New limits on the 17-keV neutrino

We present results of new measurements of the β spectrum of 35S, using the Caltech double-focusing, iron-free β spectrometer. Our data show no evidence for a heavy neutrino with a mass between 12 and 22 keV admixed to the usual light neutrino. In particular, we rule out, at the 6σ level, a 17 keV neutrino admixed at 0.85%, and give an upper limit (90% C.L.) of 0.2% for such a neutrino admixture. To demonstrate that our experiment is sensitive to spectral features such as those from heavy neutrinos we have induced an artificial kink by means of an absorber foil covering part of the source.