Double Electron Capture Processes Research Articles

A Systematic Study of Two-Neutrino Double Electron Capture

In this paper, we update the phase-space factors for all two-neutrino double electron capture processes. The Dirac–Hartree–Fock–Slater self-consistent method is employed to describe the bound states of captured electrons, enabling a more realistic treatment of atomic screening and more precise binding energies of the captured electrons compared to previous investigations. Additionally, we consider all s-wave electrons available for capture, expanding beyond the K and L1 orbitals considered in prior studies. For light atoms, the increase associated with additional captures compensates for the decrease in decay rate caused by the more precise atomic screening. However, for medium and heavy atoms, an increase in the decay rate, up to 10% for the heaviest atoms, is observed due to the combination of these two effects. In the systematic analysis, we also include capture fractions for the first few dominant partial captures. Our precise model enables a close examination of low Q-value double electron capture in 152Gd, 164Er, and 242Cm, where partial KK captures are energetically forbidden. Finally, with the updated phase-space values, we recalculate the effective nuclear matrix elements and compare their spread with those associated with 2νβ−β− decay.

Production of autoionizing states by double-electron capture in intermediate-energy C4+ + He collisions

We have performed a kinematically complete experiment for state-selective double-electron capture occurring in 15-keV/u ${\mathrm{C}}^{4+}$ $+$ He collisions by means of cold-target recoil-ion momentum spectroscopy. It was shown that capture into ground and low excited states of the ${\mathrm{C}}^{2+}$ ions is overwhelmingly dominant. Besides, a small fraction of autoionization decays from doubly excited states of symmetric as well as asymmetric electron configurations following endothermic double-electron capture were clearly observed. Emphasis was given to the population mechanisms of the different electron configurations. Our analysis indicates that electron-electron correlation effects may play a major role in the double-electron capture process. In addition, the large transverse recoil-ion momentum of symmetric configurations as compared to asymmetric configurations for the doubly excited autoionizing states may be attributed to the different number of steps involved in the primary capture process, with the former likely produced by a two-step mechanism and the latter by a one-step mechanism.

Ab Initio Study of Single- and Double-Electron Capture Processes in Collisions of He2+ Ions and Ne AtomsSupported by the National Natural Science Foundation of China (Grant Nos. 11774344 and 11474033), and the National Key Research and Development Program of China (Grant No. 2017YFA0402300).

The single- and double-electron capture (SEC, DEC) processes of He2+ ions colliding with Ne atoms are studied by utilizing the full quantum-mechanical molecular-orbital close-coupling method. Total and state-selective SEC and DEC cross sections are presented in the energy region of 2 eV/u to 20 keV/u. Results show that the dominant reaction channel is Ne+(2s2p 6 2 S) + He+(1s) in the considered energy region due to strong couplings with the initial state Ne(2s 22p 6 1 S) + He2+ around the internuclear distance of 4.6 a.u. In our calculations, the SEC cross sections decrease initially and then increase whereby, the minimum point is around 0.38 keV/u with the increase of collision energies. After considering the effects of the electron translation factor (ETF), the SEC cross sections are increased by 15%–25% nearby the energy region of keV/u and agree better with the available results. The DEC cross sections are smaller than those of SEC because of the larger energy gaps and no strong couplings with the initial state. Due to the Demkov-type couplings between DEC channel Ne2+(2s22p 4 1 S) + He(1s 2) and the dominating SEC channel Ne+(2s2p 6 2 S) + He+(1s), the DEC cross sections increase with increasing impact energies. Good consistency can also be found between the present DEC and the experimental measurements in the overlapping energy region.

Single- and Double-Electron Capture Processes in Low-Energy Collisions of N4+ Ions with He**Supported by the National Natural Science Foundation of China under Grant Nos. 11774344, 11474033 and 11574326, and the National Key Research and Development Program of China under Grant No. 2017YFA0402300.

