Isotopes of hydrogen used for deuterium and tritium. The IUPAC accepts the D and T symbols, but recommends instead using standard isotopic symbols (H and H) to avoid confusion in the alphabetic sorting of chemical formulas. The ordinary isotope of hydrogen, with no neutrons, is sometimes called "protium". (During the early study of radioactivity, some other heavy radioactive isotopes were given names, H (atomic mass u) is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the formal name "protium". The proton has never been
Tritium tritium on the Earth's surface; but these amounts rose by 2 or 3 orders of magnitude during the post-test period. Some sources reported natural background levels were exceeded by approximately 1,000 TU in 1963 and 1964 and the isotope is used in the northern hemisphere to estimate the age of groundwater and construct hydrogeologic simulation models. Recent scientific sources have estimated atmospheric levels at the height of weapons testing to approach 1,000 TU and pre-fallout levels of rainwater to be between 5 and 10 TU. In 1963 Valentia Island Ireland recorded 2,000 TU in precipitation. While in the stratosphere (post-test