Editing Alpha particle X-ray spectrometer (section)

== Alpha particles ==
[[File:Pathfinder01.jpg|thumb|''[[Sojourner (rover)|Sojourner]]'' takes its APXS measurement of the [[Yogi Rock]].]]

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.