UNC-CH Physics and Astronomy Thesis Defense

Joule Othman
Advisor: Prof. Reyco Henning

“The CAGE Scanner: Investigating Surface Backgrounds in High-Purity Germanium Detectors”

The neutrino is an elusive particle that has challenged our models of the universe. With the discovery of neutrino oscillations, we know that neutrinos have mass, which disagrees with the Standard Model (SM) of particle physics. However, we still do not know the mechanism by which neutrinos obtain their mass. The discovery of neutrinoless double-beta decay would have a profound impact on our understanding of neutrinos, and of the universe. It would show that the neutrino is its own antiparticle, ie. a Majorana particle, that lepton number is not a conserved quantity, and would give us insight into the matter-antimatter asymmetry. Next-generation searches for neutrinoless double-beta decay, such as LEGEND, are working to build ton-scale experiments with the goal of discovering neutrinoless double-beta decay. To discover such a rare process, experiments must be extremely low-background to mitigate unwanted signals that may obscure the signal of interest from neutrinoless double-beta decay. This is accomplished primarily by locating experiments underground to shield against cosmic rays, using very radiopure materials, and using pulse shape discrimination in analysis. The LEGEND experiment will operate 76Ge-enriched point-contact High-Purity germanium (HPGe) detectors directly immersed in a liquid argon (LAr) active veto. One of the largest expected backgrounds in LEGEND is from radiation interacting near the surfaces of the detectors. Thin passivated surfaces are particularly susceptible to shallowly impinging alpha and beta radiation. To help further mitigate against surface backgrounds on passivated surfaces, dedicated test stands can help us understand the detector response to surface backgrounds and develop cuts to remove them from our data, maximizing our discovery sensitivity to neutrinoless double-beta decay. In this dissertation, we introduce the Collimated Alphas, Gammas, and Electrons (CAGE) test stand, which we built to study passivated surfaces for HPGe detector geometries that will be used in LEGEND. CAGE utilizes collimated radiation sources to study the effect of shallowly impinging radiation on specific locations on the passivated surfaces of HPGe detectors. We improve on previous surface scanning systems by offering more protection from infrared (IR) shine on passivated surfaces and more flexibility in positioning the collimated source beam, most notably the ability to change the incidence angle of the source beam with respect to the passivated surface of the detector. We show that CAGE is able to operate stably and show the first results from a radial scan of a P-type Point-Contact detector using a 241Am alpha and gamma source. We present the results of a study of the risetimes of the 59.5 keV gamma from 241Am and show that certain risetime parameters can be useful in discriminating against surface backgrounds in LEGEND. We conclude by discussing the future goals of the CAGE test stand.

This defense will also be available remotely via Zoom: https://unc.zoom.us/j/97361233465?pwd=ZEUrS0xnc3JCVFovb1FFMUJtNFBnZz09