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Helium-4Helium-4 is a non-radioactive and light isotope of helium. It is by far the most abundant of the two naturally-occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is an alpha particle, having two protons and two neutrons. Since the alpha decay is a common decay mode for many radioactive isotopes, this probably explains its abundance. In fact, alpha decay of heavy elements is the source of most naturally occurring helium-4 on earth. When helium-4 is cooled to below 2.17 Kelvins, it becomes a superfluid, with properties that are very unlike those of an ordinary liquid. For example, if helium-4 is kept in an open vessel, a thin film will climb up the sides of the vessel and overflow. This strange behaviour is not explained by the chemistry of helium, but it is explained by quantum mechanics. Helium-3 does not display this behaviour, except at much much lower temperatures, because helium-3 is a fermion while helium-4 is a boson.
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