Ion-Beam Analysis

includes proton or particle-induced X-ray emission (PIXE), proton induced gamma-ray emission (PIGE), Rutherford backscattering (RBS), and proton elastic scattering analysis (PESA).

The sample of interest is bombarded with a beam of ions with energies of a few MeV. These energies require an accelerator such as the Tandem Pelletron Accelerator at LAC. As the particles (protons or other ions) penetrate the sample some of them interact with electrons and nuclei of the elements that make up the surface of the sample. These interactions result in characteristic emissions of X-rays or gamma rays and are measured with suitable detectors, such as Si(Li) Detectors or more modern Silicon Drift Detectors such as the liquid nitrogen free Bruker XFlash available at LAC. The energy spectra of the interaction products provide information on the elemental composition of the sample.

PIXE is based on the ejection of an inner-shell electron out of its orbit. As the vacancy is filled by an outer-shell electron an element and shell-specific X-ray is emitted. The energies of the X-rays are characteristic of the elements in the sample and the intensities of the X-rays can be used to calculate the abundance of an element. PIXE provides information on elements from Si to U.

PIGE involves nuclear excitation that result in the emission of γ-rays. Because nuclear interactions are rarer than whole atom interactions, PIGE is less sensitive than PIXE. However, PIGE is useful for the analysis of lighter elements.

RBS measures the energy of elastically backscattered particles from nuclei. The mass of the target nucleus is determined from the energy of the scattered ions. Information on the concentration of elements (or isotopes) can be obtained from the distribution of scattered particles. The RBS can determine C, N, and O.

All of the above described methods are typically performed under high vacuum. A recent modification of the PIXE beamline provides proton irradiation of samples under normal atmospheric conditions. This modification (a 12 μm thick Ti window) enables the study of biological material after exposure to ionizing radiation.

Other research activities include ion beam lithography and ion implantation. Ion beam lithography is driving current progress in nanotechnology and microfabrication (see examples under images). Ion implantation modifies the surface of a variety of materials without traditional heat treatment.