Nuclear Physics Seminar with Dan Mulrow and Natália Calleya

"Temperature Dependence of Triplet Diffusion in Solid-State Scintillators" presented by Dan Mulrow from Washington University in St. Louis

Organic scintillators can discriminate between incident gamma rays and neutrons from different densities of triplet states generated by recoiled electrons and protons (or other charged ions), respectively. Recoiled electrons predominantly produce excited singlet states in organic scintillators that produce a prompt "fast" decay signal. Triplet states have a naturally slow decay because the transition to the ground-state singlet is forbidden. This aspect of a triplet's decay makes it susceptible to a mechanism known as triplet-triplet annihilation, by which two triplets, with appropriate spin alignments, convert to an excited singlet plus a ground-state singlet. The excited singlet promptly decays. The time delay of the singlet de-excitation depends on the density of triplets and the spatiotemporal overlap of their wavefunctions.  Temperature dependence is expected to influence the rate of diffusion of triplets and thus the rate of triplet-triplet annihilation. In this study we investigate the influence of correlated excitation from neutrons/gammas versus single excitation events from photon absorption.

"Nuclear Binding Energies and the Saturation Energy of Nuclear Matter" presented by Natália Calleya from Washington University in St. Louis

For years many nuclear models have tried to reproduce the nuclear matter density and energy at saturation, with varying degrees of success. While one of these numbers (ρₒ) has been determined experimentally via elastic electron-scattering, the other (Eₒ) still relies on an extrapolation of the empirical mass formula. Although very successful at describing nuclear binding energies, the empirical mass formula may not contain all the necessary ingredients to determine the the binding energy at nuclear saturation, and recent calculations have  questioned the validity of said extrapolation by resulting in an  Eₒ closer to 14 MeV. A new energy functional that incorporates nuclear structure information was developed to generate binding energies and provide a better understanding of an extrapolated Eₒ value. New neutron skin data was incorporated in the calculations and will be part of the discussion.