Measurement of the $$β$$-energy spectrum of $^3\mathrmH$ to determine the antineutrino mass

An experiment to measure the β-energy spectrum of tritium implanted at high energy into a Si(Li) x-ray detector has been carried out primarily with the aim of determining the mass of the electron antineutrino, but also the extrapolated end-point energy of the β spectrum. The measurement implies a mass <65 eV with 95% confidence and a best value of 20 eV which is however only 0.2 standard deviations from zero mass. The end-point energy is 18 567±5 eV. A discussion of atomic effects which are important for calculating the 3H-3He atomic-mass difference from the end-point energy is presented, as well as a discussion on the possibility of improving the limit on the antineutrino mass in the present type of experiment.