A gamma-ray spectrometer was an instrument that analyzed gamma rays to determine the abundance and distribution of radioactive elements, usually on the surface of a planet. Since the radioactive isotopes of many elements normally made up a certain set percentage of the nonradioactive isotope, these readings could be used to infer the amount of the ordinary elements present. The instrument was therefore useful to determine the state of about twenty elements on the periodic table including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon.
A gamma-ray spectrometer was among the tools included on the United States of America space probe, Ranger 5. The spectrometer was labeled in a schematic of the Ranger 5 probe stored in the USS Enterprise's library computer, which was among the materials viewed by the Talosians in 2254. (TOS: "The Cage")