and 90% for the RPCs. The combined efficiency was aimed to be ($99.5\pm0.25$)%. As it turned out, due to high reflectivity of the custom-made Tyvek^^ composite film lining the partitions and the well-designed water purification system, the efficiency of the inner Cherenkov detector was 99.98% [@DB_muon]. Again, the multiple-detector design provides robust tagging of the cosmic-ray muons, which is essential for rejecting muon-induced background.

The geometry of the AD was optimized with extensive Monte Carlo simulation. The MO shielding is thin and the distance from the apex of the PMT to the liquid scintillator is only 20 cm. This distance is driven by the requirement of uniform detector response instead of radiation shielding. As a result, Daya Bay has a relatively high singles rate and accidental coincidence to maximize the target mass. Since the accidental-coincidence background can be determined accurately to high precision, it has negligible impact to the sensitivity or precision.

To obtain good light yield with fewer number of PMTs, reflective panels are used. A specular reflective film with reflectivity $>98\%$ in the scintillation light region, ESR (3M^^), is sandwiched between two 1-cm-thick acrylic panels 4.5 m in diameter. The space between the two acrylic panels is evacuated 