An individual medium energy proportional counter on EXOSAT had a front window of beryllium with aluminized kapton foil for thermal protection, a front chamber filled with an argon/CO2 mixture, a rear chamber with xenon/CO2, and a beryllium window separating the two chambers.[9] The argon portion of the detector was optimized for 2-6 keV and the total energy ranges for both detectors was 1.5-15 keV and 5-50 keV, respectively.