262 research outputs found
The hyperfine structure of highly charged U ions with rotationally excited nuclei
The hyperfine structure (hfs) of electron levels of U ions with
the nucleus excited in the low-lying rotational state with an energy
keV is investigated. In hydrogenlike uranium, the hfs
splitting for the -ground state of the electron constitutes 1.8 eV.
The hyperfine-quenched (hfq) lifetime of the state has been
calculated for heliumlike U and was found to be two orders of
magnitude smaller than for the ion with the nucleus in the ground state. The
possibility of a precise determination of the nuclear factor for the
rotational state by measurements of the hfq lifetime is discussed.Comment: 10 LaTex page
Electron gas polarization effect induced by heavy H-like ions of moderate velocities channeled in a silicon crystal
We report on the observation of a strong perturbation of the electron gas
induced by 20 MeV/u U ions and 13 MeV/u Pb ions channeled in
silicon crystals. This collective response (wake effect) in-duces a shift of
the continuum energy level by more than 100 eV, which is observed by means of
Radiative Electron Capture into the K and L-shells of the projectiles. We also
observe an increase of the REC probability by 20-50% relative to the
probability in a non-perturbed electron gas. The energy shift is in agreement
with calculations using the linear response theory, whereas the local electron
density enhancement is much smaller than predicted by the same model. This
shows that, for the small values of the adiabaticity parameter achieved in our
experiments, the density fluctuations are not strongly localized at the
vicinity of the heavy ions
Refined saddle-point preconditioners for discretized Stokes problems
This paper is concerned with the implementation of efficient solution algorithms for elliptic problems with constraints. We establish theory which shows that including a simple scaling within well-established block diagonal preconditioners for Stokes problems can result in significantly faster convergence when applying the preconditioned MINRES method. The codes used in the numerical studies are available online
Subshell-selective x-ray studies of radiative recombination of ions with electrons for very low relative energies
Radiative recombination (RR) into the K shell and L subshells of U92+ ions interacting with cooling electrons has been studied in an x-ray RR experiment at the electron cooler of the Experimental Storage Ring at GSI. The measured radiative recombination rate coefficients for electron-ion relative energies in the range 0â1000 meV demonstrate the importance of relativistic effects. The observed asymmetry of the measured K-RR x-ray emission with respect to the cooling energy, i.e., zero average relative velocity (âšvrelâ©=0), are explained by fully relativistic RR calculations. With our new approach, we show that the study of the angular distribution of RR photons for different relative energies opens new perspectives for detailed understanding of the RR of ions with cooling electrons in cold magnetized plasma
Ion slowing down and charge exchange at small impact parameters selected by channeling: superdensity effects
CASInternational audienceIn two experiments performed with 20-30 MeV/u highly charged heavy ions (Pb56+, U91+) channeled through thin silicon crystals, we observed the original features of superdensity, associated to the glancing collisions with atomic rows undergone by part of the incident projectiles. In particular the very high collision rate yields a quite specific charge exchange regime, that leads to a higher ionization probability than in random conditions. X-ray measurements show that electrons captured in outershells are prevented from being stabilized, which enhances the lifetime of the projectile innershell vacancies. The charge state distributions and the energy loss spectra are compared to Monte-Carlo simulations. These simulations confirm, extend and illustrate the qualitative analysis of the experimental results
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