Radionuclides in the environment lead to plants and animals being exposed both externally and internally to ionising radiation. Internal exposure arises from radionuclides incorporated into the organism by the processes described

Amphibian Annelid Arthropod Bird Grasses & Mammal Reptile Tree

Herbs

Figure 1 Example CRwo values for different radionuclide-wildlife group combina­tions (data reproduced from draft IAEA TRS).41 Am: reptile — carnivorous only, Co: mammal — omnivorous only, Pu: grasses only, no herbs.

in the previous section. In addition to the activity concentration in an organ­ism, internal exposure depends upon the organism size and the type and energy of emitted radiation. External exposure is largely determined by the con­tamination levels in the environment, habitat (the geometric relationship between the radiation source and the organism and the shielding properties of the medium), organism size, and the physical properties of the radionuclides.

The interaction of radiation with matter leads to the excitation and ionisa­tion of the target material (tissue). The unit of absorbed dose is the Gray (Gy), where one Gy = one Joule of absorbed energy per kg material (Jkg-1). Dose conversion coefficients (DCCs), defined as absorbed dose rate (pGyh-1) per unit activity concentration in an organism (Bqkg-1 fw; where fw = fresh weight) or medium (Bq per unit media fw), are used to relate organism and medium activity concentrations of an absorbed dose.

In the simplest case, an organism is assumed to be in an infinite homogeneous medium with the same density as itself, with the radionuclide distributed homogenously throughout all its tissues. Under these conditions, both internal (DCCint) and external (DCCext) dose conversion coefficients for mono-energetic radiation can be expressed as a function of the absorbed fraction, as follows:

Where E (eV) is the energy of a mono-energetic source and f(E) is the absorbed fraction for the energy E.

The equations assume that the organism and the surrounding medium are of similar density and elemental composition. If the radiation is not mono­energetic, the above definition can be generalised by summing the terms over the different radionuclide decay energies, weighted by the branching ratios of each transition. For external exposure, if the organism receives contributions from various environmental media (which may not always be assumed to have the same density as the organism), the equation also needs to sum these individual contributions.

The key quantity for estimating internal absorbed doses is the absorbed fraction (f), defined as the fraction of energy emitted by a radiation source that is absorbed by an organism. The uncertainty associated with the heterogeneous distribution of some radionuclides in organisms has been assessed.45 The conclusions were that: (i) for photons, the uncertainty due to a possible non­homogeneous radionuclide distribution is lower than 20-25% in the considered cases; and (ii) for electrons, uncertainty is below 30% and likely to be negligible below an energy of 0.5 MeV.