Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P36: Focus Session: X-ray and Neutron Instrumentation and Science |
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Sponsoring Units: GIMS Chair: George Srajer, Argonne National Laboratory Room: Morial Convention Center 228 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P36.00001: Advances in Neutron Scattering Instrumentation at Oak Ridge National Laboratory Invited Speaker: Oak Ridge National Laboratory hosts two world-class neutron sources for condensed matter research. The High Flux Isotope Reactor (HFIR) has recently added a cold source to augment the existing high-intensity thermal beams, and the Spallation Neutron Source (SNS) is the most powerful pulsed spallation neutron source in the world. Investments in neutron scattering instrumentation have come to fruition in a current set of 9 high-performance scattering stations in the user program, covering a wide range of diffraction and spectroscopic techniques. Examples of current scientific results include reflectometry from bio-responsive polymer films and the spectroscopy of slow spin dynamics in frustrated magnetic systems. In 2008 an additional 6 instruments will start user operation or commissioning at the SNS, increasing the breadth and depth of scientific possibilities. Novel techniques and new devices in such areas as sample environment and neutron polarization are also under development. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P36.00002: Recent developments on polarized neutron scattering at NIST Wangchun Chen, Julie Borchers, Ross Erwin, James McIver, Thomas Gentile, Jeffrey Lynn, Gordon Jones $^{3}$He neutron spin filters (NSF) employ nuclear spin-polarized $^{3}$He gas, produced by optical pumping, and can be used to polarize or analyze neutron beams because of the strong spin dependence of the absorption cross section for neutrons by $^{3}$He. At the NIST center for Neutron Research, the polarized $^{3}$He NSF program has been developed to enhance the measurement capability in polarized neutron scattering. This technique has been applied in a number of neutron scattering instruments for user experiments and instrumentation development. Here we will discuss applications of the $^{3}$He NSF devices in polarized small-angle neutron scattering (SANS), polarized neutron reflectometry (PNR), and polarized triple-axis spectrometry (TAS). For these applications, we employ $^{3}$He NSFs as both neutron polarizers and neutron flippers that are achieved using the adiabatic fast passage nuclear magnetic resonance technique. We will present the results in each of these applications from magnetic nanoparticles on SANS, patterned magnetic thin films on PNR, and the multiferroelectric system on TAS. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P36.00003: Neutron detection in boron carbide/Si heterojunctions as functions of time constants and bias voltage Nina Hong, John Mullins, S. Adenwalla True solid-state neutron detectors have the potential to achieve high efficiencies at low mass, size and power. [1,2] Such detectors made from semiconducting boron carbide (BC) allow for neutron capture and charge collection in the same layer. Here we report neutron detection results from p-n heterojunction diodes of boron carbide on n-type Si. Neutron capture efficiency increases with time constant and reverse bias, from 0.15{\%} at 0 bias and short time constant to 0.46{\%} at 19 V and long time constant. Increasing reverse bias increases the depletion width in the BC layer, leading to a higher proportion of charge capture. The long time constants allow for the detection of charge capture in the BC-scope traces show charge capture times of $\sim $ 30 $\mu$s (as compared to $<$20 ns in Si). These results indicate that the BC layer is playing an active role in neutron detection, capturing neutrons as well as charge. [1] B. W. Robertson, S. Adenwalla, et al., APL, 80, 3644 (2002). [2] E. Day M. J. Diaz, and S. Adenwalla, J. Phys. D: Appl. Phys. \textbf{39, }2920 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P36.00004: The effect of Gd doping on the atomic and electronic structure of HfO$_{2}$ thin films. Ihor Ketsman, Andrei Sokolov, Kirill Belashchenko, Peter Dowben, Yaroslav Losovyj, Jinke Tang, Zhenjun Wang HfO$_{2}$ is a promising oxide for many applications, including high-$k$ gate dielectric for CMOS devices. In addition, Gd-doped HfO$_{2}$ could lead to a dilute magnetic semiconductor and provide an efficient neutron detection medium due to huge neutron absorption cross section of Gd. Gd-doped HfO$_{2 }$films deposited on both $p-$type and $n$-type silicon by PLD retain monoclinic phase at small doping levels, but can be stabilized in fluorite phase by