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Results for “NEUTRON SPECTRA”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

New prompt fission neutron spectra measurements in the 238 U(n,f) reaction with a dedicated setup at LANSCE/WNR

A new prompt fission neutron spectra (PFNS) measurement in the 238 U(n,f) reaction was performed at LANSCE/WNR facility. Evaluated data show discrepancies on the low (below 1 MeV) and high (above 5 MeV) energy parts in the PFNS for different major and minor actinides. The goal is to improve these measurements in a wide range of incident energy. The energy of the incoming neutron, inducing the fission, and the prompt neutron energies, are measured by time-of-flight method. A dedicated fission chamber was developed, in order to improve alpha-fission discrimination, timing resolution, actinide mass, and to reduce the amount of neutron scattering. To detect prompt neutrons, the 54 Chi-Nu scintillator cells array were surrounding the fission chamber. High statistics were recorded during this experiment, allowing a precise study of PFNS behavior as a function of incident neutron energy, from 1 MeV to 200 MeV. In conclusion, this experiment also showed that all the new tools developed to improve PFNS measurements are performing. Therefore, measurements of PFNS with others actinides such as 239 Pu are planned.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Machine learning-assisted identification of potential sources of bias in measurements of prompt-fission neutron spectra

Unrecognized sources of uncertainty (USU) can bias the reported mean and/or covariance of experimental nuclear data. These biases, in turn, can propagate through evaluated nuclear data to application simulations or may poorly inform nuclear theory that is fitted to the experimental data. Such unknown sources of bias must be tied to the inherent physical constituents of the measurements such as the characteristics of a detector response or a background reduction technique. Here, in this article, a sparse Bayesian learning model is used to support experts in their efforts to identify and characterize USU in experimental prompt fission neutron spectra (PFNS) for spontaneous fissioning of 252 Cf by linking observed biases to features of the measurement system. Three different bias components were found. The first acts as a verification case for the algorithm as it identifies a bias coming from a well-known source related to the use of 6 Li in the neutron detection system. The second two cases demonstrate how this method can benefit the evaluation of experimental nuclear data by identifying, quantifying, and relating unknown biases to potential causes.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Templates of expected measurement uncertainties for prompt fission neutron spectra

In this paper, we provide templates of uncertainty sources expected to appear for three measurement types of prompt fission neutron spectra (PFNS): (1) shape measurements, (2) clean-ratio shape, that is the monitor PFNS are measured in nearly exactly the same surrounding as the PFNS of interest, and (3) indirect ratios, where the detector efficiency is backed out from PFNS monitor measurements. Information is also listed that is needed to faithfully include PFNS in nuclear data evaluations to guide experimenters on how to best report data and metadata for their measurements. These templates also suggest a typical range of pertinent uncertainty values and their correlations in case realistic uncertainties cannot be estimated from information on the measurement itself. The templates were based on a literature review, information found in EXFOR for 252 Cf, 235, 238 U, and 239 Pu PFNS, and enhanced by expertise from experimenters contributing to these PFNS templates.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1974

Measured and Calculated Neutron Spectra and Dose Equivalent Rates at High Altitudes; Relevance to SST Operations and Space Research

Results of the NASA Langley-New York University high-altitude radiation study are presented. Measurements of the absorbed dose rate and of secondary fast neutrons (1 to 10 MeV energy) during the years 1965 to 1971 are used to determine the maximum radiation exposure from galactic and solar cosmic rays of supersonic transport (SST) and subsonic jet occupants. The maximum dose equivalent rates that the SST crews might receive turn out to be 13 to 20 percent of the maximum permissible dose rate (MPD) for radiation workers (5 rem/yr). The exposure of passengers encountering an intense giant-energy solar particle event could exceed the MPD for the general population (0.5 rem/yr), but would be within these permissible limits if in such rare cases the transport descends to subsonic altitude; it is in general less than 12 percent of the MPD. By Monte Carlo calculations of the transport and buildup of nucleons in air for incident proton energies E of 0.02 to 10 GeV, the measured neutron spectra were extrapolated to lower and higher energies and for galactic cosmic rays were found to continue with a relatively high intensity to energies greater than 400 MeV, in a wide altitude range. This condition, together with the measured intensity profiles of fast neutrons, revealed that the biologically important fast and energetic neutrons penetrate deep into the atmosphere and contribute approximately 50 percent of the dose equivalant rates at SST and present subsonic jet altitudes.

