Activity of a Material Containing Natural U and Th

Natural uranium consists of the three radioactive isotopes (see Uranium) namely U, and to a lesser extent with both uranium-238 and uranium-235 being the parent radionuclides of the two independent radioactive decay series (4n + 3) and (4n + 2) respectively, while uranium-234 is a decay product of the uranium-238 series. Therefore, the specific activity of natural uranium (U, ) corresponds to the activities of the three isotopes including all the individual activities of all their decaying radionuclides. Therefore for a naturally occurring radioactive material containing a mass fraction of natural uranium, the specific activities of the two parent radionuclides are given by  

Assuming secular equilibrium for the two decay chains, the activity of U is equal to that of its parent radionuclide, i.e., and is simply given by  

Therefore, for natural uranium, the specific activities of the three isotopes are contrib- 

Therefore, the total specific activity of natural uranium metal in secular equilibrium and considering all the activities of its daughter radionuclides is 179.414 MBq.kg .  

Similarly, natural thorium is a mononuclidic element, i.e., it has only one radioactive isotope thorium-232, parent radionuclide of the natural decay chain (4n), therefore, for a naturally occurring radioactive material containing a mass fraction of natural thorium, the specific activity of the parent radionuclide is given by  

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