Nuclear technology applications, including reactor modeling, accelerator design, and isotope production, strongly depend on evaluated nuclear data libraries and their uncertainty information for the assessment of predictive accuracy of calculated quantities. Major nuclear data libraries such as JENDL-5, JEFF-3.3, and ENDF/B-VIII.0 lack uncertainty information for n+181Ta reactions. In addition to the lack of evaluated uncertainty information even in major nuclear data library releases, the most current US ENDF/B-VIII.0 evaluation of the unresolved resonance region (URR) does not extend to high enough energies to appropriately account for resonance self-shielding effects. This work addresses these shortcomings through anew evaluation of the URR, performed with the SAMMY evaluation tool, which extends the evaluation of the URR to encompass neutron energies of 2.5 keV to 100 keV. This study reports evaluated covariances and includes newly measured data in the evaluation analysis that were unavailable to previous evaluators. The new evaluation was designed to be closely coupled to the resolved resonance region evaluation to improve consistency across multiple evaluation regions. The updated cross sections in the URR have reduced capture and total cross sections, which improve agreement with differential measurements compared to ENDF/B-VIII.0, but they deviate slightly further from integral benchmarks.1

A new 181 Ta neutron resolved resonance region evaluation has been performed from the thermal energy range up to approximately 2.5 keV. The R-matrix SAMMY code was used with the Reich-Moore approximation to evaluate resonance parameters from several experimental data sets. A Monte Carlo approach was used for resonance spin assignments and generating 59 small fictitious resonance levels which were shown to improve the cumulative level, Porter-Thomas, and Wigner distributions as compared to theoretical predictions. Covariance information was also generated for the entire resolved resonance region. The positive impact of the new evaluation was validated through benchmark calculations which were sensitive to the 181 Ta cross section and showed improvement in the reactivity bias for several benchmark cases.

To resolve discrepancies in evaluated cross sections among major nuclear data libraries, energy-differential neutron transmission and radiative capture yield of Ta-181 were measured from 0.15 to 100 keV using multiple sample thicknesses. The new measurements provide resolution such that the resolved resonance region (RRR) can be evaluated up to at least 2.5 keV and the unresolved resonance region can be evaluated up to at least 100 keV. The transmission and capture yield measurements were modeled using resonance parameters from three major libraries to assess the predictive capability of each. It was found that JENDL-5.0 performed best in the RRR. Because of the poor performance of the U.S. ENDF/B evaluation, it is recommended that ENDF/B be reevaluated for Ta-181.

A novel method was developed to separate the 238U fission contribution measured in quasi-differential time-of-flight scattering experiments in order to isolate the elastic and inelastic events. Pulse height distributions from in-beam measurements were used to generate response functions, which were used to reconstruct the 238U prompt fission neutron spectra. This method was validated by reconstructing the measured 252Cf spontaneous fission pulse height distribution. Monte Carlo calculations were used to model the experiment. Good agreement was observed between the measured and calculated 238U fission contribution.

This paper provides a template of expected uncertainties and correlations for measurements of neutron-induced capture and charged-particle production cross sections. Measurements performed in-beam include total absorption spectroscopy, total energy detection,γ-ray spectroscopy, and direct charged-particle detection. Offline measurements include activation analysis and accelerator mass spectrometry. The information needed for proper use of the datasets in resonance region and high energy region evaluations is described, and recommended uncertainties are provided when specific values are not available for a dataset.

This paper provides a template of expected uncertainties and correlations for measurements of total neutron cross-section observables by transmission. Measurements with time-of-flight and mono-energetic neutron sources are covered. The information required for evaluations in the resonance region and high energy region is detailed, along with the template of uncertainties and correlations that can be used in the absence of other information.

A new experiment configuration was designed, developed, and implemented to measure photoneutron yields using the Rensselaer Polytechnic Institute (RPI) electron linear accelerator (LINAC) at the RPI Gaerttner LINAC Center. The experiment configuration includes a new target assembly that converts the LINAC electron beam into a high energy bremsstrahlung photon flux incident upon a sample material of interest. The photons excite nuclei in the sample of interest, which can subsequently emit neutrons (photoneutrons). The photoneutrons emitted in the direction of the detector system travel through a series of collimated vacuum pipes before reaching a pair of proton-recoil high-energy neutron detectors. The signals generated by the neutron detectors are processed using a digital data acquisition system and subsequently analyzed to determine the energy-dependent photoneutron yield from the sample of interest. The new experiment configuration was used to perform proof-of-concept experiments to measure the photoneutron yields from samples of tantalum and beryllium. The measured results were then compared against the results from Monte Carlo simulations of the detailed experiment configurations to perform preliminary assessments of evaluated photoneutron data libraries.

