Helium (He) is the most common element in the Universe after hydrogen. In the atmosphere of the earth its concentration is only 0.0005%, while in natural gas from the interior of the earth the concentration varies from 0.4% to 8%. Natural helium exists in two isotopic forms, 4He (99.999%) and 3He (0.001%). 3He can be produced in appreciable amounts in nuclear reactors.




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