keV Regime Research Articles

Stimulated Raman scattering in hydrogen by ultrashort laser pulse in the keV regime

This letter addresses the problem of stimulated Raman excitation of a hydrogen atom submitted to an ultrashort and intense laser pulse in the keV regime. The pulse central frequency ω of 55 a.u. (about 1.5 keV) is in the weakly relativistic regime, (c is the speed of light in vacuum and a0 the Bohr radius) and the pulse duration is τ ≈ 18.85 a.u. (about 456 attoseconds). We solve the corresponding time-dependent Schrödinger equation (TDSE) using a spectral approach, retardation (or nondipole) effects are included up to , breaking the conservation of the magnetic quantum number m and forcing the resolution of the TDSE in a three-dimensional space. Due to the laser bandwidth, which is of the order of the ionization potential of hydrogen, stimulated Raman scattering populates nlm excited states (n and l are the principal and azimuthal quantum numbers, respectively). The populations of these excited states are calculated and analyzed in terms of l and m quantum numbers, this showing the contributions of the retardation effects and their relative importance.

How well can we measure supermassive black hole spin?

Being one of only two fundamental properties black holes possess, the spin of supermassive black holes (SMBHs) is of great interest for understanding accretion processes and galaxy evolution. However, in these early days of spin measurements, consistency and reproducibility of spin constraints have been a challenge. Here we focus on X-ray spectral modelling of active galactic nuclei (AGN), examining how well we can truly return known reflection parameters such as spin under standard conditions. We have created and fit over 4000 simulated Seyfert 1 spectra each with 375$\pm$1k counts. We assess the fits with reflection fraction of $R$ = 1 as well as reflection-dominated AGN with $R$ = 5. We also examine the consequence of permitting fits to search for retrograde spin. In general, we discover that most parameters are over-estimated when spectroscopy is restricted to the 2.5 - 10.0 keV regime and that models are insensitive to inner emissivity index and ionization. When the bandpass is extended out to 70keV, parameters are more accurately estimated. Repeating the process for $R$ = 5 reduces our ability to measure photon index ($\sim$3 to 8 per cent error and overestimated), but increases precision in all other parameters -- most notably ionization, which becomes better constrained ($\pm$45 erg cm $\rm{s^{-1}}$) for low ionization parameters ($\xi$$<$200 erg cm $\rm{s^{-1}}$). In all cases, we find the spin parameter is only well measured for the most rapidly rotating supermassive black holes (i.e. $a$ $>$ 0.8 to about $\pm$0.10) and that inner emissivity index is never well constrained. Allowing our model to search for retrograde spin did not improve the results.

Dark matter massive fermions and Einasto profiles in galactic haloes

On the basis of a fermionic dark matter model we fit rotation curves of The HI Nearby Galaxy Survey THINGS sample and compare our 3-parametric model to other models widely used in the literature: 2-parametric Navarro--Frenk--White, pseudoisothermal sphere, Burkhert models, and 3-parametric Einasto model, suggested as the new "standard dark matter profile" model in the paper by Chemin et. al., AJ 142 (2011) 109. The results from the fitting procedure provides evidence for an underlying fermionic nature of the dark matter candidate, with rest mass above the keV regime.

Tracking subsurface ion radiation damage with metal–oxide–semiconductor device encapsulation

Abstract

Design and Tests of the Hard X-ray Polarimeter X-Calibur

X-ray polarimetry promises to give qualitatively new information bout high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested ahard X-ray polarimeter, X-&lt;em&gt;Calibur&lt;/em&gt;, to be used in the focal plane of the InFOCuS grazing incidence hard X-ray telescope.X-&lt;em&gt;Calibur&lt;/em&gt; combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 20−60 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation; in principal, a similar space-borne experiment could be operated in the 5−100 keV regime. X-&lt;em&gt;Calibur&lt;/em&gt; achieves a high detection efficiency of order unity.

Are the most super-massive dark compact objects harbored at the center of dark matter halos?

