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)