RADIVriOX DOSE 



Dr. Gray: The approximate values of -q were calculated separately for photo- 

 electrons and recoil electrons. The values shown in Figure 7 arc weighted means which 

 allow for the proportion of energy absorbed by each process in the atoms of hydrogen, 

 oxygen, and chlorine, contained in the molecules hexane, water, and chloroform. 



Dr. Jackson: Dr. Gray, having in mind Alexander's idea of direct action in dry tissue, 

 I am wondering whether you think, from the physical standpoint, that the presence 

 or absence of water in the medium can have any influence on 8-ray production. 



Dr. Gray: I do not think that the presence or absence of water in the medium is 

 likely to have any great influence on the frequency with which 8-rays of any given 

 energy occur along the track of the primary particle. It may well, however, have an 

 important influence on the distance from the axis of the track at which negative ions 

 are formed by electron attachment, and at wh'ch the charge neutralization occurs. 

 These factors will in turn influence the initial distribution of radicals relative to one 

 another, and the chances of recombination as opposed to other forms of interaction. 

 The water may thus, ind'rectly, have a considerable influence on the limits between 

 which Q must lie in order to achieve a given type of chemical or biological reaction. 



Dr. Vogel: Is there any place on the LET spectrum where the dose-rate indepen- 

 dence is lost, and is it, in your opinion, specifically related to the number of ion-pairs 

 per micron of tissue ? 



Dr. Gray: It seems that most biological effects which are initiated by the combined 

 action of more than one ionizing particle are dose-rate dependent. If this is so, then 

 the answer to your question depends on three things: (/) the dose, (2) the LET of the 

 ionizing particles, and (5) the size of the structure initially damaged. The higher the 

 LET, the larger the dose at which the transition from dose-rate dependence to dose- 

 rate independence would be expected to occur. Figure 3 gives information for struc- 

 tures of different size exposed to X-rays, neutrons, and a-particles, when in each case 

 the dose is 100 rad. By application of the Poisson formula corresponding figures may 

 be derived for any other dose. More accurately, you may use the Poisson formula and 

 Lea's Table 18 Mo calculate the probability that at any given dose level there will be 

 one particle or more than one particle incident on the structure which you have in 

 mind. 



Dr. Segal: In the experiments where a plateau of radio-sensitivity was attained at 

 12 to 24 hours, I take it that the roots were irradiated continuously. How is this 

 experiment aflfected if the total time were the same, but the dose were dehvered in, 

 say, 1 2 separate periods at hourly intervals ? 



Dr. Gray: In our experiments the roots were, as you say, irradiated continuously. 

 Twelve separate fractions at hourly intervals should approximate to a 12-hour 

 continuous irradiation, irrespective of the dose rate during each irradiation, since each 

 of the separate doses will be only one twelfth of the total dose, which in the case of 

 growth inhibition in roots is small enough to be in the dose-rate independent region. 

 I would, however, not like to hazard a guess as to the closeness of the identity between 

 highly fractionated and continuous irradiation of other materials. I believe such 

 experiments have been carried out, but I cannot recall a literature reference to quote 

 to you for this information. 



REFERENCE 



^ Lea, D. E. Actions of Radiations on Living Cells : Cambridge University Press, London, 

 1946, p. 32 



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