The Gamma-Ray Burst Monitor

The BeppoSAX Gamma-Ray Burst Monitor ([Costa et al. 1998,Frontera et al. 1997a,Pamini et al. 1990]) is an experiment derived from a secondary function of the four CsI(Na) scintillators assembly described in previous sub-section acting as active lateral coincidence of the PDS experiment and forming a square box around the main PDS detectors (Fig. 1.6). Each detector is 1 cm thick and has a geometric area of about 1136 cm² and an open field of view. The choice of CsI(Na) as the constituting and detecting material of the shields depended on standard considerations on its physical properties, mainly the high efficiency in stopping high energy photons, its resistance to thermal and mechanical shocks and its plasticity, that makes it easy to fabricate it into the desired detector geometry. The four detecting units are optically independent. Each one is composed by two optically coupled halves, whose light is seen by two independent photomultipliers (Fig. 1.7) Hamamatsu R2238 through Quartz light pipes. A light source obtained with an Am²⁴¹ source embedded in a NaI crystal is attached to each of the four detectors. Its light is then seen as a permanent source in the detector through a grey filter and its function is to help in monitoring detectors PMT gain variations.

The GRBM units are named LS1, LS2, LS3 and LS4, where LS stands for Lateral Shield. Due to the geometry of the PDS experiment and the BeppoSAX scientific payload (see Fig. 1.3), the axis of the slabs are orthogonal to the NFI axis (+Z). In particular, LS1 is co-aligned with the -Y axis, LS2 with the +X axis, LS3 with the +Y axis and LS4 with the -X axis. Therefore, LS1 and LS3 are co-aligned with WFC1(-Y) and WFC2(+Y) respectively.

The signals coming from each of the lateral shield are multiplexed and fed to a dedicated ADC converter , after a set of analog thresholds has been satisfied. In particular, for each of the four lateral shields a Low Level Threshold (LLT) can be adjusted by telecommand among 16 steps, nominally ranging from 20 to 90 keV, and an Upper Level Threshold (ULT) can be adjusted in 8 steps, from nominal 200 to 700 keV. In addition, the GRBM has been equipped with specific electronics for GRB detection. Due to the unpredictability of GRBs, an on-board specific trigger criterion is needed. The GRBM trigger operates on the signals detected between the LLT and ULT. With a time resolution on 7.8125 ms a moving average is continuously computed on a Long Integration Time (LIT) that is adjustable between 8 and 128 s. The counts in a Short Integration Time (SIT, adjustable between 7.8125 ms and 4 s) are compared to the moving average, and if they exceed of n$\sigma$(where $\sigma$ is the Poissonian standard deviation and n can be 2, 4, 8 or 16) this average then the trigger condition is satisfied for that shield. If the same condition is simultaneously active, within the SIT, for at least two shields, then the GRBM trigger condition is satisfied, and the relevant scientific data are produced.

The main characteristics of the GRBM are summarized in Tab.1.4, where we anticipate properties (e.g. effective area, energy resolution) that will be the subject of chapter 3.

  

Figure 1.6: PDS/GRBM design