D. A. Matters | Searcharxiv

arXiv2026

Analytic calculator for determination of $\gamma$-ray angular distribution coefficients and tensors in aligned and partially-aligned nuclei

A program has been developed to calculate a complete set of $\gamma$-ray angular distribution coefficients and statistical tensors in maximally- and partially-aligned nuclei. For practical nuclear structure and reaction purposes, there is no imposed constraint on any arguments that are likely to arise in the determination of these quantities. The program can also be used as a stand-alone vector-coupling calculator for the exact evaluation of Clebsch-Gordan and Racah coefficients, the closely-related Wigner 3-$j$, 6-$j$, and 9-$j$ symbols, as well as Gaunt coefficients. These quantities, which frequently arise in quantum mechanical applications involving angular momentum coupling and recoupling schemes, provide the underlying machinery in angular distribution calculations.

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arXiv2026

Westcott $g$ Factors Extended to Arbitrary Neutron Energy Spectra

Westcott $g$ factors are used in Neutron Activation Analysis (NAA) and Prompt Gamma-ray Activation Analysis (PGAA) to evaluate the impact of non-$1/v$ behavior in the neutron-capture cross sections of certain nuclei on activation product yields. This non-$1/v$ behavior arises from the presence of neutron resonances in the neutron-capture cross sections that overlap with the source neutron spectrum at low ($<5$~eV) energies. Historically, Westcott $g$ factors that have been cataloged for NAA and PGAA applications are the result of calculations that assume a Maxwellian neutron velocity distribution with a given average temperature. In this study, we use this approach with updated neutron-capture cross sections from the Evaluated Nuclear Data File, version VIII.1 (ENDF/B-VIII.1) to tabulate Westcott $g$ factor values for a broad range of Maxwellian distribution temperatures, comparing the results against currently-available $g$ factors from International Atomic Energy Agency tables and other sources. It was discovered during this analysis that the use of guided thermal and cold-neutron beams at certain facilities necessitates an approach for evaluating Westcott $g$ factors based on arbitrary non-Maxwellian spectra. In this paper, we present an approach for calculating $g$ factors with user-specified neutron spectra, and we apply these methods to obtain Westcott $g$-factors for guided- and cold-neutron beams at the Budapest Research Reactor and the Forschungsreaktor M{\"u}nchen II reactor. Open-source software has been developed as part of this study that can be used to perform these calculations for applications in PGAA and NAA experiments

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