The BeppoSAX High Energy Instruments

The PDS and HPGSPC

The high energy instruments onboard SAX are respectively a 5 atm Xenon filled Gas Scintillation Proportional Counter (High Pressure GSPC) and a NaI/CsI phoswich scintillator ( Phoswich Detector System, PDS) A great care has been put in the design to control main source of systematic effects. Both instruments adopt the technique of rocking collimators, continuously pointing on and off source (with a period of the order of a minute) to monitor the background. Furthermore, in the case of PDS, which is composed of four units, the two halves of the experiment will alternatively point source and background regions, thus providing a continuous monitoring of the background. The two experiments have also an equalization gain system, based on tagged radioactive source (time-tagged for the PDS, position tagged for the HPGSPC), that will automatically adjust the high voltage in order to keep the gain of the detectors within 0.25-0.5%.

The Gamma-Ray Burst Monitor(GRBM)

The lateral shields of the PDS are also used as detectors of gamma-ray bursts of cosmic origin (GRB) with fluence greater that 10-6 erg/cm2. The signals from each of the slabs in a programmable energy band, nominally 60-600 keV, are sent to both a Pulse Height Analyzer and the GRBM logic unit, where the trigger condition is checked. A GRB event accumulation is triggered when at least two of the four lateral shields meet the trigger condition. In this way we expect to reduce the rate of false triggers caused by interactions of high energy particles in the shields. If a GRB condition is verified, the burst time profiles in the entire energy band from each of the four detectors are stored. The integration time for count accumulation will be 10 ms for 10 s preceding the trigger, 0.5 ms for 10 s immediately following the trigger and 10 ms from 10 s to 2 minutes after the trigger time.

	Questions should be addressed to: helpdesk@sax.sdc.asi.it