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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

The DD Neutron Generator as an Alternative to Am(Li) Isotopic Neutron Source in the Uranium Neutron Coincidence Collar

The 235 U mass assay of bulk uranium items, such as oxide canisters, fuel pellets, and fuel assemblies, is not achievable by traditional gamma-ray assay techniques because of the limited penetration of the item by the characteristic 235 U gamma rays. Instead, fast neutron interrogation methods such as active neutron coincidence counting must be used. For international safeguards applications, the most commonly used active neutron systems, the Active Well Coincidence Counter (AWCC), Uranium Neutron Coincidence Collar (UNCL), and 252 Cf Shuffler, rely on fast neutron interrogation using an isotopic neutron source [i.e., 252 Cf or Am(Li)] to achieve better measurement accuracies than are possible using gamma-ray techniques for high-mass, high-density items. However, the Am(Li) sources required for the AWCC and UNCL systems are no longer manufactured, and newly produced systems rely on limited supplies of sources salvaged from disused instruments. The 252 Cf shuffler systems rely on the use of high-output 252 Cf sources, which while still available have become extremely costly for use in routine operations and require replacement every 5–7 years. Lack of a suitable alternative neutron interrogation source would leave a potentially significant gap in the safeguarding of uranium processing facilities. Previously we examined the performance of Oak Ridge National Laboratory’s Large Volume Active Well Coincidence Counter modified to accept a commercially available deuterium-deuterium (DD) neutron generator to examine the potential of the DD neutron generator as an alternative to the isotopic sources [1]. That work indicated that the DD generator was a viable replacement for the AWCC applications, but because of the large dimensions of that generator, the DD/AWCC would have to be run as a delayed neutron counting system to provide equivalent measurement precision. In this work we examine the performance of a standard UNCL modified to accept the DD neutron generator for the assay of 235 U based on the results of Monte Carlo N-Particle (MCNP) simulations and measurements of depleted uranium and highly enriched uranium items. The projected performance of the modified UNCL for fresh pressurized water reactor fuel assemblies operated in both traditional coincidence counting and delayed neutron counting modes is presented.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2022

The development of a 6 Li-based pixelated neutron detector for neutron reflectometry at the Spallation Neutron Source

We present a high-rate 6 Li-based pixelated neutron detector developed for neutron reflectometry instruments at the Spallation Neutron Source (SNS). The neutron detector has a pixelated design: each 6 Li scintillator element has its own photosensor and independent channel readout. This paper focuses on the general overview of the detector design and construction, the characterization of the pixelated detector, and the results of the first neutron reflectivity experiments conducted using the pixelated neutron detector at the SNS Liquid Reflectometer (BL-4B). The pixelated neutron detector demonstrated a global time-average count rate of ≥ 1.8 x 10 6 cps, at least 3 orders of magnitude higher than that of the existing neutron detector ( 3 He-based Multi-Wire Proportional Counter), and a local instantaneous count rate of ≥ 1.73 x 10 6 cps / cm 2 . The maximum counting rate of the detector has not yet been determined as the detector is capable of handling the maximum flux available at the beamline. Furthermore, the outcome of the neutron reflectivity experiments showed that the pixelated neutron detector is a promising candidate for next-generation neutron reflectometry instruments at the SNS.

6Li, silicon photomultiplier↗
DOE OSTI2019

Characterization of the neutron flux during production of 18 F at a medical cyclotron and evaluation of the incidental neutron spectrum for neutron damage studies

The fast neutron spectrum was measured an irradiation position adjacent to two 18 F (fluorine-18) production targets at a medical cyclotron. The neutron spectrum was modeled using MCNP6 and experimentally determined using activation foils. A three-group neutron spectrum was determined using an over-determined least squares fitting method. The MCNP6 model results and measurements of the activation foils were used as input to unfold the neutron spectrum using the PNNL-STAYSL code. The neutron spectrum adjacent to the 18 F production target was evaluated for use in incidental experiments to evaluate neutron induced radiolysis of polyurethane and radiation hardness testing of electronics. The fast neutron flux near the 18 F target between the energies of 0.4 – 16.5 MeV was 1.8 – 3.0x10 9 n cm -2 s -1 . The neutron dose rate in polyurethane at the irradiation position was 2.8 cGy s -1 – 5.8 cGys -1 . The 1 MeV equivalent neutron flux was 2.4x10 9 n cm -2 s -1 – 4.0x10 9 n cm -2 s -1 .