We study an isothermal system of semi-degenerate self-gravitating fermions in general relativity (GR). The most general solutions present is mass density profiles with a central degenerate compact core governed by quantum statistics followed by an extended plateau, and ending in a power law behavior r-2. By fixing the fermion mass m in the keV regime, the different solutions depending on the free parameters of the model: the degeneracy and temperature parameters at the center, are systematically constructed along the one-parameter sequences of equilibrium configurations up to the critical point, which is represented by the maximum in a central density (ρ0) versus core mass (Mc) diagram. We show that for fully degenerate cores, the Oppenheimer–Volkoff (OV) mass limit [Formula: see text] is obtained, while instead for low degenerate cores, the critical core mass increases showing the temperature effects in a nonlinear way. The main result of this work is that when applying this theory to model the distribution of dark matter (DM) in big elliptical galaxies from miliparsec distance-scales up to 102Kpc, we do not find any critical core-halo configuration of self-gravitating fermions, able to explain both the most super-massive dark object at their center together with the DM halo simultaneously.

A new design for imaging of fast energetic electrons

We report on an essentially improved version of the classical Eppink–Parker velocity map imaging spectrometer design (Rev. Sci. Instrum. 68, 3477 (1997)). By adding electrostatic lenses with an opposite polarity to the extraction system we succeeded in extending the range of detection of energetic particles up to the keV regime at moderate (<20kV) extraction voltage conditions. Simulations show that the electrostatic lens system acts in analogy to an achromatic lens in optics and leads to a reduction in the chromatic energy aberration. For comparison to other setups a transmission parameter of the extraction system is defined denoting the maximum kinetic energies of particles which can be analyzed. Detector size and spectrometer length only enter via geometry, that is the straight trajectories in the subsequent field-free particle drift. With respect to Eppink–Parker the energy range has been extended by a factor of 2.5. Moreover, particle trajectory simulations demonstrate that the energy resolution can be improved by about 20%. To test the performance, photoemission studies have been conducted to resolve above-threshold-ionization patterns from Xe atoms exposed to intense ultrashort laser pulses as well as single photon ionization of Ne atoms using tunable synchrotron radiation with photon energies up to 600eV.

The effect of bound states on X-ray Thomson scattering for partially ionized plasmas

X-ray Thomson scattering is being developed as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. X-ray laser sources have always been of interest because of the need to have a bright monochromatic X-ray source to overcome plasma emission and eliminate other lines in the background that complicate the analysis. With the advent of the X-ray free electron laser (X-FEL) at the SNAL Linac Coherent Light Source (LCLS) and other facilities coming online worldwide, we now have such a source available in the keV regime. An important challenge with X-ray Thomson scattering experiments is understanding how to model the scattering for partially ionized plasmas. Most Thomson scattering codes used to model experimental data greatly simplify or neglect the contributions of the bound electrons to the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion–ion scattering and inelastic electron–electron scattering and add the contribution of bound electrons in the partially ionized plasmas. Except for hydrogen plasmas, most plasmas studied today have bound electrons and it is important to understand their contribution to the Thomson scattering, especially as new X-ray sources such as an X-FEL will allow us to study much higher Z plasmas. To date, most experiments have studied hydrogen or beryllium plasmas. We first analyze existing experimental data for beryllium to validate the code. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that require new computational tools to understand. For a Sn plasma, we show that bound contributions change the shape of the scattered spectrum in a way that would change the plasma temperature and density inferred from experiment.

Coherent Storage and Phase Modulation of Single Hard-X-Ray Photons Using Nuclear Excitons

The coherent storage and phase modulation of x-ray single-photon wave packets in the resonant scattering of light off nuclei is theoretically investigated. We show that by switching off and on again the magnetic field in the nuclear sample, phase-sensitive storage of photons in the keV regime can be achieved. Corresponding π phase modulation of the stored photon can be accomplished if the retrieving magnetic field is rotated by 180°. The development of such x-ray single-photon control techniques is a first step towards forwarding quantum optics and quantum information to shorter wavelengths and more compact photonic devices.

