MEASUREMENT OF IONIZING RADIATIONS 147 



energy absorbed in irradiated biological systems by calorimetric means is 

 yet to be reported. This is not altogether surprising since the expected 

 rise in temperature in water is about 2.25 X 10^'^ C per roentgen — a tem- 

 perature variation difficult enough to detect under ideal circumstances, 

 uncomplicated by the thermochemical changes ever present in living 

 matter. A possible exception to these statements is to be found in some 

 calorimetric measurements of /3-decay energy where reported heat inputs 

 of the order of 7 X 10 "^ cal/sec have been measured with accuracies of 

 better than 1 per cent (Cannon, 1950). Despite its valuable contribution 

 to the problem of absolute standardization of |8-ray sources, this sensitive 

 method, based on the use of a liquid-nitrogen isothermal calorimeter and 

 of long times of observation, does not seem to be easily adaptable to the 

 routine measurements of radiation doses received from external sources. 



Most methods of approach to the problem have made use instead of 

 physical or chemical systems in which the response obtained conforms 

 with Eq. (1) and in which e^ and e„ are either calculable or remain essen- 

 tially proportional to those values pertaining to biological systems for 

 wide conditions of irradiation. 



The majority of methods have met with limited success and remain 

 useful only in particular circumstances. Photographic methods, besides 

 being complicated by the laws of photographic blackening and by the 

 involved techniques necessary to control adequately the emulsion expo- 

 sure and the conditions of development, are essentially limited because 

 the factor e^ of the emulsion does not remain proportional to the ec per- 

 taining to biological systems under different qualities of radiation ( Wilsey, 

 1951; Erlich and Fitch, 1951; Bromley and Herz, 1950; Deal and Hober- 

 son, 1948). The otherwise easily measurable photoelectric effects are 

 similarly limited. 



The many chemical effects produced by radiation have, in general, 

 proved to be unsuitable for this application because the effect is often- 

 times not proportional to the dose and the basic mechanisms involved are 

 usually not well understood. As early as 1902, Holzknecht introduced 

 pastilles which, when exposed to X rays, changed color to a degree depend- 

 ing upon the quantity of radiation absorbed. Attendant color charts 

 were calibrated in arbitrary units. Although of considerable historical 

 interest, the method was soon discarded because of practical difficulties 

 and because it suffered essentially from the same limitations as the photo- 

 graphic method. 



Obviously, these objections do not apply to solutions of organic com- 

 pounds where Cc is expected to follow closely the desired course (Day and 

 Stein, 1951). In view of the considerable effort being expended in this 

 direction, it is hoped that the determination of energy absorbed in irra- 

 diated liquid systems by chemical means may prove feasible and thus 

 contribute to satisfactory dosimetry in cases for which physical methods 



