The isotope $^{163}$Ho undergoes an electron capture process with a recommended value for the energy available to the decay, $Q_{\rm EC}$, of about 2.5 keV. According to the present knowledge, this is the lowest $Q_{\rm EC}$ value for electron capture processes. Because of that, $^{163}$Ho is the best candidate to perform experiments to investigate the value of the electron neutrino mass based on the analysis of the calorimetrically measured spectrum.
We present for the first time the calorimetric measurement of the atomic de-excitation of the $^{163}$Dy daughter atom upon the capture of an electron from the 5s shell in $^{163}$Ho, OI-line. The measured peak energy is 48 eV. This measurement was performed using low temperature metallic magnetic calorimeters with the $^{163}$Ho ion implanted in the absorber.
We demonstrate that the calorimetric spectrum of $^{163}$Ho can be measured with high precision and that the parameters describing the spectrum can be learned from the analysis of the data. Finally, we discuss the implications of this result for the Electron Capture $^{163}$Ho experiment, ECHo, aiming to reach sub-eV sensitivity on the electron neutrino mass by a high precision and high statistics calorimetric measurement of the $^{163}$Ho spectrum.
Phenomenological Modeling of the $^{163}$Ho Calorimetric Electron Capture Spectrum from the HOLMES Experiment
Fondazione Bruno Kessler (FBK)（布鲁诺·基斯勒基金会）；Trento Institute for Fundamental Physics and Applications (TIFPA), INFN（特伦托基础物理与应用研究所（TIFPA），意大利国家核物理研究所）；National Institute of Standards and Technology (NIST)（美国国家标准与技术研究院）；University of Colorado（科罗拉多大学）；Dipartimento di Fisica, Università di Milano-Bicocca（米兰比可卡大学物理系）；Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Milano-Bicocca（意大利国家核物理研究所（INFN）米兰比可卡分部）
