The shortest-lived known isotope of lithium is 4Li, which decays by proton emission with a half-life of about 91(9) yoctoseconds (9.1(9)×10−23 s), although the half-life of 3Li is yet to be determined, and is likely to be much shorter, like 2He (helium-2, diproton) which undergoes proton emission within 10−9 s. Both 7Li and 6Li are two of the primordial nuclides that were produced in the Big Bang, with 7Li to be 10−9 of all primordial nuclides, and 6Li around 10−13.
The isotopes of lithium separate somewhat during a variety of geological processes, including mineral formation (chemical precipitation and ion exchange).
Lithium ions replace magnesium or iron in certain octahedral locations in clays, and lithium-6 is sometimes preferred over 7Li.
The colex (column exchange) separation method makes use of this by passing a counter-flow of amalgam and hydroxide through a cascade of stages.
At the bottom of the column, the lithium (enriched with lithium-6) is separated from the amalgam, and the mercury is recovered to be reused with fresh raw material.
Lithium-6 is valuable as the source material for the production of tritium (hydrogen-3) and as an absorber of neutrons in nuclear fusion reactions.
The separation of lithium-6 has by now ceased in the large thermonuclear powers[citation needed], but stockpiles of it remain in these countries.
Mineral and brine lithium resources are a potential limiting factor in this scenario, but seawater can eventually also be used.
[13] Pressurized heavy-water reactors such as the CANDU produce small quantities of tritium in their coolant/moderator from neutron absorption and this is sometimes extracted as an alternative to the use of Lithium-6.
Because of its nuclear properties, lithium-7 is less common than helium, carbon, nitrogen, or oxygen in the Universe, even though the latter three all have heavier nuclei.
The Castle Bravo thermonuclear test greatly exceeded its expected yield due to incorrect assumptions about the nuclear properties of lithium-7.
A relative abundance of lithium-7, as high as 35 percent greater than the natural value, has been measured in the ground water in a carbonate aquifer underneath the West Valley Creek in Pennsylvania, which is downstream from a lithium processing plant.
The large neutron absorption cross section of lithium-6 (about 940 barns[16]) as compared with the very small neutron cross section of lithium-7 (about 45 millibarns) makes high separation of lithium-7 from natural lithium a strong requirement for the possible use in lithium fluoride reactors.
Having a magic number of 8 neutrons, Lithium-11 sits on the first of five known islands of inversion, which explains its longer half-life compared to adjacent nuclei.