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X-ray spectra from convective photospheres of neutron stars

Description: We present first results of modeling convective photospheres of neutron stars. We show that in photospheres composed of the light elements convection arises only at relatively low effective temperatures ({le}3 - 5 x 10{sup 4} K), whereas in the case of iron composition it arises at T{sub eff}{le} 3 x 10{sup 5}K. Convection changes the depth dependence of the photosphere temperature and the shapes of the emergent spectra. Thus, it should be taken into account for the proper interpretation of EUV/soft-X-ray observations of the thermal radiation from neutron stars.
Date: January 17, 1996
Creator: Zavlin, V.E.; Pavlov, G.G.; Shibanov, Yu.A.; Rogers, F.J. & Iglesias, C.A.
Partner: UNT Libraries Government Documents Department

Comprehensive x-ray spectral code for high energy astrophysics

Description: The aim of this project has been to develop a spectral analysis tool with a level of quality and completeness commensurate to that expected in data from the current generation of X-ray observatories. The code is called LXSS (Livermore X-Ray Spectral Synthesizer). X-ray-emitting astrophysical plasmas are rarely, if ever, in LTE, so they have adopted the detailed level accounting approach, in which rates for processes that populate or depopulate atomic energy levels are treated explicitly. This entails the generation of a large quantity of atomic data, most of which is calculated using ''in-house'' computer codes. Calculations are benchmarked against laboratory data, and spectral models have been used to provide first-time interpretations of astrophysical X-ray spectra. The design of a versatile graphical user interface that allows access to and manipulation of the atomic database comprises the second major part of the project.
Date: August 18, 2000
Creator: Liedahl, D A; Fournier, K B & Mauche, C W
Partner: UNT Libraries Government Documents Department

Excitation states of projectiles moving through solids

Description: From 3rd international conference on atomic collisions in solids; Gatlinburg, Tennessee, USA (23 Sep 1973). X-ray spectral measurements of S, Cl, and Ar atoms moving in solid carbon targets are used to extract the equilibrium distribution of vacancies in the valence and L-shells of the projectiles. It is found that the state of excitation is much higher than the mean charge measured after the projectile has left the solid. A simple model is used to deduce the expected final charge state distribution, and these are compared to thin foil measurements. (auth)
Date: September 1, 1973
Creator: Fortner, R.J. & Garcia, J.D.
Partner: UNT Libraries Government Documents Department

Characterization Studies and Performance of Half-strip High-speed X-ray Microchannel Plate Imager

Description: High-speed microchannel plate (MCP)–based imagers are critical detectors for x-ray diagnostics employed on Z-experiments at Sandia National Laboratories (SNL) to measure time-resolved x-ray spectra and to image dynamic hohlraums. A design using eight half-strip x-ray photocathodes in one imager permits recordings of radiation events in discrete temporal snapshots to yield a time-evolved movie. We present data using various facilities to characterize the performance of this design. These characterization studies include DC and pulsed-voltage biased measurements in both saturated and linear operational regimes using an intense, short-pulsed UV laser and Manson source. Surface voltage profile measurements using a picoprobe help to determine the gain variation across the strips. Test data from a recent SNL ZR-experiment demonstrates the flexibility and high-quality images obtained by this MCP imager.
Date: March 1, 2008
Creator: Moy, Kenneth & Wu, Ming
Partner: UNT Libraries Government Documents Department

Computer analysis of four channel x-ray microscopy images to obtain source and spectral emission data on laser fusion

Description: It is possible to analyze the images obtained from the four- channel x- ray microscope to obtain reasonable estimates of source spatial and energy emission. The technique shown here is particularly useful when relative comparisons are desired in which, from shot to shot, few parameters are changed. These data are of use in fuel pellet design and in checking design code predictions. The technique should also apply to pinhole camera data. Largest uncertainties appear to be due to film energy/handling calibration and mirror efficiency measurements. (auth)
Date: October 1, 1975
Creator: Harper, T.L.
Partner: UNT Libraries Government Documents Department

Milestones in EBIT Spectroscopy and Why it Almost Didn't Work

Description: The EBIT spectroscopy that now seems routine would not be possible without considerable good luck in several areas of EBIT technology. Among these are x-ray background, ion cooling, neutral gas density, and electron current density and energy control. A favourable outcome in these areas has enabled clean x-ray spectra, sufficient intensity for high resolution spectroscopy, production of very high charge states, and a remarkable variety of spectroscopic measurements. During construction of the first EBIT 20 years ago, it was not clear that any of this was possible.
Date: July 17, 2007
Creator: Marrs, R E
Partner: UNT Libraries Government Documents Department

Anisotropic x ray magnetic linear dichroism - Its importance for the analysis of soft x ray spectra of magnetic oxides

Description: Using spectroscopic information for x ray magnetometry and magnetic microscopy requires detailed theoretical understanding of spectral shape and magnitude of dichroism signals. We have shown unambiguously that--contrary to common belief--spectral shape and magnitude of x ray magnetic linear dichroism (XMLD) are not only determined by the relative orientation of magnetic moments and x ray polarization, but also their orientations relative to the crystallographic axes must be taken into account for accurate interpretation of XMLD data.
Date: July 2, 2008
Creator: van der Laan, G. & Arenholz, Elke
Partner: UNT Libraries Government Documents Department

