5. For the moment one would be inclined to prefer the hit theory 

 for simultaneous explanation of haploid- and diploid-cell-survival curves 

 if it were not for the fact that the comparison of haploid and diploid 

 survival repeated at different EEL yielded data which were not in good 

 agreement with the classical target idea. The radiations at high REL 

 were more effective than those at low REL. The shape of the survival 



100 



80 



> 60 



40 



20 



20 



40 



60 



100 



Krep 



Fig. 5. Survival of diploid yeast cells exposed to deuterons, x-rays, and alpha par- 

 ticles. Cells are taken as survivals if they divide more than once after exposure to 

 radiation. Multiple-hit curves are obtained. 



curve for both haploid and diploid cells, however, remained approxi- 

 mately the same irrespective of the REL within the entire ionization 

 region tested. In the course of this investigation it became clear that 

 the two different theories may both apply to certain aspects of the same 

 irradiation problem. In fact it seemed desirable to attempt the simul- 

 taneous use of both models of the radiation effect. In some respects this 

 view is similar to some of the concepts advanced by others, particularly 

 by Barron and Zirkle. 



Diffusion Model 



The physicist in radiobiology should be interested in the dynamics of 

 all processes involved, even beyond the mechanism of formation of ion 

 pairs. He should be able to make contributions to our knowledge of the 

 formation, distribution, and absorption of ion products, and the fate of 

 excited molecules. In a complex system such as the cell, one has to be 

 satisfied with the crudest of approximations. The inhibition of cell 



