Unlike helium-4, it cannot be produced by radioactive decay.

与氦- 4不同之处是，氦- 3不能由放射性衰变产生。

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Because of this, the quality of helium-4 nucleus has been reduced.

正因为这样，氦4原子核的质量减少了。

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Like deuterium, it is rare relative to helium-4, but, unlike hydrogen, helium doesn't form chemical compounds as abundant as water.

象氘一样，相对于氦- 4它是非常罕见的，但是不同于氢，氦不是来自化学混合物象水那样丰富。

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Terrestrial helium, for example, consists of a small amount of helium-3 (the nucleus of which has two protons and one neutron), and mostly helium-4 (which has 2 protons and 2 neutrons).

BBC: ET gases caged on Earth