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.

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)