of mineral oil (MO) to provide optical homogeneity and to shield the inner volumes against radiation from the detector components. There are 192 20-cm PMTs (Hamamatsu R5912) installed in the MO volume and around the circumference of the stainless steel vessel (SSV). The top and the bottom surfaces are not instrumented with PMTs; instead, there are two highly reflective panels. The PMTs are recessed in a 3-mm-thick black cylindrical shield located at the equator of the PMT bulb. In each hall, the ADs are submerged in a water pool that provides at least 2.5 m of water to degrade radiation from the rock. The water pool is optically divided into the inner (IWS) and outer (OWS) regions, both equipped with 20-cm PMTs to serve as water Cherenkov detectors. On the top of the water pool, there are Resistive Plate Chambers (RPCs) serving as another muon detector.

![Schematic diagram of the Daya Bay detectors. \[fig:det\]](AD+muon-PRL_pub.pdf){width="60.00000%"}

The multiple-module scheme is the most prominent feature of Daya Bay. The current generation of reactor neutrino experiments for determining $\theta_{13}$ planned to achieve a relative detector uncertainty of (0.38-0.6)%. Such uncertainties need very careful validation. With at least two ADs in each experimental hall, comparison 