Abstract Neutron capture and transmission measurements were carried out from thermal to 2000 eV on both solid and liquid samples containing elemental cesium (133Cs). This work describes the extension of the R-matrix analysis of these data from 600 to 2000 eV by correcting the capture data for false capture in the NaI detector. These false capture–corrected capture and transmission data were analyzed for resonance parameters utilizing the SAMMY Bayesian analysis code to simultaneously fit both the capture and transmission data. Parameters were obtained for 53 cesium resonances over the 600- to 2000-eV energy range. The s-wave strength function was determined over the energy range from 0 to 1800 eV for both spin J = 3 and J = 4 resonances.

Neutron scattering from a copper sample was measured at Rensselaer Polytechnic Institute utilizing the quasi-differential method. The measurement spanned the energy range from 0.5 to 20 MeV using the high-energy scattering system and from 2 keV to 0.5 MeV using the new mid-energy scattering system. Copper was selected as a material of interest to measure due to large discrepancies between experiments and simulations of the Zeus benchmark. The Zeus benchmark consists of a copper reflected highly enriched uranium system, and the angular distribution of copper scattering was thought to potentially be the cause of the discrepancy. The copper measurements found differences in the scattering response particularly in the incident energy region from 1 to 2 MeV for the high-energy measurement and from 2 to 4 keV in the mid-energy system. These differences are particularly noticeable at angles near 90 deg in the high-energy system and back angles in the mid-energy system. Additionally, for ENDF/B-VIII.0 there is a large discrepancy at the forward angle in the energy range around 0.5 MeV. For these reasons, a new evaluation of copper scattering utilizing these results is recommended and perhaps could help to improve the agreement with the Zeus benchmarks.

Often discrepancies can be found in the corresponding cross sections of different evaluated nuclear data libraries. Traditional integral benchmarks that are used to validate such libraries are sensitive to cross-section values across many different energies. This means an erroneously low cross section at one energy may compensate for an erroneously high cross section at another energy, and the integral benchmark value may still be met. While the evaluated cross section may agree with that single benchmark, it could affect other systems differently. To reduce the potential for this error, an energy differential validation method is proposed herein for continuous energy Monte Carlo neutron transport models in the resolved resonance region and the unresolved resonance region (URR). The proposed method exposes the underlying physics of the URR and validates both the average cross section and resonance self-shielding effect driven by the fluctuations in that cross section. This is done by measuring the neutron transmission of a thick sample that, by its nature, exaggerates the resonance self-shielding effect. This validation method is shown to be very sensitive to the cross-section model used (resolved versus unresolved) and the fluctuation correction employed, allowing it to probe the validity of the previously mentioned cross-section evaluations. Tantalum-181 is used as an example to demonstrate the impact of different resonance evaluations. It was found that the JEFF-3.3 and JENDL-4.0u evaluations made reasonable choices for cross-section models of Ta-181; none of the current evaluations, however, can be used to properly model the validation transmission over all energies. It was also found that updating resonance parameters in the URR provided better agreement with the validation transmission.

Abstract Neutron capture and transmission measurements were carried out from 0.01 to 600 eV on both solid and liquid samples containing elemental cesium (133Cs). Only s-wave resonances were observed in these measurements. These data were analyzed for resonance parameters utilizing the SAMMY Bayesian analysis code to simultaneously fit both the capture and transmission data. Parameters were obtained for 31 cesium resonances up to 600 eV. The thermal capture cross section and capture resonance integral were determined. The thermal capture cross section is 10% larger than the ENDF, JENDL, and JEFF evaluated values but lies within the uncertainty of the most recent measurement by Yoon and Lee [New Phys.: Sae Mulli (Korean Phys. Soc.)., Vol. 61, p. 7 (2011)]. The capture resonance integral has a statistical 1σ error of 2% and lies 1.4σ above the JENDL value, 5.5σ above the ENDF value, and 3.9σ above the JEFF value. The s-wave strength function was determined.

The electron linear accelerator housed in the Gaerttner Linear Accelerator Center at Rensselaer Polytechnic Institute was used to generate a pulsed neutron source to measure the neutron total cross section of tantalum, titanium, and zirconium from 0.4 to 25 MeV. Neutron transmission measurements were made using the time-of-flight method with neutron flight paths of approximately 100 and 250 m. The long flight paths combined with narrow neutron pulse widths, fast detector responses, fast electronics, and data collection system provide good energy resolution for the measurements. A high signal-to-background ratio through much of the energy range combined with low statistical errors resulted in low uncertainties on cross sections. The results are presented and compared with the major nuclear data evaluations. Each measurement identifies regions where the neutron total cross sections could be reevaluated. The total cross-section measurements presented here can help nuclear data evaluators improve neutron total cross-section data in future evaluations.

Accurate measurement of the prompt fission neutron spectrum is extremely important to several aspects of nuclear engineering. The spectrum for the spontaneous fission of Cf-252 is particularly of use since other spectra are often measured as a ratio to Californium and the efficiency of neutron detectors can be measured using it. A measurement was performed at Rensselaer Polytechnic Institute in order to provide a new high accuracy measurement of the spectrum particularly in the regions of little data below 0.5 MeV and above 10 MeV. A measurement was performed using the multiple gamma tagging method and provided a new dataset in the region from 50 keV to 7 MeV which agreed well with previous measurements and theory. Preliminary measurements have also been performed to test the feasibility of using this method to measure the high portion of the spectrum from 5 MeV up to 15 MeV.

