This is a pretty sophisticated question- not that many people out there
are aware that helium has a rare light isotope (He3) as well as the
much more common isotope (He4). He4 has two protons, two electrons and
two neutrons. (The four comes from two protons plus two neutrons, the
number of particles in the nucleus and the atomic mass of the atom.)
He3 has two protons, one neutron, and two electrons. In nature, there
is only about one He3 atom for every million He4 atoms, so it's
expensive to separate the rare He3 from He4.
Helium 3 is also
the decay product of the radioactive isotope of hydrogen called tritium
(one proton and two neutrons). Tritium has a half-life of about 12.5
years and decays by emitting an electron and an electron antineutrino,
changing one of the neutrons into a proton in the process. No He4 is
made by tritium decay, since each He4 nucleus actually is more massive
than a tritium nucleus. The He3 can easily be separated chemically from
the tritium (for example, it will not react with oxygen to form water
as tritium will). Most, if not all, of the purified He3 available for
scientific use is made this way, primarily from tritium produced in
reactors run to make materials for nuclear weapons.
If you're
interested in more about the uses of He3, or about the origins and
distribution of natural He3 and He4, or how the two can be separated,
or something else, please let us know.
Tim and Tom and Mike
(published on 10/22/2007)
