Beta Decay of Molecular Tritium

The beta decay of tritium in the form of molecular ${\mathrm{T}}_2$ is the basis of sensitive experiments to measure neutrino mass. The final-state electronic, vibrational, and rotational excitations modify the beta spectrum significantly and are obtained from theory. We report measurements of the branching ratios to specific ionization states for the isotopolog HT. Two earlier, concordant measurements gave branching ratios of HT to the bound ${\mathrm{HHe}}^{+}$ ion of 89.5% and 93.2%, in sharp disagreement with the theoretical prediction of 55%–57%, raising concerns about the theory’s reliability in neutrino mass experiments. Our result, 56.5(6)%, is compatible with the theoretical expectation and disagrees strongly with the previous measurements.

Figure 1

Cutaway view of the decay chamber inside the insulating silica tube, aluminum tube, and magnet coils. The 11-gap acceleration structure is 234 mm long. The beta detector is on the right behind the gold-plated high-voltage electrode; the ion detector is to the left behind the biased mesh.

Figure 2

Raw data from HT-rich source. (Top) (a) and (d)–(f) The main ion bands associated with masses 1, 3, 4, and 6. (b) Doubly charged ${\mathrm{He}}^{++}$ band. (c) Two-ion band with ${\mathrm{He}}^{+}$, ${\mathrm{T}}^{+}$, and ${\mathrm{H}}^{+}$. Bands to the left of time zero are secondary-emission bands. Bin size: 1 ns, 0.4 keV. (Center) Derived values of charge and mass for each event. Bin size: (0.01, 0.01). (Bottom) Mass spectrum of the main charge-1 bands after removing contributions from TT (solid green) and from two-ion detection branches 6e and 6f (hatched magenta). The mass-3 and 4 peaks are each fit with three Gaussians; the results are shown. Bin size: 0.01.