We investigate the electron capture processes of N4+(1s22s) colliding with He(1s2) in the energy range of 10–1700 eV/amu using the quantum-mechanical molecular-orbital close-coupling (QMOCC) method. Total and state-selective single-electron capture and double-electron capture (SEC and DEC) cross sections are obtained and compared with other available studies. The results agree better with the experimental data in both trend and magnitude when the electron translation factor (ETF) effects are included. Our results indicate that both the SEC and DEC processes play important roles in the considered energy region. For the SEC processes, the N3+(1s22p2) + He+(1s) states are the dominant capture states, and the N2+(1s22s2p2) + He2+ states are the main DEC states.

Observation of two-neutrino double electron capture in 124Xe with XENON1T

Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude1. Until now, indications of 2νECEC decays have only been seen for two isotopes2-5, 78Kr and 130Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance6,7. The 2νECEC half-life is an important observable for nuclear structure models8-14 and its measurement represents a meaningful step in the search for neutrinoless double electron capture-the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass15-17. Here we report the direct observation of 2νECEC in 124Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8×1022years(statistical uncertainty, 0.5×1022years; systematic uncertainty, 0.1×1022years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments18-20.

Double Electron Capture in H^{+}+H^{-} Collisions.

We have investigated the double electron capture process in the H^{+}+H^{-} collision system for energies from 60eV to 20keV. Despite the apparent simplicity of this highly correlated system, all previous calculations fail to reproduce the experimental total cross sections. Moreover, the latter exhibit oscillations that have been previously attributed to quantum interferences between the gerade and ungerade ionic states of the transient molecule formed during the collision. For this process, we present the absolute cross sections obtained from a fully correlated two-active-electron semiclassical atomic-orbital close-coupling approach. Our results reproduce well the experimental data in both magnitude and shape. Furthermore, we demonstrate that the oscillations stem from coherence effects between double electron capture and other two-electron inelastic channels, namely the transfer-excitation processes. This alternative interpretation is supported by a Rosenthal-like model based on a molecular treatment of the collision. Our results shed new light on this old but challenging problem.

Search for double β-decays of 124Xe with XENON100 & XENON1T

The rare nuclear process of two-neutrino double electron capture, where two electrons are simultaneously captured from the atomic shell, has not yet been observed for 124Xe. A detection of this decay would provide a new reference for nuclear matrix element calculations. Moreover, if a neutrinoless mode were discovered, it would prove a Majorana nature of neutrinos and would shed light on the effective neutrino mass. The XENON dark matter project, with its dual-phase xenon time projection chambers XENON100 and XENON1T, is well suited for this rare event searches with signatures in the keV-region. The search with the XENON100 detector, containing 29 g of 124Xe, is explained as well as the outlook of its successor XENON1T, which contains 2 kg of the isotope in its active volume.

Development of the ultra-low background HPGe spectrometer OBELIX at Modane underground laboratory

A new ultra low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm3 was developed to investigate rare nuclear processes, such as resonant neutrino-less double electron capture (0νEC/EC) and double beta decay processes (2ν2β−, 2νβ+EC, 2νEC/EC) to the excited states of daughter nuclei. The spectrometer was installed at the Modane underground laboratory (LSM, France, 4800 m w.e.). Sensitivity of the spectrometer and its background were tested. A new method for the efficiency calibration in measurements of low-active samples was developed. The spectrometer was used for the measurements of low active materials and samples. Results obtained in 395 h investigation of resonant 0νEC/EC decay of 106Cd to the 2718 keV and 2741 keV excited states of 106Pd with ∼23.2 g of enriched 106Cd and 2ν2β− decay of 100Mo sample with a mass of 2588 g to the 0+, 1130 keV and 2+, 539.5 keV excited states of 100Ru are presented.

New limits on double electron capture of 40Ca and 180W

We analyzed low-background data from the CRESST-II experiment with a total net exposure of 730 kg days to extract limits on double electron capture processes. We established new limits for 40Ca with y and y and for 180W with y and y at 90% CL. Depending on the process, these values improve the currently best limits by a factor of .

