Release Notes

International Neutron Cross Section Standards 2006

Introduction

Neutron cross-section standards are important in the measurement and evaluation of all other neutron reaction cross-sections. Not many cross-sections can be defined as absolute - most cross-sections are measured relative to the cross-section standards for normalization to absolute values. Previous evaluations of the neutron cross-section standards were completed in 1987 and disseminated as both NEANDC/INDC ( NEANDC-311 ) and ENDF/B standards. R-matrix model fits for the light elements and non-model least-squares fits for the heavy elements were the basis of the combined fits for all of the data. Some important reactions and constants are not standards, but assist greatly in the determination of the standard cross-sections and reduce their uncertainties - these data were also included in the combined fits.

The need to re-evaluate the cross-section standards at the beginning of the 21st century is based on the appearance of a significant amount of precise experimental data and developments in the methodology of analysis and evaluation. An IAEA Consultants' Meeting was held in 2001 to consider the major tasks to be undertaken in order to improve the 1987 standards evaluation ( Summary Report of the Consultants' Meeting on Improvement of the Standard Cross Sections for Light Elements, Vienna, 2-4 April 2001, INDC(NDS)- 425, prepared by A.D. Carlson, D.W. Muir and V.G. Pronyaev, June 2001 ). Thus, an IAEA Co-ordinated Research Project (CRP) entitled "Improvement of Standards Cross-Sections for Light Elements" was formulated, and this work was substantially extended through the course of these multinational studies by the inclusion of tasks to evaluate the cross-section standards for heavy elements.

Neutron Cross-section Standards Reactions

Reaction	Neutron Energy Range
	1987	2002-2005/06
			ENDF-6 Format	Free text Format
H(n,n)	1 keV to 20 MeV	1 keV to 20 MeV	std-001_H_001.endf	not available
3He(n,p)	0.0253 eV to 50 keV	0.0253 eV to 50 keV (1987 adopted)	std-002_He_003.endf	not available
6Li(n,t)	0.0253 eV to 1 MeV	0.0253 eV to 1 MeV	std-003_Li_006.endf	standards-6Li_xs-data.txt
10B(n,α)	0.0253 eV to 250 keV	0.0253 eV to 1 MeV	std-005_B_010.endf	standards-10B_na-xs-data.txt
10B(n,α1γ)	0.0253 eV to 250 keV	0.0253 eV to 1 MeV	std-005_B_010.endf	standards-10B_na1-xs-data.txt
C(n,n)	up to 1.8 MeV	up to 1.8 MeV (1987 adopted)	std-006_C_000.endf	not available
Au(n,γ)	0.0253 eV, and 0.2 to 2.5 MeV	0.0253 eV, and 0.2 to 2.5 MeV	std-079_Au_197.endf	standards-197Au_xs-data.txt
235U(n,f)	0.0253 eV, and 0.15 to 20 MeV	0.0253 eV, and 0.15 to 200 MeV	std-092_U_235.endf	standards-235U_xs-data.txt
238U(n,f)	threshold to 20 MeV	2 to 200 MeV	std-092_U_238.endf	standards-238U_xs-data.txt

The evaluations of the neutron cross-section standards were finalized in October 2005. Previous difficulties experienced with a data evaluation problem known as "Peelle's Pertinent Puzzle" create biases in the fit of correlated data, and were addressed to reduce this phenomenon. The new evaluations of the cross-section standards also include covariance matrices of the uncertainties that contain fully justifiable values. Significant contributions to the experimental database were made by participants of Subgroup 7 of the NEA Working Party on International Nuclear Data Evaluation Co-operation (WPEC). Furthermore, the evaluations could not have been carried out without access to the original GMA database and related computer codes given to staff of the IAEA Nuclear Data Section by the US Cross Section Evaluation Working Group (CSEWG).

Last updated: 13 Dec 2007.