N. M. Larson

R-matrix analysis of 235U neutron transmission and cross-section measurements in the 0- to 2.25-keV energy range

A new R-matrix analysis of the {sup 235}U cross-section data in the 0- to 2250-eV energy region is presented. The analysis was performed with the SAMMY computer code that has recently been updated to permit, for the first time, inclusion of both differential and integral data within the analysis process. Fourteen differential data sets and six integral quantities were used in this evaluation: two measurements of fission plus capture, one of fission plus absorption, six of fission alone, two of transmission, and one of eta, plus standard values of thermal cross sections for fission and capture, and of K1 and the Westcott g factors for both fission and absorption. An excellent representation was obtained for the high-resolution transmission, fission, and capture cross-section data as well for the integral quantities. The result is a single set of resonance parameters spanning the entire range up to 2250 eV, a decided improvement over the present ENDF/B-VI evaluation, in which 11 discrete resonance parameter sets are required to cover that same energy range. This new evaluation is expected to greatly improve predictability of the criticality safety margins for nuclear systems in which {sup 235}U is present.

An International Evaluation of the Neutron Cross Section Standards

Work is reported here on the process and present results of an international evaluation of the neutron cross section standards. The evaluations include the H(n,n), 3He(n,p), 6Li(n,t), 10B(n,α), 10B(n,α1γ), 197Au(n,γ), 235U(n,f), and 238U(n,f) standard reactions as well as the 238U(n,γ) and 239Pu(n,f) reactions. This evaluation was performed to include new experiments on the standards that have been made since the ENDF/B-VI evaluation was completed and to improve the evaluation process. Evaluations have been completed for the 6Li(n,t), 197Au(n,γ) and 238U(n,γ) cross sections. Also below 20 MeV the H(n,n), 235U(n,f), 238U(n,f) and 239Pu(n,f) cross sections are completed. Many of the cross sections being evaluated are used in neutron dosimetry for fluence determination. The general trend observed for the evaluations is an increase in the cross sections for most of the reactions from fractions of a percent to several percent compared with the ENDF/B-VI results.

Evaluation of 238U Resonance Parameters from 0 to 20 keV

The neutron resonance parameters of 238U were obtained in the energy range 0 to 20 keV from a sequential SAMMY analysis of the most recent high‐resolution neutron transmission and neutron capture cross‐section measurements. Special care was taken in the analysis of the lowest s‐wave resonances leading to resonance parameters slightly different from those of ENDF/B‐VI (Moxon‐Sowerby resonance parameters). The resolved‐resonance range was extended to 20 keV, taking advantage of the high‐resolution neutron transmission data of Harvey and neutron capture data of Macklin et al. Preliminary integral tests were performed with the new resonance parameters; thermal low‐enriched benchmark calculations show an improvement of the keff prediction, mainly due to a 1.5% decrease of the capture cross section at 0.0253 eV and about a 0.4% decrease of the effective shielded resonance capture integral.

R-Matrix Analysis of 232Th Neutron Transmissions and Capture Cross Sections in the Energy Range Thermal to 4 keV

Abstract Neutron resonance parameters of 232Th were obtained from the Reich-Moore SAMMY analysis of high-resolution neutron transmission measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) by Olsen in 1981, along with the high-resolution neutron capture measurements performed in 2005 at the Geel Linear Accelerator (GELINA, Belgium) by Schillebeeckx and at the n-TOF facility (CERN, Switzerland) by Aerts. The ORELA data were analyzed previously by Olsen with the Breit-Wigner multilevel code SIOB, and the results were used in the ENDF/B-VI evaluation. In the new analysis of the Olsen neutron transmissions by the modern computer code SAMMY, better accuracy is obtained for the resonance parameters by including in the experimental database the recent experimental neutron capture data. The experimental database and the method of analysis are described in the report. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared to the experimental values. A description is given of the statistical properties of the resonance parameters. The new evaluation results in a decrease in the capture resonance integral.

56Fe and 60Ni Resonance Parameters

High-resolution neutron transmission and differential elastic-scattering measurements were made for a 56Fe-enriched iron target at the Oak Ridge Electron Linear Accelerator (ORELA). A natural iron target was used for transmission measurements below 160 keV. The data were analyzed from 5 to 850 keV. Parameters were obtained for 33 l = 0 and 242 l > 0 resonances. New 6Li-glass transmission data were acquired for two 60Ni-enriched sample thicknesses. The neutron width for the 2.253-keV resonance was determined to be 59.3 ± 0.6 meV and the radiation width 553 ± 50 meV.

