Isotopes of neutronium

Neutronium is a hypothetical substance that consists almost entirely of neutrons. It is often discussed in the context of astrophysics and is theorized to be found in the cores of neutron stars, where extreme gravitational pressures force neutrons together in massive quantities. Since neutronium is made up entirely of neutrons, it doesn't have isotopes in the traditional sense as isotopes refer to variants of a chemical element that have the same number of protons but different numbers of neutrons.

A tetraneutron is a hypothetical nuclear structure consisting of four neutron particles bound together. Neutrons are subatomic particles that are found in the nucleus of atoms, and they have no electric charge. The concept of the tetraneutron arises in the field of nuclear physics and represents an attempt to understand how neutrons can interact with each other through the strong nuclear force.