Ginga was the third Japanese X-ray astronomy satellite. It was launched into low Earth orbit on 5th February 1987 and re-entered the atmosphere on 1st November 1991. The scientific payload consisted of the Large Area Counter (LAC; Turner et al. 1989), the All-Sky Monitor (ASM; Tsunemi et al. 1989) and the Gamma-ray Burst Detector (GBD; Murakami et al. 1989). A full description of the satellite is given in Makino et al. (1987). During its lifetime Ginga performed over 1000 pointed observations of approximately 350 different targets, covering all then known classes of cosmic X-ray sources.
The LAC experiment1, sensitive to X-rays with energy 1.5-37 keV, consisted of an array of eight collimated co-aligned proportional counters with a total effective area of approximately 4000 cm2 and energy resolution of 18% at 6 keV, scaling as throughout the full energy range. In each counter the anode structure was of a multi-layer and multi-cell design which provided both gain uniformity and low internal background through the use of anticoincidence. The high voltage supply was normally operated at V, but was reduced occasionally to V to achieve a larger energy range. Steel collimators restricted the field of view to 1.1 x 2.0 degrees (FWHM); the top and bottom 15 mm were coated with silver paint to prevent contamination through iron, nickel and chrome fluorescent lines. The fluorescent line of silver at 22.1 keV can be visible at high energy but it is well away from lines of astrophysical importance and can be used for calibration.
The origin and behaviour of the LAC background is described in Hayashida et al. (1989). The main sources of background include the internal component generated after passage through the Earth's radiation belts, in particular the South Atlantic Anomaly (SAA), the high- and low-energy particles in the Earth's magnetosphere, and the diffuse Cosmic X-ray Background (CXB). The first two sources generate a background which is a strong function of time and energy. Summed over the top- and mid-layer electrodes as well as over the full energy range (1.5-37 keV), this varies between 50 and 100 counts sec-1. The extragalactic component of the CXB contributes approximately 18 counts sec-1 to the background, which varies as a function of position in the sky but is constant in time.