Direct decay-energy measurement as a route to the neutrino mass

A high-precision measurement of the \(^{131}\)Cs\( \rightarrow ^{131}\)Xe ground-to-ground-state electron-capture \(Q_{\textrm{EC}}\)-value was performed using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The novel PI-ICR technique allowed to reach a relative mass precision \(\delta m/m\) of \(1.4\cdot10^{-9}\). A mass resolving power \(m/\Delta m\) exceeding \(1\cdot10^7\) was obtained in only \(1\,\)s trapping time. Allowed electron-capture transitions with sub-keV or lower decay energies are of high interest for the direct determination of the \(\nu_e\) mass. The new measurement improves the uncertainty on the ground-to-ground-state \(Q_{\textrm{EC}}\)-value by a factor 25 precluding the \(^{131}\)Cs\( \rightarrow ^{131}\)Xe pair as a feasible candidate for the direct determination of the \(\nu_e\) mass.