Analysis of cosmic-ray-muon induced spallation neutrons in Aberdeen Tunnel experiment in Hong Kong

Abstract

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.