Backup for the gamma-ray detector installed in the "wheel" section on the OSO-1 satellite launched on 7 March 1962. The detector, built at the University of Rochester, includes two scintillators and a Cerenkov counter. Entering gamma-rays impinge on a thin lead sheet in the instrument; the collision converts the high energy photon to an electron-positron pair. Those charged particles then lead to a flash of light in the Lucite Cerenkov counter. The resulting flashes are amplified and counted by a photomultiplier tube. The scintillators form part of the anti-coincidence circuit that accepts only those signals that enter the detector from the front. The instrument on OSO-1 detected the cosmic ray background but did not provide clear evidence for high energy solar events. The detector was transferred to NASM from the Smithsonian Astrophysical Observatory in December 1982.