The BeppoSAX High Pressure GSPC (HPGSPC)

The High Pressure Gas Scintillation Proportional Counter is a new development by IFCAI of a GSPC filled with 5 atm. Xenon/He mixture, which allows the detector to be sensitive up to 120 keV. The cell of the HPGSPC is seen by an Anger camera arrangement of seven PMT. The event position is used to correct the event energy with an overall improvement of a factor of two in energy resolution compared to proportional counters (Giarrusso et al. 1989). The main characteristics of the detector are summarized in the Table.

The general operating principle is similar to that of a lower pressure GSPC. This detector employs the technique of fluorescence gate discrimination to reduce the background at high energies. For incident X-ray with E >34.6 keV (the energy of the K shell of Xenon) the probability to interact with the K shell is 86%, while the probability that atom relaxes via a Ka (29.7 keV) or Kb (33.8 keV) fluorescence photon is 87.5%. Because the main penetration depth of the K fluorescence photon is 4 cm (in 5 atm Xenon), a pair of separate, but correlated, electron clouds (or VUV light bursts) can be produced. In this case the HPGSPC electronics can detect both "single" and "double correlated" events. The HPGSPC can be operated in three different "scientific" modes: In "Single Event" (SE) mode only X-ray photons detected as a single VUV light burst are collected. The SE mode can be used in the full HPGSPC energy range and is the only mode for X-ray events with energy below 34.6 keV.

In "Fluorescent Gated" (FG) mode only incident X-ray photons producing "double correlated" events are accumulated. Two VUV light bursts are detected as "double correlated" if they differ in time less than 40 msec (that is the maximum transit time in the Drift Region) and at least one is due to a Ka or Kb fluorescence photon. The FG mode is used only above 34.6 keV. In the FG mode a great improvement in the Signal to Noise ratio is obtained: the detection of a "double correlated" event provide an unique signature for a true X-ray event and, in this sense, can be used to discriminate against a non X-ray background event.

In the "All Events" mode both "single" and "double correlated" events are accumulated. Defined in the entire energy band of the HPGSPC, the "All Events" mode maximizes the detection efficiency of the Instrument. Four fixed calibration sources of mixed Fe55 and Cd109 will be used to monitor the detector gain and to calibrate the energy response for long term variation. Each source is mounted on a single hexagonal cell on the collimator. The 22 keV line of the Cd109 source in the central position is also used to automatically control the gain within the 0.5% FWHM by acting on the high voltage supplies of photo multipliers.

The HPGSPC detector characteristics

Energy range 4-120 keV
Geometrical area 450 cm2
Window: 1.3 mm Be
FOV (rocking collimator) 1.1 (FWHM)
Gas 5 atm (90% Xenon, 10% He)
Depth of drift region 10 cm
Scintillation region 1 cm
HV drift 8 kV
HV scintillation 12 kV
Read UV Anger camera with 7 PMT
Effective area 200 cm2 @ 6 keV
240 cm2 @ 60 keV
Energy resolution (FWHM) 10% @ 6 keV
4% @ 60 keV
Temporal resolution 15 us
Energy spectra 4096 channels
Burst length distribution 256 channels
In flight expected background E<35 keV 10-3 cts/s/keV/cm2
(after rejection BL Discr., side E>35 keV 10-4 cts/s/keV/cm2
via K Fluoresc, Escape gate anti)

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

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