The Apollo gamma-ray spectrometer

A gamma-ray spectrometer has been flown on the Apollo 15 and 16 spacecraft to determine the lunar-surface composition and measure the cosmic gamma-ray flux. The instrument included a NaI(Tl) scintillation crystal coupled to a 7.6-cm photomultiplier tube, a plastic mantle for anti-coincidence rejection of charged particles, and 511 channels of analysis. Boom-mounted operation permitted a significant reduction in the background. The data were transmitted on an event-by-event basis. About 22% of the lunar surface was mapped and spectra of the cosmic gamma-ray flux over an energy range of 0.065-27.5 MeV have been obtained.