Hydrogen-4 is a highly unstable isotope of hydrogen. The nucleus consists of a proton and three neutrons. It has been synthesised in the laboratory by bombarding tritium with fast-moving deuterium nuclei (see this article). In this experiment, the tritium nuclei captured neutrons from the fast-moving deuterium nucleus. The presence of the hydrogen-4 was deduced by detecting the emitted protons. Its atomic weight is 4.0279121. It decays through neutron emission and has a half-life of 9.93696x10-23seconds.