45 Matching Results

Search Results

Advanced search parameters have been applied.
in creator/contributor: "McNabb, D."  
Results: 1 - 24 next

Comparison of Techniques to Reduce Bremsstrahlung Background Radiation from Monoenergetic Photon Beams

Description: An important applied technology is a tunable mono-energetic photon source [1]. These sources are made of relativistic electron accelerators coupled to low-energy lasers, which produce high-energy, mono-energetic-rays. One challenge associated with systems such as this is a continuum of bremsstrahlung background created when an electron beam passes through an aperture of some sort and the electron bunch or its halo impinges on the aperture pictured in figure 1. For instance, in the current T-REX [1] design for the interaction point between the laser- and electron-beam, the electron-beam passes through the center of a mirror used to reflect the laser. There is a potential with this design that bremsstrahlung radiation may be produced at the edges of the mirror openings and contaminate the mono-energetic photon beam. Certain applications [2] may be sensitive to this contamination. To reduce the bremsstrahlung contaminate a collimator (thickness {approx}24in. (calculated from XCOM database [3]) to attenuate by a factor of 10{sup -3} the 112MeV photons expected in the T-REX demonstration [1]) is situated between the aperture and target. To maximize the brightness of the photon-beam, the collimator opening must be no less than the size of the photon-beam spot size expected to be about 1mm. This fixes the collimator opening. a priori the aperture size must be greater than the collimator opening and is a function distance between the aperture and collimator. In this paper we focus on two approaches to estimate the aperture size, given a collimator and a target whose sizes and distances from the aperture are given. In the next section we will discuss these approaches.
Date: June 29, 2006
Creator: Johnson, M & McNabb, D
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

T-REX Design Considerations for Detection of Concealed 238U

Description: Here they outline considerations that might inform choices for the design of a laser/linac-based light source used to detect {sup 238}U via excitation of the resonance at 680.11 keV in this isotope. They assume that the principal concern is speed of interrogation and not, e.g., how much radiological dose is imparted during a scan. It is found that if the photon detectors used in the system have an energy resolution better than or comparable to that of the interrogation beam, then to a first approximation the light source should be designed to have the highest possible specific fluence (photons per unit energy per unit time). there is also a weak dependence of scan time on the number of photons emitted per pulse of the light source. A simple formula describing the tradeoff between specific fluence and number of photons per pulse is presented.
Date: February 7, 2006
Creator: Pruet, J & McNabb, D P
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

The prediction of Neutron Elastic Scattering from Tritium for E(n) = 6-14 MeV

Description: In a recent report Navratil et al. evaluated the angle-integrated cross section and the angular distribution for 14-MeV n+T elastic scattering by inferring these cross sections from accurately measured p+3He angular distributions. This evaluation used a combination of two theoretical treatments, based on the no-core shell model and resonating-group method (NCSM/RGM) and on the R-matrix formalism, to connect the two charge-symmetric reactions n+T and p+{sup 3}He. In this report we extend this treatment to cover the neutron incident energy range 6-14 MeV. To do this, we evaluate angle-dependent correction factors for the NCSM/RGM calculations so that they agree with the p+{sup 3}He data near 6 MeV, and using the results found earlier near 14 MeV we interpolate these correction factors to obtain correction factors throughout the 6-14 MeV energy range. The agreement between the corrected NCSM/RGM and R-Matrix values for the integral elastic cross sections is excellent ({+-}1%), and these are in very good agreement with total cross section experiments. This result can be attributed to the nearly constant correction factors at forward angles, and to the evidently satisfactory physics content of the two calculations. The difference in angular shape, obtained by comparing values of the scattering probability distribution P({mu}) vs. {mu}(the cosine of the c.m. scattering angle), is about {+-}4% and appears to be related to differences in the two theoretical calculations. Averaging the calculations yields P({mu}) values with errors of {+-}2 1/2 % or less. These averaged values, along with the corresponding quantities for the differential cross sections, will form the basis of a new evaluation of n+T elastic scattering. Computer files of the results discussed in this report will be supplied upon request.
Date: June 14, 2010
Creator: Anderson, J D; Dietrich, F S; Luu, T; McNabb, D P; Navratil, P & Quaglioni, S
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Preliminary Study of the Efficacy of Using Nuclear Resonance Fluorescence with Quasi-Monoenergetic Gamma-Ray Sources for Nuclear Safeguards Assay