Two-electron transfer and ionization mechanism in 80-keV/uNe8+on He collisions

Autoionization decay from doubly excited states of ${\mathrm{Ne}}^{6+}[1{s}^{2}3lnl$ ($n=3,4,5$)] (symmetric configurations) as well as the Coster-Kronig transition from doubly excited states of ${\mathrm{Ne}}^{6+}[1{s}^{2}2pnl(n\ensuremath{\geqslant}7)]$ (asymmetric configurations) are observed in the transfer ionization reaction channel of 80 keV/u ${\mathrm{Ne}}^{8+}$-He collisions. It has been predicted that the formation of symmetric configurations results from uncorrelated double-electron capture processes [Z. Chen and C. D. Lin, Phys. Rev. A 48, 1298 (1993)], and the formation of asymmetric configurations probably results from correlated double-electron capture caused by the dynamical electron-correlation effects which are attributed to small internuclear distances. However, previous experimental measurements were not able to obtain information about the dependence on internuclear distance which can be reflected in the transversal recoil-ion momentum. In this work, we measure the recoil-ion momentum in coincidence with the ejected electron velocity and find that the observed transverse recoil-ion momentum is smaller for the formation of symmetric ${\mathrm{Ne}}^{6+}[1{s}^{2}3lnl(n=3,4,5)]$ states than the formation of asymmetric ${\mathrm{Ne}}^{6+}[1{s}^{2}2pnl(n\ensuremath{\geqslant}7)]$ states. Since large momentum transfer occurs for small internuclear distances (strong electron-electron interactions) and small momentum transfer occurs for large internuclear distances (weak electron-electron interactions), the results indicate that dynamical electron correlation is important for the formation of the asymmetric states.

State-selected studies of electron capture by low-energy Neq+ (q = 3 - 5) ions from CO2 and H2O molecules

SynopsisTranslational energy-gain spectroscopy has been used to measure energy spectra and absolute total cross sections for single- and double-electron capture processes in low-energy collisions of Neq+ (q = 3 - 5) ions with H2O and CO2 at collision energies between 15 and 500 qeV.

Electron capture and deprotonation processes observed in collisions between Xe8+and multiply protonated cytochrome-C

Electron-transfer processes in interaction between highly charged ions and multiply protonated proteins have been studied. Collisions between Xe${}^{8+}$ at 96 keV and protonated cytochrome-C at selected charge state ($q$ from 15+ to 19+) result in mass spectra composed mainly of intact molecular ions. From the spectra, single and double electron capture processes by Xe${}^{8+}$ from the protonated molecular ions were identified and the relative cross sections were measured. An unexpected process, the deprotonation process, was also observed. It is tentatively attributed to the loss of a proton induced by the strong electric field carried by the projectile ion in long-distance collisions. Upon charge variation of the molecular target from 15 to 19, the single and double electron capture cross sections remain nearly constant, while the relative cross section of the deprotonation process increases dramatically from 0.8% (\ifmmode\pm\else\textpm\fi{}0.1%) to 17% (\ifmmode\pm\else\textpm\fi{}1%). This strong charge dependency is explained by the decrease of the proton affinities with the charge. This proton removal process has not been observed previously. It seems to be specific to the long-distance Coulomb interactions between protons bound along the protein chain and the highly charged atomic ions.

Single-photon emission correlated to double-electron capture by bare ions: background processes

Radiative single- and double-electron capture are one-step processes where a single target electron or two target electrons, respectively, are captured to a bound state of a highly charged projectile with the simultaneous emission of a single photon. In ion–atom collisions, several background processes are likely to contribute to these processes and may interfere with the measured x-rays due to radiative single and double capture. In this study, possible contributions from radiative electron capture to the continuum, secondary electron bremsstrahlung, the two-step process of independent double radiative electron capture, as well as radiative- combined with nonradiative-electron capture are taken into account based on our analysis of the data for 2.21 MeV u−1 F9+ ions colliding with a thin carbon foil.

