Nuclear Physics Seminar with Dan Mulrow and Mack Atkinson

"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.

"Dispersive optical model analysis of the p2p knockout reaction" presented by Mack Atkinson from TRIUMF

Nucleon knockout reaction experiments are among the best tools to investigate deviations from the independent particle picture of the atomic nucleus.  The nonlocal implementation of the dispersive optical model (DOM) provides all the ingredients for distorted-wave impulse-approximation (DWIA) calculations of knockout reactions. Previously, a DOM analysis of the electron-induced ⁴⁰Ca(e,e'p)³⁹K reaction provided a consistent description of the experimental cross sections, verifying the DWIA. In this presentation, I will discuss a similar implementation of the DOM ingredients to describe the proton-induced ⁴⁰Ca(p,2p)³⁹K reaction. Since the same DOM ingredients and reaction framework (DWIA) are being used, this analysis sheds light on the differences that arise when using a proton to induce knockout. The results are not consistent with the ⁴⁰Ca(p,2p)³⁹K data, indicating there is a problem with the typical t- or g-matrix approach to proton-proton scattering inside the nucleus.