Measurement of the Electron Antineutrino Mass from the Beta Spectrum of Gaseous Tritium

A measurement has been made of the mass of the electron antineutrino using the beta spectrum from a source of gaseous molecular tritium, and an upper limit of 36 eV/c('2) has been set on this mass. This measurement is the first upper limit on the neutrino mass which does not rely on assumptions about the atomic configuration after the beta decay, and it has significantly smaller systematic errors associated with it than do previous measurements. The mass of the electron antineutrino manifests itself as a small decrease in the available phase space for beta particles emerging from a nucleus. This change in the phase space is observable as a small change in the shape of the beta spectrum of that nucleus. Tritium, with its simple nuclear structure, short half-life, and low decay energy, is the ideal nucleus to use for these measurements. Corrections to the spectrum must be made for the atomic final-state effects, the experimental resolution, and energy loss in the source. The intensity as a function of energy for electrons with kinetic energies near the endpoint of the tritium beta spectrum (18.6 keV) has been measured by extracting the electrons from a low-pressure gaseous source of tritium into a iron-free toroidal-field beta spectrometer with an energy resolution of 37 eV FWHM. The data is normalized to the activity in the source, and the neutrino mass is determined by fitting the observed spectrum to a theoretical spectrum using a maximum likelihood estimator. The apparatus for this measurement consists of a gaseous source, a differential pumping restriction, a nonadiabatic magnetic electron focusing system, and a toroidal -field beta spectrometer. The electrons emitted from tritium gas in the source are extracted from it by means of a solenoidal magnetic field, and then accelerated to an energy of 26 keV. They are then transported through the differential pumping region and focused through a collimator which defines the source of the spectrometer. After transport through the spectrometer, they are detected in a position sensitive resistive-wire proportional counter. The tritium gas itself is recirculated through the source, so that only a small inventory is required. Measurements continue using this apparatus, and an eventual sensitivity to a neutrino mass on the order of 10 eV/c('2) is expected.