The Phoswich Detection System

The PDS ([Frontera et al. 1997a]) is the high energy experiment of BeppoSAX, extending to 300 keV the NFI energy range and including as a subsystem the GRBM, which extends to 700 keV the WFI energy band. The main features of the experiment are reported in Tab.1.3 and a sketch of the detector and electronic units are shown in Fig. 1.4. The experiment was conceived and designed at ITESRE/CNR, Bologna (PI Prof. F. Frontera, deputy PIs D. Dal Fiume and E. Costa), was assembled at Laben and tested and calibrated at Laben, ESA/ESTEC, ITESRE/CNR,Universitá di Ferrara and IAS/CNR (Rome). The crystals constituting the active part of the PDS main detectors and of the PDS/GRBM have been manufactured and tested by Bicron Corporation, USA. The PDS/GRBM hardware team is composed by scientists at Istituto TESRE/CNR, Bologna, Universitá di Ferrara and Ferrara, IAS/CNR, Roma.

The detection system of the PDS is based on the PHOSWICH (acronym of PHOSphor sandWICH technique (e.g. [Knoll 1989]), in which the detector units are made of two layers of different crystals optically coupled. The PHOSWICH techniques exploits the different decay constants of the crystals to perform background rejection by means of Pulse Shape Analysis (PSA). In the case of PDS the detection plane (Fig. 1.5) is constituted by a square array of four phoswich units made of NaI(Tl) and CsI(Na), whose thickness are reported in Tab.1.3. Each detector unit is viewed by a PMT through a quartz light pipe. The entrance window is made of 1500 $\mu$m of Be, 800 $\mu$m of Silicon rubber, 76.1 $\mu$m of AL-coated Kapton and 100 $\mu$m of Teflon. The detectors FOV is limited by two rocking collimators made of Tantalum tubes with 1.3$\rm ^{\circ}$ hexagonal aperture, internally partially covered by a Sn/Cu gradual radiation shield. During operation, a collimators rocking strategy allows a simultaneous monitoring of source and background ([Frontera et al. 1997a]).

Besides the PSA, the background radiation rejection is obtained through a system of active anti-coincidence. From the side it is provided by four optically independent rectangular slabs of CsI(Na) scintillators 10 mm thick, 275 mm high and 402 mm wide. If a signal in one of the slab exceeds a programmable Anti Coincidence (AC) threshold then a veto signal (AC flag) is generated by the PDS analog processor. As anticipated, these detectors constitute also the Gamma-Ray Burst Monitor, which will be described in full detail in next section.

The active shielding in the +Z direction is provided by a top shield constituted by a 1mm thick plastic scintillator covered with Al-coated Kapton and located above the collimator assembly. It works for charged particles, in particular high energy electrons produced by interaction of cosmic rays with the satellite structure.

Other relevant features that make the PDS a high quality experiment are: an Automatic Gain Control (AGC) system, a Movable Calibration System (MCS, made of a 32 cm long wire of radioactive ⁵⁷Co contained in a cylinder of lead with a small aperture towards the detectors located above the top shield and which can scan by command the whole FOV to check in-flight calibration), a Light Emitting Device (LED) encapsulated in each phoswich to simulate an energy loss in the crystal and monitor gain stability and a Particle Monitor (PM) located near the instrument which monitors the level of environmental particle fluxes and turns off the High Voltage (HV) if the flux exceeds a programmable threshold in order to prevent damage to the experiment.

The electronic unit and its main functions are sketched in Fig. 1.4. In addition to the power supply module, it includes an analog processor, devoted to analog data processing and digital conversion, and a digital processor, for data management and instrument control. The ADC uses the slading scale technique ([Gatti et al. 1970,Frontera et al. 1992]) and has a conversion time of 5$\mu$s.

Finally, the on-board data are formatted in different transmission modes, which can be direct or indirect. Direct modes are used to transmit to telemetry events from the phoswiches and include information for each individual event on energy, pulse shape, time , detecting unit and the AC flag. With indirect modes, data are accumulated in histograms before being transmitted (they are used for observation of strong sources, for not saturating the experiment telemetry). Housekeeping (HK) information is provided in indirect mode and consists of spectra accumulated over 128 s time intervals (e.g. phoswiches pulse shape spectra), 1 s ratemeters (e.g. dead time counters, AC ratemeters, GRBM ratemeters, etc.) and engeenering data (e.g. HV, temperatures, status of collimators) which can be accumulated over time intervals ranging from 16 to 64 s.

 

 

Energy range 15-300 keV

Window 1.5 mm Be

NaI(Tl) phoswich thickness 3 mm

CsI(Na) phoswich thickness 50 mm

Geometrical area 795 cm²

Total area through collimator 640 cm²

Effective area 600 cm²@20 keV 500 cm²@60 keV 500 cm²@100 keV 140 cm²@200 keV

$$^\sim$$ Field of view (FWHM)

$$\leq$$ Energy resolution at 60 keV

$$\mu$$ Maximum time resolution

Gain control accuracy 0.25%

In-flight calibration accuracy 0.1%

Energy spectra 512 channels

Rise time distribution 512 channels Minimum channel width

     of energy spectra 0.3 keV

Maximum throughput 4000 events s^-1

  

Figure 1.4: Scheme of the PDS detecting and electronic units

  

Figure 1.5: PDS detector plane view