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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Inelastic neutron spectra of polyacrylonitrile-based carbon fibers

We present the vibrational spectra of polyacrylonitrile-based carbon fibers collected using inelastic neutron scattering. We ascertain the behavior of a broad range of vibrational spectra that are optically silent, and demonstrate a direct connection between these modes and thermomechanical properties of the fibers. We show directionally dependent coupling of hydrogen in the carbon fiber matrix, which is directly linked to mechanical properties. Further, we show hydrogen preferentially couples to the midband of the vibrational spectrum, and that there are higher overall mode populations in the traditional Raman D–G intervalley region, suggesting involvement of these modes in tensile strength reduction and transport properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Retrospective neutron spectrum determination of a (30 MeV D, Be) source using the multi-foil activation technique and STAYSL-PNNL [Characterization of neutron spectra from a (30 MeV d, Be) source using the multi-foil activation technique]

Retrospective characterization of a (30 MeV D, Be) neutron source was performed employing multi-foil activation and STAYSL-PNNL. Experimental reaction rates were calculated from gamma spectroscopy measurements of irradiated foils and MCNP provided the guess spectrum. Adjusted spectra were evaluated through activation calculations for a stainless-steel target using FISPACT-II. Adjusted spectra showed limited dependence on the dosimetry reactions and provided minor improvements in activation calculations. Omitting reflected neutrons in the guess spectrum generated poor activation results and the limited number of dosimetry reactions introduced doubt in the adjusted spectra. Furthermore, a dedicated neutron spectrometry experiment and a more detailed simulation is required.

42 ENGINEERING↗
DOE OSTI2024

Physics-informed Estimation of the Covariance Matrix for Various Neutron Spectra

A method for estimating covariance matrices which capture the uncertainties in calculated reactor spectra has been developed. This method is based on perturbing the parameters of a physics-based analytic model fitted to a calculated spectrum. The covariance of the perturbed analytic spectra imposes energy-dependent correlations due to the physics of the neutron processes in the reactor, i.e., a fission component, a 1/E down-scatting component, and a thermal Maxwellian component. An analytic model is developed which is shown to produce good fits to several reactor environments. The covariance matrices produced via this method are then used as the prior spectrum in STAYSL least squares spectrum adjustment where it is combined with integral metrics, such as activation measurements, to produce a high-fidelity neutron spectrum characterization. It was concluded that the methodology showed agreeable results for the ACRR free-field spectrum adjustment in STAYSL resulting in a 𝜒 2 value of 2.21 (per degree of freedom), but further work is needed to describe scattering and interface regions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Impact of the Normalization Condition and Model Information on Evaluated Prompt Fission Neutron Spectra and Associated Uncertainties

Low evaluated uncertainties compared to experimental information and a strong model impact were observed in some prompt fission neutron spectrum (PFNS) evaluations that include mean values and covariances stemming from a rigid model. Here, we show by studying the (PU)-P-239 PFNS ENDF/B-VII.1 evaluation via generalized least-squares analyses that strong model correlations in combination with the normalization condition on the estimated PFNS and its covariances result in surprisingly low evaluated uncertainties. Furthermore, the model changes the evaluated results by > 1 sigma of combined experimental uncertainties near the average outgoing neutron energy (similar to 2 MeV). We show both analytically and by means of representative numerical examples that the normalization condition on the spectrum and its covariances naturally leads to uncertainties reduced by a fully positively correlated scaling uncertainty

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Inelastic Neutron Spectra of Uranium Tetrafluoride Hydrate, UF 4 (H 2 O) 2.5

Uranium tetrafluoride hydrate (UFH) is formed by immersing anhydrous UF4 under water for 12 h. UFH is therefore clearly a chemical species of environmental concern, as anhydrous UF4 is an intermediate uranium form in the nuclear fuel cycle. We use inelastic neutron scattering (INS) to probe the full vibrational spectra of UFH and its deuterated analogue in an effort to improve the fundamental understanding of its vibrational spectra. Coupled with density functional theory (DFT) calculations, the first for this compound, and full spectral modeling, we generate the complete vibrational spectra of UFH and compare them to prior optical spectroscopic results. In particular, the combination of DFT with INS allows us to identify multiple distinct chemical environments in the water bending and OH stretching regions. Whereas the water molecules directly bound to the U atoms execute OH stretching around 3600 cm –1 , a second class of H-bonded waters vibrate below 3000 cm –1 , an indicator of strong H bonding. In addition, a class of librational water modes are observed between 400 and 900 cm–1, which themselves can be separated in energy according to their chemical environments. Furthermore, measurements presented herein directly assist in the assignment of certain spectral features in the infrared spectrum and will inform future investigations of UFH for environmental or forensics purposes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Measurement of the 240 Pu(n,f) Prompt Fission Neutron Spectra with Chi-Nu

The DOE Nuclear Criticality Safety Program has funded a multi-year effort to measure the 240 Pu(n,f) Prompt Fission Neutron Spectrum at Chi-Nu. This project is a joint LANL-LLNL effort, involving the preparation of 12 240 Pu foils and construction of a Parallel-Plate Avalanche Counter at LLNL, with subsequent measurements of the 240 Pu(n,f) PFNS at the Chi-Nu beamline at the Los Alamos Neutron Science Center’s Weapons Neutron Research facility (LANSCE/WNR). We expect the PPAC to be ready in time to perform the measurements this run cycle.

240Pu Foils↗
DOE OSTI2018

A fused silica Cherenkov radiator for high precision time-of-flight measurement of DT γ and neutron spectra (invited)

A fused silica Cherenkov radiator has been utilized at the National Ignition Facility to provide a new high precision measurement of the time-of-flight spectrum of 14.1 MeV DT fusion neutrons. This detector allows a high precision (<30 ps) co-registered measurement of both a thresholded γ-ray and a neutron spectrum on a single record. Other approaches typically require γ and neutron signals to be co-registered via other diagnostics and/or dedicated timing experiments. Analysis of the co-registered γ and neutron signals allows precise extraction of the mean neutron energy and bulk hot-spot velocity, both of which were not possible with prior scintillator technologies. Initial measurements demonstrate the feasibility of this measurement and indicate that combined detection of neutrons and γ-rays on multiple lines-of-sight should enable the bulk vector velocity of the implosion hot-spot to be determined to ≈5 km/s and reduced uncertainty in the spectral width ≈0.1 keV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Introducing Nuclear Data Evaluations of Prompt Fission Neutron Spectra [Slides]

Nuclear data evaluations provide recommended data sets for nuclear data applications such as reactor physics, stockpile stewardship or nuclear medicine. The evaluated data are often based on information from multiple experimental data sets and nuclear theory using statistical methods. Therefore, they are collaborative efforts of evaluators, theoreticians, experimentalists, benchmark experts, statisticians and application area scientists. In this talk, an introductions is given to the field of nuclear data evaluation at the specific example of a recent evaluation of the outgoing neutron energy spectrum emitted promptly after fission from 239 Pu and induced by neutrons from thermal to 30 MeV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