362 ENERGY LOSS AND BIOLOGICAL EFFECTS 



By placing absorbers in the beam, one obtains particles of lower energy 

 emerging from the absorber. The mean REL of these will increase in 

 measurable fashion. 



Figure 2 shows the Bragg ionization curve and the range curve for 

 deuterons and alpha particles. The mean specific ionization per particle 

 is the ratio of these two quantities. The particular points used for 

 experiments are indicated on the graphs. A typical schematic layout 

 of the experiments is indicated in Fig. 3. 



Biological Experiments in Progress Regarding 

 THE Effect of the Rate of Energy Loss 



With the fine equipment available a number of experiments were 

 recently performed or are being performed to provide new information 

 or to verify old data. Among these are studies of inactivation of enzymes 

 (Barron, Dale, Gray, Pollard), lethal effects on bacteria (Pollard, 

 Dobson), sex-linked lethal mutations on Drosophila (Stern), chromosome 

 breakages in Tradescantia (Giles), and skin effects on mice (Tobias). 

 Fairly extensive information was obtained on the inhibition of cell 

 division by radiations in yeast cells (Saccharomyces cerevisiae) by Zirkle 

 and Tobias (17). These cells were particularly interesting because much 

 earlier work was done with them by Holweck and Lacassagne (13) and 

 Latarjet (14), who in 1940 conclusively showed that at least one strain, 

 S. ellipsoideus, had multiple-hit ultraviolet survival curves. Henshaw 

 and Turkowitz (15) showed the formation of microcolonies inS. cerevisiae, 

 the same strain used in the present experiments. Saccharomyces cerevisiae 

 was chosen by Zirkle for a series of tests concerning the RBE of various 

 radiations because, through the courtesy of Lindegren, two haploid 

 (SC7 and SC8) and a diploid (SC6) vegetative strains were available. 

 It was thought that a comparative study of the radiation effect would be 

 a good test for the premises of the hit theory of the biological effect. 

 Latarjet and Euphrussi, working along similar lines, have irradiated 

 independently similar yeast colonies (16). Much of the discussion that 

 follows was obtained in the course of the yeast studies and thus applies 

 chiefly to the inhibition of cell division on these organisms. It is hoped, 

 however, that some of the reasoning may be applied to other types of 

 biological effects and to other organisms when sufficient data are avail- 

 able. 



The Hit Theory 



As is well known, the hit theory of biological effect of ionizing radia- 

 tions is based on the idea that there exists a sensitive volume somewhere 



