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

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

Figure 1

The amplitude of the events in channel 1 plotted versus the amplitude of the events in channel 0. The number of counts per bin for all possible combinations in the defined plane is given by the color code shown on the right.

Figure 2

Measured ${}^{163}\mathrm{Ho}$ EC spectrum (black histogram) and the theoretical description (green filled area). The background contribution that survived the cuts is shown in grey. In addition to the five lines of the ${}^{163}\mathrm{Ho}$ spectrum, three lines due to ${}^{144}\mathrm{Pm}$ EC can be seen. The latter was coimplanted with the desired ${}^{163}\mathrm{Ho}$. The insert shows a magnification of the N-lines region which emphasizes the presence of additional structure in the spectrum due to higher-order excited states in ${}^{163}\mathrm{Dy}$.

Figure 3

The calculated phase-space factors for the $Q_{\mathrm{EC}}$ evaluation. Only the black data points enter into the linear fit. The lower plot shows the residuals from the fit and the statistical errors.