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, ${{U}^{2}}$, 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.