RBS Instrumentation Tutorial

History

By 1909, Ernest Rutherford had established that alpha particles consisted of helium with +2 charge. The backscattering experiment that bears Rutherford's name was suggested by Hans Geiger (of Geiger counter fame). However, it remained for twenty-year old undergraduate Ernest Marsden to actually do the first measurements. Marsden observed that the vast majority of alpha particles (He++) passed cleanly through a thin gold foil, but that some were scattered at all angles from the incoming He++ beam. Rutherford proceeded from this observation to propose the existence of the atomic nucleus. The key feature of Rutherford's nucleus proposal is that a very small volume contains most of the atomic mass. The alpha particles scatter from nuclei as a horde of billiard balls would scatter if propelled at a bowling ball.

Rutherford backscattering spectrometry (RBS) is the measurement of energies of these backscattered particles. These energies depend on the identity of the atom from which the alpha particle scatters, the angle of scatter, and the depth into the sample to which the particle travels before scattering. Thus, RBS can be used for elemental analysis, especially of surfaces.

One early use of RBS (called the alpha-scattering experiment at the time) was elemental analysis of lunar soils as part of the Surveyor V scientific payload in 1967. Most early RBS experiments used radioactive sources of alpha particles. Today, the intense pencil like beam of alpha particles required to produce a modest backscattered signal is most commonly provided by a charged particle accelerator.

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