Upper limits on the mixing of heavy neutrinos in the beta decay of /sup 63/Ni

The beta spectrum of /sup 63/Ni was studied with a ..pi.. ..sqrt..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 +- 0.020 keV, about 1 keV higher than the value derived from the 1983 atomic mass table, 65.92 +- 0.15 keV.