Upper Limits On The Mixing Of Heavy Neutrinos In The Beta-Decay Of Ni-63

The beta spectrum of $^{63}\\mathrm{Ni}$ was studied with a \\ensuremath{\\pi} \\ensuremath{\\surd}2 beta spectrometer, a multistrip source, and multidetector array. No evidence was found for heavy neutrinos with masses ranging from 4 to 40 keV. The upper limit, including numerical evaluation of systematic errors, for the mixing of a 17 keV neutrino was set at 0.3% (90% confidence level). The beta spectrum endpoint was found to be 66.946\\ifmmode\\pm\\else\\textpm\\fi{}0.020 keV, about 1 keV higher than the value derived from the 1983 atomic mass table, 65.92\\ifmmode\\pm\\else\\textpm\\fi{}0.15 keV.