Absorbed Fraction

For small organisms, some of the energy emitted from internally deposited radionuclides will escape from the organism and thus the absorbed fraction will in general be less than unity. For alpha radiation, the absorbed fraction is normally assumed to be unity unless the dimensions of the organism are very small, for example, phytoplankton, zooplankton, or fish eggs.

The National Council on Radiation Protection and Measurements (NCRP) reports absorbed fractions for beta radiation and gamma radiation for several reference biota species calculated using the point source dose distribution method. The absorbed fractions for beta radiation are very close to unity except for small organisms such as insects and larvae and for beta (max) energies greater than 1 MeV. However, for gamma radiation, the absorbed fractions are less than 0.1 for the species modeled at gamma energies above 0.1 MeV.

A simple approach that is sometimes used for screening purposes is to assume that all the energies emitted by the radionuclide are absorbed within the organism's tissue or organ under consideration, that is, assume the absorbed fractions for all radiation (alpha, electrons, gamma) to be unity. While this approach is reasonable for a conservative screen for beta radiation, it leads to a significant overestimate for gamma radiation. For radionuclides that emit high-energy gamma such as 60Co, the internal dose calculated using this approach can be overestimated by an order of magnitude or more if this assumption is used.

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