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Alpha particle X-ray spectrometer

APXS is also an abbreviation for APache eXtenSion tool, an extension for Apache web servers.
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.[1] 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,[2] Mars Pathfinder,[3] Mars 96,[4] Mars Exploration Rover,[5] Phobos,[6] Mars Science Laboratory and the Philae comet lander.[7][8] APS/APXS devices will be included on several upcoming missions including the Chandrayaan-2 lunar rover.[9]


  • Sources 1
  • 2 Alpha particles
  • Protons 3
  • X-ray 4
  • Specific instruments 5
  • Gallery 6
  • References 7
  • External links 8


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).[10]

Alpha particles

Sojourner takes its APXS measurement of the Yogi Rock.

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 prolonged irradiation, approximately 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.[6][11]
  • ALPHA, for Mars 96 landers. Collaboration between Germany, Russia, and USA.[12]



  1. ^ Economou, T.E. ; Turkevich, A.L. ; Sowinski, K.P. ; Patterson, J.H. ; Franzgrote, E.J. (1970). "The Alpha-Scattering Technique of Chemical Analysis". J. Geophysical Research 75 (32): 6514.  
  2. ^ Patterson, J.H. ; Franzgrote, E.J. ; Turkevich, A.L. ; Anderson, W.A. ; Economou, T.E. ; Griffin, H.E. ; Grotch, S.L. ; Sowinski, K.P. (1969). "Alpha-scattering experiment on Surveyor 7 - Comparison with Surveyors 5 and 6". J. Geophysical Research 74 (25): 6120–48.  
  3. ^ R. Rieder, H. Wänke, T. Economou, A. Turkevich (1997). "Determination of the chemical composition of Martian soil and rocks:The alpha proton X ray spectrometer". J. Geophysical Research 102 (E2): 4027–4044.  
  4. ^ Rieder, R.; Wanke, H.; Economou, T.; Wanke; Economou (1997). "An Alpha Proton X-Ray Spectrometer for Mars-96 and Mars Pathfinder". American Astronomical Society 28: 1062.  
  5. ^ R. Rieder, R. Gellert, J. Brückner, G. Klingelhöfer, G. Dreibus, A. Yen, S. W. Squyres (2003). "The new Athena alpha particle X-ray spectrometer for the Mars Exploration Rovers". J. Geophysical Research 108 (E12): 8066.  
  6. ^ a b Hovestadt, D.; Andreichikov, B.; Bruckner, J.; Economou, T.; Klecker, B.; Kunneth, E.; Laeverenz, P.; Mukhin, L.; et al. (1988). "In-Situ Measurement of the Surface Composition of the Mars Moon Phobos: The Alpha-X Experiment on the Phobos Mission". Abstracts of the Lunar and Planetary Science Conference 19: 511.  
  7. ^ a b Johannes Gutenberg Universität Mainz, Alpha particle x-ray spectrometer developed in Mainz to be used on comet Churyumov–Gerasimenko, 10 April 2014
  8. ^ "Alpha Proton X-ray Spectrometer (APXS) - Mission Name: Philae". NASA. 26 August 2014. 
  9. ^ "Payloads for Chandrayaan-2 Mission Finalised".  
  10. ^ unknown. "Alpha Particle X-Ray Spectrometer (APXS) (2 pages)" (PDF). 
  11. ^ JPL QuickLook - Phobos 1, 2
  13. ^ Mars Pathfinder Instrument Descriptions - NASA
  14. ^ ATHENA - Cornell University
  15. ^ "Mars Exploration Rovers: Spacecraft: Surface Operations: Instruments: Alpha Particle X-Ray Spectrometer (APXS)". NASA JPL. 
  16. ^ NASA - Alpha Particle X-ray Spectrometer (APXS)

External links

  • 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.  
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