increased doping [1]. At small doping levels, photoemission measurements indicate $n$-type character of the films as a result of overcompensation with oxygen vacancies. Depending on a doping level, the films form heterojunctions with good rectifying properties on $n$- or $p$-type silicon. Preliminary results show the potential ability of the formed diode structures to detect neutrons. [1] Ya.B.Losovyj, I.Ketsman et al.,APL, 91, 132908, (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P36.00005: Time-dependent small-angle X-ray and neutron scattering studies of solution-mediated nanoparticulate nucleation, growth and alignment. Andrew J. Allen, Vincent A. Hackley A remote-controlled, isothermal, circulating fluid flow cell has been developed for small-angle X-ray and neutron scattering (SAXS and SANS) studies of suspensions with online monitoring of flow rate, temperature or conditions, e.g., pH. Used with the small X-ray beams available at 3rd generation synchrotron sources and the nanometer-to-micrometer scale range accessible in ultrasmall-angle X-ray scattering studies, structural characteristics ranging from 1 nm to several micrometers can be measured, in situ and in real time, as a function of changing physical or chemical conditions. Used with time-resolved pinhole SAXS instruments, rapid reaction precursor phenomena at the nanoscale level can be similarly studied. Used in SANS studies, a neutron-adapted version of the flow cell allows real-time contrast variation techniques to further elucidate the structural evolution. Applications will be discussed for real-time studies of solution-mediated nanocrystalline ceramic oxide formation, and Au nanowire alignment in extensional flow. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P36.00006: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P36.00007: Dynamical reconstruction of the valence exciton in LiF Peter Abbamonte, Wei Ku, Tim Graber, James Reed, Serban Smadici, Abhay Shukla, Jean-Pascal Rueff We have used inelastic x-ray scattering, coupled with recently developed inversion techniques, to reconstruct the structure and dynamics of the valence exciton in the prototype alkali halide LiF. Our inversions, which yield resolutions $\Delta x = 0.533 \AA$ and $\Delta t = 20.67 as (2.067 \times 10^{-17} s)$, reveal that the exciton forms in less than $50 as$, oscillates with a period of $283 as$, and decays after approximately $5 fs$. It contains a pronounced $a/3$ internal periodicity, where $a = 4.027 \AA$ is the crystal lattice parameter, that changes little during the course of its life, indicating that this exciton lies very close to the Frenkel limit. Our results resolve a 70 year old contraversy about the valence exciton in alkali halides and, when compared to {\it ab initio} calculations, demonstrate a simplified theoretical approach to describing excitons in the limit of strong binding energy. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P36.00008: Core-hole propagator and resonant inelastic X-ray scattering: exact results within a Baym-Kadanoff-Keldysh approach Andrij Shvaika, Taras Mysakovych, James Freericks We solve for the core-hole propagator and the resonant inelastic X-ray scattering (RIXS) response functions in correlated materials by employing dynamical mean-field theory. We focus our attention on the spinless Falicov-Kimball model, where the problem can be solved exactly, and the system can be tuned to go through a Mott-Hubbard-like metal-insulator transition (the coupling with the core hole is also a Falicov-Kimball type of interaction). The core-hole propagator is expressed in terms of a continuous fermionic Toeplitz determinant defined only on the upper real-time branch of the Keldysh contour. We have derived exact large-time asymptotics for the core-hole propagator using the Wiener-Hopf finite sum equation technique which produces an efficient algorithm to obtain the density of states of the X-ray edge problem for any temperature and any interaction strength. We have also derived the two-particle vertices in a diagrammatic representation of the scattering processes (using a Baym-Kadanoff-Keldysh approach). As an example of this formalism, we show results for the L-edge contributions to RIXS. \textsl{This work was supported by Award No.~UKP2-2697-LV-06 of the U.S.~Civilian Research and Development Foundation.} [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P36.00009: Identification of Mn site in Pb(Zr,Ti)O$_{3}$ A. Boonchun, M.F. Smith, S. Rujirawat, B. Cherdhirunkorn, S. Limpijumnong The impurity Mn in PbTiO$_{3}$ and PbZrO$_{3}$ has been studied by mean of first-principles spin density functional theory.