KADIOBIOI.OGICAI. MI-CHANISM AT 11 IK CIKI.IAI.AK I.IAI.L 



cells in which something happens, not the event itself. The energy delivered 

 to those cells which are affected is determined solely by the size of the cell 

 and the finality of radiation. Since dose rate (or intensity; can only have 

 meaning in relation to effects produced by more than one particle, situations 

 which satisfy the criteria for single particle action necessarily satisfy the 

 criteria for dose-rate independence. We already have the information 

 necessary for an approximate calculation of the dose of radiation D^ of given 

 quality (y, X, neutron, or a radiation) corresponding to a given volume of 

 tissue, which must, of course, be taken as the volume within which irradia- 

 tion-induced disturbances may contribute to the biological response. In 

 special circumstances this volume may be larger than the cell itself— as when 

 an irradiated cell is exposed to an 'after-effect' due to toxic products pro- 

 duced in the surrounding medium. In many cases such extraneous influence 

 can be excluded on experimental grounds, and the volume to be considered 

 is the volume of the cell. It may even be possible on experimental grounds to 

 narrow the volume further to that of a cell constituent, or organelle such as 

 the nucleus, with a corresponding increase in the magnitude of D^. 



It is believed that D^ has been reached experimentally in the study of the 

 induction of lysogeny in E. coli by X radiation, in which linearity was esta- 

 blished down to less than 1 r^^, and for the induction of chromosome 

 structural damage by a radiation in which linearity was established down 

 to 4 rad24. 



Figure 1 attempts to put together the meagre information at present 

 available into a scheme representing salient features in the development of 

 radiobiological damage. End results are shown as capitals and observable 

 intermediate stages in ordinary type. Stages which are essentially conjectural 

 are shown in italics, and it is perhaps the most significant thing about the 

 Figure that the italics predominate. They fall broadly into a group concerned 

 with primary physical and chemical processes, and a group concerned with 

 metabolism. 



The remainder of this discussion will be concerned with the possibility of 

 obtaining information about the former group by direct experiment. 



Since we must accept that, in general, within the living cell all the species 

 with which we are concerned — ions, free electrons, and free radicals — will be 

 short-lived, the optimal conditions for detection are likely to be achieved by 

 compressing the total dose into the shortest possible time, and by using pulsed 

 methods of detection. The combination of a source which delivers large doses 

 in single pulses and pulsed detection methods not only increases the chance of 

 successful detection by increasing the instantaneous concentration of the 

 species concerned, but also introduces the possibility of measuring lifetimes 

 direcdy. It may be useful to discuss practical possibilities and limitations by 

 reference to the techniques being developed in our laboratory by my 

 colleague Dr. J. W. Boag. 



Electron Pulse Generator 

 Technical aspects of our high current electron pulse generator have been 

 reported at the Second International Conference on the Peaceful Uses of 

 Atomic Energy by Miller and Boag". This instrument, which was designed 

 in the Research Department of Messrs. Metropolitan Vickers Electrical Co. 



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