A rare
isotope of
helium, with only one
neutron, instead of the two
neutrons in
helium-4, the dominant
isotope. It is a
byproduct of a number of
fusion reactions, and is emitted in quantity by the
Sun. It can be found in
lunar soil or
regolith and as a very small component of
terrestrial helium. It often is proposed as a
fusion fuel.
See also: