helium-4

…to the nucleus of the helium-4 atom, spontaneously emitted by some radioactive substances, consisting of two protons and two neutrons bound together, thus having a mass of four units and a positive charge of two. Discovered and named (1899) by Ernest Rutherford, alpha

, helium-4), and the particles required to embody the fields of quantum field theory (e.g., photons and gluons). Bosons differ significantly from a group of subatomic particles known as fermions in that there is no limit to the number that can occupy the same quantum state.…

…or ³He) and four (helium-4, or ⁴He) are stable; all the others are radioactive, decaying very rapidly into other substances. The helium that is present on Earth is not a primordial component but has been generated by radioactive decay. Alpha particles, ejected from the nuclei of heavier radioactive substances,…

…protons and one neutron, and helium-4 (or ⁴He), with two protons and two neutrons. ⁴He forms the bulk of naturally occurring helium, but the lighter isotope ³He has been formed, since about 1950, in experimentally useful quantities by the decay of tritium produced in nuclear reactors.