Sec. 16.2] INTERNAL DOSIMETRY 409 



organs and in the metabolic rates of uptake and elimination. The simplest 

 problem, for example, is encountered when the isotope is rapidly accumulated 

 and fixed in one or more organs, but instances of this are few, and calculations 

 based on this assumption usually do not serve even as a first approximation 

 for most of the problems met with in practice. 



The rate at which energy is made available by the decay of a known con- 

 centration or total quantity of administered isotope is easily calculated if the 

 decay scheme for the isotope is known. The distinction between available 

 energy and actual decay energy released per disintegration should, however, 

 always be kept in mind. For example, in simple beta decay the available 

 energy per disintegration is not the beta maximum energy given in isotope 

 tables but an average value which is often about one-third of Em^. The 

 remainder of the decay energy is carried off by neutrinos which do not con- 

 tribute to the tissue dose. Also in the process of pair production the gamma- 

 ray energy transferred to kinetic energy of secondary electrons is not E y but 

 E y — 2m c 2 , where 2m c 2 is twice the rest energy of the electron. 



Although the total available energy is easily determined, the fraction of this 

 energy actually absorbed within a specific mass of tissue (usually the tissue 

 containing the active material) and, therefore, contributing to the dose is 

 often subject to considerable uncertainty. Gamma rays, except possibly 

 those in the soft x-ray region, are never completely absorbed in the organ 

 from which they originate since the absorption (aside from a geometrical 

 factor of 1/V 2 ) is exponential with a half-value thickness greater than the 

 linear dimensions of large organs. The same difficulty affects beta emitters 

 as well. When the dimensions of the organ containing the isotope are 

 comparable to or smaller than the range of beta particles emitted, a con- 

 siderable fraction of the radiation is only partially absorbed and, con- 

 sequently, cannot contribute its entire energy to the organ under considera- 

 tion. Even in very large organs some loss of energy may be sustained due to 

 escape of radiation from active material lying near the surface. At the 

 present time the fraction of available energy contributing to the dose received 

 by a specified mass of tissue can be calculated for only those cases involving 

 the simplest physical and geometrical considerations. In other instances 

 estimates of the dose received can often be made by graphical methods- 

 They may also be made empirically with the aid of phantoms and ionization 

 chamber probes. 



16.2. Units of Dose. Many units of dose have been proposed in order to 

 circumvent the limitations of the roentgen. Thus far, however, only the 

 roentgen has been accepted by international agreement, while the various 

 proposed units have met with varying degrees of success through common 

 usage. Those units which have found the most frequent use in the United 

 States and Great Britain are described below. 



