146 RADIATION BIOLOGY 



expected, however, that a simple statement of the energy absorbed will 

 prove sufficient to predict a change in a biological system in all circum- 

 stances (Mayneord, 1950). 



Survey of Dosimetric Methods. Ionization in matter is caused directly 

 by a few types of energetic charged particles, such as electrons, protons, 

 and a particles, and, if they were used as primary sources of radiation, the 

 task of measuring energy absorbed would be simplified considerably in 

 theory because these particles lose energy in analogous ways, mostly by 

 interaction with the electrons of matter. However, the release of ion- 

 izing particles in biological objects is accomplished very frequently as the 

 result of an exposure to intrinsically nonionizing radiations (e.g., photons, 

 neutrons) or as a result of nuclear disintegration or decay of atomic nuclei 

 within the system. In these cases the task is somewhat more complex, 

 since it consists in assessing correctly both the amount of energy imparted 

 to the ionizing particles and the fraction of this energy absorbed by the 

 object under investigation. In practice both estimates must extend not 

 only to the experimental object itself but also to the immediate surround- 

 ings, which may affect the absorption of energy in the former. With 

 these premises in mind, the dose D (energy absorbed in ergs per gram) 

 received by the biological object may be represented by an expression 

 such as 



Z) = F • 6. ■ €„ (1) 



where F is the primary energy flux density in ergs per square centimeter, 

 €c is the fraction of the flux converted or released into energy of the ion- 

 izing particles per square centimeter per gram of material, and €„ is the 

 numerical fraction of this energy transferred to the biological object.^ 



Determination of D by this procedure clearly involves the separate 

 determination of F, €c, and e„. Moreover, since both ec and e^ depend on 

 the energy of the individual components of primary radiation, an inquiry 

 as to the "quality" of F must be added to what is already a very com- 

 plicated and impractical task. Ideally, this situation could be circum- 

 vented by the use of calorimetric methods designed to measure the quan- 

 tity of heat produced in a biological system by a given radiation exposure. 

 Although X-ray intensities of the order of 10^^ cal/cm'/sec have been 

 measured with acceptable accuracy (Laughlin and Beattie, 1951; 

 Laughhn, Beattie, and Ovadia, 1951), the successful determination of 



1 Coefficients ec and e„ are introduced in this chapter for didactic purposes only, in 

 order to emphasize the fact that the energy gained by an irradiated object is not 

 necessarily equal to the energy lost by the impinging beam of ionizing radiation. 

 6c may be easily identified with the true absorption coefficient for the case of photon 

 irradiation and with the specific energy loss for the case of charged particles. Implicit 

 or explicit expressions for e<; are found in the physical literature; computation of e„ 

 instead is clearly a task of dosimetry, requiring, in general, tedious and laborious 

 calculations. 