18F-fluorodeoxyglucose↗
DOE OSTI2018

Detailed Investigations of Neutron–Neutron Angular Correlations in Slow-Neutron-Induced Fission of {sup 233}U, {sup 235}U, and {sup 239}Pu

An experiment devoted to studying neutron–neutron angular correlations in the slow-neutroninduced fission of {sup 233}U, {sup 235}U and {sup 239}Pu nuclei was performed. The experimentally determined angular dependence of the number of neutron–neutron coincidences was compared with the results of a Monte Carlo simulation for various values of the neutron-detection threshold in the range between 490 and 2080 keV. It was found that the experimental angular distributions in question can be described well under the assumption that 2% to 5% of all prompt fission neutrons are emitted isotropically in the laboratory frame. Probably, such neutrons can be interpreted as so-called scission neutrons directly associated with the nuclear-rupture instant. Energy distributions of this component were also obtained from the present analysis.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Fusion-neutron measurements for magnetized liner inertial fusion experiments on the Z accelerator

Several magnetized liner inertial fusion (MagLIF) experiments have been conducted on the Z accelerator at Sandia National Laboratories since late 2013. Measurements of the primary DD (2.45 MeV) neutrons for these experiments suggest that the neutron production is thermonuclear. Primary DD yields up to 3e12 with ion temperatures ~2-3 keV have been achieved. Measurements of the secondary DT (14 MeV) neutrons indicate that the fuel is significantly magnetized. Measurements of down-scattered neutrons from the beryllium liner suggest ρR liner ~ 1g/cm 2 . Neutron bang times, estimated from neutron time-of-flight (nTOF) measurements, coincide with peak x-ray production. Furthermore, plans to improve and expand the Z neutron diagnostic suite include neutron burn-history diagnostics, increased sensitivity and higher precision nTOF detectors, and neutron recoil-based yield and spectral measurements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Neutron Tunneling: A New Mechanism to Power Explosive Phenomena in Neutron Stars, Magnetars, and Neutron Star Mergers

Neutron tunneling between neutron-rich nuclei in inhomogeneous dense matter encountered in neutron star crusts can release enormous energy on a short timescale to power explosive phenomena in neutron stars. In this work, we clarify aspects of this process that can occur in the outer regions of neutron stars when oscillations or cataclysmic events increase the ambient density. We use a time-dependent Hartree–Fock–Bogoliubov formalism to determine the rate of neutron diffusion and find that large amounts of energy can be released rapidly. The roles of nuclear binding, two-body interaction, and pairing in neutron diffusion times are investigated. We consider a one-dimensional quantum diffusion model and extend our analysis to study the impact of diffusion in three dimensions. Here, we find that these novel neutron transfer reactions can generate energy in the amount of ≃ 10 40 –10 44 erg under suitable conditions and assumptions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Neutron-neutron quasifree scattering in neutron-deuteron breakup at 10 MeV

New measurements of the neutron-neutron quasifree scattering cross section in neutron-deuteron breakup at an incident neutron energy of 10.0 MeV are reported. The experiment setup was optimized to evaluate the technique for determining the integrated beam-target luminosity in neutron-neutron coincidence cross-section measurements in neutron-deuteron breakup. The measurements were carried out with a systematic uncertainty of ± 5.6%. Our data are in agreement with theoretical calculations performed using the CD-Bonn nucleon-nucleon potential in the Faddeev formalism. The measured integrated cross section over the quasifree peak is 20.5 ± 0.5(stat) ± 1.1 (sys) mb/sr 2 in comparison with the theory prediction of 20.1 mb/sr 2 . Overall, these results validate our technique for determining the beam-target luminosity in neutron-deuteron breakup measurements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Experimental Search for Neutron to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly

