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Register a new userCan one determine the neutrino mass by electron capture?
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Amand Faessler
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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.
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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.
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.
The rate for the photon emission accompanying orbital 1S electron capture by the atomic nucleus is recalculated. While a photon can be emitted by the electron or by the nucleus, the use of the length gauge significantly suppresses the nuclear contribution. Our calculations resolve the long standing discrepancy of theoretical predictions with experimental data for $Delta J=2$ forbidden transitions. We illustrate the results by comparison with the data established experimentally for the first forbidden unique decays of $^{41}$Ca and $^{204}$Tl.
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.
Solar neutrino capture cross-section by 127I nucleus has been studied with taking into account the influence of the resonance structure of the nuclear strength function S(E). Three types of isobaric resonances: giant Gamow-Teller, analog resonance and low-lying Gamow-Teller pigmy resonances has been investigated on the framework of self-consistent theory of finite Fermi systems. The calculations have been performed considering the resonance structure of the charge-exchange strength function S(E). We analyze the effect of each resonance on the energy dependence of the cross-section. It has been shown that all high-lying resonances should be considered. Neutron emission process for high energy nuclear excitation leads to formation 126Xe isotope. We evaluate contribution from various sources of solar neutrinos to the 126Xe/127Xe isotopes ratio formed by energetic neutrinos. 126Xe/127Xe isotope ratio could be an indicator of high-energy boron neutrinos in the solar spectrum. We also discuss the uncertainties in the often used Fermi-functions calculations.
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