Tritium is an isotope of hydrogen, with a nucleus composed of a proton and two neutrons. It is very rare on Earth, as it is only naturally produced by cosmic rays. Tritium is radioactive and decays into helium-3 by beta decay, with a half-life of 4500 days. It has applications in nuclear fusion because tritium can undergo fusion with deuterium to produce helium and large amounts of energy. As it is so rare in nature, it is produced for use in nuclear weapons and for research by bombarding heavy water with neutrons.