A high energy beam ( 2 - 4 MeV ) of low mass ions ( e.g. He ++ ) is directed at a sample. A detector is placed such that particles which scatter from the sample at close to a 180 degree angle will be collected. The energy of these ions will depend on their incident energy and on the mass of the sample atom which they hit, because the amount of energy transferred to the sample atom in the collision depends on the ratio of masses between the ion and the sample atom. Thus, measuring the energy of scattered ions indicates the chemical composition of the sample.
Additionally, in the case that the incident ion doesn't hit any of the atoms near the surface of the sample, but instead hits an atom deeper in , the incident ion loses energy gradually as it passes through the solid, and again as it leaves the solid. This means that RBS can be used as a means to perform a depth profile of the composition of a sample. This is especially useful in analysis of thin-film materials.