{title2=$^3\mathrm H$}
{wiki}

<Tritium> is the <hydrogen> <isotope> $^{3}_{1}\mathrm H$: its <nucleus> contains one <proton> and two <neutrons>. It undergoes <beta decay> to <helium-3>, emitting an <electron> and an <electron antineutrino>:
$$
{}^{3}_{1}\mathrm H\longrightarrow{}^{3}_{2}\mathrm{He}+e^-+\bar\nu_e.
$$
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.<DOE Fundamentals Handbook Chemistry Volume 2 of 2>[CH-03, page24, Table2 and Disintegration]{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 $(4504\pm9)$ 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.<Calibration of the National Institute of Standards and Technology tritiated-water standards by M. P. Unterweger and others>[Abstract, calibration methods and38-year SRM4927 record]{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.


 Tritium (source code)