Helium-3

Helium-3 is a non -radioactive and light isotope of Helium. It has two protons but only one neutron in contrast to two neutrons in ordinary Helium. Helium-3 is hard to come by on Earth and sought after to use in fusion. Relatively abundant Helium-3 is thought to exist on the Moon (embedded in the upper layer of regiolith from solar wind over billions of years) and the gas giants (left over from the original solar nebula) in our solar system.

Helium-3 is used in cryogenics to achieve temperatures as low as a few thousandths of a Kelvin.