Tuesday, July 2, 2013

EDS Pulse Processing Time Constants

The fundamental principle of EDS is energy dispersion. By that it is meant that X-ray energies are measured by directly measuring their energy. In EDS detectors, X-rays generate an electrical cascade in a solid state detector where the resultant pulse height is proportional to the X-ray energy.

The pulse processing electronics of the EDS detector can be modified to change the performance of the detector. The most important parameter is the pulse shaping time constant, or as it is indicated in the software-- TC. The top graph shows the Mn Kα peak width as estimated by the FWHM as a function of the time constant, TC.  The smaller TC's result in wider peaks and thus lower spectral resolution.  As the TC is reduced the Mn Kα width reaches a floor of ~ 142 eV which represents the resolution of the EDS detector.  It should be noted that it is convention to measure the resolution of an EDS detector at the Mn Kα line: 5.893 keV.

An obvious question is why would anyone then want to increase the TC and thus reduce the resolution of the EDS detector given the potential problem of overlaps?

The second graph shows the maximum detected rate-- defined as the maximum rate at ~ 28% dead time-- on a Mn sample with a 30 kV beam. Note that empirically there is a power-law that relates TC to maximum rate through a negative exponent close to -1.  As such the maximum rate is nearly inversely  proportional (but not exactly) to TC.  At at 0.3 µs time constant the maximum rate is about 90 kc/s, while at the 32 µs time constant the maximum rate is 1.5 kc/s.

As such the smaller time constants are best for high throughput applications. These would include EDS mapping and the detection of trace elements in the absence of potential overlaps. The larger time constants are best suited to higher resolution applications, such as quantitation that requires deconvolution of overlapping peaks. The larger time constants are also useful for very light elements as will be demonstrated in a future application note.

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