428 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



The product A I in the dosage-rate formula has the following important 

 physical significance: it is the dose in roentgens per second at a point 1 cm 

 distant from a point source of 1 microcurie of radioactive isotope for a 

 particular gamma ray (or x-ray) emitted by the isotope. Its value in air 

 calculated by Marinelli, et al. [13], as a function of energy is plotted in Fig. 

 118, assuming one gamma ray per disintegration and a 1-millicurie point 

 source. If the decay scheme is complex or if strong internal conversion 

 occurs, the value of roentgens per millicurie at 1 cm taken from the graph 

 should be multiplied by the factor A/3.7 X 10 7 , which is the fraction of the 

 disintegrations in which the gamma ray appears. 



Experimental values of I A for air are given in Table 37, but it should 

 be remembered that they represent the dose per hour per millicurie at 1 cm 

 due to all gamma and x-radiation emitted by the substance. In making 

 gamma-ray dose calculations for any isotope the values of I A given in Table 

 37 or the graph may be used, or they can be calculated as indicated above. 



The foregoing discussion may at first glance appear to place dose calcu- 

 lations generally beyond the scope of anyone but the physicist or radiologist 

 who is thoroughly familiar with the subject. For the most exacting calcu- 

 lations involving complicated distributions of active material and inhomogen- 

 eous absorbing material this may perhaps be true, but in many kinds of 

 problems of practical importance even relatively complicated dose calcu- 

 lations can be carried out quickly and with the exercise of nothing more 

 difficult than ordinary arithmetic. Furthermore it is usually unnecessary 

 to consult elaborate tables and charts in order to compute a reasonable 

 approximation of the radiation dose resulting from internal irradiation. The 

 only really essential physical data for this purpose are the energies and per- 

 centages of the radiations emitted by the isotope, and the absorption coeffi- 

 cient of tissue for the gamma rays in question. The energies and per- 

 centages are given in the table of isotopes (Sec. 7.12) and by the decay 

 scheme (Sec. 7.10) if it is known for the isotope. The absorption coefficient 

 of soft tissue has a value of about 0.03 per cm (or cm 2 per gm), which is 

 sufficiently accurate for most calculations involving gamma rays with energies 

 from 0.08 to 2.0 mev. The remaining information that is required is more 

 or less physiological and anatomical; at the very least, this must include 

 some information concerning the amount of the isotope taken up in various 

 tissues and the changes in concentration with time. 



The comparative simplicity of most dose calculations, as well as the gen- 

 eral procedure to follow, is best demonstrated by a typical, although ele- 

 mentary, gamma-ray dose problem worked out in detail. The method 

 described below is easily extended to more involved problems with the aid 

 of formulas discussed in Sees. 16.6 and 16.7. The example to be considered 

 here is the calculation of the gamma-ray dosage rate and the accumulated 



