Search for -transitions with the lowest decay energy for a determination of the neutrino mass

From a variety of -transitions only those with decay energies of a few keV and smaller are considered suitable for a determination of the neutrino mass on a sub-eV level. The decay energy of a transition can be very small, if, e.g., in an allowed -decay or electron-capture transition, a nuclear excited state of the daughter nuclide is populated whose energy is very close to the mass difference of the transition nuclides. Investigation of these transitions can also be useful for the assessment of a validity of the current -decay theory in the region of vanishingly small decay energies. The authors of this proposal have found several such -transitions whose decay energies are expected to be extremely small. In order to assess the suitability of these -transitions for the determination of the neutrino mass, measurements of the mass differences of the transition nuclides must be carried out with a sub-keV uncertainty. Presently, only high-precision Penning-trap mass spectrometry is capable of achieving this goal, and ISOLTRAP is superior to other installations with respect to the production and investigation of the proposed