Alpha particle X-ray spectrometer


Alpha particle X-ray spectrometer

:"APXS is also an abbreviation for APache eXtenSion tool, an extension for Apache web servers."
[
Mars Pathfinder Sojourner rover] An Alpha Particle X-Ray Spectrometer (APXS) (or Alpha Proton X-Ray Spectrometer for the Pathfinder mission) is a device that analyses the chemical element composition of a sample from the scattered alpha particles, emitted protons (for Pathfinder only), and fluorescent X-rays after the sample is irradidated with alpha particles and X-rays from radioactive sources. [cite journal
title = The Alpha-Scattering Technique of Chemical Analysis
author = Economou, T.E. ; Turkevich, A.L. ; Sowinski, K.P. ; Patterson, J.H. ; Franzgrote, E.J.
journal = J. Geophysical Research
year = 1970
volume = 75
doi = 10.1029/JB075i032p06514| pages = 6514
] As there are faster methods available for the analyses the chemical element composition which do not use radioactive material this compact and low power consuming methode is only used on space missions. Over the years several modified versions of the APS (without X-ray spectrometer) or APXS were used or are planned of the Surveyor 5-7 [cite journal
title = Alpha-scattering experiment on Surveyor 7 - Comparison with Surveyors 5 and 6
author = Patterson, J.H. ; Franzgrote, E.J. ; Turkevich, A.L. ; Anderson, W.A. ; Economou, T.E. ; Griffin, H.E. ; Grotch, S.L. ; Sowinski, K.P.
journal = J. Geophysical Research
year = 1969
volume = 74
pages = 6120–48| doi = 10.1029/JB074i025p06120
] Mars Pathfinder [cite journal
title=Determination of the chemical composition of Martian soil and rocks:The alpha proton X ray spectrometer
author= R. Rieder, H. Wänke, T. Economou, A. Turkevich
journal= J. Geophysical Research
year= 1997
volume= 102
pages=4027–4044
doi=10.1029/96JE03918
] , Mars 96 [cite journal
title=An Alpha Proton X-Ray Spectrometer for Mars-96 and Mars Pathfinder
author= Rieder, R.; Wanke, H.; Economou, T.
journal= American Astronomical Society
year= 1997
volume= 28
pages=1062
url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1996DPS....28.0221R
] , Mars Exploration Rover [cite journal
title =The new Athena alpha particle X-ray spectrometer for the Mars Exploration Rovers
author = R. Rieder, R. Gellert, J. Brückner, G. Klingelhöfer, G. Dreibus, A. Yen, S. W. Squyres
journal = J. Geophysical Research
year = 2003
volume = 108
pages = 8066
doi = 10.1029/2003JE002150
] , Phobos [cite journal
title =In-Situ Measurement of the Surface Composition of the Mars Moon Phobos: The Alpha-X Experiment on the Phobos Mission
author = Hovestadt, D.; Andreichikov, B.; Brückner, J.; Economou, T.; Klecker, B.; Kunneth, E.; Laeverenz, P.; Mukhin, L.; Prilutskii, A.; Radchenko, V.; REppin, C.; Rieder, R.; Sagdeev, R.; Sastri, C.S.; Turkevich, A.; Vasiliev, V.; Wänke, H.
journal = Abstracts of the Lunar and Planetary Science Conference
year = 1988
volume = 19
pages = 511
url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1988LPI....19..511H
] , Rosetta/Philae, Mars Science Laboratory and Exomars missions.

ource

The alpha particles of 5.1 MeV are emitted by curium 244, whereas the X-ray of 14 and 18 keV is provided by the decay product plutonium 240. For the Mars Exploration Rovers' Athena payload the activity is 30 mCi.

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 spectra makes it possible to determine the composition of the sample, especially for the lighter elements. The low backscattering rate makes it necessary for elongate irradiation

Protons

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.

X-ray

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.

References

* [http://marsrovers.jpl.nasa.gov/mission/spacecraft_instru_apxs.html NASA page of the APXS]
*cite journal|title= Chemical Composition of Rocks and Soils at the Pathfinder Site |author=H. Wänke, J. Brückner, G. Dreibus, R. Rieder, I. Ryabchikov|journal= Space Science Reviews| volume= 96|pages= 317–330| year= 2001|doi=10.1023/A:1011961725645


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