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An Alpha Particle X-Ray Spectrometer (APXS) or Alpha Proton X-Ray Spectrometer (APXS) for the Pathfinder mission, is a device that analyses the elements of a sample from the scattered Alpha particles and the emitted Protons (for Pathfinder) and X-Rays afer the sample is iridated with alpha particle and x-rays.


The alpha particles of 5.1 MeV are emitted by curium 244, where as the x-ray of 14 and 18 keV is provided by the decay product plutonium 240. For the MER Rover the activity is 30 mCi.

Alpha particles

Some of the alpha particle of a defined energy are backscaterd to the detector if they hit direct 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 nuleous hit by the alpha particle. Light elements absorbe more energy of the alpha particle while alpha particles ar 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 spectra makes it possible to determin the composition of the sample, especially for the lighter elements. The low backscatering rate makes it necessary for elongate irradiation.


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 methode. This method was only used in the Pathfiner APXS for the Mer Rovers the proton detector was substituted by a second alpha particle sensor.


The alpha particles are also able to kick out electrons of lower shell (K- and L-shell). This vacancies are filled by electrons of upper shells by emitting a charactaristic X-ray fluorescence. This characteristic X-ray fluorescence is easy to detect and has its best resulution for the heavier elements.


  • NASA page of the APXS
  • 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: 4027-4044.
  • 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: 8066. DOI:10.1029/2003JE002150. Research Blogging.