In conventional imaging multiwire proportional counters, the event position is usually derived from the avalanche charge distribution, which has a rather limited spatial extent. The statistical spatial accuracy of the avalanche charge distribution is not the limiting factor for the spatial resolution of a proportional counter. In gas scintillation proportional counters (GSPC), X-ray photons trigger UV light bursts, which carry information about the energy and the location of the events (see Chap. 2.5). The energy resolution of a GSPC is about a factor two better compared with proportional counters. In contrast to proportional counters, where the charge distribution is confined to a small region, the UV photons in the GSPC are distributed over the whole sensitive area of the UV photon read out device. Solid angle and UV detection efficiency reduce the fraction of the registered photons. The large distribution area of the UV photons in combination with the reduced number of photons limit the statistical accuracy of the center of gravity determination of the UV light burst to several 100 jam. The spatial resolution of a GSPC can be expressed as follows:
ox = spatial resolution oP = rms width of UV photon distribution at the photon detector oe = rms width of the electron cloud NP = number of detected UV photons
Ne = number of electrons generated by the absorption of the X-ray photon
Several methods can be employed in GSPCs to localize the event position via a center of gravity determination of the UV light burst. By measuring the UV light burst with an array of photomultipliers the relative signal amplitudes can be used to extract the event position. Further methods are to image the light output of GSPCs
Table 4.2 Characteristics of gas scintillation proportional counters for imaging X-ray telescopes
Focal length Detector Active area (diameter) Energy range Window
Gas cell Gas
Pressure Drift region Scintillation reg. aE/E %(FWHM) aX mm (FWHM) Ang. res. (HPD*) Det. + Mirror NXB
arcmin~2 s_1 keV" NXB x RE
350 cm GIS 50 mm
185 cm LEGSPC 20 mm
1.2 bar Driftless 49 mm
PI, polyimide; AlN, aluminiumnitride * = half power diameter at 6 keV
with continuous UV sensitive instruments like micro channel plates or position sensitive proportional counters. The imaging GSPCs aboard the X-ray satellites ASCA (GIS) and BeppoSAX (GSPCs of LECS and MECS) had multianode photomulti-plier for the readout of the detectors. Table 4.2 gives an overview of the characteristics of these detectors.
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