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
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/146140 |
| Date | January 2012 |
| Creators | Ngai, Ho-yin., 倪浩然. |
| Contributors | Pun, JCS, Leung, JKC |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Source | http://hub.hku.hk/bib/B47249250 |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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