The main function of the ACS is to shield the Germanium detectors against background (photons and particles) from sources outside the FoV. The ACS system consists of 91 Bismuth Germanate scintillator crystals in combination with photo multiplier tubes.
The scintillator crystals are used to convert all incoming events into photons in the 480 nm region (visible light). Photo-multiplier tubes are used to detect these photons and convert them into electrical pulses which are sorted, normalized and summed up by the ACS electronics. Each photon registered by ACS induces a time tagged veto signal directed to the Digital Front End Electronics (DFEE) which reject any simultaneous detector events.