Please note that the content of this book primarily consists of articles
available from Wikipedia or other free sources online.Although there are
eight known isotopes of helium, only helium-3 and helium-4 are stable.
In the Earth's atmosphere, there is one 3He atom for every million 4He
atoms. However, helium is unusual in that its isotopic abundance varies
greatly depending on its origin. In the interstellar medium, the
proportion of 3He is around a hundred times higher. Rocks from the
Earth's crust have isotope ratios varying by as much as a factor of ten;
this is used in geology to investigate the origin of rocks and the
composition of the Earth's mantle. The most common isotope, 4He, is
produced on Earth by alpha decay of heavier radioactive elements; the
alpha particles that emerge are fully ionized 4He nuclei. 4He is an
unusually stable nucleus because its nucleons are arranged into complete
shells. Equal mixtures of liquid 3He and 4He below 0.8 K will separate
into two immiscible phases due to their dissimilarity. Dilution
refrigerators take advantage of the immiscibility of these two isotopes
to achieve temperatures of a few millikelvins. There is only a trace
amount of 3He on Earth, primarily present since the formation of the
Earth, although some falls to Earth trapped in cosmic dust
