EDAX EDS Manual

THE OPTIMIZATION OF X-RAY COUNT THROUGHPUT
The x-ray count rate is certainly one our most fundamental parameters for determining
optimal spectrum conditions. It is counter-intuitive for most users that increasing the
count rate does not automatically result in a better spectrum or at a minimum in
collecting more counts. The EDAX detector amplifier has two time constants (20 µs and
40 µs) which control the amount of time taken to process an x-ray event. While the x
ray is being processed, the detector is “dead” and will not process any additional events.
For each time constant there is a maximum throughput. When it is desirable to have
more counts in a spectrum or in a map, it is always possible to collect the signal for a
longer interval of time, but it is also possible to use a shorter time constant. When the
shorter time constant is employed, the spectrum resolution degrades but for many
applications this is not a serious drawback.
The figure below provides an example of how many counts can be processed in a fixed
interval of time (20 clock seconds). With the longer time constant (“40 tc” or 40 µs),
there are actually fewer counts processed when the count rate was increased from 2000
to 10,000 cps. The same 10,000 cps count rate yielded 6 to 7 times more processed
counts when the shorter time constant (“20 tc” or 20 µs). With the shorter time constant,
doubling the count rate only increased the processed counts by about ¼.
The resolution of the detector degrades noticeably when using the longer time constant
(see figure on the next page), but resolution is not normally a fundamental concern
when we do certain procedures such as collecting x-ray maps, or when the spectra
collected do not have significant overlaps and we are not concerned with low-energy
peaks or their analysis.
EDAX Training Course - X-Ray Count Optimization - page 1