Penning‐trap mass spectrometry and neutrino physics

Rapidly developing neutrino physics has found in Penning‐trap mass spectrometry a staunch ally in investigating a variety of fundamental problems. The most familiar are the absolute neutrino mass, possible existence of resonant neutrinoless double‐electron capture and of keV‐sterile neutrinos, and investigation of neutrino oscillations. This article is a brief review of the latest achievements and future perspectives of Penning‐trap mass spectrometry in the exploration of these problems with a focus on electron capture and double electron capture processes.

Calculation of the cross section of radiative double-electron capture by a bare nucleus with emission of one photon

We present a calculation of the cross section for the process of double-electron capture by a bare nucleus with emission of a single photon. The calculation is performed within the framework of quantum electrodynamics. The cross section is presented as a function of the incident-electron energy for ions with the nuclear charge within the interval $8\ensuremath{\le}Z\ensuremath{\le}92$. The available experiments on the double-electron capture by a bare nucleus are discussed.

Single- and double-charge transfer in slow He2+–He collisions

The single- and double-electron capture processes in He2+–He collisions are investigated by the fully quantum-mechanical molecular orbital close-coupling method employing a basis containing 15 gerade and 14 ungerade molecular states. The energies and wavefunctions of He22+ molecular states included in the study are determined ab initio by the multireference single- and double-excitation configuration interaction method. The dominant capture mechanisms are discussed and the integral and differential charge transfer cross sections are calculated in the energy range of 0.0005–17.5 keV/u and compared with other available experimental and theoretical results.

Cross section of double electron capture by bare nucleus

Calculation of cross section for process of double electron capture by bare nucleus with emission of a single photon is presented.

Matrix Elements for Neutrinoless Double Beta Decay

The status of calculation of the neutrinoless double beta decay (0νββ-decay) nuclear matrix elements (NMEs) is reviewed. The 0νββ-decay NMEs obtained in the framework of the Quasiparticle Random Phase Approximation (QRPA), Large Scale Shell Model (LSSM), Interacting Boson Model (IBM), Projected Hartree-Fock Bogoliubov approach (P-HFB) and Energy Density Functional Method (EDF) are presented and discussed. The problem of reliable determination of the 0νββ-decay NMEs is addressed. It is manifested that the uncertainty associated with the calculation of the 0νββ-decay NMEs can be diminished by suitable chosen nuclear probes. Further, the recent development in the field of double beta decay showed that the search for the neutrinoless double electron capture process (0νECEC) may establish the Majorana nature of neutrinos. This possibility should be considered as alternative and complementary to searches for the 0νββ-decay.

Radiative double-electron capture by bare nucleus with emission of one photon

Calculation of the cross-section for the process of double-electron capture by a bare nucleus with emission of a single photon is presented. The double-electron capture is evaluated within the framework of quantum electrodynamics. The line-profile approach is employed. Since the radiative double-electron capture is governed by the electron correlation, corrections to the interelectron interaction were calculated with high accuracy, partly to all orders of the perturbation theory.

Nonperturbative treatment of multielectron processes in ion-molecule scattering: Application to He2+-H2collisions

We present a nonperturbative theory to describe multielectronic processes occurring in the course of collisions between an ion and a molecule. The approach is based on the expansion of the electronic scattering wave function onto asymptotic mono- or multicenter states with proper translational conditions and includes both static and dynamical electronic correlations. Therefore, it has a wide application range around intermediate impact velocities $v\ensuremath{\approx}{v}_{e}$, where ${v}_{e}$ is the averaged electron velocity in the initial state. As a first application, we report results on single- and double-electron capture processes in He${}^{2+}$-H${}_{2}$ collisions for impact energies ranging from 0.01 to 25 keV/u. Special emphasis on the prediction of cross sections for double-electron capture into doubly excited states of helium is addressed.