No 17 keV neutrino: Admixture < 0.073% (95% C.L.)

To solve the controversial issue concerning the possible existence of a 17 keV neutrino with a 1% admixture in nuclear β decay, we searched directly for any evidence of a production-threshold effect. The Ni63 β spectrum was measured with a magnetic spectrometer, with very high statistics along with a fine energy scan over a narrow energy region around the expected threshold. The obtained mixing strength was |U|2=[-0.011±0.033(stat)±0.030(syst)]%, very consistent with zero, and decisively excluding the existence of a 17 keV neutrino admixing at the 1% level with the electron neutrino. The corresponding upper limit was set at |U|2<0.073% (95% C.L.). A new limit was also obtained for a wider mass range: |U|2<0.15% (95% C.L.) for 10.5 to 25.0 keV neutrinos.