The Gamma-Ray Burst Monitor

The GRBM ([Pamini et al., 1990,Feroci et al., 1997,Amati, et al., 1997,Costa et al., 1998a]), consists of four optically independent slabs of CsI(Na) scintillators (10 mm x 275 mm x 413 mm, geometric area of 1136 cm$^2$), each of which is composed by two optically coupled halves viewed by two independent 3 $^{\prime\prime}$PMTs (Hamamatsu R-2238), through a quartz light pipe with independent high voltage power supplies (fig. ). Between the two PMTs of each detector unit a light pulser, made of an Am$^{241}$ source inside a small NaI(Tl) crystal, is mounted in order to monitor the detectors' gain during the mission.

As previously mentioned, the GRBM is derived from a secondary function of the AC detectors of the PDS, originally called Lateral Shields (hereafter LSs); the large area of the LSs and the energy band (40-700 keV) suggested the possibility of using them as monitors of GRBs.