HEAO-1, the first of three High Energy Astronomical Observatories, was launched on 1977 August 12, returned its first useful data a week following launch, and returned its last data 1979 January 9. It carried four experiments for non-imaging observations of X-ray sources; these had varying energy ranges extending from < 1 keV to ~ 10 MeV, and varying fields-of-view from half a degree to many degrees.
The A4 experiment, also known as the UCSD/MIT Hard X-ray/Low-Energy Gamma-Ray Experiment, consisted of seven inorganic phoswich scintillator detectors surrounded by massive scintillators which served as active anti-coincidence against ambient radiations. The two Low Energy Detectors, optimized for the energy range 15 - 200 keV, had area 100 cm2 each and were collimated with slats to a fan beam of 1.7° x 20° FWHM. The slats were inclined at ±30° to the satellite scan direction. The four Medium Energy Detectors, with a nominal energy range of 80 keV to 2 MeV, each had area 45 cm2 and a circular beam of 17° FWHM. The High Energy Detector had a nominal range of 120 keV - 10 MeV, area 100 cm2, and circular beam 37° FWHM.