20 Matching Results

Search Results

Advanced search parameters have been applied.

Axions from cosmic string and wall decay

Description: If inflation occurred with a reheat temperature > T{sub PQ}, axions from the decay of global axion strings and domain walls would make an important contribution to the cosmological energy density, comparable to that from vacuum misalignment. Several groups have numerically studied the evolution of axion strings and walls in the past, however substantial uncertainties remain in their contribution to the present density {Omega}{sub a,string+wall} {approx} 1-100 (f{sub a}/10{sup 12} GeV){sup 7/6}, where f{sub a} is the axion decay constant. I will describe the numerical methods used in our simulations and show results for several string and wall configurations.
Date: March 10, 2010
Creator: Hagmann, C A
Partner: UNT Libraries Government Documents Department

Two-Phase Emission Detector for Measuring Coherent Neutrino-Nucleus Scattering

Description: Coherent scattering is a flavor-blind, high-rate, as yet undetected neutrino interaction predicted by the Standard Model. We propose to use a compact (kg-scale), two-phase (liquid-gas) argon ionization detector to measure coherent neutrino scattering off nuclei. In our approach, neutrino-induced nuclear recoils in the liquid produce a weak ionization signal, which is transported into a gas under the influence of an electric field, amplified via electroluminescence, and detected by phototubes or avalanche diodes. This paper describes the features of the detector, and estimates signal and background rates for a reactor neutrino source. Relatively compact detectors of this type, capable of detecting coherent scattering, offer a new approach to flavor-blind detection of man-made and astronomical neutrinos, and may allow development of compact neutrino detectors capable of nonintrusive real-time monitoring of fissile material in reactors.
Date: November 26, 2003
Creator: Bernstein, A & Hagmann, C A
Partner: UNT Libraries Government Documents Department

Verification and Validation of MERCURY: A Modern, Monte Carlo Particle Transport Code

Description: Verification and Validation (V&V) is a critical phase in the development cycle of any scientific code. The aim of the V&V process is to determine whether or not the code fulfills and complies with the requirements that were defined prior to the start of the development process. While code V&V can take many forms, this paper concentrates on validation of the results obtained from a modern code against those produced by a validated, legacy code. In particular, the neutron transport capabilities of the modern Monte Carlo code MERCURY are validated against those in the legacy Monte Carlo code TART. The results from each code are compared for a series of basic transport and criticality calculations which are designed to check a variety of code modules. These include the definition of the problem geometry, particle tracking, collisional kinematics, sampling of secondary particle distributions, and nuclear data. The metrics that form the basis for comparison of the codes include both integral quantities and particle spectra. The use of integral results, such as eigenvalues obtained from criticality calculations, is shown to be necessary, but not sufficient, for a comprehensive validation of the code. This process has uncovered problems in both the transport code and the nuclear data processing codes which have since been rectified.
Date: December 9, 2004
Creator: Procassini, R J; Cullen, D E; Greenman, G M & Hagmann, C A
Partner: UNT Libraries Government Documents Department

Monte Carlo Simulation of Proton-induced Cosimc Ray Cascades in the Atmosphere

Description: We have developed a Monte Carlo model of the Earth's atmosphere and implemented it in three different codes (GEANT4, MCNPX, and FLUKA). Primary protons in the energy range of 1 GeV-100 TeV are injected at the top of the atmosphere. The codes follow the tracks of all relevant secondary particles (neutrons, muons, gammas, electrons, and pions) and tally their fluxes at selectable altitudes. Comparisons with cosmic ray data at sea level show good agreement.
Date: March 14, 2007
Creator: Hagmann, C A; Lange, D J & Wright, D M
Partner: UNT Libraries Government Documents Department

