The Veto shield is crucial to the operation of IBIS. IBIS uses anticoincidence logic to accept or reject detected events as real photons in the field of view, or background particles or photons propagating through, or induced in, the spacecraft.
The sides, up to the ISGRI bottom level, and rear of the stack of detector planes are surrounded by an active Bismuth Germanate (BGO) veto shield. Like the detector array, the Veto shield is modular in construction. There are 8 lateral shields, i.e., 2 modules per side, and 8 bottom modules.
The high density and mean of BGO ensures that a thickness of 20 mm is sufficient to reduce the detector background due to leakage through the shielding of cosmic diffuse gamma-ray background and gamma-rays produced in the spacecraft, to less than the sum of all other background components.
IBIS VETO count rate is transmitted to ground each 8s in dedicated housekeeping packets and can be used as detectors for gamma-ray bursts, without directional information. Its geometrical configuration complements the coverage of the SPI anti coincidence shield, making INTEGRAL an almost omni-directional detector of gamma-ray burst [12].