Measuring mass of neutrinos with β-decays of tritium and rhenium

Already long time ago the shape of the electron spectrum in β‐decays of 3H and 187Re has been recognized as an important tool for understanding of neutrino masses. The sensitivity of KATRIN (in preparation, tritium β‐decay) and the MARE (under consideration, 187Re β‐decay) experiments to neutrino mass will reach the sub eV domain. In view of this experimental progress there is a request for a highly accurate theoretical description of the electron endpoint spectra. By taking the advantage of the elementary particle treatment of 3H and 3He the relativistic form for β‐decay endpoint spectrum of tritium is obtained by taking into account also the effect of nuclear recoil. Further, the currently unknown shape of the electron spectrum for the β‐decay of 187Re is presented. It is found that the first forbidden 187Re(5/2+)→187Os(1/2−)β‐decay transition is accompanied with emission of mostly p3/2‐state electrons.