Helium is the second most common element in the universe, and the lightest of the inert gases. Unlike helium-4, the most common isotope of helium, which contains two protons and two neutrons, helium-5 is a radioactive isotope of helium which contains one extra neutron.
To date, owing to its extremely short half-life (7.6 x 10^-22 seconds), the chemical properties of helium-5 have not been established. Helium generally is an inert gas and thus minimally reactive with other elements, making it much more desirable to work with in nuclear contexts than hydrogen. It has very low water solubility. Uniquely among elements, even at absolute zero it remains liquid at standard atmospheric pressures, which suggests directions for future cryogenic and superconductivity research. Plasmic helium, from which the electrons have been stripped, has very different chemical properties from the familiar atomic helium; but although plasmic helium is the most common state of helium in the universe, it is almost never found on earth outside fissile conditions.
Helium-5 does not occur in nature. Rather, it is created artificially by nuclear fusion between deuterium (H2) and tritium (H3), which have particularly low Coulomb barriers. Consequently the H2-H3 (usually referred to as DT) fusion reaction rate peaks at the lowest temperature and highest value of all nuclear reactions under practical consideration for fusion energy. Because He5 is the shortest-lived of the helium isotopes, the resulting He5 almost immediately decays into high energy free neutrons and He4 atoms. In time, the expelled neutron will also decay into a single proton and electron: a standard hydrogen atom (H1) with a small charge: its half-life is just under fifteen minutes. Alternately, the free neutron itself may sustain other forms of nuclear reaction, although this is much more common with nuclear fission.
The DT fusion reaction is just about the most efficient mass-energy exchange currently believed possible. Only matter-antimatter direct mass conversion produces more energy.
The products of the DT reaction, including the intermediate He5, are either stable (He4) or extremely short-lived. This means that storage of nuclear waste from this form of fusion reaction is much less of a concern than fissile waste.
This is not the stellar version of fusion reaction, which involves stripped proton chains instead. Stellar energy release rates are actually much lower than the volumetric rate at which a human body generates heat while at rest. Thus, to replicate stellar core conditions and the stellar fusion reaction for the purpose of generating nuclear fusion power is not practical.
