Home Search Collections Journals About Contact us My IOPscience The neutron cross section standards, evaluations and applications This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Metrologia 48 S328 (http://iopscience.iop.org/0026-1394/48/6/S09) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 129.6.99.26 The article was downloaded on 28/11/2012 at 19:44 Please note that terms and conditions apply. IOP PUBLISHING METROLOGIA Metrologia 48 (2011) S328–S345 doi:10.1088/0026-1394/48/6/S09 The neutron cross section standards, evaluations and applications A D Carlson National Institute of Standards and Technology, STOP 8463, Gaithersburg, MD 20899, USA E-mail: [email protected] Received 21 April 2011, in final form 19 June 2011 Published 28 October 2011 Online at stacks.iop.org/Met/48/S328 Abstract The measurement of a neutron cross section can be simplified if it is measured relative to a neutron cross section standard. Then it is not necessary to measure the neutron fluence. The standards in effect provide determinations of the neutron fluence. These standards are not defined standards but instead working standards. As such it is important to make measurements to improve the quality of the standards. Also better techniques for evaluating the standards must be promoted. The history of the evaluation process for the standards is discussed from the first rather primitive one to the latest very sophisticated evaluation that was an international cooperative effort. An overview of the main detectors used for applications of the standards is given. (Some figures in this article are in colour only in the electronic version) 1. Introduction section or fluence measurements that can be made without using a standard. These measurements are either independent In the measurement of a cross section, for thin samples, the or only very weakly dependent on any cross sections. These event counting rate for the cross section to be measured is include those using transmission (to obtain the total cross given by section), certain types of spherical shell transmission (to obtain R = εφNσ (1) the inelastic cross section), associated particle and associated where ε is the efficiency for detecting the event, φ is the neutron activity measurements. Also certain detectors such as the fluence, N is the number of atoms in the sample and σ is the ‘Black Detector’ [POE72] allow accurate determinations of cross section being measured. the neutron fluence. The measurements of neutron cross sections are Neutron cross section standards were used very early in significantly easier if a neutron cross section standard can be the cross section measurement process. Unfortunately many used. Then if a detector that implements the standard reaction of the ‘standards’ were not appropriate. And in some cases, is placed in the same neutron beam where the detector for which improved candidates were suggested [CAR70, CIE77] but not a cross section to be measured is located, for the conditions of adopted, largely as a result of the need to significantly improve equation (1), one obtains their databases which would have required significant effort. An idealized standard should have the following R ε N σ = x s s σ (2) characteristics: x R ε N s s x x • It should be possible to use the nuclide in elemental where the subscript x refers to the detector for which a cross form (not in a compound), be chemically inert and not section is to be measured and subscript s refers to the standard radioactive. detector. Using the standard it is not necessary to make a direct • It should be easy to fabricate into various shapes. measurement of the neutron fluence. The standards in effect • It should be readily available; not expensive. remove the need for measuring the fluence. However, then it is • It should have few (or no) other channels open that could necessary to measure the standards very accurately since any cause interference with the reaction of interest. measurement relative to a standard is limited in accuracy to • For activation standards, there should be suitable half-lives that of the standard. There are, however, a few types of cross and decay schemes for the product. 0026-1394/11/060328+18$33.00 © 2011 BIPM & IOP Publishing Ltd Printed in the UK & the USA S328 The neutron cross section standards, evaluations and applications • Monotopes are preferred. For the various evaluations in the library, the data had been • For capture standards, the total gamma-ray energy should measured relative to a standard cross section, but different be large with high multiplicity yet a hard capture gamma- evaluators were not necessarily using the same values for the ray spectrum. standard cross sections. Thus the term standard had a different • In the standards energy region, the cross section should be meaning for that work compared with what it means now. It large with a minimal amount of structure. was rapidly recognized that proper and consistent standards • For capture standards, preferably one resonance should must be used in the evaluation process. The evaluations for have appropriate resonance parameters so the saturated ENDF/B-I were basically used to check the processing codes resonance technique can be used for neutron fluence which had been written. determination. Normally the conversion of a cross section ratio measurement 2.2. ENDF/B-II to the cross section of interest is not done with a single measurement of that standard. Instead the available data on the Greater attention was given to the standards used for cross standards are evaluated to improve the accuracy and provide section evaluations in this version. A Normalization and uncertainty information that is generally much better than can Standards Subcommittee was established to work on the be obtained from a single standards measurement. These standards. The ENDF/B-II library did not predict several standards become the basis for the evaluation of cross sections integral benchmark measurements as well as the ENDF/B-I for the neutron cross section libraries. library, even though much of the ENDF/B-II data were believed to be appreciably ‘better’ than the ENDF/B-I data. Neither 2. Historical perspective of these libraries was believed adequate for reactor design applications. There were many early standards but as the need for better cross sections, hence better standards arose, the standards with less desirable features were removed. Except for the most recent 2.3. ENDF/B-III evaluation of the standards, the responsibility for evaluating the standards was the US Cross Section Evaluation Working The scope of responsibilities of the Normalization and Group (CSEWG) that produces the ENDF library. Within that Standards Subcommittee had become very broad. All Group the actual evaluation of the standards was usually done activities within CSEWG Subcommittees involving standards by a Standards Subcommittee. The standards were always and dosimetry cross sections required interaction with this made available internationally for all versions of the ENDF/B Subcommittee. Any interaction outside the CSEWG, e.g. with standards. The evaluations were accepted internationally to the International Atomic Energy Agency (IAEA), on standards ensure that the same standards are used worldwide in all efforts was also handled by this Subcommittee. It appeared as major evaluation projects. The ENDF/B standards were made though anything that was known relatively well had to have available outside of America even for ENDF/B-V while other some sort of approval from the Normalization and Standards ENDF/B-V evaluations were not. Seldom is a single standards Subcommittee. measurement used in a critical calculation. Instead all the The ENDF/B-III efforts led to laboratories/individuals standards data are evaluated to provide the best results. The taking on the responsibility for specific evaluations for which standards have an important role in the definition of a data they had expertise and interest. library. Almost all the cross sections in a given data library The ENDF/B-III evaluation process for the 235U(n, f) cross depend on the standard cross sections. There is therefore section had a problem since the database showed a trend very large leverage for improvements to the standards since in which the cross section appeared to be decreasing with an improvement in a standard causes an improvement in every time. This is illustrated in [POE70]. Actually the trend was cross section relative to that standard. This applies to cross not unreasonable. The earlier measurements were subject to section measurements that have been made as well as those large backgrounds which were difficult to remove completely. that will be made. These backgrounds add signal (counts) to the apparent fission When ENDF/B was in its infancy, the number of standards, response, thus making the cross section appear too high. their energy ranges of applicability and their accuracy were However, there were many discussions with experimenters not well established. The need for better standards has led who had the larger cross sections and were convinced that they to significant improvements and very sophisticated evaluation had the ‘correct’ values for the cross sections. All the data procedures such as those that were used for the ENDF/B-VI were used in the evaluation process. and ENDF/B-VII standards evaluations. For ENDF/B-III, there were a number of reactions that were very seriously considered for standards but 2.1. ENDF/B-I not accepted (e.g. 233U(n, f), and a number of capture The initial evaluation efforts for the standards were for the reactions). The ENDF/B-III standard cross sections were for ENDF/B-I library. The standards could not be used properly H(n, n), 3He(n, p), 6Li(n, t), 10B(n, α), 12C(n, n), Au(n, γ)and for this library.