Abstract
Composition analyses for all of the elements in the periodic table can be performed through a combin¬ation of techniques using ion beams at MeV energies (MeV-IBA: see INTRODUCTION TO ION BEAM TECHNIQUES) including PIXE, RBS, EBS, ERD, NRA. See also PARTICLE SCATTERING and ATOMIC EXCITATION METHODS in the COMMON CONCEPTS chapter. In this unit we consider the MeV elastic backscattering techniques: RBS, Rutherford backscattering spectrometry; and EBS, elastic (non-Rutherford) backscattering. RBS, following Rutherford's treatment in 1911 of Geiger & Marsden's 1909 alpha-scattering experiment, approximates the scattering cross-section by that expected for the Coulomb interaction of point charges. This approximation is valid providing the interacting nuclei do not come too close during the interaction. As the energy is increased this approximation fails, and quantum mechanical effects become visible: then the scattering is called "EBS". BS (elastic backscattering spectrometry, either RBS or EBS) using MeV beams is used to obtain elemental depth profiles of thin films up to ~10 m thick. Depth resolution degrades with depth but can be ~1 nm at the surface. Various ion beams and various beam energies can be selected to obtain the optimal analytical conditions for particular samples. Barbour's article was on "Elastic Scattering", which included the important ERD technique now covered separately (see: ELASTIC RECOIL DETECTION ANALYSIS). We will mention the use of microbeams since many samples are small or laterally non-homogeneous, but microbeam IBA is reviewed in ION BEAM TOMOGRAPHY. We will also mention the use of ion channelling geometries for characterising defects in single crystal samples, but this is reviewed extensively in MEDIUM-ENERGY ION BEAM ANALYSIS. We should also mention that LEIS and MEIS are both RBS techniques, but they use low energy beams and will not be covered in this article (see, respectively, LOW-ENERGY ION SCATTERING and MEDIUM-ENERGY ION BEAM ANALYSIS). The Wiley Characterisation of Materials book of which this article is part has a section on Ion Beam Analysis (MeV-IBA: part of the ION BEAM TECHNIQUES section). The 2002 edition treated all the IBA techniques independently, but this 2012 edition will treat them synergistically. The present article considers the details of analysis using a particle detector placed in the backscattering direction. We explicitly distinguish between RBS and EBS, even though in any particular BS spectrum there may be (and often are) both RBS and EBS signals: there is a philosophical difference of treatment between a signal resulting from an interaction that can be accurately approximated by Coulomb scattering and one resulting from an interaction that must be treated quantum mechanically.