Experimental limits on heavy neutrinos in tritium beta decay

A search for a 17 keV neutrino kink in tritium beta decay has been made on a new sample of 430 million decays per keV at electron kinetic energies near 1.5 keV. The new sample contains 40% less wall background than our earlier one. This significantly reduces the wall background systematics. No 17 keV neutrino signal was observed. The 99% confidence level upper limit obtained for the mixing probability is 0.28%. The combined upper limit from both samples is 0.24% (99% CL). In addition, limits less than 0.5% (99% CL) are obtained for other heavy neutrino mass values between 13.5 and 17.5 keV.