The BeppoSAX Low Energy GSPC (LECS)

One of the four BeppoSAX GSPCs has been developed by SSD/ESTEC (Parmar et al. 1990). This detector is similar to the other SAX GSPCs ( MECS), except that a thin window (Bavdaz et al. 1994) allows transmission of low-energy X-rays into a detector that does not have separate drift and scintillation regions. This is to minimize the loss of electrons to the window in the low-field drift region of a conventional GSPC (the so-called "Inoue effect").

In this configuration a uniform electric field is maintained across the gas cell and this causes scintillation of the xenon gas as the electrons travel towards the grid. The amount of light produced is proportional to the energy of a detected X-ray but also depends on the depth the X-ray penetrated into the gas cell before being photoabsorbed. This depth can be determined by measuring the duration of the scintillation light that is produced and is referred to as the burst length. This means that in order to determine the true energy of an X-ray event, a burst length dependant compensation must be applied to the energy data.

The gain of the 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 unit of the integrated optics + focal plane LE detector have been calibrated in the Panter facility in November 1994. The analysis of calibration data confirm the expected performance of the system (Parmar et al 1996). Above 1 keV the energy and the position resolution are the same as the ME units, following the expected E-0.5 law.

LECS characteristics

Window thickness 1.25 um multilayer polyamide on W grid 25 *m thick
Window radius 10 mm = 20 arcmin
Window support structure transmission 80 % (on average)
Gas 1.2 atm Xenon
Cell thickness
(driftless configuration) 49 mm
HV 4 KV/cm
UV readout PMT Hamamatsu position sensitive
Energy range 0.1-10 keV
Effective area 20 cm2 @ 0.28 keV
38 cm2 @ 1 keV
56 cm2 @ 2 keV
50 cm2 @ 6.4 keV
27 cm2 @ 9 keV
Energy resolution (FWHM) 8.0 (E/6)0.5 %
Position resolution(FWHM) 1.6 (E/6)0.5 arcmin
Time Resolution >16 us
Maximum throughput 2000 cts/s
Image size 256x256 pixels
Original pixel dimension 17.4"
Energy spectra 1024 channels
Veto signal distribution 256 channels
Burst length distribution 256 channels
Background rejection 98 %
Particle background < 1.6E-4 cts/s/arcmin2
Cosmic X-ray background < 0.8E-4 cts/s/arcmin2

More detailed information about this instrument is available from the LECS pages at ESTEC

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