The X-ray spectrometer was designed to study both solar flares and cosmic gamma-ray bursts. The detector was intended to be a part of a long baseline interferometry network of widely separated spacecraft. The efforts were aimed primarily at determining the origin of the bursts through precise directional information established by such a network. The experiment consisted of 2 cylindrical X-ray detectors: a Xenon filled proportional counter covering 5–14 keV, and a NaI(Tl) scintillator covering 12–1250 keV. The proportional counter was 1.27 cm in diameter and was filled with a mixture of 97% Xenon and 3% carbon dioxide. The central part of the counter body was made of 0.51 mm thick beryllium and served as the X-ray entrance window. The scintillator consisted of a 1.0 cm thick cylindrical shell of NaI(Tl) crystal surrounded on all sides by 0.3 cm thick plastic scintillator. The central region, 4.1 cm in diameter, was filled by a quartz light pipe. The whole assembly was enclosed (except for one end) in a 0.1 cm thick beryllium container. The energy channel resolution and timing resolution could be selected by commands sent to the spacecraft. The proportional counter could have up to 9 channels with 0.5 s resolution; the NaI scintillator could have up to 16 channels and 0.00025 s resolution. The gamma-ray burst mode was triggered by two simultaneous conditions: the count rates in certain PHA channels rose above a selected level and the spectral hardness of incident photons as determined by the ratio of 2 selected PHA channels exceeded a certain value.

The germanium detector was a high purity 4.02 × 2.9 cm right circular cylinder. The total germanium volume was 35 cu-cm. The detector was hermetically sealed in a Mg enclosure, which also provided the radiative surface for cooling. This structure reached a temperature of 130K roughly 3 days after launch and remained stable at that value for over a year. The detector system had a resolution of 10 keV at 570 keV and had 4096 energy channels.