RECENT DEVELOPMENTS IN DATA-ACQUISITION AND PROCESSING ON THE OXFORD SCANNING PROTON MICROPROBE

A new data acquisition system has been developed for the Oxford SPM facility, This is based on a single PC-compatible computer with a custom designed interface unit and permits the recording of several channels of pulsed detector data together with x and y scanning voltages, beam charge and deadtime. In addition, the system will also control the beam position allowing the generation of irregular scan areas, line scans at any angle and the selection of point analyses using the mouse. The hardware and software of the system are described and some examples of data collected using the system are presented. One problem of quantitative microanalysis using focused beam PIXE is the fact that the local matrix composition and secondary electron emission characteristics may change on a scale comparable with the beam size across the sample. This leads to uncertainties in the measured charge (especially at the low currents found in microbeams) and local self-absorption correction which can introduce significant inaccuracy into the analysis. This problem has been largely solved at Oxford by the simultaneous use of RBS analysis. The shape of the RBS spectrum yields the local matrix composition and its area gives a measure of the total charge, so that the dependence on precise charge measurement is much reduced. This ''Q factor'' technique is described in detail and it is shown that once the system has been calibrated routine standardless accuracy of the order of 5-10% can be obtained.