In the Local Interstellar Cloud, the proportion of 3He to 4He is 1.62(29)×10−4,[6] which is ~121 times higher than in Earth's atmosphere.
[8] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvin.
According to theoretical calculations, it would be much more stable (but still β+ decay to deuterium) if the strong force were 2% greater.
[18] Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state; so 2He's two protons have opposite-aligned spins and the diproton itself has negative binding energy.
In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps 2He.
[20][21] The team led by Alfredo Galindo-Uribarri of Oak Ridge National Laboratory announced that the discovery will help understand the strong nuclear force and provide fresh insights into stellar nucleosynthesis.
Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time.
The team fired a beam of fluorine ions at a proton-rich target to produce 18Ne, which then decayed into oxygen and two protons.
Further evidence comes from Riken in Japan and Joint Institute for Nuclear Research in Dubna, Russia, where beams of 6He nuclei were directed at a cryogenic hydrogen target to produce 5H.
[18] Some models suggest that variations in the strong force allowing a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, which would be catastrophic for the development of stars and life.
However, a 2009 study suggests that such a conclusion can't be drawn, as the formed diproton would still decay to deuterium, whose binding energy would also increase.
In some scenarios, it is postulated that hydrogen (in the form of 2H) could still survive in large amounts, rebutting arguments that the strong force is tuned within a precise anthropic limit.
Extraplanetary material, such as lunar and asteroid regolith, has traces of 3He from solar wind bombardment.
The unusual nuclear structures of such isotopes may offer insights into the isolated properties of neutrons and physics beyond the Standard Model.