Ab Initio Study of Single- and Double-Electron Capture Processes in Collisions of He2+ Ions and Ne Atoms

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.

Charge-exchange soft x-ray emission of highly charged ions with inclusion of multiple-electron capture

Charge-exchange has been recognized as a primary source of soft x-ray emission in many astrophysical outflow environments, including cometary and planetary exospheres impacted by solar wind. Some models have been setup by using different data collections of charge-exchange cross-sections. However, multiple electron transfer has not been included in these models. In this paper, we setup a charge exchange model with the inclusion of double-electron capture, and make a detailed investigation of this process on x-ray emissions of highly charged carbon, nitrogen, oxygen and neon ions by using available experimental cross-sections. We also study the effect of different n −selective cross-sections on soft x-ray emission by using available experimental n −distributions. This work reveals that double electron capture enhancement on line intensity is basically linearly proportional to the ratio of ion abundance in the solar wind. It is more obvious for soft x-rays from carbon ions (C4 +) in collision with CO2, and the enhancement on line intensity can be up to 53 per cent with typical ion abundances (ace observation) in solar wind. The synthetic spectra with parameters from the Ulysses mission for solar wind reveals velocity-dependence, target-dependence, as well as the non-negligible contribution from the double-electron capture.

Sensitivity of the NEXT experiment to Xe-124 double electron capture

Double electron capture by proton-rich nuclei is a second-order nuclear process analogous to double beta decay. Despite their similarities, the decay signature is quite different, potentially providing a new channel to measure the hypothesized neutrinoless mode of these decays. The Standard-Model-allowed two-neutrino double electron capture (2νEC EC) has been predicted for a number of isotopes, but only observed in 78Kr, 130Ba and, recently, 124Xe. The sensitivity to this decay establishes a benchmark for the ultimate experimental goal, namely the potential to discover also the lepton-number-violating neutrinoless version of this process, 0νEC EC. Here we report on the current sensitivity of the NEXT-White detector to 124Xe 2νEC EC and on the extrapolation to NEXT-100. Using simulated data for the 2νEC EC signal and real data from NEXT-White operated with 124Xe-depleted gas as background, we define an optimal event selection that maximizes the NEXT-White sensitivity. We estimate that, for NEXT-100 operated with xenon gas isotopically enriched with 1 kg of 124Xe and for a 5-year run, a sensitivity to the 2νEC EC half-life of 6 × 1022 y (at 90% confidence level) or better can be reached.