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This chapter provides a brief review of missions using X-ray, gamma-ray, and neutron spectroscopy to determine the chemical composition of planetary surfaces. This chapter presents the history of planetary radiation measurements, including significant discoveries. Summary tables with links to the archived data provide a resource for readers interested in working in this field. Upcoming missions and possible future directions are described.
Neutrons, gamma rays, and X-rays are used to measure the subsurface elemental composition of Solar System bodies, providing insights into their formation and evolution. Neutrons and gamma rays are highly penetrating particles made by the steady bombardment of the regolith of airless bodies by galactic cosmic rays. Gamma rays are also made by the decay of natural radioelements. The escaping radiation can be detected in close-proximity orbits and analyzed to determine subsurface elemental composition to depths of a few decimeters. Because the radiation sensors have nearly omnidirectional response, spatial resolution depends on orbital altitude. X-ray fluorescence is induced by solar X-rays. Consequently, X-ray spectroscopy is most useful for studies of objects in the inner Solar System. Characteristic elemental X-rays are made within the uppermost ~100 micrometers of the surface. The suite of elements analyzed overlaps that of nuclear spectroscopy, providing complementary geochemical information. Because X-rays are easily collimated, relatively high spatial resolution measurements are possible. This chapter presents the fundamentals of neutron, gamma-ray, and X-ray production, transport, and detection along with an overview of the measurement principles, including modeling, analysis, and mapping methods.