160 RADIATION BIOLOGY 



respectively. It should be mentioned at this point that recognition of 

 the general validity of Eq. (10) is implicit in the latest International 

 Commission Recommendations 1951), although no generally accepted 

 value of W and p^ as variables characterizing the radiation have been 

 proposed (see pp. 165ff). 



An excellent method leading to the experimental evaluation of pm • W 

 for electron groups of widely different energy, and independent of the 

 laws of conversion of photon energy, consists in the measurement of 

 ionization in chambers having thick^ walls containing pure /3-ray emitters. 

 In this case the energy imparted to the medium can be calculated from a 

 knowledge of the radioelement concentration and average energy E^ 

 released per disintegration (see p. 167). The experiments already 

 reported (Gray, 1949; Sheppard and Abele, 1949) are consistent with 

 Gray's principle, although the precision attained is somewhat lower than 

 one would wish to find in standardization measurements. Since most of 

 the discrepancies reported include errors in E^ and in the absolute 

 determination of the disintegration rate of the radioelements incorporated 

 in the wall, no final statement can be made as to the variation of the 

 product Pm • W with electron energy. It is gratifying to know, therefore, 

 that recently greater accuracy in the standardization of radioactive 

 sources has been attained by the use of absolute methods, such as At count- 

 ing (Sehger and Cavallo, 1951), loss of charge (Clark, 1950), and calorim- 

 etry (Cannon, 1950). 



A question raised by the international recommendations and not 

 answered by the method just described is the determination of the dose at 

 points in a medium closer to a discontinuity (such as a boundary charac- 

 terized by a change in density and/or a change in atomic number within 

 the medium) than the range of fastest electron available. Theoretically, 

 if the hnear path of the electrons in the ionized gas is negligible, Eq. (10) 

 still applies, provided that p,„ • W is strictly independent of electron 

 velocity. If it is not, pm • W will be different from the value appropriate 

 to the thick and homogeneous wall cavity since the spectrum of electron 

 velocities would not be expected to be identical in both cases. From 

 what is known of the relative stopping power in hght elements (namely, 

 it does not seem to vary greatly with the speed of the particle), no errors 

 of a basic nature are expected in this extension of the "cavity" method. 

 However, the experimental conditions necessary for the accurate deter- 

 mination of energy absorbed in a medium may not prove to be easily 

 predictable and might have to be established directly according to the 

 nature of the problem, by extrapolation to zero volume or pressure (see 

 p. 176). Pending a clarification of this state of affairs it is to be hoped 

 that the biologist will follow as much as possible the principle of conduct- 



7 That is, walls of thickness greater than the range of the fastest /3 particles available. 



