High sensitivity search for a 17 keV neutrino. Negative indication with an upper limit of 0.095%

We have performed a search for a 17 keV neutrino by looking for a kink in the 63Ni β-spectrum near the expected energy threshold. Using a large number of events (2.4 × 109) accumulated in a narrow energy region with the INS iron-free π√2 β-spectrometer, we obtain a mixing strength of [1.8±3.3(stat.)±3.3(syst.)]×10−4, and no indication of the presence of a 17 keV neutrino with the order of 1% mixing. We set an upper limit of 0.095% for the admixture of a neutrino with a mass of 17 keV and of 0.1% with a mass range of 10–24 keV, at the 95% confidence level.