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Consider a sample of uranium-238 that is undergoing decay to thorium-234. During the transmutation, a gamma ray is emitted from the nucleus. However, the thorium-234 daughter particle does not decay quickly enough to be detected.
What kind of storage container should the uranium sample be stored in to minimise radiation exposure?
Due to its very long half-life, U-238 is not particularly radioactive; the specific activity is only about 12 kBq/g. The emission probability of gamma radiation is less than 0.1 % per disintegration of U-238. Furthermore, the gamma energy is relatively low. Therefore, the direct gamma radiation from pure U-238 (i.e. disregarding Th-234 and further decay products) is usually insignificant.
For example, the gamma dose rate in 1 m distance from 1 kg of pure U-238 (about 12 MBq) is only approximately 0.1 nSv/h – probably less due to self-absorption. By way of comparison, the dose rate from natural background radiation amounts to about 100 nSv/h.
Nearly all of the alpha radiation from U-238 is absorbed within the sample. Only a small fraction originating from the surface of the sample can escape but is stopped by air within a distance of about 3 cm.
Therefore, you do not need any shielding. Just use a suitable closed container to prevent contamination or incorporation.
