Reduced Order Modelling for Nuclear Linear Response and the Incompressibility of Pb-208

Abstract

Linear response theory provides essential information regarding the excitations of many-body systems, such as atomic nuclei. It yields ground state transition probabilities, or strength functions, from which reaction rates and cross-sections can be derived. These quantities are for example a critical input for astrophysical simulations and modelling of beta-decay. Currently, the most general theoretical framework for modelling global nuclear properties is Energy Density Functional (EDF) theory. Modern approaches for linear response employ the quasiparticle random-phase approximation (QRPA) on top of a mean-field vacuum. This can be done by using conventional matrix QRPA formulations, but can be sped up substantially by using the finite amplitude method (FAM). Nevertheless, obtaining highly-resolved response functions over the complete nuclear chart remains computationally demanding, which limits large-scale applications. This work introduces a Reduced Order Modeling (ROM) approach to emulate Finite Amplitude Method (FAM-QRPA) calculations, significantly reducing the computational cost of obtaining nuclear response functions. By employing a 2D-greedy strategy to interpolate from a small set of snapshots, the emulator achieves a x20 speed-up while maintaining high accuracy across various nuclei, operators, and energy density functionals (EDFs). A second objective of this work is to investigate the correlation between the infinite nuclear matter incompressibility and the ISGMR centroid position of Pb-208, specifically for EDF forms and parametrisations developed in Brussels: the BSk(G)-family. Our results indicate that the correlation does not persist.

Comments: Dissertation presented in fulfillment of the requirements for the degree of Master of Physics. Supervised by Dr. W. Ryssens. Corrected version containing minor corrections to the original (https://lib.is/lbsn9994932375301471)