Is the Daya Bay Reactor Neutrino Experiment an experiment on nuclear energy?

No, instead the proposed experiment at Daya Bay aims to study neutrino oscillation and obtain a precise measurement of the mixing angle θ13.

Instead of being the subject of the study, the nuclear reactors at Daya Bay act as predictable and steady sources of raw materials - electron antineutrinos - for the experiment. Using knowledge about the fission process, neutrino flux through the detector can be predicted and compared to the measured value. Any difference would be a consequence of neutrino oscillation.

The site of the detector has been carefully chosen such that effect of neutrino oscillation is maximum while overburden of rock provides greatest shielding from background radiations.

Daya Bay Nuclear Power Plant LingAo Nuclear Power Plant

The photos above show the two nuclear power plants - Daya Bay and LingAo - which are currently in operation at the site.

Where does the Aberdeen Tunnel Lab come in?

The Aberdeen Tunnel experiment is a study of cosmic radiation. One major component of cosmic rays are cosmic muons, which can interact with rock and other laboratory components to generate neutrons.

These cosmic muon-induced neutrons happen to be a major source of background error in the Daya Bay experiment because they generate the same responses as the electron antineutrinos. By analysing the characteristics of signals due to the induced neutrons, we can provide useful information to the Daya Bay experiment for differentiating between the real and "fake" experimental events.

Inside the lab Inside cross tunnel 5

Why has gadolinium in particular been chosen for the neutron detector?

Gadolinium has one of the highest 'neutron capture cross-section' of all the elements.

In a nuclear reaction, 'cross-section' refers to the rate of particle production per rate of particle input per target nucleus density. Roughly speaking, it measures the ease with which a nuclear reaction occurs.

As there is no output particle during neutron capture, the 'neutron capture cross-section' becomes the probability of capture per target atom per incident neutron.