Atomic Masses of Tritium and Helium-3

By measuring the cyclotron frequency ratios of He-3(+) to HD+ and T+ to HD+, and using HD+ as a mass reference, we obtain new atomic masses for He-3 and T. Our results are M[He-3] = 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 beta decay to be (M[T] - M[He-3])c(2) = 18 592.01(7) eV. This allows an improved test of systematics in measurements of tritium beta decay that set limits on neutrino mass.