The BGO Inner Ball

The 4pi calorimeter is composed of 80 BGO crystals which cover 80% of the solid angle.
The geometry of the GASP array is based on a polyhedron with 122 faces. 40 faces are used by the Germanium detectors (28 in the so called "hexagonal" positions and 12 in the "pentagonal" ones), 2 are devoted to the beam pipe in and out ports and the remaining 80 to the inner BGO ball.
The BGO detector thickness (65 mm) is sufficient to absorb 95% of gamma-rays of 1~MeV.
The resulting total efficiency for is 70% . In the case of high multiplicity events, like in standard fusion reactions, the total inner ball efficiency is very close to 100%.
The read-out of the crystal is made with standard PMT's and the electronic treatment of the signals is such that the energy and time information of each individual BGO detector can be recorded.
The BGO ball adds a background reduction factor that is reaction dependent, but can be conservatively estimated to be about 2.
The BGO ball is operational since the beginning of the GASP activity (1992)