Tritium (T, or H 3 ) is a radioactive isotope of hydrogen with a mass number of 3. The tritium core consists of 1 proton and 2 neutrons. Tritium can be obtained at an accelerator (see Accelerators of charged particles ), bombarding with neutrons or deuterons the atoms of heavy hydrogen, lithium, and beryllium. The half-life of tritium is 12.4 years. Tritium has a very mild beta radiation (see), which is completely absorbed by a layer of 5.7 mm of air or 6.5 μ of tissue. In nature, tritium is constantly formed from the nitrogen of air under the action of cosmic rays (see Cosmic Radiation). In the entire earth's atmosphere, it contains 18 grams, and in the water of the oceans - about 800 g. Tritium can serve as a radioactive marker for studying the synthesis and decomposition of DNA in a cell. It is used in thermonuclear bombs. The maximum permissible content of tritium in the human body is 2000 mcuries. See also Isotopes.