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The pulse
processor
Processors
which use adaptive pulse shaping
It is possible to use variable
averaging times to measure the size of each step. Events that arrive far
apart can be measured with more noise averaging than events arriving close
together (Fig. 18b). This produces a spectrum where each peak is built up of
many Gaussian shapes with a distribution of resolutions determined by the
distribution of the timing of the arrival of events on the voltage ramp.
At low input rates this type of
processor will deliver similar resolution to a fixed processor using a long
process time. However, as the input rate is increased the adaptive shaper
begins to use shorter averaging times and the resolution becomes worse,
although the acquisition rate is better than with a fixed process time. The
adaptive pulse shaper thus ensures efficient measurement of as many X-rays
as possible. The maximum acquisition rate that this type of processor can
achieve will still be controlled by the shortest time allowed to measure a
voltage step. This will be similar to the maximum count rate achieved by a
fixed process time processor using its shortest process time.
Using this type of shaping the
analyst loses control over resolution of the spectrum. The resolution varies
with count rate, for example when moving from one phase to another, or with
any change in beam current. ‘Strobing’ cannot be used to measure resolution
because the averaging time is variable. As a consequence resolution and peak
shape are poorly defined and this compromizes the accuracy of spectrum
processing.
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