A new resonance region evaluation of neutron cross sections for sup 235 U

A new evaluation of the resolved resonance range for the neutron cross sections of {sup 235}U is described. Up to 110 eV, the evaluation is based on an R-matrix analysis of several fission, capture, and transmission measurements. Levels above 110 eV are no longer resolved so that many resonances are missed; from 110 to 500 eV, most of the important resonances can be identified and analyzed so that the cross section and transmission data are well represented by the proposed parameters. From 500 to 2250 eV, fictitious parameters are provided that describe fairly well the results of thick sample transmission measurements and recent fission cross-section data. Such a parameterization is likely to yield a better approximation of resonance self-shielding than the current ENDF/B-V unresolved resonance treatment.

R-Matrix Analysis of 238U High-Resolution Neutron Transmissions and Capture Cross Sections in the Energy Range 0 to 20 keV

Abstract The neutron resonance parameters of 238U were obtained from a SAMMY analysis of high-resolution neutron transmission measurements and high-resolution capture cross-section measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) in the years 1970–1990 and from more recent transmission and capture cross-section measurements performed at the Geel Linear Accelerator. Compared with previous evaluations, the energy range for this resonance analysis was extended from 10 to 20 keV, taking advantage of the high resolution of the most recent ORELA transmission measurements. The experimental database and the method of analysis are described in this paper. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared with the experimental data. A description is given of the statistical properties of the resonance parameters and of the recommended values of the average parameters. The new evaluation results in a slight decrease of the effective capture resonance integral and improves the prediction of integral thermal benchmarks by 70 to 200 pcm.

Neutron Resonance Parameters and Covariance Matrix of 239Pu

In order to obtain the resonance parameters in a single energy range and the corresponding covariance matrix, a reevaluation of 239Pu was performed with the code SAMMY. The most recent experimental data were analyzed or reanalyzed in the energy range thermal to 2.5 keV. The normalization of the fission cross section data was reconsidered by taking into account the most recent measurements of Weston et al. and Wagemans et al. A full resonance parameter covariance matrix was generated. The method used to obtain realistic uncertainties on the average cross section calculated by SAMMY or other processing codes was examined.

Evaluation of the 238U Neutron Cross Section in the Unresolved Resonance Range

Abstract This paper presents a new analysis of the 238U cross sections in the unresolved resonance range, from 20 to 150 keV. Statistical analysis of the resonance parameters in the resolved resonance range with random-matrix theory provides accurate experimental values of strength function, average radiative width and average level spacing for s- and p-wave resonances. Above 20 keV, the simultaneous fit of selected experimental data (average transmission and capture) is performed with a statistical model of nuclear reactions as implemented in the SAMMY code. Compared to previous evaluations, such as those described by Fröhner or by Maslov et al., this work benefits from the accurate transmission data measured by Harvey et al. at Oak Ridge Electron Linear Accelerator, which have never been studied before. This new evaluation was written into the current ENDF format for use in practical applications. This work stresses the need for an improved ENDF format to store average resonance parameters and cross sections in the unresolved resonance range.

A New Approach for Nuclear Data Covariance and Sensitivity Generation

Covariance data are required to correctly assess uncertainties in design parameters in nuclear applications. The error estimation of calculated quantities relies on the nuclear data uncertainty information available in the basic nuclear data libraries, such as the U.S. Evaluated Nuclear Data File, ENDF/B. The uncertainty files in the ENDF/B library are obtained from the analysis of experimental data and are stored as variance and covariance data. The computer code SAMMY is used in the analysis of the experimental data in the resolved and unresolved resonance energy regions. The data fitting of cross sections is based on generalized least‐squares formalism (Bayes’ theory) together with the resonance formalism described by R‐matrix theory. Two approaches are used in SAMMY for the generation of resonance‐parameter covariance data. In the evaluation process SAMMY generates a set of resonance parameters that fit the data, and, in addition, it also provides the resonance‐parameter covariances. For existing reso...