Technical Description of Micro-Pixe Analysis
A small Pelletron accelerator provides a proton beam of energy 3 MeV. This beam is directed onto a small rectangular aperture defined by a set of adjustable tantalum jaws. The aperture then acts as the `object' for a magnetic focusing system, which creates a demagnified `image' of the object; this is analogous to the use of a conventional lens system in light optics. The magnetic focusing system comprises two miniature air-cooled quadrupole lenses, which bring the proton beam to a focus at the position of the specimen. The beam spot diameter is typically a few microns. The beam transport tube and the specimen chamber are under high vacuum to prevent defocusing of the beam by scattering on residual gas modules.
Specimens are housed in a standard electron-probe mount which is carried on a stepping-motor-driven XYZ stage controlled by a computer. An optical microscope with magnifications (x80, x160, x300), coupled to a CCTV camera, permits viewing of the specimen before and during analysis. Two Si(Li) X-ray detectors record characteristic X-rays emitted from the specimen. The first of these is a large-area device, close to the specimen, and having a choice of absorbers: this is the trace element detector. The second detector, equipped with an ultra-thin window, records the spectrum due to major and minor elements. Both are under control of the supervising microcomputer. Points of interest on the specimen surface may be previewed, and the spatial coordinates of the points to be analyzed can be recorded in memory. The computer will then supervise sequential PIXE analysis of each selected region of interest, recording all relevant parameters such as time and beam charge in addition to the X-ray spectrum. In geological work, a sequence of points might be chosen on an individual mineral grain, or one point might be chosen on each of many mineral grains potted in the typical resin mount.
The spectra are processed by the Guelph PIXE software package, which has been adopted for use by over 100 PIXE laboratories around the globe. GUPIX conducts a least-squares-fit to the spectrum, deduces X-ray line intensities, and converts these to concentrations using standards. A variety of standards can be used in GUPIX including NIST materials, single-element standards, the Astimex mineral standard set, CANMET synthetic sulfides, and various geochemical reference materials.
The beam may be rastered back and forth continuously along a pre-defined lines In these "linescans", the computer provides plots of the intensities of particular X-ray intensities as a function of position. Areas up to 600 microns square may be mapped by rastering the beam over the entire area. Several contiguous lines or areas can be defined in order to study much larger regions without manual intervention. The SPM thus provides imaging of elemental concentrations for up to 14 elements simultaneously in either one or two dimensions. Spot analyses can be pre-programmed at chosen points within pre-determined lines or areas in order to supplement the image data with quantitative analysis of concentrations.