Atomic masses of tritium and helium-3.

By measuring the cyclotron frequency ratios of (3)He(+) to HD(+) and T(+) to HD(+), and using HD(+) as a mass reference, we obtain new atomic masses for (3)He and T. Our results are M[(3)He]=3.016 029 322 43(19)  u and M[T]=3.016 049 281 78(19)  u, where the uncertainty includes an uncertainty of 0.12 nu in the mass reference. Allowing for cancellation of common systematic errors, we find the Q value for tritium β decay to be (M[T]-M[(3)He])c(2)=18 592.01(7)  eV. This allows an improved test of systematics in measurements of tritium β decay that set limits on neutrino mass.