Sec. 16.3] INTERNAL DOSIMETRY 415 



usually be estimated by relatively simple methods when the concentration 

 and disposition of the isotope are known. The fact that the range of beta 

 particles in soft tissue is usually only a few millimeters means that the 

 exposed tissue is confined largely to the same regions that contain the active 

 material. For this reason calculations of tissue dose from beta particles, 

 and other low-energy (< 15 mev) corpuscular radiations as well, are essen- 

 tially estimates of the energy made available by the decay of a certain 

 quantity of active isotope per gram of tissue; the result divided by an appro- 

 priate constant permits direct conversion to the desired units of dose. An 

 important factor concerning beta-particle dose should, however, be kept in 

 mind. Although the relatively short range and the consequent localized 

 absorption of beta particles provide a considerable advantage over gamma 

 rays in calculating dose, it cannot be assumed in every instance that a reason- 

 able estimate of dose will be obtained by considering the entire beta energy 

 to be absorbed solely in the tissue or organ containing the isotope. This 

 assumption is valid only when the linear dimensions of the organ are large 

 compared to the range of the particles. If thcactive material is concentrated 

 near the surface of a large organ or in tissues whose linear dimensions are 

 only a few millimeters, a considerable fraction of the more energetic particles 

 is not completely stopped in the tissue but expends part of its energy outside 

 the region. This is especially true when energetic beta emitters such as 

 P 32 (1.7 mev) are taken up in the organs of mice and similar small animals. 

 The computed dose, assuming complete absorption of beta radiation in the 

 organ under consideration, may be in error by a considerable amount. 

 This factor is less important for soft beta emitters, such as H 3 , C 14 , and S 35 , 

 since the ranges in tissue are much less than a millimeter and few organs 

 are this small. 



The range of beta particles expressed in milligrams per square centimeter 

 is very nearly constant among elements of low atomic weight. It can be 

 assumed, therefore, within the accuracy with which dose calculations are 

 possible, that the range in tissue is essentially the same as in water and 

 aluminum for which the range-energy relations are well established. Note, 

 however, that if the range in a medium is to be expressed in centimeters, the 

 range in water expressed in milligrams per square centimeter must be divided 

 by the density of the medium. 



Beta particles in the range of energies encountered from radioactive 

 isotopes (0 to 3 mev) lose energy primarily by ionization. Nevertheless, 

 some energy is also lost by radiation {Bremsstrahlung) which in turn is 

 absorbed only in large thicknesses of tissue in the same manner as con- 

 tinuous x-rays. The amount of this radiation is difficult to estimate, but 

 because it is usually small compared to energy loss by ionization, it is 

 disregarded in dosimetry. Positrons present a special problem. Each 