Cadmium has been used historically as an important component of integral experiments because of its high thermal neutron absorption cross section. Correct interpretation of such experiments depends on accurate differential neutron cross section measurements. The 60 MeV electron accelerator at the Gaerttner LINAC Center was used to generate neutrons for neutron capture and total cross section measurements of natural Cd. Measurements were performed in the thermal and epithermal resonance range with sample thicknesses ranging from 1 x 10-4 to 4 x 10-2 atoms per barn. A full resonance region analysis was performed in order to determine the thermal cross sections and resonance integrals of the cadmium isotopes. The Bayesian R-Matrix code SAMMY 8.0 was used to shape fit the data and extract the resonance parameters. Resonance parameter and cross section uncertainties were determined from their primary components: transmission background, capture normalization, experimental resolution function, burst width, sample thickness, and counting statistics. The experiments were analyzed for consistency within the measured capture and transmission using multiple sample thicknesses. Results are compared to previously published measurements and evaluated nuclear libraries. No major changes to the thermal cross section or the first resonance in Cd113 were identified from the consensus achieved from measurements and evaluations over the past decade.

Molybdenum can exist in many nuclear reactor components, including fuel, cladding, or as a high yield fission product. As a result, accurate isotopic nuclear data for molybdenum are important for reactor simulation. To this end, high-resolution time-of-flight neutron transmission measurements on highly enriched isotopic metallic samples of Mo-98 and Mo-100 were performed and data were reduced to transmission in the resolved resonance region from 10 eV to 53 keV for Mo-98 and 10 eV to 26.5 keV for Mo-100. Measurements were taken with Li-6 glass transmission detectors at 31 m and 100 m flight paths. The Bayesian R-matrix code SAMMY 8.0 was used to shape-fit the data and to extract resonance parameters from the transmission spectra. The newly fitted resonance parameters were compared with those given in ENDF/B-VII.1. The results represent a refinement of those given in the current evaluation due to the improvement in the experimental resolution in these measurements. The comparison included analysis of level statistics. The resonance parameters for Mo-98 show many differences with the current evaluations. The results of the analysis indicated missing levels in Mo-98 starting at 10 keV, which implies an inability to resolve all resonances at higher energies. The resonance parameters for Mo-100 agree well with the current evaluation. Level statistics analysis indicates there are few missing levels up to 26.5 keV. (C) 2018 Elsevier Ltd. All rights reserved.

A new array of C6D6 detectors installed at the RPI LINAC Center has enabled the capability to measure neutron capture cross sections above the 847 keV inelastic scattering threshold of 56Fe through the use of digital post-processing filters and pulse-integral discriminators, without sacrificing the statistical quality of data at lower incident neutron energies where such filtering is unnecessary. The C6D6 detectors were used to perform time-of-flight capture cross section measurements on a sample 99.87% enriched iron-56. The total-energy method, combined with the pulse height weighting technique, were then applied to the raw data to determine the energy-dependent capture yield. Above the inelastic threshold, the data were analyzed with a pulse-integral filter to reveal the capture signal, extending the the full data set to 2 MeV.

A series of neutron scattering benchmark measurements were performed on beryllium and molybdenum with the Rensselaer Polytechnic Institute's Neutron Scattering System. The pulsed neutron source was produced by the Rensselaer Polytechnic Institute's Linear Accelerator and a well collimated neutron beam was incident onto the samples located at a distance of 30.07 m. Neutrons that scattered from the sample were measured using the time-of-flight by eight EJ-301 liquid scintillator detectors positioned 0.5m from the sample of interest. A total of eight experiments were performed with two sample thicknesses each, measured by detectors placed at two sets of angles. All data were processed using pulse shape analysis that separated the neutron and gamma ray events and included a gamma misclassification correction to account for erroneously identified gamma rays. A detailed model of the neutron scattering system simulated each experiment with several current evaluated nuclear data libraries and their predecessors. Results for each evaluation were compared to the experimental data using a figure-of-merit. The neutron scattering system has been used as a means to quantify a library's performance.

A new array of four Deuterated Benzene (C6D6) detectors has been installed at the Gaerttner Linear Accelerator Center at Rensselaer Polytechnic Institute for the purpose of measuring neutron capture cross sections in the keV region. Measurements were performed on samples of Ta-181 in the unresolved resonance region (URR) using a filtered-beam technique, by which a 30 cm iron filter was placed in a white-spectrum neutron beam to remove all time-dependent. gamma-ray background and all neutrons except those transmitted through resonance-potential interference "windows" in the iron. The resulting filtered beam was effectively a quasimonoenergetic neutron source, which was used for performing measurements on isotopes with narrow level spacings in the URR. The capture cross-section results obtained for two thicknesses of tantalum are in agreement with those documented in the JEFF-3.2 library, as are the average resonance parameters obtained via a fit to the data using the SAMMY-FITACS code.