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(top right), MSL
Curiosity's alpha particle X-ray spectrometer, with a ruler (bottom).
An alpha particle X-ray spectrometer (APXS) is a
spectrometer that analyses the chemical element composition of a sample from scattered
alpha particles and fluorescent
X-rays after a 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 greater power requirements. A variation is the alpha proton X-ray spectrometer, such as on the
Pathfinder mission, which also detects
protons.
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
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 prolonged irradiation necessary, approximately 10 hours.
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. So it is also called alpha particle X-ray spectrometer.
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.
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]
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.[16]
^
Rieder, R.; Wanke, H.; Economou, T. (1997). "An Alpha Proton X-Ray Spectrometer for Mars-96 and Mars Pathfinder". American Astronomical Society. 28: 1062.
Bibcode:
1996DPS....28.0221R.
^
ab
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.
Bibcode:
1988LPI....19..511H.
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.
Bibcode:
2001SSRv...96..317W.
doi:
10.1023/A:1011961725645.