186 Applied Biophysics 



it was only after Gray's detailed treatment that an adequate 

 insight into the problem was attained, and the idea of radiation- 

 dose advanced a stage further than the rontgen unit. 



It must be borne in mind that the rontgen is solely a measure 

 of exposure to radiation — it merely describes what the beam 

 of radiation will do in air, and not what it will do in any other 

 medium, although it gives a good approximate guide to the 

 latter in the case of light elements, such as occur, for example, 

 in tissue. Furthermore, the energy absorption in a medium 

 other than air cannot in general be calculated from the rontgen 

 dose by correcting with the ratio of the absorption ( "energy 

 conversion") coefificients of the medium and air, because nor- 

 mally the quality of the radiation, on which these coefficients 

 depend, is unknown. 



Gray's theory removes this element of vagueness, for it enables 

 the actual energy communicated to any medium to be deduced 

 from measurements of the ionization produced in a small gas- 

 filled cavity in that medium. If E is the energy communicated 

 to the medium per unit volume, J the ionization per unit volume 

 of the gas-filled cavity, and q the ratio of the rates at which 

 a secondary particle loses energy in the medium and in the 

 gas of the cavity, and W is the average energy expended by the 

 secondary particles in producing an ion pair in the gas of the 

 cavity, then 



E = eWJ 



The detailed derivation and exposition of this relationship, 

 called by Gray the "principle of equivalence" must be sought 

 in the original publication. There are certain restrictions : (1 ) the 

 fraction of their energy lost by the secondary particles in cross- 

 ing the cavity must be negligible;* (2) the cavity must be 

 surrounded on all sides by a thickness of the medium at least 

 equal to the maximum range of the secondary particles ; (3) the 



• Restriction (1) is unnecessary if the gas in the cavity is of the same constitu- 

 tion as the walls. 



