Tritium (T) is a naturally occurring radioactive isotope (3H) of hydrogen (2H), having two neutrons and one proton in the nucleus. The isotope decays (half-life = 12.26 years, see decay constant) by beta emission (see beta decay) to stable helium (3He) and this can be used to measure the age of water samples back to approximately 30 years. Tritium is produced in the upper atmosphere by the interaction of fast cosmic-ray neutrons with stable 14N. The tritium combines with hydrogen and oxygen to form HTO, which is then dispersed throughout the hydrosphere. The natural production rate of tritium is 15–45 atoms/min/cm2 of the Earth’s surface. In some experiments, artificially produced tritium has been introduced into groundwater and used to trace and time underground movements. It can also be used to measure and time rates of mixing in oceanic current systems.