almost all of the kinetic energy of the antineutrino, thus the positron energy deposited in the liquid scintillator is highly correlated with the antineutrino energy. The neutron thermalizes before being captured on either a proton or a gadolinium nucleus with a mean capture time of $\sim$30 $\mu$s in Gd-LS or $\sim$200 $\mu$s in normal LS. When a neutron is captured on Gd, it releases several gamma-rays with a total energy of $\sim$8 MeV, and is thus easily distinguished from the background coming from natural radioactivity. The capture on H suffers from a larger background but provides an independent measurement and can improve the precision of the $\theta_{13}$ measurement.

The ADs are calibrated with sources in the ACUs weekly, and with spallation products, IBDs, and natural radiation from materials inside the detectors. Two independent calibration algorithms are utilized. The energy scale is determined using the $^{60}$Co or Am-C neutron source in the ACUs, or spallation neutrons. The uncertainty in the energy scale is determined by comparing more than 10 known references in the 8 ADs and by studying their stabilities over time. The energy scale uncertainty has reduced from 0.5% reported in the initial publications [@ad12; @DB_discovery] to 0.2% in the 