a radionuclide-specific coefficient called specific energy, or SE. For any radionuclide, source region S, and target region T, the specific energy at age t is defined as
where Yi is the yield of radiation of type i per nuclear transformation, Ei is the average or unique energy of radiation type i, AFi (TS;t) is the fraction of energy emitted in source region S that is absorbed within target region T at age t, and MT(t) is the mass of target region T at age t. Age dependence in SE arises from the changes with age in both the absorbed fraction and the mass of the target region. The quantity AFi (TS;t) is called the absorbed fraction (AF) and, when divided by the mass of the target region, MT, is called the specific absorbed fraction (SAF). Information on the energies and intensities of the radiation emitted by the members of the radon series is tabulated in ICRP Publication 38 (1983).
The SE values used here were. computed with the SEECAL code of Cristy and Eckerman (1993). The calculations use files (electronic libraries) of the nuclear-decay data, SAFs for the emitted radiation, and values for the masses of the organs in people of various ages. The nuclear-decay data files and SAFs are those now used by ICRP (Cristy and Eckerman 1993; 1987). Organ masses for adults are taken from ICRP Publication 23 (1975). For children, age-specific organ masses are taken from Cristy and Eckerman (1987).
The absorbed-dose rate in target region T includes contributions from each radionuclide in the body and from each region in which radionuclides are present. The absorbed-dose rate, DT(t, t0), at age t in region T of a person of age to at the time of intake, can be expressed as:
where qs,j(t) is the activity of radionuclide j present in source region S at age t, SE(TS;t)j is the specific energy deposited in target region T per nuclear transformation of radionuclide j in source region S at age t, and c is any numerical constant required by the units of q and SE. The absorbed dose is the time integral of the absorbed-dose rate.
The equivalent dose is the absorbed dose of the various kinds of radiation weighted by a factor that represents their relative contributions to the biologic insult. The weighting factor, referred to as the radiation weighting factor (earlier called the quality factor), represents a judgment of the relative biologic effectiveness of the different radiations (ICRP 1991). In the context of radon, the equivalent dose, H, is given as