HEXE was one of the four X-ray instruments on board the Kvant module, docked to the Soviet space station Mir since April 1987 (Reppin et al. 1985). HEXE consisted of four individual phoswich detectors sensitive in the energy range 20-200 keV, providing a total effective area of 750 cm2. 3.2 mm thick NaI(Tl) crystals are backed by 50 mm thick CsI(Tl) scintillators. In addition the whole detector was surrounded by plastic anticoincidence shields. The energy resolution was approximately 30% (FWHM) at 60 keV. The field of view is defined by two honeycomb tungsten collimators with a triangular response function of 1.6x1.6 deg FWHM. Normal HEXE observations consist of intervals of data acquisition of about 10-25 minutes, called sessions. The sessions are subdivided into alternating 240 s background and 270 s on-source pointings. The HEXE field of view is changed periodically from a background field - which is several degrees off the source - to the source and back.
The space station Mir was deorbited on March 23, 2001. Its unburned fragments fell into the South Pacific Ocean.