Characterization of the ${}^{163}$Ho Electron Capture Spectrum: A Step Towards the Electron Neutrino Mass Determination

The isotope ${}^{163}$Ho 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 ${}^{163}$Dy with an energy available to the decay, Q${}_{EC}$, of about 2.8 keV. According to the present knowledge, this is the lowest Q${}_{EC}$ value for such transitions. Here we discuss a newly obtained spectrum of ${}^{163}$Ho, taken by cryogenic metallic magnetic calorimeters with ${}^{163}$Ho implanted in the absorbers and operated in anticoincident mode for background reduction. For the first time, the atomic deexcitation of the ${}^{163}$Dy daughter atom following the capture of electrons from the 5s shell in ${}^{163}$Ho, 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 Q${}_{EC}$=(2.858±0.010${}_{stat}$±0.05${}_{syst}$)  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 ${}^{163}$Ho and ${}^{163}$Dy obtained by Penning-trap mass spectrometry, demonstrating the reliability of the calorimetric technique.