Hard X-ray and Hot Electron Environment in Vacuum Hohlraums at NIF

Description: Time resolved hard x-ray images (hv > 9 keV) and time integrated hard x-ray spectra (hv = 18-150 keV) from vacuum hohlraums irradiated with four 351 nm wavelength NIF laser beams are presented as a function of hohlraum size and laser power and duration. The hard x-ray images and spectra provide insight into the time evolution of the hohlraum plasma filling and the production of hot electrons. The fraction of laser energy detected as hot electrons (f{sub hot}) and a comparison to a filling model are presented.
Date: September 22, 2005
Creator: McDonald, J. W.; Suter, L. J.; Landen, O. L.; Foster, J. M.; Celeste, J. R.; Holder, J. P. et al.
Partner: UNT Libraries Government Documents Department

EUV/soft x-ray spectra for low B neutron stars

Description: Recent ROSAT and EUVE detections of spin-powered neutron stars suggest that many emit ``thermal`` radiation, peaking in the EUV/soft X-ray band. These data constrain the neutron stars` thermal history, but interpretation requires comparison with model atmosphere computations, since emergent spectra depend strongly on the surface composition and magnetic field. As recent opacity computations show substantial change to absorption cross sections at neutron star photospheric conditions, we report here on new model atmosphere computations employing such data. The results are compared with magnetic atmosphere models and applied to PSR J0437-4715, a low field neutron star.
Date: May 23, 1995
Creator: Romani, R.W.; Rajagopal, M.; Rogers, F.J. & Iglesias, C.A.
Partner: UNT Libraries Government Documents Department

Synergistic approach to modeling X-ray spectra

Description: Plasma emission models used in X-ray astronomy need to simulate X-ray spectra from at least thirteen elements. Development of comprehensive models requires large-scale calculations; for example, Fe M-shell spectra, K{alpha} fluorescence from near-neutral ions, and dielectronic recombination satellite spectra from L-shell ions. Current and recent missions (EUVE, ASCA, DXS, etc.) have already demonstrated the need for major, rapid improvements in spectral models. The high-resolution spectra to be acquired with the next generation of X- ray observatories (AXAF, XMM, Astro-E) promise to push spectral models to their limits. Essential to ensuring the quality of calculations used in spectral codes is corroboration in the laboratory, where controlled and precisely measured plasma conditions can be attained. To this end, we are capitalizing on a three-way synergistic relationship that links astrophysical observations, atomic modeling, and experiments using the LLNL Electron Beam Ion Trap (EBIT). After providing a brief orientation concerning the role of plasma emission models in X-ray astronomy, we discuss one example of this interplay.
Date: July 1, 1998
Creator: Liedahl, D.A., LLNL
Partner: UNT Libraries Government Documents Department

ALCHEMI of niobium dichrome/vanadium C15 Laves phase

Description: 33Nb42Cr25V was prepared by arc melting and annealing at 1400 C for 120 hr. Energy-dispersive x-ray spectra were collected over a range of {l_brace}400{l_brace} excitations between symmetry and beyond {l_brace}12 0 0{l_brace}. Results show that at least qualitatively V substitutes for Cr. Therefore, electronic effects must be more important than size effects in this case.
Date: May 1, 1996
Creator: Kotula, P.G.; Chu, Fuming; Mitchell, T.E.; Anderson, I.M. & Bentley, J.
Partner: UNT Libraries Government Documents Department

Interpreting X-ray and auger resonant Raman spectra

Description: We discuss resonant Raman Scattering in atoms within the context of conservation of energy, arguing that energy conservation determines the principal features of spectra taken at a resolution comparable to the natural widths of the transitions involved. Example systems applicable to atoms or solids are calculated using the model by Tulkki and Aberg, and the model is discussed in terms of energy conservation. Finally, results for X-ray resonant Raman scattering in Xe and Auger resonant Raman scattering in Ar are presented and the two processes are contrasted.
Date: December 31, 1996
Creator: Lebrun, T.
Partner: UNT Libraries Government Documents Department

Higher order parametric x-ray spectra in mosaic graphite and single silicon crystals

Description: We have observed up to eight orders (n) in the spectra of parametric x-radiation, in the range 5-40 keV, produced by interaction of a 90-MeV electron beam with mosaic graphite and 90 and 35 MeV beams with single Si crystals, Measured yields and intensity ratios, I(n{ge}2)/I(n=1), in graphite are not in agreement with PXR theory for mosaic crystals. In comparison, yield and ratios of intensities in Si are close to preductions for perfect crystals. Bandwidths of spectral lines measured in both Si and graphite are in good agreement with theory and are determined by the angular field of view of the detector.
Date: December 31, 1993
Creator: Fiorito, R.B.; Rule, D.W. & Maruyama, X.K.
Partner: UNT Libraries Government Documents Department

