|The Lab||The Detector||The Collaboration|
Located inside cross-tunnel No.5 of the Hong Kong Aberdeen Tunnel, the lab is 22m above sea level and has an overburden of ~250m of rock (corresponding to ~660 m.w.e.). The adjacent Mounts Cameron (420m) and Nicholson (430m) created effective shielding against background radiations.
Since its reopening in 2005, the lab has been fully refurbished. Wireless LAN allows Internet access from laptop computers. A real-time audio-visual monitoring system is installed recently and can be accessed online. The high voltage supply for proportional counter tubes are remote-controllable. Walls are coated with anti-radon paint to reduce natural emanation and dust drop-out from rocks. Fully air-conditioned, temperature and humidity are constantly monitored and maintained below 22 degrees C and 50-60% respectively.
As the lab is inside a functioning traffic tunnel with a flow of >100,000 cars per day, dust presents a constant challenge for the electronic components. At present, all incoming air is filtered and the lab is cleaned regularly. There are also plans to install clean-room facilities in future.
The detector has two main components - a muon tracker and a neutron detector.
The muon tracker consists of four alternating horizontal layers of cylindrical proportional counters (PropTubes) and plastic scintillators at the top of a movable steel frame and two layers at the bottom. Tubes in adjacent layers point to mutually perpendicular directions. Passage of energetic cosmic muons will produce simultaneous signals in multiple layers of counters (a "coincidence"). The track of incoming muons can then be reconstructed using the coordinates of the counters that fired during the coincidence.
The neutron detector is still under construction. It is expected to hold about 1 tonne of gadolinium-doped organic liquid scintillator using a cyclindrical acrylic tank. A larger steel tank contains the acrylic tank and a mineral oil buffer, which serves to absorb background gamma radiation. Scintillation light is detected by PMTs located at the corners inside the rectangular steel tank. Signals from the neutron detector will only be taken into account when triggered by a coincidence at the muon tracker. This ensures that the neutron signal is due to an incoming muon. Lead sheets will be placed around the detector to minimise background radiation. Click here to know more about the current design of the neutron detector.
This detector provides a means to measure the flux and angular distribution of cosmic muons and study the characteristic scintillation signal due to muon-induced neutrons.
The Daya Bay experiments aim to measure the neutrino mixing angle θ13 using the inverse β decay process. During inverse β decay, an electron antineutrino collides with a proton to give out a neutron and a positron. The positron then annihilates on collision with an electron, generating two γ photons while the neutron produces a scintillation light signal with a characeristic time delay of ~180μs.
When a cosmic muon passes through a scintillator, there are two possibilities that it can mimic an antineutrino signal:
Given the similar rock structure between the Aberdeen Tunnel and the Daya Bay detector site, data from the Aberdeen experiment can provide good estimations for the background neutron counts at Daya Bay.
The Daya Bay site currently consists of two operating nuclear power plants - Daya Bay and LingAo, shown below - and plans for a third plant have already begun.