Description: We have studied the efficacy of using nuclear resonance fluorescence (NRF)-based techniques to assay spent nuclear fuel for Pu content using quasi-monoenergetic sources. We have developed two techniques to precisely determine the Pu content in a fuel rod/pin. One of our approaches is virtually free of systematic uncertainties. Using analytical models, we have determined the amount of time required to measure the Pu content in spent nuclear fuel rods and spent fuel assemblies to within 1% precision. We note that Pu content can be determined in a fuel assembly about as fast as in a single fuel pin. The performance of NRF-based assay techniques with improved photon sources, which are currently under development, will also estimated. For follow-on research we propose to: (1) Construct research prototype detection systems for both of the NRF-based assay systems proposed in this paper and measure their calibration curves; (2) Determine the systematic errors associated with both assay methods, explore ways to reduce the errors and fold the results into future performance calculations; (3) Develop an algorithm to assay a fuel assembly; (4) Perform validation measurements using a single pin and scaled assemblies; (5) Research and develop current-mode detection and/or threshold detection techniques to improve assay times; (6) Characterize the flux of newly constructed sources and fold the results into the calculations presented here to determine the feasibility of a variety of proposed sources; and (7) Collaborate with others in the safeguards community to build a prototype system and perform an NRF-based assay demonstration on spent fuel.
Date: February 17, 2011
Creator: Johnson, M S; McNabb, D P; Hall, J M & Gonzalez, J J
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Alternative Approach to Nuclear Data Representation: Building the infrastructure to support QMU and next-generation simulations

Description: The nuclear data infrastructure currently relies on punch-card era formats designed some five decades ago. Though this system has worked well, recent interest in non-traditional and complicated physics processes has demanded a change. Here we present an alternative approach under development at LLNL. In this approach data is described through collections of distinct and self-contained simple data structures. This structure-based format is compared with traditional ENDF and ENDL, which can roughly be characterized as dictionary-based representations.
Date: January 17, 2006
Creator: Pruet, J; Brown, D A; Beck, B & McNabb, D P
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Final Task Report on NRF Measurements of Photon Scattering Resonances in Plutonium at the High Voltage Research Laboratory of MIT

Description: Conclusions of this report are that there are 13 new transitions associated with NRF states in {sup 239}Pu that have been discovered. These resonances are between 2- and 2.5-MeV relative to the ground state in {sup 239}Pu. The strengths of most these resonances are between 15 and 20 eV b. This approximately is the strength required for using the transmission detection method for NRF [Pruet et al J. Appl. Phys. 99 12310 (2006)] for six-sigma alarm confidence levels. Future measurements on {sup 239}Pu at higher photon energies are necessary to probe for NRF strengths at higher energies. Such resonances may be more advantageous to the NRF technique with FINDER if they are stronger or if the mean free path of these energetic photons is longer.
Date: February 23, 2007
Creator: Johnson, M S; McNabb, D P & Norman, E B
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Alternative Approach to Nuclear Data Representation

Description: This paper considers an approach for representing nuclear data that is qualitatively different from the approach currently adopted by the nuclear science community. Specifically, they examine a representation in which complicated data is described through collections of distinct and self contained simple data structures. This structure-based representation is compared with the ENDF and ENDL formats, which can be roughly characterized as dictionary-based representations. A pilot data representation for replacing the format currently used at LLNL is presented. Examples are given as is a discussion of promises and shortcomings associated with moving from traditional dictionary-based formats to a structure-rich or class-like representation.
Date: July 27, 2005
Creator: Pruet, J; Brown, D; Beck, B & McNabb, D P
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Calculation of 239Pu(n,2n) Cross Section by the Subtraction and the Ratio Methods

