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AbstractAbstract

[en] Evidence is presented that H

^{-}ions are the main electron traps responsible for the long-lived luminescence of the F centers observed at 2.3 eV in thermochemically reduced MgO crystalsPrimary Subject

Source

Aug 1982; 19 p; US/Japan seminar on recombination induced defect formation in crystals; Urbana, IL (USA); 2-5 Jun 1982; Available from NTIS, PC A02/MF A01 as DE83014058

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[en] Refractory oxide crystals suitable for use in tunable leasers and a method for preparing the same are presented. The crystals are characterized by high quantum efficiency, high thermal stability, good crystal transparency, and a high percentage of useful luminescence. The method for preparation of the crystals involves removing substantially all the hydrogen, thermochemically reducing the crystal's oxygen content to produce oxygen (anion) vacancy defects, and subsequently irradiating the crystal with electrons to inactivate trace H - ions so that an increased amount of short lived F+ luminescence is produced when the crystal is optically excited

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16 Oct 1990; 12 Sep 1988; 4 p; US PATENT DOCUMENT 4,963,755/A/; Patent and Trademark Office, Box 9, Washington, DC 20232 (USA); ?: 12 Sep 1988

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Patent

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AbstractAbstract

[en] Shielding calculations of advanced nuclear facilities such as accelerator based neutron sources or fusion devices of the tokamak type are complicated due to their complex geometries and their large dimensions, including bulk shields of several meters thickness. While the complexity of the geometry in the shielding calculation can be hardly handled by the discrete ordinates method, the deep penetration of radiation through bulk shields is a severe challenge for the Monte Carlo particle transport simulation technique. This work proposes a dedicated computational approach for coupled Monte Carlo - deterministic transport calculations to handle this kind of shielding problems. The Monte Carlo technique is used to simulate the particle generation and transport in the target region with both complex geometry and reaction physics, and the discrete ordinates method is used to treat the deep penetration problem in the bulk shield. To enable the coupling of these two different computational methods, a mapping approach has been developed for calculating the discrete ordinates angular flux distribution from the scored data of the Monte Carlo particle tracks crossing a specified surface. The approach has been implemented in an interface program and validated by means of test calculations using a simplified three-dimensional geometric model. Satisfactory agreement was obtained for the angular fluxes calculated by the mapping approach using the MCNP code for the Monte Carlo calculations and direct three-dimensional discrete ordinates calculations using the TORT code. In the next step, a complete program system has been developed for coupled three-dimensional Monte Carlo deterministic transport calculations by integrating the Monte Carlo transport code MCNP, the three-dimensional discrete ordinates code TORT and the mapping interface program. Test calculations with two simple models have been performed to validate the program system by means of comparison calculations using the Monte Carlo technique directly. The good agreement of the results obtained demonstrates that the program system is suitable to treat three-dimensional shielding problems with satisfactory accuracy. Finally the program system has been applied to the shielding analysis of the accelerator based IFMIF (International Fusion Materials Irradiation Facility) neutron source facility. In this application, the IFMIF-dedicated Monte Carlo code McDeLicious was used for the neutron generation and transport simulation in the target and the test cell region using a detailed geometrical model. The neutron/photon fluxes, spectra and dose rates across the back wall and in the access/maintenance room were calculated and are discussed. (orig.)

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Apr 2005; 105 p; ISSN 0947-8620; ; Available from TIB Hannover: ZA 5141(7075); Diss.

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Thesis/Dissertation

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AbstractAbstract

[en] Steady-state Ginzburg-Landau equations with arbitrary parameter λ > 0 in two-dimensional Euclidean space were studied. The existence of finite-energy symmetric and nonsymmetric solutions with arbitrary vortex number is demonstrated by direct method of the calculus of variations via a Sobolev space argument. Moreover, the important properties of symmetric solutions are determined, and it is proved that as λ → ∞, nonlinear desingularization occurs for the symmetric solutions

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1985; 102 p; University Microfilms Order No. 86-02,620; Thesis (Ph. D.).

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AbstractAbstract

[en] We have characterized the parameters which control the stability of amorphous phases under ball milling on the model alloys (Ni

_{x}Zr_{y}). We have improved a vibrating frame grinder to perform experiments at several controlled temperatures under static vacuum. Based on analyzing the movement of the milling ball, we have evaluated the frequency of impact (f) and the relative velocity of the ball at the time impact (V_{m}ax) for different vibration amplitudes of the frame. The kinetic of amorphization by ball milling, studied by x-ray diffraction and transmission electron microscopy, shows that: - a fully amorphous phase is obtained provided that the 'specific milling intensity' is larger than a threshold value. The specific milling intensity is defined as the impact momentum times the impact frequency divided by the mass of powder (M_{b}V_{m}axf/M_{p}): M_{p}, M_{b}are the mass of the ball and the powder). The threshold is composition and temperature dependent; - below the amorphization threshold, a two-phase microstructure (crystalline phase imbedded in an amorphous cement) is stabilized. The alloy achieves a steady state fraction of amorphous phase, which increases with the milling intensity and decreases with increasing the milling temperature. A study of the mechanical alloying of NbTiAl_{2}starting from elemental powders is also presented. (Author). refs., figs., tabsOriginal Title

Contribution a la physique du procede de mecanosynthese

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Oct 1992; 151 p; These (D. es Sc.).