Looking Deeper: Angle-Resolved Photoemission with Soft and Hard X-rays

Traditional angle-resolved photoemission (ARPES) with excitation in the ca. 20 to 150 eV range has clearly evolved to be the technique of choice for studying the electronic structure of surfaces and complex new strongly correlated and magnetic materials. However, it is clear that ARPES with excitation only up to 150 eV or so remains a very surface-sensitive probe, thus necessitating careful in-situ sample treatment, cleaving, or even synthesis to avoid the measurement of surface-associated artifacts. A key measure of this surface sensitivity is the electron inelastic mean free path (IMFP orΛe), which measures the mean depth of electron emission without inelastic scattering, and both experimental [1, 2] and theoretical [3] IMFP studies showing that the only reliable way to increase bulk or buried layer/interface sensitivity for all material types is to go to higher photon energies in the soft X-ray (ca. 0.5–2 keV) or hard X-ray (ca. 2–10 keV) regime.

Study of slow and fast extraction for the ultralow energy storage ring (USR)

The Facility for Antiproton and Ion Research (FAIR, GSI) will not only provide future users with high fluxes of antiprotons in the high-energy range, but it is also intended to include a dedicated program for low-energy antiproton research in the keV regime, realized with the FLAIR project. The deceleration of antiprotons with an initial energy of 30 MeV down to ultralow energies of 20 keV will be realized in two steps. First, the beam is cooled and slowed down to an energy of 300 keV in a conventional magnetic ring, the Low energy Storage Ring (LSR) before being transferred into the electrostatic Ultralow energy Storage Ring (USR). In this synchrotron the deceleration to a final energy of 20 keV will be realized. This paper describes the ion-beam optical and mechanical layout of the beam extraction from the USR and summarizes the expected beam qualities of extracted beams.

Coherent control of nuclear forward scattering

The possibility to control the coherent decay of resonant excitations in nuclear forward scattering is investigated. By changing abruptly the direction of the nuclear hyperfine magnetic field, the coherent scattering of photons can be manipulated and even completely suppressed via quantum interference effects between the nuclear transition currents. The efficiency of the coherent decay suppression and the dependence of the scattered light polarization on the specific switching parameters is analyzed in detail. Using a sophisticated magnetic switching sequence involving four rotations of the hyperfine magnetic field, two correlated coherent decay pulses with different polarizations can be generated out of one excitation, providing single-photon entanglement in the keV regime. The verification of the generated entanglement by testing a single-particle version of Bell's inequality in an X-ray optics experimental setup is put forward.

Effect of substrate temperature on implantation doping of Co in CdS nanocrystalline thin films

We demonstrate doping of nanocrystalline CdS thin films with Co ions by ion implantation at an elevated temperature of 573 K. The modifications caused in structural and optical properties of these films are investigated. Co-doping does not lead to amorphization or formation of any secondary phase precipitate for dopant concentrations in the range of 0.34-10.8 at.% used in the present study. However, we observe a systematic reduction in the d-spacing with increasing cobalt concentration. Optical band gap of CdS does not show any obvious change upon Co-doping. In addition, implantation gives rise to grain growth and increase in the surface roughness. The results are discussed in the light of ion-matter interaction in the keV regime.

Double electron capture cross-sections by protons in collisions with He

Double electron capture (DEC) by protons from He was studied in collisions at energies in the keV regime, theoretically and experimentally. Theoretical cross-sections were calculated using the electron nuclear dynamic approach and a binomial distribution method in the energy range 1–100 keV. Experimental cross-sections were measured by means of the grow-rate method in the energy range of 4–10 keV. The cross-sections measured are consistent with those of the previous data at the high energies of the measured interval and show a different trend for the low energy. This behavior is consistent with those of other DEC cross-sections measured by protons from He-like targets. With the two collision models, it is proven that the reference data for this system are consistent only with the assumption of uncorrelated electrons and with independent target-projectile nuclei dynamics.

Implantation-assisted Co-doped CdS thin films: Structural, optical, and vibrational properties

This paper reports on structural, optical, vibrational, and morphological properties of cobalt-doped CdS thin films, prepared by 90 keV Co+ implantation at room temperature. In this work, we have used cobalt concentration in the range of 0.34–10.8 at. %. Cobalt doping does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. However, with increasing cobalt concentration a decrease in the optical band gap, from 2.39 to 2.26 eV, is observed. This reduction is addressed on the basis of band tailing due to the creation of localized energy states in association with Urbach energy calculations. In addition, implantation gives rise to grain growth and increase in the surface roughness. Size and shape fluctuations of individual CdS grains, at higher fluences, give rise to inhomogeneity in strain. The results are discussed in the light of ion-matter interaction in the keV regime.