We report an unexplained >4σ discrepancy persists between “beam” and “bottle” measurements of the neutron lifetime. A new model proposed that conversions of neutrons n into mirror neutrons n', part of a dark mirror sector, can increase the apparent neutron lifetime by 1% via a small mass splitting Δm between n and n' inside the 4.6 T magnetic field of the National Institute of Standards and Technology Beam Lifetime experiment. A search for neutron conversions in a 6.6 T magnetic field was performed at the Spallation Neutron Source which excludes this explanation for the neutron lifetime discrepancy.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

A Search for Neutron to Mirror Neutron Oscillation Using Neutron Electric Dipole Moment Measurements

Baryon number violation is a key ingredient of baryogenesis. It has been hypothesized that there could also be a parity-conjugated copy of the standard model particles, called mirror particles. The existence of such a mirror universe has specific testable implications, especially in the domain of neutral particle oscillation, viz. the baryon number violating neutron to mirror-neutron (n−n′) oscillation. Consequently, there were many experiments that have searched for n−n′ oscillation, and imposed constraints upon the parameters that describe it. Recently, further analysis on some of these results have identified anomalies which could point to the detection of n−n′ oscillation. All the previous efforts searched for n−n′ oscillation by comparing the relative number of ultracold neutrons that survive after a period of storage for one or both of the two cases: (i) comparison of zero applied magnetic field to a non-zero applied magnetic field, and (ii) comparison where the orientation of the applied magnetic field was reversed. However, n−n′ oscillations also lead to variations in the precession frequency of polarized neutrons upon flipping the direction of the applied magnetic field. Precession frequencies are measured, very precisely, by experiments searching for the electric dipole moment. For the first time, we used the data from the latest search for the neutron electric dipole moment to constrain n−n′ oscillation. After compensating for the systematic effects that affect the ratio of precession frequencies of ultracold neutrons and cohabiting 199Hg-atoms, chief among which was due to their motion in non-uniform magnetic field, we constrained any further perturbations due to n−n′ oscillation. We thereby provide a lower limit on the n−n′ oscillation time constant of τnn′/|cos(β)|>5.7s,0.36T′

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Neutron tomography of axially symmetric objects using 14 MeV neutrons from a portable neutron generator

In nuclear boiling water reactor cores, the distribution of water and steam (void) is essential for both safety and efficiency reasons. In order to enhance predictive capabilities, void distribution assessment is performed in two-phase test-loops under reactor-relevant conditions. This article proposes the novel technique of fast-neutron tomography using a portable deuterium-tritium neutron generator to determine the time-averaged void distribution in these loops. Fast neutrons have the advantage of high transmission through the metallic structures and pipes typically concealing a thermal-hydraulic test loop, while still being fairly sensitive to the water/void content. However, commercially available fast-neutron generators also have the disadvantage of a relatively low yield and fast-neutron detection also suffers from relatively low detection efficiency. Fortunately, some loops are axially symmetric, a property which can be exploited to reduce the amount of data needed for tomographic measurement, thus limiting the interrogation time needed. In this article, three axially symmetric test objects depicting a thermal-hydraulic test loop have been examined; steel pipes with outer diameter 24 mm, thickness 1.5 mm, and with three different distributions of the plastic material POM inside the pipes. Data recorded with the FANTOM fast-neutron tomography instrument have been used to perform tomographic reconstructions to assess their radial material distribution. Here, a dedicated tomographic algorithm that exploits the symmetry of these objects has been applied, which is described in the paper. Results are demonstrated in 20 rixel (radial pixel) reconstructions of the interior constitution and 2D visualization of the pipe interior is demonstrated. The local POM attenuation coefficients in the rixels were measured with errors (RMS) of 0.025, 0.020, and 0.022 cm{sup −1}, solid POM attenuation coefficient. The accuracy and precision is high enough to provide a useful indication on the flow mode, and a visualization of the radial material distribution can be obtained. A benefit of this system is its potential to be mounted at any axial height of a two-phase test section without requirements for pre-fabricated entrances or windows. This could mean a significant increase in flexibility of the void distribution assessment capability at many existing two-phase test loops.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