Photon beam polarization and non-dipolar angular distributions

Description: Angular distributions of ejecta from unoriented atoms and molecules depend upon the polarization state of the incident x-rays as well as upon the dynamics of the physical systems being studied. I recommend a simple geometrical way of looking at the polarization and its effects upon angular distributions. The polarization is represented as a vector in a parameter space that faithfully represents the polarization of the beam. The simple dependence of the angular dependence of the angular distributions on the polarization vector enables easy extraction of the dynamical information contained in those angular distributions. No new physical results emerge from this geometrical approach, but known consequences of the symmetries appear in an easily visualized form that I find pleasing and that has proved to be useful for planning experiments and for analyzing data.
Date: August 1, 1996
Creator: Peshkin, M
Partner: UNT Libraries Government Documents Department

EUVE spectroscopy of polars

Description: An admittedly pedantic but hopefully useful and informative analysis is presented of the <I>EUVE</I> 70-180 Å spectra of nine polars. These spectra are fit with three different models - a blackbody, a pure-H stellar atmosphere, and a solar abundance stellar atmosphere|to reveal the presence of spectral features such as absorption lines and edges, and to investigate the sensitivity of the derived (<i>kT, N</i><sub>H</sub>, solid angle) and inferred (fractional emitting area, bolometric luminosity) parameters to the model assumptions. Among the models tested, the blackbody model best describes the observed spectra, although the untested irradiated solar abundance stellar atmosphere model is likely a better overall description of the EUV/soft X-ray spectra
Date: October 13, 1998
Creator: Mauche, C W
Partner: UNT Libraries Government Documents Department

Application of spectral line shapes to the study of high density ICF plasmas

Description: Spectral line broadening manifests itself in the study of high density inertial confinement fusion (ICF) plasmas in two important ways. First, comparison between measured and calculated lineshapes of individual spectral lines or groups of lines is used to diagnose plasma conditions in dense ICF plasmas, particularly in implosions. Secondly, through the emission and absorption coefficients spectral lineshapes serve as important inputs to plasma spectroscopy simulation codes which calculate simulated spectra from ICF targets. We discuss recent results from each of these areas. With regard to lineshape diagnostics, the advent of generalized line broadening codes has allowed the line profiles of complex multielectron emitters to be considered for diagnostic purposes. Particular example of this is the use of Ar He-{beta} and its associated dielectronic satellites as a diagnostic of T{sub e} and N{sub e}, as well as the development of Ne-like Xe line broadening as a density diagnostic. With respect to simulation codes, the implementation of detailed lineshapes in calculations of this type is in many ways in its infancy. We present here examples of cases where effects related to spectral lineshapes such as continuum lowering and line transfer of Stark broadened lines are important so as to provide a stimulus for future work in this field. 34 refs., 9 figs., 1 tab.
Date: September 1, 1994
Creator: Keane, C.J.; Hammel, B.A.; Langer, S.H.; Lee, R.W.; Calisti, A.; Godbert, L. et al.
Partner: UNT Libraries Government Documents Department

Calculation of electron emission from a tantalum foil irradiated by 100-kV and 50-kV x-rays

Description: Two Monte Carlo programs, XITRAN and XMTRAN, were developed for calculating the emission of electrons from high-Z foils irradiated with x rays. XITRAN follows all individual elastic collisions of electrons with atoms, whereas XMTRAN uses the condensed-random-walk model. Both codes take into account photo-electrons, fluorescence radiation, and Auger electrons. Comparisons are made with an experiment by Dolan at Sandia Laboratories involving the backward and forward emission of electrons from a tantalum foil irradiated by 100-kV and 50-kV x-ray beams. There is good agreement between results from the XITRAN and XMTRAN codes. There emitted per incident x-ray photon, and in regard to the angular distribution of the emerging electrons. In regard to the electron energy spectra, there is fair agreement down to a spectral energy of 20 keV, whereas below 20 keV the calculated spectra lie considerably below the measurements.
Date: March 1, 1998
Creator: Berger, M.J.
Partner: UNT Libraries Government Documents Department

X-ray spectral properties of {gamma}-ray bursts

Description: The authors summarize the spectral characteristics of a sample of 22 bright gamma-ray bursts detected with the gamma-ray burst sensors aboard the satellite Ginga. This instrument employed a proportional and scintillation counter to provide sensitivity to photons in the 2--400 keV range, providing a unique opportunity to characterize the largely unexplored X-ray properties of gamma-ray bursts. The photon spectra of the Ginga bursts are well described by a low energy slope, a bend energy, and a high energy slope. In the energy range where they can be compared, this result is consistent with burst spectral analyses obtained from the BATSE experiment aboard the Compton Observatory. However, below 20 keV they find evidence for a positive spectral number index in approximately 40% of their burst sample, with some evidence for a strong rolloff at lower energies in a few events. They find that the distribution of spectral bend energies extends below 10 keV. The observed ratio of energy emitted in the X-rays relative to the gamma-rays can be much larger than a few percent and, in fact, is sometimes larger than unity. The average for their sample is 24%.
Date: September 1, 1997
Creator: Strohmayer, T.E.; Fenimore, E.E.; Murakami, Toshio & Yoshida, Atsumasa
Partner: UNT Libraries Government Documents Department