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RADIATION DOSE— MACROSCOPIC, MICROSCOPIC 

 AND SUB-MICROSCOPIC ASPECTS 



L. H. Gray 



British Empire Cancer Campaign Research Unit in Radiobiology, 

 Mount Vernon Hospital, jS'orthwood, England 



For the purpose of evaluating the physical, chemical, and biological effects 

 of the interaction of radiation with matter, it is necessary to have some 

 measure of the radiation, or its interaction, in physical terms. 



The effects of radiation on a cell, or a tissue, are, of course, due to the 

 radiation which is absorbed by the cell and independent of the radiation 

 which happens to pass through the cell. It follows that for biological purposes 

 the most suitable physical quantity can be defined in terms of energy 

 imparted by ionizing particles to matter at the place of interest. It is in 

 terms of this quantity that absorbed dose has been defined internationally^. 



'Report of the International Commission on Radiological Units and 

 Measurements (I.C.R.U.) 1956. Handbook 62. 



1.1. Absorbed dose of any ionizing radiation is the energy imparted to 

 matter by ionizing particles per unit mass of irradiated material at the 

 place of interest. 



1.2. The unit of absorbed dose is the rad. 1 rad is 100 ergs/g.' 



There is another physical quantity which is useful for the description of 

 radiological phenomena, namely the photon or neutron energy which 

 would, through interaction with matter, become transformed into the energy 

 of ionizing particles originating in a given quantity of matter, divided by its 

 mass. This quantity has not hitherto been explicitly defined or named by the 

 International Commission. There might be certain advantages in intro- 

 ducing such a definition to describe the interaction between radiation and 

 matter quite generally, in terms of energy and mass, in some such manner as 

 that suggested above. As this is at present under consideration by the Com- 

 mission, it would be inappropriate to discuss it further here. It will readily 

 be seen that in the restricted case of the interaction of photon radiation with 

 air, this quantity is essentially that which has been defined by the Commis- 

 sion as 'exposure dose'. It is the quantity of which the roentgen is a unit. 

 The relation between absorbed dose and exposure dose, as at present defined, 

 has been discussed elsewhere'^. 



The energy defined by the absorbed dose is imparted to matter in the 

 form of energy transfers between the moving charged particle and orbital 

 electrons. These transfers result, in the first instance, in molecular excitation 

 and ionization, and secondarily, in chemical change and heat. 



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