Measurement of the indoor and outdoor 220Rn (thoron) equilibrium factor: Application to lung dose

A miniature four-chamber alpha track detector was developed that measures both 222Rn (radon) and 220Rn (thoron), in duplicate. Using this detector and the previous long-term measurements of the 220Rn decay products 212Pb, and 212Bi, an equilibrium factor, Feq, is derived for both outdoor and indoor 220Rn environments (0.004+0.001 outdoors and 0.04+0.01 indoors). The lung airway dose can then be calculated from a dose factor from UNSCEAR that requires the equilibrium equivalent thoron concentration (EEC), i.e. the product of Feq and the 220Rn gas concentration. The lung dose from thoron in domestic or occupational surveys is often overlooked. The values of Feq for thoron in several published studies are in general agreement with the values reported here. Thus, a long-term alpha track measurement of thoron multiplied by an appropriate indoor or outdoor equilibrium factor yields the EEC, which can be used to assess bronchial lung dose. (authors)