The BeppoSAX Phoswich Detector System (PDS)

The Phoswich Detector System (PDS) consists of a square array of four independent NaI(Tl)/CsI(Na) phoswich scintillation detectors. Each of the four detectors is made of two crystals of NaI(Tl) and CsI(Na) optically coupled and forming what is known as PHOSWICH (acronym of PHOsphor sandWICH). The scintillation light produced in each phoswich is viewed, through a light guide of quartz, by a photomultiplier tube (PMT). The NaI(Tl) acts as X-ray detector, while the CsI(Na) scintillator acts as an active shield .

The events (due to photons or charged particles) detected in each phoswich can be divided in three classes: those that deposit energy only in the NaI crystal (NaI events or ``good events''); those that deposit energy only in the CsI crystal (CsI events); those that deposit energy in both crystals (mixed events). The events of the first class are accepted, while the events of the other two classes are rejected. The rejection is performed using the different decay constants of the scintillations produced in the NaI (about 0.25 microsec) and CsI (0.6 microsec and about 7 microsec for CsI).

The analysis electronics, via its Pulse Shape Analyzer (PSA), can discriminate events of the three classes described above. The phoswich configuration is well known to provide a detector with a high efficiency at hard X-ray energies and a very low background level. In order to further reduce the phoswich background level, both a lateral and a top anticoincidence shield are provided. The lateral shielding system is made of 4 CsI(Na) scintillators 10 mm thick that rejects a relevant fraction of the unwanted X-ray photons and charged particles impinging on the sides of the instrument.

The top shield is made of an organic scintillator 1 mm thick that covers the X-ray entrance aperture. It rejects the background events due to charged particles, in particular high energy electrons generated by interactions of Cosmic Rays with the materials of the instrument or with the satellite. Two calibration systems are provided. One is a gain control of each detection unit which is performed by using a time-tagged calibration source. This is a small cylinder made of plastic scintillator doped with Am241, which is placed close to the X-ray entrance windows of the four phoswich units. The 59.6 keV photons produced by Am241 are emitted in temporal coincidence with alpha particles of about 5 MeV.

These particles lose their energy in the plastic scintillator, giving light pulses easily detected by a small PMT coupled with the scintillator. This signal is then used by PDS electronics to tag the photons simultaneously detected in one of the phoswiches. The tagged photons are used by the PDS Instrument Controller to detect gain variations. A continuous and automatic correction is performed by acting on the high voltage supplies of the phoswich PMTs. The other calibration system consists of two radioactive sources of Co57 ( lines at 14, 122, 136 keV ) distributed along a wire 141 mm long. Upon command, the sources, one for each two detectors, cross the field of view of the instrument above the collimators and top shield. This monitoring allows to evaluate the absolute gain of the instrument and recalibrate it for long term variations.

The collimators can be independently rocked back and forth to allow the simultaneous monitoring of the source and background. Finally a particle monitor provides information on high environmental particle fluxes and triggers the action to prevent the damage of the detectors PMT's. This instrument, developed by ITESRE and IAS (Frontera et al. 1991), and tested by balloon experiments, will extend the range of SAX up to 300 keV. The flight units have been calibrated in November 1994 and August 1995.

Main characteristics of the PDS detector

Energy range 15-300 keV
Geometrical area 800 cm2
Window: 1.5 mm Be
FOV (rocking collimator) FWHM 1.4
Collimator trasmission 80%
Effective area 600 cm2 @ 20 keV
500 cm2 @ 60 keV
500 cm2 @ 100 keV
140 cm2 @ 200 keV
Energy resolution (FWHM) 15% @ 60 keV
Energy spectra 512 channels
Rise time distribution 512 channels
Temporal resolution 16 usec
In flight expected background 30-40 keV 2.5 10-4 cts/s/keV/cm2
(after shape discrim. anticoincidence) 40-80 keV 1.8 10-4 cts/s/keV/cm2
80-200 keV 1.1 10-4 cts/s/keV/cm2

More detailed information about this instrument is available from the PDS pages at TESRE

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