JETP Letters: issues online

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.