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Register a new user$^{159}$Dy electron-capture: a strong new candidate for neutrino mass determination
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Solving the puzzle of the absolute mass scale of neutrinos is an outstanding issue of paramount importance. Current approaches that can directly pinpoint the (anti)neutrino mass in a precise and model-independent way are based on beta decay and electron capture experiments. Such experiments focus on decays that have the smallest decay energy -- $Q$-value -- to maximize the sensitivity to the neutrino mass. Here we report the ground-to-ground state electron-capture $Q$-value of $^{159}$Dy, measured directly for the first time using high-precision Penning trap mass spectrometry. The result, 364.73(19)~keV, reveals a decay channel to a state with spin-parity $5/2^-$ with the smallest $Q$-value of any known electron capture, 1.18(19)~keV. Investigation of the spectrum shape unveiled an order-of-magnitude enhancement in the event rate near the endpoint for $^{159}$Dy compared to $^{163}$Ho, which is so far the only nucleus used for direct neutrino mass determination. $^{159}$Dy is not only suitable but, by far, the best of any currently known ground-to-excited state decay candidate to pursue a neutrino mass measurement.
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Electron capture can determine the electron neutrino mass, while the beta decay of Tritium measures the electron antineutrino mass and the neutrinoless double beta decay observes the Majorana neutrino mass. Electron capture e. g. on 163Ho plus bound electron to 163Dy* plus neutrino can determine the electron neutrino mass from the upper end of the decay spectrum of the excited Dy*, which is given by the Q-Value minus the neutrino mass. The Dy* states decay by X-ray and Auger electron emissions. The total decay energy is measured in a bolometer. These excitations have been studied by Robertson and by Faessler et al.. In addition the daughter atom Dy can also be excited by moving in the capture process one electron into the continuum. The escape of these continuum electrons is automatically included in the experimental bolometer spectrum. Recently a method developed by Intemann and Pollock was used by DeRujula and Lusignoli for a rough estimate of this shake-off process for s wave electrons in capture on 163Ho. The purpose of the present work is to give a more reliable description of s wave shake-off in electron capture on Holmium. For that one needs very accurate atomic wave functions of Ho in its ground state and excited atomic wave functions of Dy* including a description of the continuum electrons. In the present approach the wave functions of Ho and Dy* are determined selfconsistently with the antisymmetrized relativistic Dirac-Hartree-Fock approach. The relativistic continuum electron wave functions for the ionized Dy* are obtained in the corresponding selfconsistent Dirac-Hartree-Fock-Potential. In this improved approach shake-off can hardly be seen after electron capture in 163Ho and thus can probably not affect the determination of the electron neutrino mass.
There are three different methods used to search the neutrino mass: - The electron antineutrino mass can probably best be determined by the Triton decay. - The neutrinoless Double Beta Decay yields information, if the neutrino is a Dirac or a Majorana particle. It can also determine the Majorana neutrino mass. - Electron capture of an atomic bound electron by a proton in a nucleus bound electron plus proton to neutron plus electron-neutrino can give the mass of the electron neutrino. This contribution summarizes our theoretical work on the possibility to determine the electron neutrino mass by electron capture. One expects the largest influence of the neutrino mass on this decay for a small Q = 2.8 keV for electron capture in Holmium. The energy of the Q value is distributed to the emitted neutrino and the excitation of the Dy atom. Thus the energy difference between the Q value and the upper end of the deexcitation spectrum is the electron neutrino mass. The excitation spectrum of Dy is calculate by one-, two- and three-electron hole excitations, and by the shake-off process. The electron wave functions are calculated selfconsistently by the Dirac-Hartree-Fock approach for the bound and the continuum states. To extract the neutrino mass from the spectrum one must adjust simultaneously the neutrino mass, the Q value, the position, the relative strength and the width of the highest resonance. This fit is only possible, if the background is reduced relative to the present situation. In case of a drastically reduced background a fit of the Q-value and the neutrino mass only seems also to be possible. The analysis presented here shows, that the determination of the electron neutrino mass by electron capture is difficult, but seems not to be impossible.
Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For $^{124}$Xe this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K-shell of $^{124}$Xe using 7636 kg$cdot$d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90 % credibility limit on the half-life $T_{1/2}>6.5times10^{20}$ yr. We also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and find a sensitivity of $T_{1/2}>6.1times10^{22}$ yr after an exposure of 2 t$cdot$yr.
Using the recent shell model evaluation of stellar weak interaction rates we have calculated the neutrino spectra arising from electron capture on pf-shell nuclei under presupernova conditions. We present a simple parametrization of the spectra which allows for an easy implementation into collapse simulations. We discuss that the explicit consideration of thermal ensembles in the parent nucleus broadens the neutrino spectra and results in larger average neutrino energies. The capture rates and neutrino spectra can be easily modified to account for phase space blocking by neutrinos which becomes increasingly important during the final stellar collapse.
Double electron capture is a rare nuclear decay process in which two orbital electrons are captured simultaneously in the same nucleus. Measurement of its two-neutrino mode would provide a new reference for the calculation of nuclear matrix elements whereas observation of its neutrinoless mode would demonstrate lepton number violation. A search for two-neutrino double electron capture on $^{124}$Xe is performed using 165.9 days of data collected with the XMASS-I liquid xenon detector. No significant excess above background was observed and we set a lower limit on the half-life as $4.7 times 10^{21}$ years at 90% confidence level. The obtained limit has ruled out parts of some theoretical expectations. We obtain a lower limit on the $^{126}$Xe two-neutrino double electron capture half-life of $4.3 times 10^{21}$ years at 90% confidence level as well.
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