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alpha—proton—X-ray spectrometer (APXS) A set of instruments carried on Russian Vega and Phobos missions and by Sojourner, the rover vehicle carried on the 1997 Mars Pathfinder mission, that measures the elemental chemistry of surface materials. The sensor head of the instrument contains curium, as a source of alpha particles, an alpha particle detector, a proton detector, and an X-ray detector. The head is placed in contact with a sample and remains there for 10 hours. Alpha particles of known energy bombard the sample. Scattered alpha particles, protons from alpha-proton reactions, and X-rays produced by excitation by the alpha particles of the atomic structure of the sample are measured by the detectors. The energy spectrum of detections by all three instruments is then recorded and transmitted to Earth.