422 IS0T0P1C TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



ponents can usually be lumped together and an average value of n used 

 without introducing errors greater than the other uncertainties in dose 

 calculations. 



The media for which dose calculations are usually made, chiefly air, water, 

 and tissue, or substances of similar atomic composition, are not very different 

 from one another in their effect on gamma-ray absorption. Hydrogen alone 

 is anomalous in its photoelectric effect and scattering. It has an absorption 

 coefficient twice that of other light elements, and the presence of hydrogen 

 in a medium will affect correspondingly the energy absorbed for a given 

 amount of radiation. Thus, the energy absorbed per gram of water (93 

 ergs) exposed to lr is 11 per cent greater than that absorbed per gram of air 

 (83 ergs). This effect does not occur in pair production, and consequently 

 hydrogenous substances exposed to very high-energy gamma rays absorb 

 energy as would be expected for an element of unit atomic number. 



16.5. Gamma-ray Dose Calculations. The calculation of internal dose 

 delivered to tissue by gamma rays emitted from radioactive material dis- 

 tributed within an organ or animal is made extremely difficult by the many 

 physical factors affecting the radiation intensity in various parts of the 

 animal, including the organ containing the active materials. Most important 

 among these is the slow rate of absorption of gamma rays in tissue. The 

 energy made available per disintegration in the form of electromagnetic 

 radiation is readily calculated, but this amount of energy is never completely 

 absorbed in the organ containing the radioactive isotope, and indeed, it is 

 usually not completely absorbed by the entire animal. This is especially 

 true in small laboratory animals. An estimate of dose in this instance, 

 based on complete absorption of gamma-ray energy released by u micro- 

 curies of administered radioactive isotope, will be in error by a very con- 

 siderable factor. The dosage rate at any point in an animal often depends, 

 therefore, on how the active material is distributed throughout the whole 

 body. The distinction between gamma rays and beta particles in this regard 

 is illustrated by considering a spherical mass of tissue containing uniformly 

 distributed active material at a density of u microcuries per gm of tissue. 

 The dose delivered by beta particles under these conditions depends only 

 on u, but that delivered by gamma rays depends upon both u and the total 

 mass in which the isotope is distributed; the greater the mass, the more 

 intense is the gamma radiation at each point. 



The dose received at a point P, fig. 117, due to a small volume dV of active 

 material of density u microcuries per gm of tissue at O depends on the 

 geometrical reduction in energy flux over the distance R and absorption of 

 the gamma rays by intervening tissue. In principle, the dose received per 

 unit time at P due to active material distributed throughout the volume V 

 is given by the integral 