Foelsche, T.↗
DOE OSTI2019

Analysis of Water Coupling in Inelastic Neutron Spectra of Uranyl Fluoride

Inelastic neutron scattering (INS) is uniquely sensitive to hydrogen due to its comparatively large thermal neutron scattering cross-section (82 b). Consequently, the inclusion of water in real samples presents significant challenges to INS data analysis due directly to the scattering strength of hydrogen. Here, we investigate uranyl fluoride (UO 2 F 2 ) with inelastic neutron scattering. UO 2 F 2 is the hydrolysis product of uranium hexafluoride (UF 6 ), and is a hygroscopic, uranyl-ion containing particulate. Raman spectral signatures are commonly used for inferential understanding of the chemical environment for the uranyl ion in UO 2 F 2 , but no direct measurement of the influence of absorbed water molecules on the overall lattice dynamics has been performed until now. To deconvolute the influence of waters on the observed INS spectra, we use density functional theory with full spectral modeling to separate lattice motion from water coupling. In particular, we present a careful and novel analysis of the Q-dependent Debye–Waller factor, allowing us to separate spectral contributions by mass, which reveals preferential water coupling to the uranyl stretching vibrations. Coupled with the detailed partial phonon densities of states calculated via DFT, we infer the probable adsorption locations of interlayer waters. We explain that a common spectral feature in Raman spectra of uranyl fluoride originates from the interaction of water molecules with the uranyl ion based on this analysis. In conclusion, the Debye–Waller analysis is applicable to all INS spectra and could be used to identify light element contributions in other systems.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2015

Measurement of neutron spectra in the experimental reactor LR-0

The measurement of fast neutron fluxes is important in many areas of nuclear technology. It affects the stability of the reactor structural components, performance of fuel, and also the fuel manner. The experiments performed at the LR-0 reactor were in the past focused on the measurement of neutron field far from the core, in reactor pressure vessel simulator or in biological shielding simulator. In the present the measurement in closer regions to core became more important, especially measurements in structural components like reactor baffle. This importance increases with both reactor power increase and also long term operation. Other important task is an increasing need for the measurement close to the fuel. The spectra near the fuel are aimed due to the planned measurements with the FLIBE salt, in FHR / MSR research, where one of the task is the measurement of the neutron spectra in it. In both types of experiments there is strong demand for high working count rate. The high count rate is caused mainly by high gamma background and by high fluxes. The fluxes in core or in its vicinity are relatively high to ensure safe reactor operation. This request is met in the digital spectroscopic apparatus. All experiments were realized in the LR-0 reactor. It is an extremely flexible light water zero-power research reactor, operated by the Research Center Rez (Czech Republic). (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

Neutron Spectra, Fluence and Dose Rates from Bare and Moderated Cf-252 Sources

A new, stronger 252 Cf source (serial number SR-CF-3050-OR) was obtained from Oak Ridge National Laboratory (ORNL) in 2014 to supplement the existing 252 Cf sources which had significantly decayed. A new instrument positioning track system was designed and installed by Hopewell Designs, Inc. in 2011. The neutron field from the new, stronger 252 Cf source in the modified calibration environment needed to be characterized as well as the modified neutron fields produced by the new source and seven different neutron moderators. Comprehensive information about our 252 Cf source, its origin, production, and isotopic content and decay characteristics needed to be compiled as well. This technical report is intended to address these issues.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2016

Interpreting inertial fusion neutron spectra

A burning laser fusion plasma produces a neutron spectrum first described by Brysk (1973 Plasma Phys. Control. Fusion 15 611). This and more recent work deals with the spectrum produced by a single fluid element. The distribution of temperatures and velocities in multiple fluid elements combine in any real spectrum; here we derive formulas for how the neutron spectrum averages these contributions. The single element momentum spectrum is accurately Gaussian, but the multi-element spectrum exhibits higher moments. In particular, the skew and kurtosis are likely to be large enough to measure. Even the single fluid element spectrum may exhibit measurable directional anisotropy, so that instruments with different lines of sight should see different yields, mean velocities, mean temperatures, and higher moments. Finally, we briefly discuss how scattering in the imploded core modifies the neutron spectrum by changing the relative weighting of fuel regions with different temperatures and velocities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
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↗