The Daya Bay reactor neutrino experiment aims to determine sin2 2θ13 with a sensitivity of 0.01 or better at 90% confidence level. One of the major backgrounds to neutrino measurements is the muon-induced neutrons. An ex- periment had been set up inside the Aberdeen Tunnel laboratory, Hong Kong, to study spallation neutrons induced by cosmic-ray muons in an underground environment similar to the Daya Bay experiment. The Aberdeen Tunnel laboratory is 22 m above sea level at 22:23?N and 114:6?E. The amount of overburden is approximately 235 m of rocks, which is equivalent to 611 m.w.e. Rock compositions in the Aberdeen Tunnel area is similar to that in Daya Bay. MUSIC simulation results showed that in the laboratory the mean energy of muons 〈Eμ〉= 122 GeV and the integrated muon intensity I = 9:64 X10??6 cm??2 s??1. A Bonner Spheres Neutron Spectrometer (BSS) was developed to measure the ambient neutron energy spectrum. The BSS consists of a thermal neutron detector and a set of eight polyethylene spherical shells. The overall detection efficiency of the BSS was (96:7 +3:3 ??13:1)% with a detector background rate of (1:96_0:03)_10??3 s??1. The total neutron fluence rate measured at the Surface Assembly Building (SAB) of the Daya Bay experiment was (5:20 +0:81 ??0:44) _ 10??3 cm??2 s??1, which agreed with the neutron fluence rate measured in the air/ground interface in Taiwan. The unfolded SAB neutron energy spectrum showed a clear thermal-neutron peak around 20 meV and a cascade peak around 100 MeV. Detectable number of neutrons could be seen at 1 GeV. The neutron fluence rate measured at the Aberdeen Tunnel (ABT) laboratory was significantly higher then some other underground laboratories. The unfolded ABT neutron energy spectrum showed a pronounced evaporation peak around 1 MeV, and a sup- pression in the cascade peak. Detections of muon-induced neutrons inside the Aberdeen Tunnel laboratory is achieved by a Muon Tracker and a Neutron Detector. The Muon Tracker consists of three main layers of crossed plastic scintillator hodoscopes capable of determining the incoming direction of muons. The average efficiency for most of the hodoscopes was above 95%. The Neutron Detector consists of about 760 L of gadolinium-doped liquid scintillator and sixteen photomultiplier tubes. The liquid scintillator target is shield by about 1900 L of mineral oil from external radiations. The overall average detection efficiency of muon-induced neutrons was about 16%. The measurement of muon-induced neutrons in the Aberdeen Tunnel lab- oratory started from June 2011, with a total live time of about 30 days. The average rate of the accepted muon events was 0.013 Hz. The muon-induced neutron yield was determined to be Nn = (8:5 _ 0:4(syst.) _ 1:8(stat.)) _ 10??5 neutron/(μg cm??2). This value agreed with the parametrization of FLUKA-1999 simulation results if the muon energy dependence of muon-induced neutron yields was considered. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The muon-induced radioactive isotopes, especially neutrons, are dangerous background component for rare-event detection in underground experiments, like neutrino-less double-beta decay and dark matter search. Understanding these cosmogenic backgrounds is crucial for these experiments. An underground experiment aiming at measuring the cosmic-ray muons' flux and their neutron production yield in liquid scintillator through spallation process is being carried out in the Aberdeen Tunnel laboratory located in Hong Kong with a total vertical overburden of 235 m of rocks (611 m.w.e.). The Aberdeen Tunnel detection system is constituted of a Muon Tracker (MT) for muon tagging and a Neutron Detector (ND) for neutron detection. The MT consists of 60 plastic scintillator hodoscopes to determine the incoming muon direction and the ND is a two-zone detector containing 760 L of gadolinium-doped liquid scintillator as target volume and 1900 L of mineral oil as shields. The experiment has been taking data stably since 2012. To obtain reliable results, the detector performance and the stability of the experiment have been studied in this work. Muon-induced fast neutrons can be captured in Gd-LS with characteristic energies released and the capture time follows a characteristic exponential distribution. By using the capture time and energy information, we can select the neutron candidates and thus calculate the neutron production yield. The energy of a neutron capture event is reconstructed from the calibrated photo-multiplier tube signals, while the directions of cosmic-ray muons can be reconstructed from the MT. The mean energy of the incoming muons that pass the selection criteria was estimated by a simulation code MUSIC that transported atmospheric muon spectrum through the mountains to the laboratory, and is found to be 92 GeV. The neutron production yield is calculated to be Yn = (3:28 ±0:12(sta:) ±0:24(sys:)) X 〖10〗^(-4) (n/μ〖gcm〗^(-2)) for both the showering muon and single muon events. This result is about two times higher than the expectation value from previous simulations and experiments. The neutron production yield of the single muons is calculated to be Yn = (1:04 ± 0:08(sta:) ± 0:07(sys:)) X 〖10〗^(-4) (n/μ〖gcm〗^(-2)). This reveals a enhancement of the neutron production from the muons accompanied by showers. / published_or_final_version / Physics / Master / Master of Philosophy