An apparatus for studying electrical breakdown in liquid helium at 0.4 K and testing electrode materials for the neutron electric dipole moment experiment at the Spallation Neutron Source

In this study, we have constructed an apparatus to study DC electrical breakdown in liquid helium at temperatures as low as 0.4 K and at pressures between the saturated vapor pressure and ~600 Torr. The apparatus can house a set of electrodes that are 12 cm in diameter with a gap of 1–2 cm between them, and a potential up to ±50 kV can be applied to each electrode. Initial results demonstrated that it is possible to apply fields exceeding 100 kV/cm in a 1 cm gap between two electropolished stainless steel electrodes 12 cm in diameter for a wide range of pressures at 0.4 K. We also measured the current between two electrodes. Our initial results, I < 1 pA at 45 kV, correspond to a lower bound on the effective volume resistivity of liquid helium of ρ V > 5 × 10 18 Ω cm. This lower bound is 5 times larger than the bound previously measured. Finally, we report the design, construction, and operational experience of the apparatus, as well as initial results

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Light output and neutron detection efficiency of boron-based neutron scintillator screens for neutron imaging

Recent research has explored the development of boron-based neutron scintillator screens, which potentially offer improved spatial resolution and neutron capture efficiency compared to traditional lithium-based screens. This work builds upon previous efforts to improve boron-based neutron scintillators by assessing a newer generation of boron-based scintillator screens fabricated using different compositions and fabrication approaches compared to previous generations of screens. Some of the test screens exhibit higher light output than previous efforts and higher neutron capture efficiency than lithium-based screens. This paper describes the current state of screen development, measurement results for the most recent generation of screens, and future activities.

46 - INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AN↗
DOE OSTI2015

Note: Detector collimators for the nanoscale ordered materials diffractometer instrument at the Spallation Neutron Source

Five neutron collimator designs were constructed and tested at the nanoscale ordered materials diffractometer (NOMAD) instrument. Collimators were made from High Density PolyEthylene (HDPE) or 5% borated HDPE. In all cases, collimators improved the signal to background ratio and reduced detection of secondary scattering. In the Q-range 10-20 Å{sup −1}, signal to background ratio improved by factors of approximately 1.6 and 2.0 for 50 and 100 mm deep collimators, respectively. In the Q-range 40-50 Å{sup −1}, the improvement factors were 1.8 and 2.7. Secondary scattering as measured at Q ∼ 9.5 Å{sup −1} was significantly decreased when the collimators were installed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Resolving Local Configurational Contributions to X-ray and Neutron Radial Distribution Functions Within Solutions of Concentrated Electrolytes – A Case Study of Concentrated NaOH

Extreme conditions of complex materials often lead to a manifold of local environments that challenge characterization and require new advances at the intersection of modern experimental and theoretical techniques. In this contribution, highly caustic and viscous aqueous NaOD solutions were characterized with a combination of X-ray and neutron radial distribution function (RDF) analyses, molecular dynamics simulations and sub-ensemble analysis. The interpretation of the experimental RDFs extends beyond the traditional understanding of atom-pair interactions and to the individual contributions of specific molecular-scale configurations (below 3.5 Å). This integrated approach yields unique insight into the complex and significant breath of ion solvation and pairing structural motifs present, the experimental sensitivity of RDFs to changes in local geometries, and the challenge of experimentally differentiating the ensemble of all superimposed local environments — a feature of increasing importance within the extreme condition of high ionic strength. This study was facilitated by the development of a chemically resistant sample cell for in-situ neutron total scattering measurements of highly caustic materials at elevated temperature and pressure conditions. Advancing these integrated capabilities is critical for ongoing model validation efforts and for extending the predictive capabilities needed for the extremes of chemical composition, aiding a variety of industrial processes that include advanced nuclear waste remediation and metal refining strategies.

Semrouni, David↗