Each of the 2 BGO detectors is 12.7 cm in diameter, 12.7 cm thick, and is viewed by 2 PMTs for improved resolution and redundancy. The Bismuth Germanate (BGO) detectors provide spectral coverage from about 150 keV to 30 MeV. BGO is a high-density material which provides good sensitivity over this difficult energy range. The energy resolution of the 12.7 cm by 12.7 cm cylindrical BGO crystal will be 14% at 661 keV and 4% at 10 MeV and there will be significant efficiency overlapping the lower energy range of the LAT. Each BGO detector is coupled to 2 PMTs on opposite sides, whose outputs are summed, each with its own high-voltage control. This design allows a homogeneous light collection over the detector volume and also provides redundancy should one of the PMT's fail or degrade. The BGO detectors are positioned on opposite sides of the LAT, providing nearly full sky coverage.

Engineering Quality Model BGO Detector on test bench at NSSTC June 2005.

NaI and BGO Detectors in thermal vacuum chamber at Max Planck Institute April 2005.