The BeppoSAX Medium Energy GSPC (MECS)

Three of the BeppoSAX GSPC's have 50 micron beryllium windows, thus being sensitive in the range of 1.3-10 keV. The detectors exploit the UV light produced in a high electrical field region to which the electron cloud, resulting from the X-ray photo-absorption, is drifted from the absorption region. The amplitude of the signal, detected by a position sensitive photomultiplier, is proportional to the energy of the absorbed X-ray. The duration of the signal (the burstlenght) is used to discriminate genuine X-rays against events induced by the particle background. The efficiency of the background rejection is greater than 99 % (Giarrusso et al. 1990). The gain of each detector is monitored by two Fe55 radioactive sources (which produce Mn k-alpha with energy of 5.9 keV) at the edge of the instrument.

The flight units of the MECS have been calibrated (together with the flight units of the concentrators) at the Panter X-ray facility in November 1994. The results of the calibrations confirm the expected energy resolution (FWHM) of 8 % at 6 keV and a position resolution at the same energy of 0.7 mm FWHM, corresponding to 0.7 arcmin HPR (Boella et al. 1996), therefore matching well the resolution of the concentrators.

MECS characteristics

Window thickness 50 m Beryllium
Window radius 15 mm = 30 arcmin
Gas 1.0 atm Xenon
Drift region 20 mm
Scintillation region 16 mm
HV drift region 500 V/cm
HV scintillation region 4 KV/cm
UV Readout Phototube Hamamatsu position sensitive
Energy range 1-10 keV
Effective area (3 units) 110 cm2 @ 2 keV
150 cm2 @ 6.4 keV
70 cm2 @ 9 keV
Energy resolution (FWHM) 8.0 (E/6 keV)0.5 %
Position resolution (FWHM)1.4 (E/6 keV)0.5 arcmin
Time Resolution 16 us
Maximum throughput 2000 cts/s
Image size 256x256 pixels
Original pixel dimension 20"
Energy spectra 256 channels
Burst length distribution 256 channel
Background rejection (acceptance = 87%)99.6 %
Particle background < 4.7E-5 cts/s/arcmin2/detector
Cosmic X-ray background < 2.7E-5 cts/s/arcmin2/detector

More detailed information about this instrument is available from the MECS pages at IFCAI

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