1. The Gas Scintillation Proportional Counter

EXOSAT's Gas Scintillation Proportional Counter (GSPC) is shown in Figure 2.1. It had a FWHM energy resolution of 10% at 6 keV. The energy resolution was proportional to E^-1/2. The detector's beryllium window formed a section of a sphere. In front of this was a mechanical collimator that provided a $45\hbox{$^\prime$}$
FWHM field of view. The gas cell was filled with 95% xenon and 5% helium at a pressure of 1 atmosphere.

Figure 4: The EXOSAT GSPC (Peacock et al. (1981)).