Unstable heavy → isotope of → hydrogen whose nucleus contains one → proton and two → neutrons. Tritium occurs naturally due to → cosmic rays interacting with atmospheric gases. In the most important reaction for natural tritium production, a fast neutron interacts with atmospheric nitrogen: 14N + n → 12C + 3T. Its → half-life is about 12 years. Tritium was formed in an intermediate step in light element synthesis in → early Universe.