Alpha particle X-ray spectrometer
- APXS is also an abbreviation for APache eXtenSion tool, an extension for Apache web servers.
An Alpha Particle X-Ray Spectrometer is a device 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. Since there are faster methods available which do not use radioactive material to analyse the elemental composition of a sample, this compact and low power method is most often used on space missions. A variation is the Alpha Proton X-Ray Spectrometer, such as on the Pathfinder mission, which also detects protons. The common abbreviation is APXS.
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. APS/APXS devices will be included on several upcoming missions including the Philae comet lander, and 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).
Alpha particles 
Some of the alpha particles of a defined energy are backscattered to the detector if they collide with an atomic nucleus. The physical laws for Rutherford backscattering in an angle close to 180° are conservation of energy and conservation of linear momentum. This makes it possible to calculate the mass of the nucleus hit by the alpha particle. Light elements absorb more energy of the alpha particle, while alpha particles are reflected by heavy nuclei nearly with the same energy. The energy spectrum of the scattered alpha particle shows peaks from 25% up to nearly 100% of the initial alpha particles. This spectrum makes it possible to determine the composition of the sample, especially for the lighter elements. The low backscattering rate makes it necessary for elongated irradiation, approx 10 hours.
Some of the alpha particles are absorbed by the atomic nuclei. The [alpha,proton] process produces protons of a defined energy which are detected. Sodium, magnesium, silicon, aluminium and sulfur can be detected by this method. This method was only used in the Mars Pathfinder APXS. For the Mars Exploration Rovers the proton detector was replaced by a second alpha particle sensor.
The alpha particles are also able to eject electrons from the inner shell (K- and L-shell) of an atom. These vacancies are filled by electrons from outer shells, which results in the emission of a characteristic x-ray. This process is termed Particle-Induced X-ray Emission and is relatively easy to detect and has its best sensitivity and resolution for the heavier elements.
Specific instruments 
- Alpha-X, for DAS lander on Phobos 1 and Phobos 2.
- ALPHA, for Mars 96 landers. Collaboration between Germany, Russia, and USA.
- Alpha Proton X-Ray Spectrometer, for Mars Pathfinder by the Max Planck Institute and the University of Chicago.
- Alpha Particle X-ray Spectrometer, for Spirit (MER-A) and Opportunity (MER-B) Mars Exploration Rovers.
- Alpha Particle X-ray Spectrometer, for Curiosity (MSL). The principal investigator for Curiosity's APXS is Ralf Gellert, a physicist at the University of Guelph in Ontario, Canada. It was developed and funded by the Canadian Space Agency, with operations also supported by Guelph and United States' space administration.
- Alpha Proton X-ray Spectrometer, for Philae, the European Space Agency's lander attached to Rosetta, to study the comet 67P/Churyumov–Gerasimenko.
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- "Alpha Proton X-ray Spectrometer (APXS) - Mission Name: Philae". NASA.
- "Payloads for Chandrayaan-2 Mission Finalised". isro.gov.in. Indian Space Research Organisation. 2010-08-30. Retrieved 2012-08-07.
- unknown (unknown). "Alpha Particle X-Ray Spectrometer (APXS) (2 pages)".
- JPL QuickLook - Phobos 1, 2
- SMALL AUTONOMOUS STATIONS - Mars 96
- Mars Pathfinder Instrument Descriptions - NASA
- ATHENA - Cornell University
- "Mars Exploration Rovers: Spacecraft: Surface Operations: Instruments: Alpha Particle X-Ray Spectrometer (APXS)". NASA JPL.
- NASA - Alpha Particle X-ray Spectrometer (APXS)
- H. Wänke, J. Brückner, G. Dreibus, R. Rieder, I. Ryabchikov (2001). "Chemical Composition of Rocks and Soils at the Pathfinder Site". Space Science Reviews 96 (1/4): 317–330. doi:10.1023/A:1011961725645.