The Gas Imaging Spectrometer on Board ASCA

The Gas Imaging Spectrometer (GIS) system on board ASCA is described. The experiment consists of 2 units of imaging gas scintillation proportional counters with a sealed-off gas cell equipped with an imaging phototube. The performance is characterized by the high X-ray sensitivity (from 0.7 keV to over 10 keV), good energy resolution (7.8% FWHM at 6 keV following E−0.5 as a function of X-ray energy E), moderate position resolution (0.5 mm FWHM at 6 keV with E−0.5 dependence), fast time resolution down to 61 μs, and an effective area of 50 mm diameter. The on-board signal processing system and the data transmitted to the ground are also described. The background rejection efficiency of the GIS is reaching the level achieved by the non-imaging multi-cell proportional counters.