black ABS is installed at the equator of the PMTs. This design has an unforeseen advantage of suppressing an instrumental background, the PMT flasher events, which appeared in many neutrino experiments using PMTs but become difficult to reject for relative small detectors. The electronic components or connections on the base of the Hamamatsu PMT may occasionally discharge and produce a flash of light. The detected energy of these faked events ranges from sub-MeV to a hundred MeV in Daya Bay. Although only a small fraction of the PMTs spontaneously discharge infrequently, these flasher events significantly increase the accidental-coincidence background. With the black shield, the flasher events always appear as a characteristic PMT-hit pattern; thus they can be easily identified and rejected [@DB_CPC]. Without this unique PMT-hit pattern, we would have to either turn off the flashing PMTs, or bear a larger uncertainty in the selection efficiency and a larger accidental-coincidence background.

Signal and Background {#signal}
=====================

The reactor antineutrinos are detected via the inverse $\beta$-decay (IBD) reaction, $\overline{\nu}_e + p \to e^+ + n$, in the Gd-LS. The coincidence of the prompt scintillation from the $e^+$ and the delayed neutron capture on Gd provides a distinctive signature. The positron carries 