Dosimetry

Dosimetry models used to assess the potential adverse effect of environmental radioactivity on biota and to demonstrate compliance with biota radiation dose standards are based on radioactivity concentrations in biota and in environmental media (air, water, soil, sediment) in direct contact with biota. Models for biota are not as well developed as compared to those for humans; this is in part due to the almost infinite diversity of species of nonhuman biota (anatomical, morphological, and physiological aspects). In practice, simplifying assumptions have to be made in the models to approximate the ecological system.

Models are needed to convert exposure from both external and internal radiation to absorbed dose. Factors important for external dosimetry include geometrical relations between the source of the radiation and the biota, the size of the biota, and characteristics of the radionuclides (e.g., radioactive decay characteristics). Factors important to estimating dose from internal radio-nuclides include chemical species and the possibility of a nonuniform distribution of radionuclides within the organism, the fraction of energy emitted from internally deposited radionuclides that is absorbed in the biota (a function of the size of the biota and the energy of the radiation(s)), and the relative biological effectiveness (RBE) of different kinds of radiation.

The radiation dose received by the biota (or some organ or tissue of the biota) is the sum of both external and internal exposures. In general terms, the dose rate D (mGy h~ ) can be calculated according to

D ^ ^ [DCCext,r' Qoil,sediment,water,r ^ DCCint,r' Qjiota,r] [1]

r where DCCextr = dose conversion coefficient for external exposure to radionuclide r (p,Gyh~ per Bqkg~ ); Csoil,sediment,water,r = activity concentration of radionuclide r in soil, sediment, or water (Bqkg-1); DCCintr = dose conversion coefficient for internal exposure to radionu-clide r (pGyh-1 per Bqkg-1); and Cbiota,r = internal activity concentration of radionuclide r in biota (flora or fauna) (Bq kg~ ).

Dose conversion coefficients for external exposures of biota are the result of complex and nonlinear interactions ofvarious factors, including among others, the concentrations ofradionuclides in other environmental components of the habitat, radionuclide-specific decay properties, the size of the organism, and any shielding provided by materials in the environment. Dose conversion coefficients for a wide variety of biota and radionuclides are provided in multiple sources.

Two subjects merit special consideration, namely the fraction of energy emitted from internally deposited radionuclides that is retained in the organism (absorbed fraction) and the weighting factor that should be applied to account for RBE of the various kinds of internally deposited radionuclides.

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