BzScope: an absolute cross section calculator for neutron-phonon scattering
Ming Tang, Zi-Yi Pan, Ni Yang, Thomas Kittelmann, Xiao-Xiao Cai
TL;DR
BzScope addresses the challenge of computing high-precision absolute cross sections for neutron-phonon scattering in large phase spaces by adopting an adaptive VEGAS-based integral method within a harmonic-approximation framework. It provides coherent and incoherent treatments for single- and two-phonon processes, extending numerical robustness up to $Q$ in the order of $100~\text{Å}^{-1}$ and enabling higher-order terms via incoherent approximation. The software integrates with Phonopy-formatted force constants and exports to NCrystal through a dedicated plugin, facilitating direct use in Monte Carlo simulations and experimental comparisons. Validation against NCrystal and experimental data demonstrates good agreement and reveals the tool’s ability to capture low-symmetry material effects and detailed two-phonon structure, significantly advancing neutron scattering studies and condensed-matter dynamics simulations.
Abstract
BzScope is a Python package designed for efficiently calculating absolute cross sections of neutron-phonon inelastic scattering for crystalline powders in large phase spaces, addressing the limitations of traditional histogramming techniques in reproducing sharp structures and ensuring convergence. The package employs an adapted integral method and supports calculations of single- and two-phonon scattering functions in ideal crystalline powders, with numerical robustness up to a momentum transfer of 100 Ang^-1. Higher order scatterings up to several hundred orders are calculated by incoherent approximation in a well-established thermal neutron scattering physics package, NCrystal. In addition, a NCrystal plugin is made available for NCrystal-enabled Monte Carlo packages, facilitating direct comparison between the new physics and experimental data. Validation against NCrystal demonstrates good agreement in incoherent scattering for cubic systems Ni. In addition, it shows improved accuracy for low-symmetry materials $NiP_2$ by avoiding the isotropic atomic displacement approximations in NCrystal. Benchmarks the experimental differential cross section of LiH and total cross section of Be confirm its reliability. BzScope integrates with NCrystal via a plugin and therefore can be directly used in any NCrystal-enabled Monte Carlo package. This tool enhances the efficiency and accuracy of neutron scattering simulations, advancing the study of condensed matter dynamics.