THEORY, PRACTICE AND APPLICATION OF MICRO-PIXE ANALYSIS AND ELEMENT-DISTRIBUTION MAPS

Micro-PIXE (Proton Induced X-ray Emission) analysis is the proton analogue of electron-probe micro-analysis. A 2-4 MeV beam of protons, produced using an electrostatic accelerator, is focused to a micrometer spot-size on a specimen, and the emitted X-ray flux is analyzed using a Si(Li) detector. The high magnetic rigidity of the beam requires magnetic or electrostatic quadrupole lenses in contrast to the cylindrically symmetrical electrostatic lenses used with electron beams. Several factors constrain the geometry of an end-station: (i) to minimize thermal effects resulting from the high densities of current required for low limits of detection, it is necessary to have the detector close to the sample; (ii) a high-magnification microscope using transmitted and reflected light is necessary to be able to direct the beam onto samples mounted on a motorized X-Y-Z stage; (iii) multi-element analysis necessitates the use of X-ray absorbing filters between the sample and detector. X-ray data are recorded in a multichannel analyzer and processed using a dedicated PIXE data-processing program. On-demand beam-deflection reduces pulse pile-up in X-ray spectra. Samples are analyzed using a dual analysis protocol; concentrations of major and minor elements are determined first, followed by those of trace elements. Analysis of homogeneous standards indicates that matrix effects are understood. Detection limits for geologically important trace elements such as Cr, Ni, Sr, Rb, Y, Nb, Zr, U and Pb are usually in the 1 - 20 ppm range, In the scanning proton microprobe (SPM), distribution maps of elements are recorded by rastering a proton beam across a sample using electrostatic or magnetic fields. Most SPM systems permit the on-line display of a number of 1-D or 2-D element images on a graphics workstation.