FREYA-a new Monte Carlo code for improved modeling of fission chains

Description: A new simulation capability for modeling of individual fission events and chains and the transport of fission products in materials is presented. FREYA ( Fission Yield Event Yield Algorithm ) is a Monte Carlo code for generating fission events providing correlated kinematic information for prompt neutrons, gammas, and fragments. As a standalone code, FREYA calculates quantities such as multiplicity-energy, angular, and gamma-neutron energy sharing correlations. To study materials with multiplication, shielding effects, and detectors, we have integrated FREYA into the general purpose Monte Carlo code MCNP. This new tool will allow more accurate modeling of detector responses including correlations and the development of SNM detectors with increased sensitivity.
Date: June 12, 2012
Creator: Hagmann, C A; Randrup, J & Vogt, R L
Partner: UNT Libraries Government Documents Department

16-APR-03 Final Release of ENDF/B-V for use with LLNL Codes

Description: The new data files were prepared in two steps. First, the ENDF/B-V database was translated to an ENDL-format ascii database. The ENDL ascii format is a point-wise tabular storage scheme where intermediate values are extracted via interpolation. Sufficient point-wise information was generated in the translation to insure an extraction tolerance of 0.1% for most of the data. The only exception is along the incident neutron energy axis of the outgoing particle energy probability density function where a 0.5% tolerance was maintained. Second, processed files were generated from the translated database. Since the translated ENDF/B-V data is in ENDL-format, the standard processing codes were used to generate the new processed data files. To the best of our knowledge, these processed data files are accurate representations of the ENDF/B-V database to within the stated tolerances. However, there are several issues that users must be aware of and they are listed in this report.
Date: April 16, 2003
Creator: Hill, T S; McNabb, D P; Hedstrom, G W; Beck, B & Hagmann, C A
Partner: UNT Libraries Government Documents Department

MCAPM-C Generator and Collision Routine Documentation

Description: This report documents the C version of the MCAPM (Monte Carlo All Particle Method) generator and collision physics library of subroutines. The Monte Carlo data generator (gen2000) reads cross sections and distributions that describe in-flight reactions from a binary library and creates an internal data library. The collision routines (bang2000) use this internal library to perform the physics of interaction of particles with the background material. Particles modeled with MCAPM-C are neutrons, charged particles (p, d, t, {sup 3}He, and {alpha}), and {gamma}'s. MCAPM-C is written in (nearly) standard C, and versions exist for Sun Solaris, Compaq Unix, IBM AIX, SGI Irix, and Linux. The library and its data files are installed on LC's Compass, TC, Forest, Blue, and Sky machines. This report describes the contents and format of the library, physics assumptions made, and the interface to the library's subroutines.
Date: November 22, 2000
Creator: Brantley, P S; Hagmann, C A & Rathkopf, J A
Partner: UNT Libraries Government Documents Department

Comparison of Ramsauer and Optical Model Neutron Angular Distributions

Description: In a recent paper it has been shown that the nuclear Ramsauer model does not do well in representing details of the angular distribution of neutron elastic scattering for incident energies of less than 60 MeV for {sup 208}Pb. We show that the default angular bin dispersion most widely used in Monte Carlo transport codes is such that the observed differences in angular shapes are on too fine a scale to affect transport calculations. The effect of increasing the number of Monte Carlo angle bins is studied to determine the dispersion necessary for calculations to be sensitive to the observed discrepancies in angular distributions. We also show that transport calculations are sensitive to differences in the elastic scattering cross section given by recent fits of {sup 208}Pb data compared with older fits.
Date: April 20, 2004
Creator: McNabb, D P; Anderson, J D; Bauer, R W; Dietrich, F S; Grimes, S M & Hagmann, C A
Partner: UNT Libraries Government Documents Department