[1] It is found that the Mn atom energetically prefers to substitute on the Ti/Zr site over other sites (i.e., Pb site, O site or interstitial) under all equilibrium growth conditions. The calculations predict that a majority of Mn atoms substitute for Ti/Zr and have neutral-charge state each with a total electron spin of M$_{z}$ = 3/2. This prediction is supported by the combination of x-ray absorption near edge structure (XANES) experiment and first-principles simulation of the spectrum. [2] The measured XANES of the Mn-doped Pb(Ti,Zr)O$_ {3}$ within the concentration range of 0.5 - 2.0 at.\% yield the exact same features, indicating that the location of Mn in the crystal is independent of Mn concentration. The measured XANES is consistent with the partial density of states simulation of Mn atom on the Ti/Zr site and inconsistent with the simulations of Mn atom on other sites.\newline [1] A. Boonchun, M. F. Smith, B. Cherdhirunkorn, and S. Limpijumnong, J. Appl. Phys. 101, 043521 (2007). [2] S. Limpijumnong, S. Rujirawat, A. Boonchun, M.F. Smith, B. Cherdhirunkorn, Appl. Phys. Lett. 90, 103113 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P36.00010: Direct mapping of phonon dispersions in copper by x-ray thermal diffuse scattering. Ruqing Xu, Hawoong Hong, Tai Chiang X-ray Thermal Diffuse Scattering (TDS) arises from lattice vibrations. Measured TDS intensities can be used to extract phonon dispersion relations, as has been demonstrated in a number of systems. But most analysis methods so far have involved data fitting based on assumed atomic force constant models, and it is difficult to determine a priori the accuracy of the procedure. Methods of direct inversion, i.e., determining phonon frequencies directly from x-ray TDS data without a presumed model, have been proposed in prior work, but most of the schemes involve absolute intensity measurements, which are difficult especially in the presence of an unknown background. Here we report an improved approach, in which phonon frequencies are determined through the temperature dependence of TDS intensities at each point in reciprocal space. Polarization selection rules are employed to disentangle contributions from different phonon branches. Results taken from a simple model material, copper, will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P36.00011: Fractionation of uranium isotopes in minerals screened by gamma spectrometry. Jeffrey L. Geiger, Austin M. Baldwin, Charles C. Blatchley At least two groups have reported finding shifts in the ratio of U-235/U-238 for sandstone, black shale, and other sedimentary samples using precision ICP-MS. These shifts were tentatively attributed to a recently predicted isotope effect based on nuclear volume that causes fractionation for U$^{IV}$-U$^{VI}$ transitions. However, fractionation of high Z elements may be less likely an explanation than U-235 depletion induced by galactic cosmic ray neutrons. Isotope depletion in marine sediments could therefore be an indicator of changes in cosmic ray flux due to nearby supernovae, gamma-ray bursts, or longer term changes during the 62 million year cycle of the Sun above and below the galactic plane. We report using a less precise approach than ICP-MS, but one which can quickly screen samples to look for anomalies in isotope ratios, namely HPGe gamma ray spectrometry. Various levels of depletion were measured for uranium rich minerals, including brannerite, carnotite, and pitchblende, as well as coal and limestone samples. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P36.00012: Electronic Structure Study of Cerium Doped Scintillators Da Gao, Michael McIlwain The interest shown in recent years related to cerium doped lanthanum halides, such as LaCl3, LaBr3, and LaI3, is mainly due to their potential applicability as gamma ray scintillation detectors. We have performed a comprehensive theoretical study of these materials to better understand the scintillation process and define the nature of the self trapped exciton (STE) associated with thermally dependent scintillation process. The present work focuses on the study of the luminescence properties of cerium doped lanthanum halide scintillators from the point view of solid state band structure calculations. Our calculated band structures are in good agreement with experimental values. For example, LaCl$_{3}$ band gap is calculated to be 6.85 eV as compared to approximately 7.0 eV for the experimentally determined value. The theoretically calculated excitation spectra are also compared with the experimental spectra. We find that scintillation efficiencies are dependent on the location of the of Ce$^{3+}$ ground 4f and excited 5d levels with respect to the fundamental band gap of the host materials. [Preview Abstract] |
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