Lifetimes of doubly K -shell ionized states

The present work provides a reliable interpretation of the Khα₁/Khα₂ intensity ratios and an explanation of the lifetime values for K-shell hollow atoms based on an advanced theoretical analysis (using extensive multiconfiguration Dirac–Fock calculations with the inclusion of the transverse Breit interaction and quantum electrodynamics corrections). It was found that, as a result of closing the Khα₁ de-excitation channel in the pure LS coupling scheme, the Khα₁/Khα₂ intensity ratio changes with the atomic number from small values (for the LS coupling limit at low Z) to about 1.5– 1.6 (for the j–j coupling limit at high Z). However, closing the Khα₁ de-excitation channel (due to the domination of the pure LS coupling for the low-Z atoms) does not enlarge the lifetimes of hollow atoms

High-resolution tungsten spectroscopy relevant to the diagnostic of high-temperature tokamak plasmas

The x-ray transitions in Cu- and Ni-like tungsten ions in the 5.19-5.26 angstrom wavelength range that are relevant as a high-temperature tokamak diagnostic, in particular for JET in the ITER-like wall configuration, have been studied. Tungsten spectra were measured at the upgraded Shanghai-Electron Beam Ion Trap operated with electron-beam energies from 3.16 to 4.55 keV. High-resolution measurements were performed by means of a flat Si 111 crystal spectrometer equipped by a CCD camera. The experimental wavelengths were determined with an accuracy of 0.3-0.4 m angstrom. The wavelength of the ground-state transition in Cu-like tungsten from the 3p(5)3d(10)4s4d [(3/2,(1/2,5/2)(2)](1/2) level was measured. All measured wavelengths were compared with those measured from JET ITER-like wall plasmas and with other experiments and various theoretical predictions including COWAN, RELAC, multiconfigurational Dirac-Fock (MCDF), and FAC calculations. To obtain a higher accuracy from theoretical predictions, the MCDF calculations were extended by taking into account correlation effects (configuration-interaction approach). It was found that such an extension brings the calculations closer to the experimental values in comparison with other calculations

Structure of Kα1,2 - And Kβ1,3 -emission x-ray spectra for Se, Y, and Zr

UID/FIS/04559/2020 UID/MULTI/04046/2020 Project No. PTDC/FIS-AQM/31969/20 Grant No. 2017/25/B/ST2/00901The Kα and Kβ x-ray spectra of Se, Y, and Zr were studied experimentally and theoretically in order to obtain information on the Kα1 line asymmetry and the spin doublet in Kβ1,3 diagram lines. Using a high-resolution antiparallel double-crystal x-ray spectrometer, we obtained the line shapes, that is, asymmetry index and natural linewidths. We found that the corrected full width at half maximum of the Kα1 and Kα2 lines as a function of Z is in good agreement with the data in the literature. Furthermore, satellite lines arising from shake-off appear in the low-energy side of the Kα1 and Kα2 lines in Se but, in Y and Zr, it was very difficult to identify the contribution of the shake process to the overall lines. The Kβ1,3 natural linewidth of these elements was also corrected using the appropriate instrumental function for this type of x-ray spectrometer, and the spin doublet energies were obtained from the peak positions. The corrected full width at half maximum (FWHM) of the Kβ1 x-ray lines increases linearly with Z, but this tendency was found to be, in general, not linear for Kβ3 x-ray lines. This behavior may be due to the existence of satellite lines originated from shake processes. Simulated line profiles, obtained using the multiconfiguration Dirac-Fock formalism, accounting for radiative and radiationless transitions and shake-off processes, show a very good agreement with the high-resolution experimental spectra.publishersversionpublishe

Development of a PbWO4 Detector for Single-Shot Positron Annihilation Lifetime Spectroscopy at the GBAR Experiment

We have developed a PbWO4 (PWO) detector with a large dynamic range to measure the intensity of a positron beam and the absolute density of the ortho-positronium (o-Ps) cloud it creates. A simulation study shows that a setup based on such detectors may be used to determine the angular distribution of the emission and reflection of o-Ps to reduce part of the uncertainties of the measurement. These will allow to improve the precision in the measurement of the cross-section for the (anti)hydrogen formation by (anti)proton-positronium charge exchange and to optimize the yield of antihydrogen ion which is an essential parameter in the GBAR experiment

High energy resolution measurements of the radiative decay of double K-shell vacancies in 20 ≤ Z ≤ 29 elements bombarded by fast C and Ne ions

We report on high energy resolution measurements of the Kα hypersatellite x-ray spectra of Ca, V, Fe and Cu induced by impact with 144 MeV C and 180 MeV Ne ions

Isomer depletion as experimental evidence of nuclear excitation by electron capture

The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a 3-ray. This 'internal conversion' has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons. In the inverse process - nuclear excitation by electron capture (NEEC) - a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted in 1976 and has not hitherto been observed. Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments, in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of 'isomer depletion', which has been investigated previously through other reactions, but is used here to obtain evidence for NEEC

Reply to: Possible overestimation of isomer depletion due to contamination

We appreciate the interest of Guo et al., the points that they raise, and the opportunity that we have to provide additional details that are not included in ref. This allows us to strengthen our experimental case while, in parallel, recent developments are improving our theoretical understanding of nuclear excitation by electron capture (NEEC), such as the exploration of a substantial increase in predicted NEEC probability when considering capture by an ion in an excited state (S. Gargiulo et al., submitted) or the impact of the momentum distribution of target electrons (J.R. et al., submitted). In the accompanying Comment, Guo et al. focus on whether potential background contributions were underestimated in our analysis. As discussed below, these concerns are mostly unwarranted; aside from a small systematic uncertainty that could possibly slightly reduce our reported NEEC excitation probability of Pexc = 0.010(3), our original conclusions still stand

Accumulation of Positrons from a LINAC Based Source

International audienc

Modelling of the effect of ELMs on fuel retention at the bulk W divertor of JET

Effect of ELMs on fuel retention at the bulk W target of JET ITER-Like Wall was studied with multi-scale calculations. Plasma input parameters were taken from ELMy H-mode plasma experiment. The energetic intra-ELM fuel particles get implanted and create near-surface defects up to depths of few tens of nm, which act as the main fuel trapping sites during ELMs. Clustering of implantation-induced vacancies were found to take place. The incoming flux of inter-ELM plasma particles increases the different filling levels of trapped fuel in defects. The temperature increase of the W target during the pulse increases the fuel detrapping rate. The inter-ELM fuel particle flux refills the partially emptied trapping sites and fills new sites. This leads to a competing effect on the retention and release rates of the implanted particles. At high temperatures the main retention appeared in larger vacancy clusters due to increased clustering rate