Comparison of Ramsauer and Optical Model Neutron Angular Distributions

Description: The nuclear Ramsauer model is a semi-classical, analytic approximation to nucleon-nucleus scattering that reproduces total cross section data at the 1% level for A > 40, E{sub n} = 5-60 MeV with 7-10 parameters. A quick overview of the model is given, demonstrating the model's utility in nuclear data evaluation. The Ramsauer model predictions for reaction cross section, elastic cross section, and elastic scattering angular distributions are considered. In a recent paper it has been shown that the nuclear Ramsauer model does not do well in predicting details of the angular distribution of neutron elastic scattering for incident energies of less than 60 MeV for {sup 208}Pb. However, in this contribution it is demonstrated that the default angular bin dispersion most widely used in Monte Carlo transport codes is such that the observed differences in angular shapes are on too fine a scale to affect transport calculations. Simple studies indicate that 512-2048 bins are necessary to achieve the dispersion required for calculations to be sensitive to the observed discrepancies in angular distributions.
Date: September 30, 2004
Creator: McNabb, D P; Anderson, J D; Bauer, R W; Dietrich, F S; Grimes, S M & Hagmann, C A
Partner: UNT Libraries Government Documents Department

New Capabilities in Mercury: A Modern, Monte Carlo Particle Transport Code

Description: The new physics, algorithmic and computer science capabilities of the Mercury general-purpose Monte Carlo particle transport code are discussed. The new physics and algorithmic features include in-line energy deposition and isotopic depletion, significant enhancements to the tally and source capabilities, diagnostic ray-traced particles, support for multi-region hybrid (mesh and combinatorial geometry) systems, and a probability of initiation method. Computer science enhancements include a second method of dynamically load-balancing parallel calculations, improved methods for visualizing 3-D combinatorial geometries and initial implementation of an in-line visualization capabilities.
Date: March 8, 2007
Creator: Procassini, R J; Cullen, D E; Greenman, G M; Hagmann, C A; Kramer, K J; McKinley, M S et al.
Partner: UNT Libraries Government Documents Department

A Proposal for First-Ever Measurement of Coherent Neutrino-Nucleus Scattering

Description: We propose to build and deploy a 10-kg dual-phase argon ionization detector for the detection of coherent neutrino-nucleus scattering, which is described by the reaction; {nu} + (Z,N) {yields} {nu} + (Z,N), where {nu} is the scattering neutrino, and (Z,N) is the target nucleus of atomic number Z and neutron number N. Its detection would validate central tenets of the Standard Model. We have built a gas-phase argon ionization detector to determine the feasibility of measuring the small recoil energies ({approx} 1keV) predicted from coherent neutrino scattering, and to characterize the recoil spectrum of the argon nuclei induced by scattering from medium-energy neutrons. We present calibrations made with 55-Fe, a low-energy X-ray source, and report on measurements to date of the recoil spectra from the 2-MeV LINAC Li-target neutron source at LLNL. A high signal-to-noise measurement of the recoil spectrum will not only serve as an important milestone in achieving the sensitivity necessary for measuring coherent neutrino-nucleus scattering, but will break new scientific ground on its own.
Date: February 5, 2008
Creator: Winant, C D; Bernstein, A; Foxe, M P; Hagmann, C A; Jovanovic, I; Kazkaz, K M et al.
Partner: UNT Libraries Government Documents Department

Update on the Development and Validation of MERCURY: A Modern, Monte Carlo Particle Transport Code

Description: An update on the development and validation of the MERCURY Monte Carlo particle transport code is presented. MERCURY is a modern, parallel, general-purpose Monte Carlo code being developed at the Lawrence Livermore National Laboratory. During the past year, several major algorithm enhancements have been completed. These include the addition of particle trackers for 3-D combinatorial geometry (CG), 1-D radial meshes, 2-D quadrilateral unstructured meshes, as well as a feature known as templates for defining recursive, repeated structures in CG. New physics capabilities include an elastic-scattering neutron thermalization model, support for continuous energy cross sections and S ({alpha}, {beta}) molecular bound scattering. Each of these new physics features has been validated through code-to-code comparisons with another Monte Carlo transport code. Several important computer science features have been developed, including an extensible input-parameter parser based upon the XML data description language, and a dynamic load-balance methodology for efficient parallel calculations. This paper discusses the recent work in each of these areas, and describes a plan for future extensions that are required to meet the needs of our ever expanding user base.
Date: June 6, 2005
Creator: Procassini, R J; Taylor, J M; McKinley, M S; Greenman, G M; Cullen, D E; O'Brien, M J et al.
Partner: UNT Libraries Government Documents Department

