Characterization of the Ho163 Electron Capture Spectrum: A Step Towards the Electron Neutrino Mass Determination

Abstract

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.

Improved description of one- and two-hole excitations after electron capture inHo163and the determination of the neutrino mass

“…2 the theoretical results for the bolometer spectrum in electron capture in l63Ho to 163Dy of this work are compared with the experimental data of Ranitzsch et al [28] and Gastaldo et al [29] in a linear plot assuming a Q value of 2.8 keV.…”

“…3 (4) and (21) for electron capture in ,63Ho to l63Dy with the Q value = 2.8 keV [see Eq. (5)] including the one-and two-hole probabilities calculated in this work and compared to the Heidelberg experiment of Ranitzsch et al [28] and Gastaldo et al [29] for the total bolometer energy from 0.02 to 2.80 keV. The theoretical spectrum is normalized to the data of the VI (4j.…”

“…1, which would show the small effects of the two-hole excitations in Dy better. The theoretical spectrum is calculated for zero neutrino mass and with the excitation and the width of the one-hole states of the ECHo Collaboration [28,29] listed in Table IV. spectrum from 2.345 to 2.355 keV for a neutrino mass of mv = 0 eV and mv = 2 eV. Figure 5 shows the upper end of a theoretical spectrum with a mixture of two mass eigenstates for the electron neutrino.…”

“…The electron binding energies and widths of hole states in l63Ho from the literature[13,[15][16][17][18] and recent ECHo data[28,29], Electrons below 3si/2 cannot be captured in l63Ho. Due to the Q value of about 2.8 keV they are energetically forbidden.…”

MetroMMC: Electron-Capture Spectrometry with Cryogenic Calorimeters for Science and Technology

“…In recent years, a few measurements of β − -decay and EC conducted by means of cryogenic detectors have shown significant discrepancies to previous experimental and/or theoretical data [8][9][10][11][12][13], which underlines the need of further accurate measurements and improvement of theoretical models. Corresponding proposals led to two projects: The recently concluded MetroBeta [14] project, which was concerned with β − -decay, and the currently ongoing project MetroMMC to scrutinize the more challenging EC decay.…”