Measurement of cosmic-ray muon induced neutrons in the Aberdeen Tunnelunderground laboratory in Hong Kong

Abstract

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.