A Metrology Approach for the Monitoring of EDX Silicon Detector Performance

摘要

Energy dispersive X-ray spectroscopy (EDX) is a routine non-destructive analytical tool for elemental analysis of materials on a micrometer scale and therefore falls into the measurement of amount of substance in the field of chemical metrology as specified under the ISO guidelines. The ISO 17025 accreditation procedure for any laboratory requires the identification and classification of the sources of uncertainty for all analysis techniques such as EDX applied within the laboratory. For the EDX technique this includes the monitoring of the detector performance parameters such as resolution. The ISO 15632 standard deals with the measurement of the semiconductor detector resolution using the full width half maximum (FWHM) of the MnK(alpha) X-ray peak. The operating conditions for the measurement of detector resolution is specified for an x-ray input count rate of 1 000cps and a minimum peak intensity of 10 000 counts. There are, however, two major disadvantages for the ISO 15632 standard. Firstly, there is no statistical foundation for the optimum resolution measurement specifications. Secondly there are no official guidelines for setting up of quality control charts for the monitoring of the detector resolution. The establishment of statistical control charts will require the EDX operator to monitor the detector resolution on the microscope column under precise and repeatable measurement conditions at regular time intervals over an extended period. It is therefore necessary to determine the optimum conditions for the measurement of the detector resolution as a function of the acquisition parameters. In this study we report the results for the resolution measurement for the MnK(alpha) peak as a function of spectrum acquisition live time and pulse processor time constant. Secondly we present results for the establishment of statistical control charts by the application of two statistical sampling strategies, e.g. CUSUM and Shewhart, for the monitoring of the detector resolution.