Tritium (${}^3\mathrm{H}$)

Tritium is the hydrogen isotope ${}_1^3\mathrm{H}$: its nucleus contains one proton and two neutrons. It undergoes beta decay to helium-3, emitting an electron and an electron antineutrino:This equation uses nuclear notation: the mass number stays3, while electric charge is conserved because the daughter nucleus has charge $+2e$ and the emitted electron has charge $-e$. The half-life is approximately12.3years.^[ref]

A motivating measurement is a long series of activity comparisons. A 2000 study calibrated tritiated-water standards with internal gas-proportional counting and liquid-scintillation intercomparisons. Activity data for one standard extending over38years gave a half-life of $(4504\pm 9)$ days, with the stated uncertainty one standard deviation. This is that study's measured result, rather than an assertion that every evaluation uses precisely the same value.^[ref]

For a closed sample with no added tritium, radioactive decay gives $N(t)=N_0{2}^{-t/t_{1/2}}$. After two half-lives, one quarter of the original tritium nuclei remain. The lost parent nuclei have become daughter nuclei; a smaller tritium activity does not mean the sample has lost the same fraction of its total atoms.