Description: The {sup 239}Pu(n,2n) and the {sup 235}U(n,2n) cross section are estimated by applying unitarity in several approaches: a subtraction method and also by using a ratio approach that relates the above cross sections to the {sup 238}U(n,2n) cross section and the {sup 239}Pu(n,2n) cross section to the {sup 235}U(n,2n) cross section, respectively. Also, a self-consistent, simultaneous analysis of the cross section data of four nuclei, {sup 239}Pu, {sup 235}U, {sup 238}U and {sup 232}Th, was undertaken to evaluate the {sup 239}Pu(n,2n) cross section at 11 MeV.
Date: September 28, 2000
Creator: Mavratil, P. & McNabb, D.P.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Level Density and Strength Function from Quasicontinuous Decay of Superdeformed Excitations in Lead Nuclei

Description: The shape of the spectrum associated with the quasicontinuous (QC) decay of superdeformed rotational bands in even- and odd-mass Pb isotopes is sensitive to the gap in level density at finite temperature and angular momentum at normal deformations. This gap in level density was deduced to be {approx}0.95 MeV at 6{Dirac_h} for {sup 194}Pb and {approx}0.4 MeV at 10{Dirac_h} for {sup 192}Pb, while the shape of the QC spectrum for {sup 195}Pb is consistent with no gap in the level density at about 11{Dirac_h}.
Date: October 4, 2002
Creator: Cizewski, J A; Johnson, M & McNabb, D P
Item Type: Refine your search to only Report
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
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Thermal Neutron Capture Cross Sections of The Palladium Isotopes

Description: We have measured precise thermal neutron capture {gamma}-ray cross sections {sigma}{sub {gamma}} for all stable Palladium isotopes with the guided thermal neutron beam from the Budapest Reactor. The data were compared with other data from the literature and have been evaluated into the Evaluated Gamma-ray Activation File (EGAF)[1]. Total radiative neutron capture cross-sections {sigma}{sub 0} can be deduced from the sum of transition cross sections feeding the ground state of each isotope if the decay scheme is complete. The Palladium isotope decay schemes are incomplete, although transitions deexciting low-lying levels are known for each isotope. We have performed Monte Carlo simulations of the Palladium thermal neutron capture de-excitation schemes using the computer code DICEBOX [2]. This program generates a level scheme where levels below a critical energy E{sub crit} are taken from experiment, and those above E{sub crit} are calculated by a random discretization of an a priori known level density formula {rho}(E, J{sup {pi}}). Level de-excitation branching intensities are taken from experiment for levels below E{sub crit} and the capture state, or calculated for levels above E{sub crit} assuming an a priori photon strength function and applying allowed selection rules and a Porter-Thomas distribution of widths. The calculated feeding to levels below E{sub crit} can then be normalized to the measured cross section deexciting those levels to determine the total radiative neutron cross-section {sigma}{sub 0}. In this paper we have measured {sigma}{sub 0}[{sup 102}Pd(n,{gamma})] = 0.9 {+-} 0.3 b, {sigma}{sub 0}[{sup 104}Pd(n,{gamma})] = 0.61 {+-} 0.11 b, {sigma}{sub 0}[{sup 105}Pd(n,{gamma})] = 21.1 {+-} 1.5 b, {sigma}{sub 0}[{sup 106}Pd(n,{gamma})] = 0.36 {+-} 0.05 b, {sigma}{sub 0}[{sup 108}Pd(n,{gamma})(0)] = 7.6 {+-} 0.6 b, {sigma}{sub 0}[{sup 108}Pd(n,{gamma})(189)] = 0.185 {+-} 0.011 b, and {sigma}{sub 0}[{sup 110}Pd(n,{gamma})] = 0.10 {+-} 0.03 b. We have also determined from our statistical calculations that the neutron ...
Date: August 30, 2005
Creator: Firestone, R B; Krticka, M; McNabb, D P; Sleaford, B; Agvaanluvsan, U; Belgya, T et al.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Modeling of High Precision Neutron Nonelastic Cross Sections

Description: A new method has been applied to the determination of neutron nonelastic cross sections for iron {sup 56}Fe and lead {sup 208}Pb for energies between 5 and 26 MeV. These data have estimated errors of only a few percent and do not suffer from the ambiguities encountered in earlier nonelastic data. We attempt to fit these high precision data using both a semiclassical single phase shift model (nuclear Ramsauer model) as well as a recent global optical model that well reproduces a wide body of neutron scattering observables. At the 5% uncertainty level, both models produce satisfactory fits. However, neither model gives satisfactory fits to these new precise data. We conclude that fitting precise data, i.e., data with errors of approximately 2% or less, may require a nuclear mass dependence of radii that reflects structure effects such as shell closures.
Date: February 5, 2007
Creator: Dietrich, F S; Anderson, J D; Bauer, R W; Grimes, S M & McNabb, D P
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Thermal Neutron Capture Cross Sections of the PalladiumIsotopes

