The purpose of the plastic scintillator subassembly (PSAC) is to reduce the 511 keV background due to particle emission by the passive mask. The detector consists of a plastic scintillator inside a light tight box, located just below the passive mask. It has a good gamma ray transparency, and actively detects particles which deposit energies in excess of 300 keV. The light flashes that are produced by the impacts of these particles are detected by four photo multiplier tubes located around the light-tight box and converted into electrical pulses which are processed by the PSAC electronics assembly. The electronics produce a veto signal associated with the detected events, which are later merged with the ACS veto signal by the ACS front end electronics.