3. Principles of operation of the ME-GSPC

The focal plane detectors are Xenon filled GSPC (Gas Scintillation Proportional Counters), working in the range 1.3-10 keV with an energy resolution of ~8% at 5.9 keV and a position resolution of ~0.5 mm (corresponding to 1 arcmin, approximately) at the same energy.
The gas cell is composed by a cylindrical ceramic body (96 mm internal diameter) closed, at the top, by a 50 micron thick entrance Beryllium window with 30 mm diameter and, on the bottom, by an UV exit window made of Suprasil quartz with 80 mm diameter and 5 mm thickness, as schematically shown in fig. 3-I below.

[GSPC sensitive picture]

Fig. 3-I : click-sensitive scheme of the MECS GSPC

A typical GSPC operates as follows (see also fig. 3-II ):

An X-ray photon absorbed in the gas cell liberates a cloud of electrons. A uniform electrical field across the cell drifts the cloud up to the scintillation region, with an higher electric field, where UV light is produced through the interaction of the accelerated electrons with the Xe ions.
The amplitude of the UV signal, detected by a photomultiplier (PMT), is proportional to the energy of the incident X-ray. The duration of the signal, the so-called Burst Length (BL), depends on the interaction point and it is used to discriminate genuine X-rays against induced background events. BL rejection may be carried out on board and/or on-ground. The BL rejection mechanism on board is based on a programmable BL acceptance window (not energy dependent).
Two grids inside the cell separate the absorption/ drift region (20 mm depth) from the scintillation region (17.5 mm depth). The UV readout system consists of a crossed-wire anode position sensitive Hamamatsu PMT with quantum efficiency of ~20%.
The high voltage nominal values are respectively -8 kV for the Be window -7,kV for the scintillation grid, while for the PMT of M1 M2,,and M3 units the ground settings at Panter were 1000, 992, and 943 V. These settings were used also in flight during the first light observations, but were updated just before the start of the commissioning phase to the present values of 979, 976 and 926 V

[GSPC interaction scheme]

Fig. 3-II : photon interactions in the MECS GSPC

The entrance window is externally supported by a Beryllium strongback structure, 0.55 mm thick, consisting of a ring (10 mm inner diameter, 1 mm width) connected to the window border by four ribs.
Two 55Fe collimated calibration sources (line at 5.894 keV), with an emission rate of ~1 count per second, are located, diametrically opposed, near the edge of the Be window. These sources, continuosly visible at the edge of the Field of View (FOV), allow the monitoring of the detector gain.
Furthermore, a passive ion shield is placed in front of the detector.

GSPC detector characteristics are listed here below.

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        GSPC characteristics
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Operative range            1.3 - 10 keV               
Energy resolution          ~8% at 5.9 keV            
Position resolution        ~0.5 mm (~1 arcmin)        
Gas cell geometry:                                    
   - internal diameter     96 mm                      
UV exit window:                                       
   - material              Suprasil quartz            
   - diameter              80 mm                      
   - thickness              5 mm                       
Gas type                   Xenon                      
Gas filling pressure       1.0 atm at 25 °C        
X-ray window:                                         
   - material              Beryllium                  
   - diameter              30 mm (FOV 56' diameter)   
   - thickness             50 um                      
   - central region        10 mm diameter             
X-ray window strongback:                               
   - material              Beryllium                  
   - thickness             0.55 mm                    
   - width                 1 mm                       
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