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The Gas Scintillation Proportional Counter

The gas scintillation proportional counter, GSPC, on EXOSAT has a deltaE/E of 4.5(E/6 keV)-0.5 % FWHM, (a factor of 2 better than the ME experiment) with a peak effective area of ~100 cm2. The energy spectra were pulse height analysed into 256 channels.

Three different electronic gain modes were used: gain 1 = 2-32 keV, gain 2 = 2-16 keV and gain 0.5 = 2-64 keV. The latter was only used on the very bright source Sco X-1. Variations in the intrinsic detector gain caused by temperature variations were removed using two background line features at 10.54 and 12.70 keV caused by fluorescence in the lead collimator and the radioactive decay of residual plutonium in the beryllium window. In addition a Xenon L feature at 4.78 keV could also be used for bright sources to calibrate the gain.

The particle background rejection used burst length discrimination, which rejects events which have exceptionally long or short duration. The background shape spectrum remains constant during the short timescale variations, but shows small changes on longer timescales. The standard OBC mode used in the GSPC gives 256 channel spectra every 8 seconds. Higher time resolution was used only for bright sources and if the telemetry needs of the other experiments was low. The GSPC worked perfectly for the entire mission.

Last Update: 24 August 2005

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