Alpha particle X-ray spectrometer
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 after irradiation with alpha particles and detected X-rays from a radioactive source. There are faster methods for analyzing the chemical composition, which require no radioactive material. Therefore, this compact, energy-saving method is only used in space missions. During the last decades, several different versions of the APS ( without X-ray spectrometer ) or APXS used ( by spectrometer ) for the following missions or their use has been planned: 5-7 Surveyor Mars Pathfinder, Mars 96, Mars Exploration Rover, Fobos, Philae lander, Mars Science Laboratory, Yutu and Exomars mission.
Radiation source
The alpha particles of 5.8 MeV are released from Curium 244cm, while the X-rays of 14 keV and 18 of its decay product of plutonium is 240. In the instrument of the Athena payload of the Mars Exploration Rovers, the activity of 30 mCi.
Alpha particles
Some of the emitted alpha particles with defined energy are reflected back to the detector, when they collide with an atomic nucleus. The Rutherford Backscattering Spectrometry for physical laws in an angle close to 180 ° correspond to the conservation of energy and conservation of momentum. This makes it possible to calculate the mass of the core adopted by the alpha particles. Light elements absorb more energy of the alpha particle. Alpha particles by heavy nuclei remained at nearly the same energy reflected. The energy spectrum of the reflected alpha shows peaks between 25% and almost 100 % of the energy of the emitted alpha particles. This range makes it possible for the composition of the sample, in particular the light elements contained therein to determine. The low reflecting rate makes a long irradiation time is required. The penetration depth of the radiation is only a few microns, so you have to be examined object eg be freed from dust.
Protons
Some of the alpha particles are absorbed by the atomic nuclei. The [ alpha, proton ] process produces protons which are measured with a certain energy. Sodium, magnesium, silicon, aluminum, and sulfur can be detected. This method was used only by the Mars Pathfinder APXS. The APXS for the Mars Exploration Rovers the proton detector was replaced by a second Alphateilchensensor.
X-rays
The alpha particles can also of the inner electron shells (K -and L-shell ) of an atom electrons forcibly throw. These holes are filled by electrons from the outer shell, which leads to emission of characteristic X-rays. This is called particle -induced X-ray emission, which can easily be registered, and their greatest sensitivity and resolution of the heavy elements has ( heavier than sodium).