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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
Kim, Samuel
Kim, Samuel
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Thesis/Dissertation
Date
2021
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Physics
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http://dx.doi.org/10.34944/dspace/6437
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.
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