Gamma Background Reduction in A Low Counting Facility At Shallow Depth, And Analysis of Gamma Emissions From Eight Beta Decay Contributors to the Reactor Antineutrino Spectrum Anomaly

Abstract

DarkSide-50 is a detector designed to directly detect a type of dark matter called WeaklyInteracting Massive Particles. Due to a very low event rate, a high level of radiopurity is required for the detector materials. This dissertation concerns a low background gamma counting facility at a shallow depth which we built at Temple University, to screen the potential construction materials for Darkside-50. The primary interest is in determining the radioactivity from 238-U and 232-Th decay chains, 40-K, and muon induced radionuclides. The gamma spectrometry system is based on a 100% relative eciency, n-type low background high purity germanium detector. Internal components of the detector were received from the vendor and assayed in the underground gamma ray counting facility at Laboratori Nazionali del Gran Sasso before its assembly, to ensure mBq levels of radiopurity. The sample chamber is lined with 2.5cm thick oxygen-free high thermal conductivity copper to shield against lead X-rays. The shielding enclosure has a glovebox and airlock to allow Rn decay before samples are inserted, within an airtight enclosure with eective Rn purging by LN boilo. A cosmic-ray anti-coincidence (veto) system is installed, along with borated and non-borated polyethylene panels surrounding the lead shielding for further background reduction. This resulting background level (which determines the radioanalytical sensitivity) is better than published results for the earth's-surface HPGe facility at Pacic Northwest National Laboratory. Further improvement is expected with imminent improvements to the cosmic ray veto system. In addition to DarkSide radioanalytical work, the facility was used to study ssion yields of interest in connection with the Reactor Neutrino Anomaly (RAA), a bump in the reactor antineutrino spectrum around 5 MeV wihch is under intense scrutiny in the community. Gamma rays from 252-Cf were initially studied, followed by analysis of new data from irradiation of a 235-U sample by collaborators at the Oak Ridge National Laboratory (ORNL), using High Flux Isotope Reactor (HFIR) in Neutron Activation Analysis facility (NAA). The analysis of these spectra showed that the expected and measured gamma emissions do agree with tabulated ssion yields within 2 standard deviations, implying that the errors in the tabulated data are not causing the RAA.