Alpha particle X-ray spectrometer

Alpha particle X-ray spectrometer (top left), APXS at the back of the Mars Pathfinder Sojourner rover (right), MSL Curiosity's alpha particle X-ray spectrometer, with a ruler (bottom left).

An alpha particle X-ray spectrometer (APXS) is a spectrometer that analyses the chemical element composition of a sample from the scattered alpha particles, and fluorescent X-rays after the sample is irradiated with alpha particles and X-rays from radioactive sources. This method of analysing the elemental composition of a sample is most often used on space missions, which require low weight, small size, and minimal power consumption. Other methods (e.g. mass spectrometry) are faster, and do not require the use of radioactive materials, but require larger equipment with less modest power requirements. A variation is the alpha proton X-ray spectrometer, such as on the Pathfinder mission, which also detects protons.

Over the years several modified versions of this type of instrument such as APS (without X-ray spectrometer) or APXS have been flown: Surveyor 5-7,Mars Pathfinder,Mars 96,Mars Exploration Rover,Phobos,Mars Science Laboratory and the Philae comet lander. APS/APXS devices will be included on several upcoming missions including the Chandrayaan-2 lunar rover.

Several forms of radiation are used in APXS. They include alpha particles, protons, and X-rays. Alpha particles, protons, and X-rays are emitted during the radioactive decay of unstable atoms. A common source of alpha particles is curium-244. It emits particles with an energy of 5.8 MeV. X-rays of 14 and 18 keV are emitted in the decay of plutonium-240. The Mars Exploration Rovers' Athena payload uses curium-244 with a source strength of approximately 30 millicuries (1.1 GBq).

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