Protons
Some of the alpha particles are absorbed by the atomic nuclei. The 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.
Read more about this topic: Alpha Particle X-ray Spectrometer