materials such as lead, actually increase the undesired background rate, due to the intense showers of secondary particles and photons produced by the extremely high energy (GeV) particles characteristic of the space radiation environment.) A plastic anti-coincidence scintillation shield, essentially transparent to gamma-ray photons, protected the detectors from high-energy charged particles entering from the front. For all seven modules, the unwanted background effects of particles or photons entering from the rear was suppressed by a "phoswich" design, in which the active NaI detecting element was optically coupled to a layer of CsI on its rear surface, which was in turn optically coupled to a single photomultiplier tube for each of the seven units. Because the NaI has a much faster response time (~0.25 μs) than the CsI (~1 μs), electronic pulse shape discriminators could distinguish good events in the NaI from mixed events accompanied by a simultaneous interaction in the CsI. The largest, or High Energy Detector (HED), occupied the central position and covered the upper range from ~120 keV to 10 MeV, with a field-of-view (FOV) collimated to 37° FWHM. Its NaI detector was in diameter by thick. The extreme penetrating power of photons in this energy range made it necessary to operate the HED in electronic anti-coincidence with the surrounding CsI and also the six other detectors of the hexagon. Two Low Energy Detectors (LEDs) were located in positions 180° apart on opposite side of the hexagon. They had thin ~3 mm thick NaI detectors, also in diameter, covering the energy range from ~10–200 keV. Their FOV was defined to fan-shaped beams of 1.7° x 20° FWHM by passive, parallel slat-plate collimators. The slats of the two LEDs were inclined to ±30° to the nominal HEAO scanning direction, crossing each other at 60°. Thus, working together, they covered a wide field of view, but could localize celestial sources with a precision determined by their 1.7° narrow fields. The four Medium Energy Detectors (MEDs), with a nominal energy range of 80 keV — 3 MeV, had dia by thick NaI detector crystals, and occupied the four remaining positions in the hexagon of modules. They had circular FOVs with a 17° FWHM. The primary data from A4 consisted of "event-by-event" telemetry, listing each good (i.e., un-vetoed) event in the NaI detectors. The experiment had the flexibility to tag each event with its pulse height (proportional to its energy), and a