L. H. GRAY 



^^Lea, D. E. Actions of Radiations on Living Cells, 2nd edn: Cambridge University 



Press, London, 1955, p. 225 

 ^2 Wolff, S. and Luippold, H. E. Progress in Radiobiology (cd. J. S. Mitchell, B. E. 



Holmes and C. L. Smith): Oliver and Boyd, Edinburgh, 1955, p. 217 

 "Beatty, a. v., Beatty, J. W. and Collins, C. Amer.J. Bot. 43 (1956) 328 

 ^*KoHN, H. I. Progress in Radiation Therapy (ed. F. Buschke); Grune and Stratton, 



New York, 1958 p. 62 

 ^*KoHN, H. I. and Gunter, S. E. Radiation Res. 5 (1956) 674 

 I6K0HN, H. I. and Kallman, R. F. Radiation Res. 5 (1956) 693 

 ^^ Christie Hospital and Holt Radium Institute Medical Research Council Unit, 



Symposium on the Relative Biological Efficiency of 4 AleV and 300 kV Radiations: pub- 

 lished in Brit. J. Radiol. N.S. 30 (1957) 337 

 I8GRAY, L. H. and Scholes, M. E. Brit. J. Radiol. N.S. 24 (1951) 82 

 ^®Lea, D. E. Actions of Radiations on Living Cells, 2nd edn: Cambridge University 



Press, London, 1955, p. 269 

 20BURCH, P.J. R. Radiation Res. 6 (1957) 289 I 



2^ Gray, L. H. Actions Chimiques et Biologiques des Radiation, vol. 1, (ed. M. Haissinsky) : 



Masson et Cie, Paris, 1955, p. 76 

 22GRAY, L. H. J. Chim.phys. 52 (1955) 519 

 23Kirby-Smith, J. S. and Daniels, G. S. Genetics 38 (1953) 375 

 24Lea, D. E. Brit. J. Radiol. N.S. Supplement 1 (1947) 59 

 25READ, J. Brit. J. Radiol. N.S. 22 (1949) 366 

 ^^Franck, J. and Platzman, R. Radiation Biology, vol. 1 (ed. A. Hollaender) : McGraw 



Hill, New York, 1954, p. 191 

 2^ Platzman, R. and Franck, J. Symposium of Information Theory in Biology (ed. H. P. 



Yockey) : Pergamon Press, London, 1958, p. 262 



DISCUSSION 



Dr. George : Just to clear up one point of my ignorance — In Lea's figure^ T = 0. 

 I do not see why T = if the negative and positive ions do not have the same 

 distribution. 



Dr. Gray: The diagram to which you refer was drawn by Lea to show the diffusion 

 of the hydroxyl radicals and hydrogen atoms which he presumed to be formed from 

 the positive and negative ions respectively. In that diagram time was measured from 

 the moment of formation of these radicals. 



Dr. Martin: Would I be correct in assuming that the hump in your efficiency curve 

 would depend upon the effective target size? It must bear some relationship to the 

 range of S-rays. 



Dr. Gray: Yes, the magnitude of the hump depends on the manner in which you 

 define the quantity 0, i.e. on the kinds of energy transfer which are assumed to have 

 unit efficiency. The definition will involve implicitly one assumption about target 

 size and one about the amount of energy which must be deposited in the target to 

 produce the particular biological eflfect under consideration. I have carried through 

 a calculation for two different definitions of 0, and obtained humps of two different 

 sizes, but located at approximately the same value of photon energy. I do not think 

 this was accidental. The hump occurs in the range of photon energies for which the 

 contribution of photo-electrons of the total dose is rapidly diminishing in size relative 

 to that of recoil electrons, and is relatively independent of the particular way in which 

 we define 0. 



Mr. Green: Dr. Gray, in Figure 7 you showed the differing effects of hexanc, water, 

 and chloroform. Did those calculations involve mean atomic numbers for he.xane? 



13 