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AbstractAbstract

[en] With the launching of the Next Linear Collider coming closer and closer, there is a pressing need for physicists to develop a fully-integrated computer simulation of e

^{+}e^{-}annihilation process at center-of-mass energy of 1TeV. A simulation program acts as the template for future experiments. Either new physics will be discovered, or current theoretical uncertainties will shrink due to more accurate higher-order radiative correction calculations. The existence of an efficient and accurate simulation will help us understand the new data and validate (or veto) some of the theoretical models developed to explain new physics. It should handle well interfaces between different sectors of physics, e.g., interactions happening at parton levels well above the QCD scale which are described by perturbative QCD, and interactions happening at much lower energy scale, which combine partons into hadrons. Also it should achieve competitive speed in real time when the complexity of the simulation increases. This thesis contributes some tools that will be useful for the development of such simulation programs. We begin our study by the development of a new Monte Carlo algorithm intended to perform efficiently in selecting weight-1 events when multiple parameter dimensions are strongly correlated. The algorithm first seeks to model the peaks of the distribution by features, adapting these features to the function using the EM algorithm. The representation of the distribution provided by these features is then improved using the VEGAS algorithm for the Monte Carlo integration. The two strategies mesh neatly into an effective multi-channel adaptive representation. We then present a new algorithm for the simulation of parton shower processes in high energy QCD. We want to find an algorithm which is free of negative weights, produces its output as a set of exclusive events, and whose total rate exactly matches the full Feynman amplitude calculation. Our strategy is to create the whole QCD shower as a tree structure generated by a multiple Poisson process. Working with the whole shower allows us to include correlations between gluon emissions from different sources. QCD destructive interference is controlled by the implementation of ''angular-ordering,'' as in the HERWIG Monte Carlo program. We discuss methods for systematic improvement of the approach to include higher order QCD effectsPrimary Subject

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2 Oct 2003; 12 p; AC03-76SF00515; Available from PURL: https://www.osti.gov/servlets/purl/826474-PJHK2L/native/

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[en] We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the process on the nucleon to the generalized parton distributions which also enter in other wide angle scattering processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2 photon correction, does reproduce the Rosenbluth data

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4 Mar 2004; [vp.]; AC03-76SF00515; Available from PURL: https://www.osti.gov/servlets/purl/826794-0MQc6Y/native/

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[en] First-principles total energy calculations have been performed for transition metal Zr in the ω, bcc, hcp, fcc structures and for Ba in the bcc, fcc structures. From our calculations we obtained equilibrium lattice constants, cohesive energies, bulk moduli and energy difference between the bcc and the hcp phases which are in good agreement with experiment. The total energy of ω phase was found to be very close to that of hcp phase in Zr. Recent neutron scattering experiments on a single crystal of bcc Ba have revealed that the longitudinal branch along the [100] direction is lower than the transverse branch. This anomalous behavior has been studied with use of the frozen-phonon method and is shown to arise from d-electron interactions. For the pressure induced bcc-hcp transition in Ba, the volume dependence of the complete energy surface from the bcc to the hcp structure has been determined as a function of the atomic displacements corresponding to the T

_{1}N-point phonon mode and a long wavelength shear. Results from frozen phonon calculations for Zr indicate an instability toward the hcp phase at t = 0 K in the bcc-hcp transition and determine the third and fourth order anharmonic coupling strengths which were used to evaluate the temperature dependence of selected frequencies. 135 refs., 29 figs., 13 tabsSecondary Subject

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Nov 1986; 112 p; Available from NTIS, PC A06/MF A01 as DE87003050; Thesis.

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Thesis/Dissertation; Numerical Data

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AbstractAbstract

No abstract available

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Oct 1971; 61 p

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AbstractAbstract

[en] The Diagnostic X (D-X) beamlines will transport the DARHT-II beam from the end of the accelerator to the Diagnostic X firing point providing four lines of sight for x-ray radiography. The design goal for the Diagnostic X beamline is to deliver four x-ray pulses with the DARHT-II dose format and time integrated spot size on each line of sight. The D-X beamline's final focus should be compatible with a range of first conjugates from 1 m-5 m. Furthermore, the D-X beamline operational parameters and the beamline layout should not preclude a possible upgrade to additional lines of sight. The DARHT-II accelerator is designed to deliver beams at a rate of 1 pulse per minute or less. Tuning the D-X beamline with several hundred optical elements would be time consuming. Therefore, minimizing the required number of tuning shots for the D-X beamline is also an important design goal. Many different beamline configurations may be able to accomplish these design objectives, and high beam quality (i.e., high current and low emittance) must be maintained throughout the chosen beamline configuration in order to achieve the DARHT-II x-ray dose format. In general, the longer the distance a beam travels, the harder it is to preserve the beam quality. Therefore, from the point of view of maintaining beam quality, it is highly desirable to minimize the beamline length. Lastly, modification to the DARHT-II building and the downstream transport should be minimized. Several processes can degrade beam quality by increasing the beam emittance, increasing the time-varying transverse beam motion, creating a beam halo, or creating a time-varying beam envelope. In this report, we consider those processes in the passive magnet lattice beamline and indicate how they constrain the beamline design. The physics design considerations for the active components such as the kicker system will be discussed in Ref. 2. In Sec. I, we discuss how beam emittance affects the x-ray forward dose. We also establish a physics design goal for the emittance growth budget. In Sec. II, we discuss how the conductivity and size of the beam pipe affects the transverse beam motion. We also discuss the emittance growth arise from the beam centroid offset. In Sec. III, we discuss the background gas focusing effects and establish the vacuum requirements. In Sec. IV, we consider the emittance growth in a bend. In Sec. V, we discuss the misalignment and corkscrew motion. The design specifications for misalignment are established. In Secs. VI and VII, we discuss the design objectives on how to extract beams from the DARHT-II beamline and how to minimize the tuning shots. The integrated spot size and final focusing are discussed in Sec. VIII. A conclusion will be presented in Sec. IX

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10 Apr 2000; 25 p; W--7405-ENG-48; Available from PURL: https://www.osti.gov/servlets/purl/15013579-biKTym/native/

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