Alpha particle X-ray spectrometer

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Alpha particle X-ray spectrometer type from Spirit and Opportunity (Photo NASA / JPL-Caltech)
The APXS on the back of Mars Pathfinder's Sojourner Rover

An alpha particle X-ray spectrometer ( APXS ) (or alpha proton X-ray spectrometer for the Pathfinder mission ) is a device that the percentages of chemical elements in a sample surface by the reflected alpha particles , emitted protons (only Pathfinder), and X-ray fluorescence determined after exposure to alpha particles and X-rays from a radioactive source. There are faster methods of chemical composition analysis that do not require radioactive material. This is why this compact and energy-saving method is only used for space missions. During the last decades several different versions of the APS (without X-ray spectrometer) or APXS (with X-ray spectrometer) have been used or planned to be used in the following missions: Surveyor 5-7 , Mars Pathfinder , Mars 96 , Mars Exploration Rover , Fobos , Lander Philae , Mars Science Laboratory , Yutu and the Exomars Mission .

Radiation source

The 5.8 MeV alpha particles are released from Curium 244 Cm, while the X-rays of 14 and 18 keV come from its decay product, Plutonium 240 . The Mars Exploration Rovers Athena payload instrument has an activity of 30 m Ci .

Alpha particles

Some of the alpha particles emitted with a defined energy are thrown back to the detector when they collide with an atomic nucleus. The laws of physics for Rutherford Backscattering Spectrometry at an angle close to 180 ° correspond to the law of conservation of energy and the conservation of momentum. This makes it possible to calculate the mass of the core hit by the alpha particle. Light elements absorb more energy from the alpha particle. Alpha particles from heavy atomic nuclei, on the other hand, are reflected with almost the same energy. The energy spectrum of the reflected alpha particles shows peaks from 25% to almost 100% of the energy of the emitted alpha particles. This spectrum makes it possible to determine the composition of the sample, especially the light elements it contains. The low rate of reflection makes a long exposure time necessary. The penetration depth of the radiation is only a few micrometers, so the object to be examined must e.g. B. be freed from dust.

Protons

Some of the alpha particles are taken up by the atomic nuclei. The [alpha, proton] process produces protons with a certain energy which are measured. Sodium , magnesium , silicon , aluminum and sulfur can be detected in this way. This method was only used by the Mars Pathfinder APXS. In the APXS for the Mars Exploration Rovers , the proton detector has been replaced by a second alpha particle sensor.

X-rays

The alpha particles can also catapult electrons away from the inner electron shells (K and L shells) of an atom. These holes are filled by electrons from outer shells, which leads to the emission of characteristic X-rays. This is called particle-induced X-ray emission , which can easily be registered and which has its greatest sensitivity and resolution with the heavy elements (heavier than sodium ).

Web links

Commons : Alpha Particle X-ray Spectrometer  - Collection of Images
  • Page about the APXS NASA
  • H. Wänke, J. Brückner, G. Dreibus, R. Rieder, I. Ryabchikov: Chemical Composition of Rocks and Soils at the Pathfinder Site . In: Space Science Reviews , Volume 96, 2001, pp. 317-330, doi : 10.1023 / A: 1011961725645

Individual evidence

  1. ^ TE Economou, AL Turkevich, KP Sowinski, JH Patterson, EJ Franzgrote: The Alpha-Scattering Technique of Chemical Analysis . In: J. Geophysical Research , Volume 75, 1970, p. 6514
  2. ↑ JH Patterson, EJ Franz Grothe, AL Turkevich, WA Anderson, TE Economou, HE Griffin, SL Grotch, KP Sowinski: alpha scattering experiment on Surveyor 7 - Comparison with Surveyors 5 and 6 . In: J. Geophysical Research , Volume 74, 1969, pp. 6120-6148
  3. ^ R. Rieder, H. Wänke, T. Economou, A. Turkevich: Determination of the chemical composition of Martian soil and rocks: The alpha proton X ray spectrometer . In: J. Geophysical Research , Volume 102, 1997, pp. 4027-4044
  4. ↑ R. Rieder, H. Wanke, T. Economou: An Alpha Proton X-Ray Spectrometer for Mars-96 and Mars Pathfinder . American Astronomical Society, Volume 28, 1997, p. 1062, bibcode : 1996DPS .... 28.0221R
  5. ^ R. Rieder, R. Gellert, J. Brückner, G. Klingelhöfer, G. Dreibus, A. Yen, SW Squyres: The new Athena alpha particle X-ray spectrometer for the Mars Exploration Rovers . In: J. Geophysical Research , Volume 108, 2003, p. 8066
  6. ↑ D. Hovestadt, B. Andreichikov, J. Brückner, T. Economou, B. Klecker, E. Kunneth, P. Laeverenz, L. Mukhin, A. Prilutskii, V. Radchenko, C. Reppin, R. Rieder, R Sagdeev, CS Sastri, A. Turkevich, V. Vasiliev, H. Wänke: In-Situ Measurement of the Surface Composition of the Mars Moon Phobos: The Alpha-X Experiment on the Phobos Mission . (PDF; 435 kB) In: Abstracts of the Lunar and Planetary Science Conference , Volume 19, 1988, p. 511
  7. ↑ Chang'e 3 (CE 3) / Yutu ( Memento of the original from November 23, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Gunters Space Page; accessed December 23, 2013 @1@ 2Template: Webachiv / IABot / space.skyrocket.de
  8. ↑ G. Audi, O. Bersillon, J. Blachot, AH Wapstra: The NUBASE evaluation of nuclear and decay properties. In: Nuclear Physics. Volume A 729, 2003, pp. 3-128. doi : 10.1016 / j.nuclphysa.2003.11.001 . ( PDF ; 1.0 MB).
  9. ^ Mars Exploration Rovers. bernd-leitenberger.de