409 research outputs found
Quantum signatures in laser-driven relativistic multiple-scattering
The dynamics of an electronic Dirac wave packet evolving under the influence
of an ultra-intense laser pulse and an ensemble of highly charged ions is
investigated numerically. Special emphasis is placed on the evolution of
quantum signatures from single to multiple scattering events. We quantify the
occurrence of quantum relativistic interference fringes in various situations
and stress their significance in multiple-particle systems, even in the
relativistic range of laser-matter interaction.Comment: 4 pages, 2 figures, LaTeX, revtex
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
Isotope shift in the dielectronic recombination of three-electron ^{A}Nd^{57+}
Isotope shifts in dielectronic recombination spectra were studied for Li-like
^{A}Nd^{57+} ions with A=142 and A=150. From the displacement of resonance
positions energy shifts \delta E^{142,150}(2s-2p_1/2)= 40.2(3)(6) meV
(stat)(sys)) and \delta E^{142,150}(2s-2p_3/2) = 42.3(12)(20) meV of 2s-2p_j
transitions were deduced. An evaluation of these values within a full QED
treatment yields a change in the mean-square charge radius of ^{142,150}\delta
= -1.36(1)(3) fm^2. The approach is conceptually new and combines the
advantage of a simple atomic structure with high sensitivity to nuclear size.Comment: 10 pages, 3 figures, accepted for publication in Physical Review
Letter
Measurements ofK-shell ionization with132Xe and208Pb projectiles in the Energy Range of 1.4?5.9 MeV/u
X-ray emission from a brown dwarf in the Pleiades
We report the first detection of X-ray emission from a brown dwarf in the
Pleiades, the M7-type Roque 14, obtained using the EPIC detectors on
XMM-Newton. This is the first X-ray detection of a brown dwarf intermediate in
age between ~12 and ~320 Myr. The emission appears persistent, although we
cannot rule out flare-like behaviour with a decay time-scale > 4 ks. The
time-averaged X-ray luminosity of Lx = (3.3 +/- 0.8) x 10^{27} erg/s, and its
ratios with the bolometric (Lx/Lbol = 10^{-3.05}) and Halpha (Lx/LHa = 4.0)
luminosities suggest magnetic activity similar to that of active main-sequence
M dwarfs, such as the M7 old-disc star VB 8, though the suspected binary nature
of Roque 14 merits further attention. No emission is detected from four
proposed later-type Pleiades brown dwarfs, with upper limits to Lx in the range
2.1-3.8 x 10^{27} erg/s and to log(Lx/Lbol) in the range -3.10 to -2.91.Comment: 8 pages, 4 figures (6 eps files), accepted for publication in MNRAS,
Na I "emission" corrected to "absorption" in description of target
A representative particle approach to coagulation and fragmentation of dust aggregates and fluid droplets
Context: There is increasing need for good algorithms for modeling the
aggregation and fragmentation of solid particles (dust grains, dust aggregates,
boulders) in various astrophysical settings, including protoplanetary disks,
planetary- and sub-stellar atmospheres and dense molecular cloud cores. Here we
describe a new algorithm that combines advantages of various standard methods
into one.
Aims: The aim is to develop a method that 1) can solve for aggregation and
fragmentation, 2) can easily include the effect and evolution of grain
properties such as compactness, composition, etc., and 3) can be built as a
coagulation/fragmentation module into a hydrodynamics simulations.
Methods: We develop a Monte-Carlo method in which we follow the 'life' of a
limited number of representative particles. Each of these particles is
associated with a certain fraction of the total dust mass and thereby
represents a large number of true particles which all are assumed to have the
same properties as their representative particle. Under the assumption that the
total number of true particles vastly exceeds the number of representative
particles, the chance of a representative particle colliding with another
representative particle is negligibly small, and we therefore ignore this
possibility. This now makes it possible to employ a statistical approach to the
evolution of the representative particles.
Results: The method reproduces the known analytic solutions of simplified
coagulation kernels, and compares well to numerical results for Brownian motion
using other methods. For reasonably well-behaved kernels it produces good
results even for moderate number of swarms.Comment: accepted for publication in A&
Relativistic quantum dynamics in strong fields: Photon emission from heavy, few-electron ions
Recent progress in the study of the photon emission from highly-charged heavy
ions is reviewed. These investigations show that high- ions provide a unique
tool for improving the understanding of the electron-electron and
electron-photon interaction in the presence of strong fields. Apart from the
bound-state transitions, which are accurately described in the framework of
Quantum Electrodynamics, much information has been obtained also from the
radiative capture of (quasi-) free electrons by high- ions. Many features in
the observed spectra hereby confirm the inherently relativistic behavior of
even the simplest compound quantum systems in Nature.Comment: Version 18/11/0
Recommended from our members
The programming of sequences of saccades
Saccadic eye movements move the high-resolution fovea to point at regions of interest. Saccades can only be generated serially (i.e., one at a time). However, what remains unclear is the extent to which saccades are programmed in parallel (i.e., a series of such moments can be planned together) and how far ahead such planning occurs. In the current experiment, we investigate this issue with a saccade contingent preview paradigm. Participants were asked to execute saccadic eye movements in response to seven small circles presented on a screen. The extent to which participants were given prior information about target locations was varied on a trial-by-trial basis: participants were aware of the location of the next target only, the next three, five, or all seven targets. The addition of new targets to the display was made during the saccade to the next target in the sequence. The overall time taken to complete the sequence was decreased as more targets were available up to all seven targets. This was a result of a reduction in the number of saccades being executed and a reduction in their saccade latencies. Surprisingly, these results suggest that, when faced with a demand to saccade to a large number of target locations, saccade preparation about all target locations is carried out in paralle
Radiative recombination of bare Bi83+: Experiment versus theory
Electron-ion recombination of completely stripped Bi83+ was investigated at
the Experimental Storage Ring (ESR) of the GSI in Darmstadt. It was the first
experiment of this kind with a bare ion heavier than argon. Absolute
recombination rate coefficients have been measured for relative energies
between ions and electrons from 0 up to about 125 eV. In the energy range from
15 meV to 125 eV a very good agreement is found between the experimental result
and theory for radiative recombination (RR). However, below 15 meV the
experimental rate increasingly exceeds the RR calculation and at Erel = 0 eV it
is a factor of 5.2 above the expected value. For further investigation of this
enhancement phenomenon the electron density in the interaction region was set
to 1.6E6/cm3, 3.2E6/cm3 and 4.7E6/cm3. This variation had no significant
influence on the recombination rate. An additional variation of the magnetic
guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted
in periodic oscillations of the rate which are accompanied by considerable
changes of the transverse electron temperature.Comment: 12 pages, 14 figures, to be published in Phys. Rev. A, see also
http://www.gsi.de/ap/ and http://www.strz.uni-giessen.de/~k
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