Characterization of the Ho 163 Electron Capture Spectrum: A Step Towards the Electron Neutrino Mass Determination

Abstract

The isotope Ho163 is in many ways the best candidate to perform experiments to investigate the value of the electron neutrino mass. It undergoes an electron capture process to Dy163 with an energy available to the decay, QEC, of about 2.8 keV. According to the present knowledge, this is the lowest QEC value for such transitions. Here we discuss a newly obtained spectrum of Ho163, taken by cryogenic metallic magnetic calorimeters with Ho163 implanted in the absorbers and operated in anticoincident mode for background reduction. For the first time, the atomic deexcitation of the Dy163 daughter atom following the capture of electrons from the 5s shell in Ho163, the OI line, was observed with a calorimetric measurement. The peak energy is determined to be 48 eV. In addition, a precise determination of the energy available for the decay QEC=(2.858±0.010stat±0.05syst) keV was obtained by analyzing the intensities of the lines in the spectrum. This value is in good agreement with the measurement of the mass difference between Ho163 and Dy163 obtained by Penning-trap mass spectrometry, demonstrating the reliability of the calorimetric technique.