The gas scintillation proportional counter on EXOSAT.

In the counter, the electrons produced by the X-ray photoabsorption are drifted into a spherical high electric field region. Here they acquire energy sufficient to excite the noble gas atoms. These excited atoms form diatomic molecules through collisions, with UV photons in the waveband 1500-1950 A emitted. This burst of UV photons is subsequently observed by a photomultiplier tube, whose integrated anode signal has an amplitude proportional to the energy of the absorbed X-ray photon. The energy resolution depends on the variance in the number of UV photons observed by the photomultiplier tube as well as the variance in the initial number of electrons produced in the photoabsorption process. The energy resolution is approximately a factor of 2 better than the conventional proportional counter, being about 10 percent full-width-half-maximum at 6 keV.