GAMMA-RAY COMPTON LIGHT SOURCE DEVELOPMENT AT LLNL

Description: A new class of tunable, monochromatic {gamma}-ray sources capable of operating at high peak and average brightness is currently being developed at LLNL for nuclear photoscience and applications. These novel systems are based on Compton scattering of laser photons by a high brightness relativistic electron beam produced by an rf photoinjector. A prototype, capable of producing > 10{sup 8} 0.7 MeV photons in a single shot, with a fractional bandwidth of 1%, and a repetition rate of 10 Hz, is currently under construction at LLNL; this system will be used to perform nuclear resonance fluorescence experiments. A new symmetrized S-band rf gun, using a Mg photocathode, will produce up to 1 nC of charge in an 8 ps bunch, with a normalized emittance modeled at 0.8 mm.mrad; electrons are subsequently accelerated up to 120 MeV to interact with a 500 mJ, 10 ps, 355 nm laser pulse and generate {gamma}-rays. The laser front end is a fiber-based system, using corrugated-fiber Bragg gratings for stretching, and drives both the frequency-quadrupled photocathode illumination laser and the Nd:YAG interaction laser. Two new technologies are used in the laser: a hyper-Michelson temporal pulse stacker capable of producing 8 ps square UV pulses, and a hyper-dispersion compressor for the interaction laser. Other key technologies, basic scaling laws, and recent experimental results will also be presented, along with an overview of future research and development directions.
Date: August 15, 2007
Creator: Hartemann, F V; Anderson, S G; Gibson, D J; Hagmann, C A; Johnson, M S; Jovanovic, I et al.
Partner: UNT Libraries Government Documents Department

Techniques and use of a tunable, laser-based, MeV-Class Compton scattering light source

Description: A Compton scattering {gamma}-ray source, capable of producing photons with energies ranging from 0.1 MeV to 0.9 MeV has been commissioned and characterized, and then used to perform nuclear resonance fluorescence (NRF) experiments. The key source parameters are the size (0.01 mm{sup 2}), horizontal and vertical divergence (6 x 10 mrad{sup 2}), duration (10 ps), spectrum and intensity (10{sup 5} photons/shot). These parameters are summarized by the peak brightness, 1.5 x 10{sup 15} photons/mm{sup 2}/mrad{sup 2}/s/0.1%bandwidth, measured at 478 keV. Additional measurements of the flux as a function of the timing difference between the drive laser pulse and the relativistic photoelectron bunch, {gamma}-ray beam profile, and background evaluations are presented. These results are systematically compared to theoretical models and computer simulations. NRF measurements performed on {sup 7}Li in LiH demonstrate the potential of Compton scattering photon sources to accurately detect isotopes in situ.
Date: June 30, 2009
Creator: Albert, F; Anderson, S G; Gibson, D J; Hagmann, C A; Johnson, M S; Messerly, M et al.
Partner: UNT Libraries Government Documents Department

Large-scale search for dark-matter axions

Description: Early results from a large-scale search for dark matter axions are presented. In this experiment, axions constituting our dark-matter halo may be resonantly converted to monochromatic microwave photons in a high-Q microwave cavity permeated by a strong magnetic field. Sensitivity at the level of one important axion model (KSVZ) has been demonstrated.
Date: January 1, 1998
Creator: Hagmann, C. A.; Kinion, D.; Stoeffl, W.; Van Bibber, K.; Daw, E. J.; McBride, J. et al.
Partner: UNT Libraries Government Documents Department