The Aberdeen Tunnel Laboratory
In the Daya Bay Project, we are trying to measure a very small quantity. Although a large amount of rock could help us reducing background in the experiment, serious trouble will occur when a cosmic ray muon passes through the detector.
Reminded that one of our neutrino signal is from the neutron. When a cosmic ray muon, which has high penetrating power, gets inside the detector, there are three possibilities that the muon will mimic the signal.
- Fast neutron, induced by cosmic ray mouns, scatters in detector and some protons will be recoiled giving out the fake prompt signal. The thermalized neutron will be captured at a later time forming the delayed signal.
- 8He/9Li, produced indirectly by the muons, they have a β-neutron decay chain. Again, they will have a fake prompt signal and a fake delayed signal such that they mimic the neutrino signal seriously.
Accidental, some neutrons may not recoil with the protons giving a prompt signal but there is a single neutron capture signal. Thus, it is possible that the single neutron signal coincides with the real prompt signal, giving out a fake neutrino event.
The ultimate goal of the Aberdeen project is to measure how many number of neutron will be produced from the cosmic ray muons. In the ideal case, the neutron production mechanism can be understood better. The neutron background at Daya Bay can thus be reduced, given the fact that the rock at Daya Bay is very similar to the rock at Aberdeen. Even consider the Aberdeen project as an independent experiment, the future experimental data would be very useful for other underground experiments which might need precise understanding of neutron background.