Observations on the Effects of 252Cf Spontaneous-Fission Neutron Spectrum Uncertainties on Uncertainties in Calculated Spectrum-Average Cross Sections for Reactions in the Neutron Dosimetry Library IRDFF-II

Abstract

The Neutron Dosimetry Library IRDFF-II was released in January 2020, and was subsequently documented by an article published in the Nuclear Data Sheets by A. Trkov et al. The development of this library, under the auspices of the IAEA Nuclear Data Section, entailed extensive effort to assemble and examine the available, relevant experimental data, as well as to select and validate the best available cross-section evaluations, for some 110 neutron reaction processes included in this library. Part of the validation process for this library entailed comparing experimental and evaluated differential cross-sections as well as corresponding integral cross sections for a variety of integral neutron spectra considered to be relevant for fission and fusion neutron dosimetry purposes. Prominent among these spectra is the well-known 252Cf spontaneous-fission (s.f.) neutron spectrum that is considered to be a standard for both IRDFF-II and the ENDF/B libraries. Experimental and calculated uncertainties for all these data were considered in this validation process. The evaluated 252Cf s.f. neutron spectrum, owing to the manner in which it was developed, is considered to be largely independent of the evaluated neutron reaction cross sections in both of these libraries. Therefore, the uncertainties in calculated spectrum-average cross sections (SACS) stemming from evaluated cross-section uncertainties are considered to be largely independent from the uncertainties in the evaluated 252Cf s.f. neutron-spectrum itself. The present investigation sought specifically to examine systematic behaviors for the uncertainties of calculated neutron reaction SACS in IRDFF-II due exclusively to uncertainties in the 252Cf spectrum. This work was enabled by the availability of extensive calculations performed and documented by A. Trkov during development of the IRDFF-II Library. It was observed from the present investigation that the 252Cf s.f. neutron spectrum component of these calculated SAC uncertainties, for threshold reactions that exhibit relatively smooth cross-section behaviors, vary quite smoothly and predictably as a function of the parameter E50% (that neutron energy at which the reaction-rate integral for this neutron spectrum reaches 50% of the integral over the full energy range) for values of E50% from around 2 MeV up to almost 17 MeV. The behaviors observed in the present investigation for those reactions in IRDFF-II involving lower and higher values of E50% are less predictable owing to factors that are discussed in this report. This report also provides numerous tables and plots based on calculated results from the work of A. Trkov to illustrate these conclusions.