R. G. H. Robertson

Limit on νe mass from observation of the beta decay of molecular tritium

Abstract We report the most sensitive upper limit set on the mass of the electron antineutrino. The upper limit of 9.4 eV (95% confidence level) was obtained from a study of the shape of the beta decay spectrum of free molecular tritium. Achieving such a level of sensitivity required precise determinations of all processes that modify the shape of the observed spectrum. This result is in clear disagreement with a reported value for the mass of 26(5) eV.

Upper limit on the neutrino mass from the Los Alamos free molecular gaseous tritium experiment

The mass of the electron antineutrino can be determined by measurement of the shape of the beta spectrum of tritium near the 18.6 keV endpoint. A measurement of this spectrum has been undertaken at the Los Alamos National Laboratory, and initial results have been obtained, allowing an upper limit of 29.7 eV to be set on the neutrino mass.

Status of the Los Alamos tritium beta decay experiment

The Los Alamos tritium experiment employs a gaseous tritium source and a magnetic spectrometer to determine the mass of the electron antineutrino from the shape of the tritium beta spectrum. Since publication of the first result from this apparatus (m/sub nu/ < 27 eV at 95% confidence), work has concentrated on improving the data rates. A 96-element Si microstrip array detector has been installed to replace the single proportional counter at the spectrometer focus, resulting in greatly increased efficiency. Measurements of the 1s photoionization spectrum of Kr now obviate the need for reliance on the theoretical shakeup and shakeoff spectrum of Kr in determining the spectrometer resolution. 19 refs., 3 figs.

A Limit on the ̄νe Mass in Free Molecular Tritium Beta Decay

The question of a nonzero neutrino mass has received considerable attention since the claims of Lyubimov et al [1] in 1980 were published which showed evidence for an electron antineutrino mass between 14 and 46 eV, with a best fit value of 35 eV. However, there are still considerable concerns about possible systematic problems in their experiment. Many of these concerns revolve around the use of a tritiated valine source, in which the energy given up in final state excitations of the molecule following the beta decay of one of the tritium atoms is comparable to the size of the neutrino mass observed. The effect of these final state effects is difficult to calculate in a molecule as complex as valine. In addition, ionization energy loss and backscattering of the betas in traversing the solid source are appreciable and must be very accurately accounted for. These concerns have led us to carry out an experiment using free molecular tritium as the source material. The final state effects have been accurately calculated for the tritium molecule [2–4] and the uncertainties in these calculations cannot generate a spurious neutrino mass greater than 1 eV. In addition, the energy loss in the source is small because the source consists of tritium only and there is no backscattering.