REFERENCES 383 



3. Evaluation of the ionic efficiency in a given example allows approxi- 

 mate calculation of the magnitude of biological effects as a function of 

 specific ionization. 



4. Experimental tests of the diffusion model are expected to clarify 

 some radiation mechanisms in detail. The formulation of the diffusion 

 model allows for variation in radiation sensitivity with substrates, state 

 of cell division, water content, etc.; it furnishes clues for study of the 

 quantitative effects of externally applied cell poisons. 



5. In the modified theory the size of the target loses significance. 

 Individual sensitive "sites" may exist in the cell. They are important 

 because of their biological role, not because they form "targets." 



6. By means of the diffusion model the mechanism of inhibition of cell 

 division by radiation may be quantitatively accounted for in haploid, 

 diploid, and possibly in multiploid yeast cells. 



7. The production of yeast cells with unpaired radiation-induced 

 genetic defects should stimulate biochemical and biophysical study of 

 the basic mechanisms involved in cell division. 



Acknowledgments 



Much of the work reported here concerning relative biological effects 

 as functions of rate of energy loss was done in collaboration with Pro- 

 fessor R. E. Zirkle, to whom the author is much indebted. Many of the 

 ideas presented were stimulated as a result of this collaboration. The 

 author wishes also to thank Professor J. H. Lawrence for his interest, 

 Professors Ernest Lawrence and Robert Thornton for the use of the 

 184-in. cyclotron, and R. Mortimer, R. Wijsman, B. Stepka, and H. 

 Anger for their collaboration. The paper is partially based on work 

 done under the auspices of the Atomic Energy Commission. Some of 

 the material in this paper has been previously reported in Am. J. 

 Roentgen., 67: 1, 1952. 



REFERENCES 



1. Zirkle, R. E., Manhattan Project Rept. CH-946, September 1943. 



2. Brobeck, W. M., E. O. Lawrence, K. R. MacKenzie, E. M. McMillan, R. Serber, 

 D. C. Sewell, K. M. Simpson, and R. L. Thornton, Phys. Rev., 71: 449-450, 1947. 



3. Powell, W. M., L. R. Henrick, Q. A. Kerns, D. Sewell, and R. L. Thornton, 

 Rev. Sci. Instr., 19: 506, 1948. BP 138. 



4. Wilson, R. R., Radiology, 47: 487, 1946. 



5. Tobias, C. A., H. Anger, P. P. Weymouth, and R. L. Dobson, Atomic Energy 

 Comm. Doc. 2099-A, May 28, 1948. See also AACR Detroit Meeting, April 1949. 



6. Tobias, C. A., P. Rosahn, M. Lewis, H. Anger, and J. H. Lawrence, Atomic 

 Energy Comm. Rept. UCRL 193. 



7. Rosahn, P., Atomic Energy Comm. Rept. UCRL 270, December 1948. 



