The correct answer is 0.25. • Tritium • It is a beta-emitting radioactive isotope of hydrogen. • Its nucleus consists of one proton and two neutrons, making it three times as heavy as a hydrogen nucleus (with its one proton) and one-and-a-half times as heavy as deuterium (which contains one proton and only one neutron). • Tritium would no longer exist in our environment if cosmic radiation produces it in the atmosphere in very small amounts. • The half-life of the unstable tritium nucleus is of 12.3 years, which is very short on the radioactive time scale. • This comparatively fast disappearance means that very little tritium can accumulate in any place. Related Concepts: • Fabrication of thermonuclear weapons requires the production of sufficient quantities of tritium. • Since natural tritium is extremely rare, this military tritium is obtained by bombarding lithium with neutrons. • In addition, tritium from military facilities generates tritium waste, problematic more because of the tritium mobility than its very low radioactive toxicity. • The nuclear fusion of deuterium and tritium will be exploited in reactors based on nuclear fusion. • This is the goal of the research carried out on a worldwide scale with the ITER project.