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.[1] 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[2]Mars Pathfinder[3], Mars 96[4], Mars Exploration Rover[5], Phobos[6]
, Rosetta/Philae, Mars Science Laboratory and Exomars missions.
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.
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