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We present a reanalysis of the experimental data of electron capture in$^7$Be embedded in Ta which have been published by other authors. Our goal isto set upper limits on a mixture of electron neutrino with a possibleright-handed heavy neutrino in the 150--800 keV mass range. In the publishedexperiment a $^7$Li recoil energy spectrum in the 20--200 eV range wasmeasured. In case of electron capture with emission of a heavy neutrino, therecoil spectrum should be shifted to the lower energies. We search for anadditional Gauss-shaped structure with the same energy width as the mainK-shell transition peak. For this we digitize the published spectrum curve,find the energy resolution, calculate the moving sum of the events along thespectrum in the energy interval of about 3 sigma of energy resolution. Then weuse the statistical error of this sum to exclude at some level the appearanceof an additional peak. Finally, 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 at least one order of magnitude lowerthan the existing data in 300--800 keV mass range.

Analysis of published data of electron capture in 7Be in the search for a heavy neutrino in the mass range under 800 keV