Rutherford backscattering

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Rutherford backscattering (or RBS, for Rutherford Backscattering Spectrometry) is an analytical technique in materials science. It is named for Ernest Rutherford who in 1911 first explained Geiger and Marsden's experimental results for alpha particle scattering from a very thin gold foil in a backward direction by using the Coulomb electrostatic force between the positively charged nucleus and the positively charged alpha particle. Rutherford first correctly described the atom as a tiny positive nucleus surrounded by negatively charged electrons (essentially the Bohr atom) on the basis of this experiment. This contradicted J.J. Thomson's "plum pudding model," the popularly accepted model of the atom at that time. Rutherford famously expressed his surprise at this experiment: "It was as though one fired a bullet at a piece of paper, and it bounced back at you!"

A high energy beam ( 2 to 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. For example, films about half a micrometre in thickness can be profiled using a 2 MeV He beam, or films up to about 10 micrometres thick can be profiled with a 2 MeV H beam.

RBS is now a very widely used analytical technique, which has the great advantage that it is absolute, requiring no standards for quantification (since the probability of interaction - the cross-section - is given by the Coulomb potential). It is one of a family of techniques, collectively known as Ion beam analysis.