when bonding together on the edge. The sandwich structure is maintained by vacuum pressure with the least mechanical connections between two panels to keep the optical surface intact. A reflector is put on the top of the gamma catcher and another at the bottom. Such a design reduces the number of PMTs by about one half while achieving almost the same energy and vertex resolution. Furthermore, the mechanical structure of the AD is simplified with the adoption of reflective panels, enabling transportable ADs.

Three water-proof automated calibration units (ACUs) are mounted on the top of each AD. Each ACU is equipped with a LED, a $^{68}$Ge source, and a composite source of $^{241}$Am-$^{13}$C and $^{60}$Co. Deployment of the source into the liquid scintillator, one at a time, is controlled remotely. With the ACUs, the AD can be fully submerged in water without a chimney for calibration. Therefore, Daya Bay does not experience backgrounds coming from external radioactivity or the Michel electrons from decays of stopped muon.

The inner wall of the stainless steel tank is painted black with a fluor-carbon paint. To reduce the stray light reflected from the PMT glass, cable, and other components, another light shield made of 