International Evaluation of Neutron Cross Section Standards
International Evaluation of Neutron Cross Section Standards
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DOI：
10.1016/j.nds.2009.11.001
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发表时间：
2009-12
影响因子：
3.7
通讯作者：
A. Carlson;V. Pronyaev;D. L. Smith;N. M. Larson;Zhen-Peng Chen;G. Hale;F. Hambsch;E. Gai;Soo-youl Oh;S. Badikov;T. Kawano;H. Hofmann;H. Vonach;S. Tagesen
中科院分区：
文献类型：
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作者：
A. Carlson;V. Pronyaev;D. L. Smith;N. M. Larson;Zhen-Peng Chen;G. Hale;F. Hambsch;E. Gai;Soo-youl Oh;S. Badikov;T. Kawano;H. Hofmann;H. Vonach;S. Tagesen
中子截面标准是大多数中子截面测定的基础。它们用于中子截面的测量和评估。没有多少横截面可以绝对获得-大多数横截面是相对于横截面标准测量的，并使用标准的评估进行转换。上一次中子截面标准的完整评估于1987年完成，并作为NEANDC/INDC和ENDF/B-VI标准分发。在评价中，轻元素的R矩阵模型拟合和所有横截面的非模型最小二乘拟合是所有数据的组合拟合的基础。一些重要的反应和常数不是标准，但它们极大地帮助了标准截面的确定，并降低了它们的不确定性-这些数据也包含在组合拟合中。在评估中使用的最大的实验数据库是由Poenitz准备的，包括约400组实验数据和不确定性的协方差矩阵，这些数据解释了所有交叉能量、交叉反应和交叉材料的相关性。对于评价GMA，Poenitz开发的最小二乘代码，用于在一个完整的分析中拟合所有类型的横截面（绝对和形状），它们的比率，光谱平均横截面和热常数。但是，以这种方式得出的不确定性，特别是在R矩阵模型拟合中获得的不确定性，被认为太低，不切实际。这些不确定性在1987年建议的数据文件中公布之前大大增加。修正后的百分比不确定度由美国横截面评估工作组的标准小组委员会重新分配，用于广泛的能量，并且当时没有提供协方差（或相关性）矩阵。重新评价横截面标准的必要性是基于大量精确实验数据的出现以及分析和评价方法的改进发展。美国跨部门评价工作组成立了一个工作队，为编写新的评价报告作出了初步努力。人们认识到，需要开展国际合作来进行评价。核能机构核科学委员会国际评价合作工作队成立了一个分组，国际原子能机构成立了一个协调研究项目。这些小组合作改进评价过程。编写评价报告的主要工作是通过CRP。2005年10月完成了对中子截面标准的评估。以前遇到的困难与被称为“皮尔的相关难题”的数据评估问题创建偏差的相关数据的拟合，他们已经解决，以减少这种现象。截面标准的新评估还包括包含完全合理值的不确定性的协方差矩阵。这一国际努力的产物已被采用作为ENDF/B-VII.0的中子标准。
Neutron cross section standards are the basis for the determination of most neutron cross sections. They are used for both measurements and evaluations of neutron cross sections. Not many cross sections can be obtained absolutely - most cross sections are measured relative to the cross section standards and converted using evaluations of the standards. The previous complete evaluation of the neutron cross section standards was finished in 1987 and disseminated as the NEANDC/INDC and ENDF/B-VI standards. R-matrix model fits for the light elements and non-model least-squares fits for all the cross sections in the evaluation were the basis of the combined fits for all of the data. Some important reactions and constants are not standards, but they assist greatly in the determination of the standard cross sections and reduce their uncertainties - these data were also included in the combined fits. The largest experimental database used in the evaluation was prepared by Poenitz and included about 400 sets of experimental data with covariance matrices of uncertainties that account for all cross-energy, cross-reaction and cross-material correlations. For the evaluation GMA, a least-squares code developed by Poenitz, was used to fit all types of cross sections (absolute and shape), their ratios, spectrum-averaged cross sections and thermal constants in one full analysis. But, the uncertainties derived in this manner, and especially those obtained in the R-matrix model fits, have been judged to be too low and unrealistic. These uncertainties were substantially increased prior to their release in the recommended data files of 1987. Modified percentage uncertainties were reassigned by the United States Cross Section Evaluation Working Group's Standards Subcommittee for a wide range of energies, and no covariance (or correlation) matrices were supplied at that time. The need to re-evaluate the cross section standards is based on the appearance of a significant amount of precise experimental data and improved developments in the methodology of analysis and evaluation. Initial efforts to produce a new evaluation were made by the United States Cross Section Evaluation Working Group which formed a Task Force. It was realized that international cooperation would be needed to produce the evaluation. The Working Party on International Evaluation Cooperation of the Nuclear Energy Agency Nuclear Science Committee formed a Subgroup, and the International Atomic Energy Agency formed a Coordinated Research Project (CRP). These groups worked cooperatively to improve the evaluation process. The major effort in producing the evaluation was through the CRP. 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 they have been 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. The product of this international effort has been adopted as the neutron standards for ENDF/B-VII.0.