High Quality Content by WIKIPEDIA articles! 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 irradiated with alpha particles and X-rays from radioactive sources. Since there are faster methods available which do not use radioactive material to analyse the elemental composition of a sample, this compact and low power method is most often used on space missions.