The β-spectrum of 35S and search for the admixture of heavy neutrinos

The β-spectrum of 35S has been measured with high statistical precision. A total of 2.2×1010 events have been accumulated with a signal to background ratio of 103 or better. A systematic search for neutrino mixing has been performed for assumed neutrino masses up to 105 keV. In the mass range 10 to 90 keV, an upper limit around 10−3 or less was set for the mixing probability.