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VOLUME 114 (2021) | ISSUE 1 | PAGE 4
Search for a heavy neutrino in the mass range under 750 keV using electron capture in 7Be
We present reanalysis of the experimental data of electron capture in 7Be embedded in Ta which have been published by other authors. Our goal is to set upper limits on a mixture of electron neutrino with a possible right handed heavy neutrino in the 300-750 keV mass range. In the published experiment a 7Li recoil energy spectrum in the 20-200 eV range was measured. In case of electron capture with emission of a heavy neutrino, the recoil spectrum should be shifted to the lower energies. We search for an additional Gauss-shaped structure with the same energy width as the main K-shell transition peak. We present the upper limits at a 95 % confidence level on electron neutrino - heavy neutrino mixing element, U2, in the mass matrix. New upper limits are in the range of (2-4)× 10-4 which are at least one order of magnitude lower than the existing data in 300-800 keV mass range.