Single-Photon Entanglement in the keV Regime via Coherent Control of Nuclear Forward Scattering

Generation of single-photon entanglement is discussed in nuclear forward scattering. Using successive switchings of the direction of the nuclear hyperfine magnetic field, the coherent scattering of photons on nuclei is controlled such that two signal pulses are generated out of one initial pump pulse. The two time-resolved correlated signal pulses have different polarizations and energy in the keV regime. Spatial separation of the entangled field modes and extraction of the signal from the background can be achieved with the help of state-of-the-art x-ray polarizers and piezoelectric fast steering mirrors.

Double capture cross sections inp−Arcollisions

$p\text{\ensuremath{-}}\mathrm{Ar}$ collisions are investigated theoretically and experimentally at impact energies in the keV regime. Total cross sections for double electron capture of $p$ on Ar are calculated by an independent-particle method in the impact energy range $3--100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. We have measured the absolute differential and total cross sections for double electron capture for $1.0--5.0\text{\ensuremath{-}}\mathrm{keV}$ $p\text{\ensuremath{-}}\mathrm{Ar}$ collisions. The absolute differential cross section (DCS) for all the collision energies considered shows a decreasing behavior with increasing angle, exhibiting an overall decrease of three orders of magnitude. The integrated DCS is found to be between the range of $0.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ and $4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{2}$, displays an increasing behavior as a function of the incident energy, and merges with previous data at the high-energy side. The theoretical total cross sections reproduce the structures around $5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and $15\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ reported by other available measurements. The analysis of the capture into the $1{s}^{1}4{s}^{1}$ hydrogen-like configuration points in the direction of a common origin for these peaks.

Ion bombardment induced smoothing of amorphous metallic surfaces: Experiments versus computer simulations

Smoothing of rough amorphous metallic surfaces by bombardment with heavy ions in the low keV regime is investigated by a combined experimental-simulational study. Vapor deposited rough amorphous ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Cu}}_{27.5}$ films are the basis for systematic in situ scanning tunneling microscopy measurements on the smoothing reaction due to $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{Kr}}^{+}$ ion bombardment. The experimental results are directly compared to the predictions of a multiscale simulation approach, which incorporates stochastic rate equations of the Langevin type in combination with previously reported classical molecular dynamics simulations [Phys. Rev. B 75, 224107 (2007)] to model surface smoothing across length and time scales. The combined approach of experiments and simulations clearly corroborates a key role of ion induced viscous flow and ballistic effects in low keV heavy ion induced smoothing of amorphous metallic surfaces at ambient temperatures.

Secondary electron yield measurements from materials with application to collectors of high-power microwave devices

An experimental test facility has been established for measuring the secondary electron yield (SEY) of materials thought to be suitable for low yield vacuum electronic applications such as collectors in high-power microwave (HPM) tubes. Experiments can be broadly divided into two energy-regimes: a high-energy (1-50 keV) and a low-energy (10 eV-1 keV) regime. Measurements of SEY at high energies are presented for the following materials: copper, titanium, and Poco graphite. Observation of time-dependent SEY behavior in these samples suggests that surface processes play an important role during measurements. In addition, SEY at low energies and as a function of the angle of incidence of primary electrons has been measured for plasma sprayed boron carbide (PSBC). The experimental results presented here are benchmarked with existing SEY data in the literature, empirically and to first principle formulae

Inner- and outer-shell electron dynamics in proton collisions with sodium atoms

p+Na collisions have been investigated theoretically and experimentally at impact energies in the keV regime. We present results for capture and ionization processes, and, in particular, analyse the role of initial inner-shell electrons, whose active participation is identified in the experiments through the analysis of recoil-ion momentum spectra. Quantum-mechanical calculations within the independent particle model have been carried out for all active electrons. A very good overall agreement between the theoretical and experimental results is found. The calculations support the observation that capture from inner shells is an important reaction channel even at relatively low impact energies, and dominates total capture above 40 keV.