426 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



roentgen, W = 32.5 ev and, hence, nW = 5.22 X 10 13 ev or 83 ergs per 

 gram of air per roentgen. Accurate values of n and W are not known 

 explicitly for various kinds of tissue, but their product nW can be deter- 

 mined experimentally. Although the value of nW for air is sometimes taken 



Table 37. Specific Gamma-ray Doses from Radioactive Isotopes 

 Dosage rate in air at 1 cm distance from unfiltered point source of radioactive isotope. 

 Units of I A are in roentgens per millicurie per hour at 1 cm, milliroentgens per microcurie 

 per hour at 1 cm, or milliroentgens per 10 millicuries per hour at 1 m. Values in paren- 

 theses are specific dosage rates due to x-radiation associated with K capture and internal 

 conversion. All other values are for gamma rays from isomeric transitions and from annihi- 

 lation of positrons. Reported by L. D. Marinelli, E. H. Quimby, and G. J. Hine, Am. J. 

 Roentgenol. Radium Therapy, 59, 260 (1948). Values for Ra are for radium in equilibrium 

 with its decay products and with platinum filtration, reported by W. V. Mayneord, and 

 J. E. Roberts, Brit. J. Radiology, 10, 365 (1937). 



to be the same for soft tissue, its true value depends on both the atomic com- 

 position of the absorbing medium and the gamma-ray energy. In soft 

 tissue (water), because of the presence of considerable amounts of hydrogen, 

 the value of nW is found to be about 93 ergs per gm per r. 