Description: Precise gamma-ray thermal neutron capture cross sectionshave been measured at the Budapest Reactor for all elements withZ=1-83,92 except for He and Pm. These measurements and additional datafrom the literature been compiled to generate the Evaluated Gamma-rayActivation File (EGAF), which is disseminated by LBNL and the IAEA. Thesedata are nearly complete for most isotopes with Z<20 so the totalradiative thermal neutron capture cross sections can be determineddirectly from the decay scheme. For light isotopes agreement with therecommended values is generally satisfactory although large discrepanciesexist for 11B, 12,13C, 15N, 28,30Si, 34S, 37Cl, and 40,41K. Neutroncapture decay data for heavier isotopes are typically incomplete due tothe contribution of unresolved continuum transitions so only partialradiative thermal neutron capture cross sections can be determined. Thecontribution of the continuum to theneutron capture decay scheme arisesfrom a large number of unresolved levels and transitions and can becalculated by assuming that the fluctuations in level densities andtransition probabilities are statistical. We have calculated thecontinuum contribution to neutron capture decay for the palladiumisotopes with the Monte Carlo code DICEBOX. These calculations werenormalized to the experimental cross sections deexciting low excitationlevels to determine the total radiative thermal neutron capture crosssection. The resulting palladium cross sections values were determinedwith a precision comparable to the recommended values even when only onegamma-ray cross section was measured. The calculated and experimentallevel feedings could also be compared to determine spin and parityassignments for low-lying levels.
Date: July 17, 2006
Creator: Firestone, R.B.; Krticka, M.; McNabb, D.P.; Sleaford, B.; Agvaanluvsan, U.; Belgya, T. et al.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Applications of Photonuclear Physics for International Safeguards and Security

Description: Studies of nuclear resonance fluorescence based applications are presented. Important for these applications are data for isotopes such as {sup 239}Pu. Nuclear resonance fluorescence measurements of {sup 239}Pu were performed at the free electron laser facility at UC Santa Barbara using photons from a bremsstrahlung beam with an endpoint energies between 4.0 MeV and 5.5 MeV. Though no discrete states with significant confidence level were measured, we have excluded the region above 27(3) eV-barns, or 4-sigma, where we would expect only a small chance of false positives. Details of the measurements and the results are presented here.
Date: April 16, 2010
Creator: Johnson, M S; Hall, J M; McNabb, D P; McFarland, J; Norman, E; Bertozzi, W et al.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements

Description: The {sup 239}Pu(n,2n{gamma}){sup 238}Pu partial reaction cross-section, {sigma}{sub (n,2n{gamma})}, has been measured as a function of neutron energy for several transitions in {sup 238}Pu. Partial {gamma}-ray cross sections for yrast, ''collector'' transitions, can provide especially valuable constraints on the magnitude and shape of the total (n,2n) reaction cross-section. In essence, nuclear reaction models will be used to infer the shape and magnitude of the total (n,2n) reaction cross-section from the measured partial {gamma}-ray cross-sections. The reason for undertaking this somewhat indirect approach is that previous measurements of the {sup 239}Pu(n,2n{gamma}) have been hampered by a variety of constraints. Activation measurements have several hurdles: (1) intense flux and long counting times are required to overcome the relatively long half-life of {sup 238}Pu (87 years) and (2) isotopically pure samples of {sup 239}Pu in an environment free of {sup 238}Pu contamination are difficult to come by. Neutron counting experiments are subject to significant uncertainties because (1) large background statistics from fission neutrons and (2) the experimental fission neutron multiplicity spectrum is subject to systematic errors because the flux of low-energy neutrons which induce fissions in thermally-fissile {sup 239}Pu is very difficult to characterize. In this measurement, spallation neutrons are provided by the LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. Neutron flux is monitored in-beam with one {sup 235}U fission chamber and one {sup 238}U fission chamber. The {sup 238}U is not sensitive to background from low-energy neutrons, whereas the {sup 235}U fission chamber has better statistics. Hence, in essence the partial {gamma}-ray cross sections are normalized to the evaluated fission cross sections of {sup 235}U and {sup 238}U. As a check of our normalization to provide additional constraints to the nuclear reaction modeling, benchmark measurements of {sup nat}Fe(n, n{prime}{gamma}) and {sup 235}U(n,2n{gamma}) have also been undertaken. The secondary {gamma}-rays ...
Date: May 1, 2000
Creator: McNabb, D.P.; Archer, D.E.; Becker, J.A.; Bernstein, L.A. & Garrett, P.E.
Item Type: Refine your search to only Report
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 &gt; 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.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Fiber-Based, Spatially and Temporally Shaped Picosecond UV Laser for Advanced RF Gun Applications

