NO 17 KEV NEUTRINO - ADMIXTURE LESS-THAN 0.073-PERCENT (95-PERCENT CL)

To solve the controversial issue concerning the possible existence of a 17 keV neutrino with a 1% admixture in nuclear beta decay, we searched directly for any evidence of a production-threshold effect. The Ni-63 beta 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.