Dineutron follows neutron
Mononeutron: An isolated neutron undergoes beta decay with a mean lifetime of approximately 15 minutes (half-life of approximately 10 minutes), becoming a proton (the nucleus of hydrogen), an electron, and an antineutrino.
Dineutron: The dineutron, containing two neutrons, was unambiguously observed in 2012 in the decay of beryllium-16. It is not a bound particle, but had been proposed as an extremely short-lived resonance state produced by nuclear reactions involving tritium. It has been suggested to have a transitory existence in nuclear reactions produced by helions (completely ionized helium-3 nuclei) that result in the formation of a proton and a nucleus having the same atomic number as the target nucleus but a mass number two units greater. The dineutron hypothesis had been used in nuclear reactions with exotic nuclei for a long time. Several applications of the dineutron in nuclear reactions can be found in review papers. Its existence has been proven to be relevant for nuclear structure of exotic nuclei. A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so. This has some consequences on nucleosynthesis and the abundance of the chemical elements.
Trineutron: A trineutron state consisting of three bound neutrons has not been detected, and is not expected to exist even for a short time.
Tetraneutron: A tetraneutron is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated.
Pentaneutron: Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound.Although not called "neutronium", the National Nuclear Data Center's Nuclear Wallet Cards lists as its first "isotope" an "element" with the symbol n and atomic number Z = 0 and mass number A = 1. This "isotope" is described as decaying to hydrogen-1 with a half life of 10.24±0.2 min.