Measurements, evaluation, and validation of Ta-181 resolved and unresolved resonance regions

Abstract

Neutron transmission and neutron capture yield measurements were made at Rensselaer Polytechnic Institute to obtain energy differential cross section data that was not previously available to the nuclear engineering and physics communities. This was done in an effort to support a new evaluation. An important feature of this evaluation dataset is that the transmission and capture yield measurements were made with the same experimental conditions. As the 181Ta nucleus only allows elastic scattering and capture reactions (with any significant probability) in the energy range of interest (1 eV to 100 keV), this combined dataset constrains the cross sections of interest much better than separate measurements. This combined dataset is a valuable resource that can be used to improve the community’s understanding of how 181Ta reacts to neutrons from approximately 1 eV to 100 keV.

This energy-differential validation method is also capable of answering the question evaluators pose about the extension of the resolved resonance region. Often evaluators wish to extend the resolved resonance region beyond the energies where conventional wisdom would advise against it (because it is partially unresolved), in an effort to improve the cross section model for applications. The transmission validation method can be used to verify that the partially resolved cross section either does or does not properly predict transmission. It was found that the extension of the 181Ta resolved resonance region to 2.4 keV predicted the thick-sample transmission measurement well and did not negatively affect the integral benchmarks tested in this work. It was also found that the unresolved resonance parameters in the major evaluated libraries did not properly model the thick-sample transmission measurement. To address this, the separate combined set of transmission and capture yield measurements were evaluated in the resolved and unresolved resonance regions and compared to the thick-sample transmission measurement. Good agreement was found between the Rensselaer Polytechnic Institute evaluation and the transmission validation measurement. The thick-sample transmission measurement demonstrated here can be applied to other isotopes to validate URR evaluated parameters, ultimately enabling more accurate modeling of nuclear applications.

Following the evaluation of 181Ta it is important to validate the final evaluated cross section. This has commonly been done with integral benchmarks. It has been shown in this work that, in addition to the well-known integral benchmarks, an energy-differential thick-sample transmission measurement can be used to better validate evaluated data. This is shown by modeling transmission with a continuous energy Monte Carlo code. The transmission validation method developed in this work is designed specifically to probe the resonance self-shielding effect in the unresolved resonance region. This is a novel application of a thick-sample transmission measurement in the unresolved resonance region, which focuses on the self-shielding effect.

181Ta is a metal which is resistant to heat and chemical reactions, making it a desirable material for nuclear applications. For any material used in a nuclear application the neutron cross section must be well known, or large uncertainties will exist in the behavior of the application. There are discrepancies between the reported neutron cross section for 181Ta in evaluated libraries. Total cross section and capture cross section measurements must be consulted to resolve the discrepancies found in the evaluations. The data available to the public, however, is inadequate in some energy regions. The lack of high-resolution energy differential data, in the keV region specifically, motivated performing high-resolution measurements. In addition to reporting discrepant nuclear parameters for 181Ta, evaluated libraries also apply resolved and unresolved resonance treatments to different energy regions, which can result in different predicted cross sections and observables such as transmission.