MEASUREMENT OF IONIZING RADIATIONS 173 



stable and present in a vniiform concentration of C txc per gram of mate- 

 rial is given by (Marinelli, Qiiimb}^, and Hine, 1948) 



D-y = KyCgy roentgens 



where 



Ky = IAU- ly- 10-^ (20) 



and where Ky expresses the number of roentgens at 1 cm distance in air 

 due to the complete disintegration of an unfiltered point source of 1 ^c, t is 

 the half-life in hours, and ly is the 7-ray dose rate in roentgens per hour 

 at 1 cm in air from an unfiltered source of 1 millicurie. Values of ly are 

 incorporated in Table 2-2. The quantity Qy is the geometrical factor 

 given by the volume integral 



where /x is the coefficient of absorption of the 7 radiation in the medium. 

 Recalling the analogous expression assumed for the case of /3 radiation, it 

 is obvious that 



gy = ^-KQ^Ip. ' (21) 



It is to be concluded, therefore, that the expressions for g derived for jS 

 radiation can be used with the aid of expression (21) for the calculation 

 of 7-ray dose in roentgens provided that the linear parameters are 

 expressed in relaxation lengths (1/m) appropriate to the photon energy 

 under consideration. These are plotted in Fig. 2-9 and have been 

 calculated in millimeters of water as l/iVo(T + o-a), where r and Oa are the 

 photoelectric and true Compton absorption coefficient per electron and 

 A^^o is the number of electrons per 0.1 ml of water. The procedure, as a 

 matter of fact, is much more justified in the case of 7 radiation because 

 the absorption coefficient \x has here a much more definite meaning than 

 for the case of ^ radiation. Whenever the dose D\ is to be expressed in 

 ergs per gram, then 



where 



D; = K',Cg\ ergs/g (21a) 



In this expression T is the half-life in days and Ey is the average 7-ray 

 energy in mega electron volts per disintegration. The expression for 

 g'^^ in this case, is identical to that of g^ provided that, as above, the linear 

 parameters are expressed in the proper way. If the 7 radiation is com- 

 posed of several 7 rays, each occurring on the average pi times per dis- 

 integration, the expression for D'^ will become obviously 



^7 = C' 2 P^^^y'^'^^^y ergs/g 



i 



where K'y{i) and g' Ji) refer to each single monochromatic component. 