Description: The fiber-based, spatially and temporally shaped, picosecond UV laser system described here has been specifically designed for advanced rf gun applications, with a special emphasis on the production of high-brightness electron beams for free-electron lasers and Compton scattering light sources. The laser pulse can be shaped to a flat-top in both space and time with a duration of 10 ps at full width of half-maximum (FWHM) and rise and fall times under 1 ps. The expected pulse energy is 50 {micro}J at 261.75 nm and the spot size diameter of the beam at the photocathode is 2 mm. A fiber oscillator and amplifier system generates a chirped pump pulse at 1047 nm; stretching is achieved in a chirped fiber Bragg grating. A single multi-layer dielectric grating based compressor recompresses the input pulse to 250 fs FWHM and a two stage harmonic converter frequency quadruples the beam. Temporal shaping is achieved with a Michelson-based ultrafast pulse stacking device with nearly 100% throughput. Spatial shaping is achieved by truncating the beam at the 20% energy level with an iris and relay-imaging the resulting beam profile onto the photocathode. The integration of the system, as well as preliminary laser measurements will be presented.
Date: June 8, 2007
Creator: Shverdin, M Y; Anderson, S G; Betts, S M; Gibson, D J; Hartemann, F V; Hernandez, J E et al.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Optimal Design of a Tunable Thomson-Scattering Based Gamma-Ray Source

Description: Thomson-Scattering based systems offer a path to high-brightness high-energy (&gt; 1 MeV) x-ray and {gamma}-ray sources due to their favorable scaling with electron energy. LLNL is currently engaged in an effort to optimize such a device, dubbed the ''Thomson-Radiated Extreme X-Ray'' (T-REX) source, targeting up to 680 keV photon energy. Such a system requires precise design of the interaction between a high-intensity laser pulse and a high-brightness electron beam. Presented here are the optimal design parameters for such an interaction, including factors such as the collision angle, focal spot size, optimal bunch charge, and laser energy. These parameters were chosen based on extensive modeling using PARMELA and in-house, well-benchmarked scattering simulation codes.
Date: June 7, 2007
Creator: Gibson, D J; Anderson, S G; Betts, S M; Hartemann, F V; Jovanovic, I; McNabb, D P et al.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Nuclear Resonance Fluorescence Excitations Near 2 MeV in 235U and 239Pu

Description: A search for nuclear resonance fluorescence excitations in {sup 235}U and {sup 239}Pu within the energy range of 1.0- to 2.5-MeV was performed using a 4-MeV continuous bremsstrahlung source at the High Voltage Research Laboratory at the Massachusetts Institute of Technology. Measurements utilizing high purity Ge detectors at backward angles identified 9 photopeaks in {sup 235}U and 12 photopeaks in {sup 239}Pu in this energy range. These resonances provide unique signatures that allow the materials to be non-intrusively detected in a variety of environments including fuel cells, waste drums, vehicles and containers. The presence and properties of these states may prove useful in understanding the mechanisms for mixing low-lying collective dipole excitations with other states at low excitations in heavy nuclei.
Date: December 27, 2006
Creator: Bertozzi, W; Caggiano, J A; Hensley, W K; Johnson, M S; Korbly, S E; Ledoux, R J et al.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department