The primary objective of HXRBS was to study hard X-ray spectra of solar flares on time scales as short as 10 ms. The Principal Investigator was K. Frost. The HXRBS instrument could also be used for observing the hard X-ray spectra of gamma-ray bursts which fell within the field of view of the detector. The HXRBS detector consisted of a CsI(Na) scintillator with a radius of 4.67 cm and thickness of 0.64 cm. The crystal was surrounded on all sides (except for the solar direction) by a CsI(Na) shield of thickness 3.2 cm. Events were pulse height analyzed in 15 channel energy spectra from 30–500 keV. The time resolution was 128 ms. However, the total count rate over the same energy ranges was recorded every 10 ms. The HXRBS threshold for detecting a gamma-ray burst was 1 × 10^-7 ergs/cm² (E > 30 keV). Since the SMM views the Earth during the nighttime portion of each orbit, only about 1 event on 40 is detected with a π/6 sr field of view for which the spectra are not badly contaminated by the shield-processed contributions. Between launch and mid-1985, more than 15 gamma-ray bursts were seen by the HXRBS.

The GRS was designed for investigation of the gamma-ray spectrum of solar flares. The Principal Investigator was E. Chupp. The main detector was an array of 7 gain-controlled 7.6 cm diameter × 7.6 cm thick NaI(Tl) detectors. A complete spectrum was obtained every 16.38 s in the energy range 0.3–9.0 MeV. The number of counts in the 4.2–6.4 MeV range was read out every 2.048 s. The number of counts in a 50 keV wide window around 300 keV was read out every 64 ms. The spectrometer was shielded by a 2.5 cm thick annulus of CsI(Na) and a 25 cm diameter × 7.6 cm thick back detector. The shield elements defined a field of view of 135 degrees (FWHM) in the solar direction. The CsI back detector and the 7 NaI detectors together provided an effective area of ∼100 cm² and 4 energy channels from 10–100 MeV. The number of counts in these energy channels was read out every 2.048 s. The experiment was complimented by a pair of 8 cm² × 0.6 cm thick NaI(Tl) detectors which measured the X-ray portion of the spectrum every 1.024 s in the energy range 13–182 keV. The GRS detection threshold for gamma-ray bursts in the field of view was ∼5 × 10^-6 ergs/cm² (E > 300 keV). Between launch and mid-1985, GRS detected at least 75 bursts.