Many retrospective studies regarding the long time exposure to radon (222Rn) are based on theaccumulation of 210Pb (T1/2=22.3y) on the surface of glass objects or other materials and measuring thespecific alpha activity of 210Po (T1/2=138 d, E?=5.3 MeV) resulted by implanting of radon daughters,due to the alpha recoil. In the case of thoron (22oRn) this method is not applicable because there is nota long life time isotope to allow a signifficant accumulation of any descendant. The longest life timeof the thoron descendants is 212Pb with a live time of T1/2= 10.6 hours.
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