Neutron Cross Section Standards Evaluations For Endf

The neutron cross section standards are now being evaluated as the initial phase in the development of the new ENDF/B-VI file. These standards evaluations are following a somewhat different process compared with that used for earlier versions of ENDF. The primary effort is concentrated on a simultaneous evaluation using a generalized least squares program, R-matrix evaluations, and a procedure for combining the results of these evaluations. The ENDF/B-VI standards evaluation procedure is outlined, and preliminary simultaneous evaluation and R-matrix results are presented. 16 refs., 7 figs.

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,?), 10B(n,?1?), 197Au(n,?), 235U(n,f), and 238U(n,f) standard reactions as well as the 238U(n,?) 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,?) and 238U(n,?) 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.

Over the past year the Cross Section Evaluation Working Group (CSEWG) has finalized and issued new evaluations for over 70 nuclides in Version 6 of ENDF/B. These data provide the foundation for a new series of reactor benchmark calculations. An integral part of the evaluation effort for ENDF/B-VI has been the most ambitious effort ever undertaken to systematically and simultaneously reevaluate the cross sections used for neutron standards. While a number of the materials and reactions identified as standards are directly involved in fission reactor calculations, the impact of the standards data is felt far beyond the specific standards reactions. The effect of the Version 6 standards data file on several evaluations of data of interest for fission reactors is discussed in this paper, together with the methodology used to utilize the standards.

This book describes the Proceedings of the International Conference on Nuclear Data for Science and Technology held at Jillich in May 1991. The conference was in a series of application oriented nuclear data conferences organized in the past under the auspices of the Nuclear Energy Agency-Nuclear Data Committee (NEANDC) and with the support of the Nuclear Energy Agency-Committee on Reactor Physics (NEACRP). It was the fIrst international conference on nuclear data held in Germany, with the scientific responsibility entrusted to the Institute of Nuclear Chemistry of the Research Centre Jillich. The scientific programme was established by the International Programme Committee in consultation with the International Advisers, and the NEA and IAEA cooperated in the organization. A total of 328 persons from 37 countries and fIve international organizations participated. The scope of these Proceedings extends to a wide range of interdisciplinary topics dealing with measu rement, calculation, evaluation and application of nuclear data, with a major emphasis on numerical data. Both energy and non-energy related applications are considered and due attention is given to some fundamental aspects relevant to the understanding of nuclear data.

Cross-section standards are required to be of high quality and accuracy to form the foundation of a wide range of important nuclear measurements and evaluations for both energy and non-energy applications. They constitute the yardstick for all other nuclear physics measurements. These standards have been re-assessed and evaluated to make significant improvements in the data, and to take into account important new experimental data and improved evaluation techniques. The methods employed were the most sophisticated ever used in cross-section evaluations. These present standards are more accurate than all earlier versions, and the given errors are well justified in the context of contemporary data and evaluation methodology. --Publisher's description.