TALENT. KWOK
Published: 2017-01-27
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This dissertation, "A Study of Background Radiation and Cosmic Muon Detection at the Aberdeen Tunnel Laboratory in Hong Kong" by Talent, Kwok, 郭天能, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled A STUDY OF BACKGROUND RADIATION AND COSMIC MUON DETECTION AT THE ABERDEEN TUNNEL LABORATORY IN HONG KONG Submitted by Talent Kwok for the degree of Master of Philosophy at The University of Hong Kong in July 2007 Background identification and suppression is one of the crucial steps for successful underground high sensitivity particle experiments. In the underground environment, natural radioactivity associated with both rocks and hard components of cosmic rays, the latter having energy in the order of 10 GeV or above, dominate. The natural radioactivities, mainly originating from radioactive isotopes in the U series, 40 Th series and K in rocks or detector materials, can create overwhelming number of chance coincidence over genuine but rarer particle events. Cosmic rays, composed of mainly energetic cosmic muons, can produce backgrounds including cosmogenic radioactive isotopes, electromagnetic showers, and neutrons, which cannot be entirely eliminated by going deeper in Earth's crust or using more passive shieldings. The Daya Bay Reactor Neutrino Experiment, which aims at establishing neutrino oscillation due to the mixing angle θ and determining sin 2θ to a sensitivity of 0.01 13 13 or better at 90% confidence level, is facing the same challenge of background suppression. Before the commencement of tunnel construction for the Daya Bay experiment, background data were obtained in the underground laboratory in the Aberdeen Tunnel in Hong Kong ( 602 m.w.e.). In the Aberdeen Tunnel laboratory, a muon tracker is constructed to be the trigger of the future neutron detector, so that multiplicity of spallation-neutron from cosmic muon can be measured. The muon tracker comprises three layers of plastic scintillator hodoscopes and gaseous proportional counters, respectively. Characterization of the detectors has been carried out to optimize the quality of their performance. Natural radioactivity measured by High Purity Germanium (HPGe) detector in 238 the Aberdeen Tunnel (20 hours) shows a concentration of (6.83 0.19) ppm for U, 232 (26.61 0.76) ppm for Th, and (3.54 0.21)% for K O. These concentrations are smaller than the numbers of Daya Bay rock sample numbers of 10.4 ppm, 33 ppm and 238 232 4.41%, for U, Th and K O, respectively. This shows the natural gamma background in the Aberdeen Tunnel laboratory due to Uranium, Thorium and Potassium is about 20% - 30% less than the rock samples from Daya Bay. Cosmic muon count rates at different zenith angles were measured at sea level and in the Aberdeen Tunnel underground laboratory using plastic scintillator hodoscopes. Sea level cosmic muon agrees with the zenith angle (θ) relation: 2 2 -2 -1 -1 Φ(θ)= 72.0 cosθ ( 1 - 0.108 sinθ ) m s sr The underground measurement and simulation results by MUSIC for the Aberdeen Tunnel landscape using modified Gaisser parameterization agree to their order of magnitude only. This is probably due to the low statistics in count rate measurement of the muon telescope, leading to percentage error ranging from 19% - 31%. DOI: 10.5353/th_b3881997 Subjects: Muons Cosmic rays