Hi Fred,
fine slicing does not alleviate the count RATE limitation of photon
counting detectors because fine slicing does not reduce the
instantaneous photon flux on the detector when you cross the diffraction
maximum. Fine slicing does help if you push the maximum counts per pixel
per frame to the counter register limit. On integrating detectors, like
CCDs, there is practically no count RATE limit. They do have a charge
(~photons) per pixel per frame limit, as well, which is mostly much
lower than for the photon counting detectors - about 1/10 even after
taking into account the diffraction spot covers many more pixels.
Different from integrating detectors where you only have to watch the
overflow, for counting detectors you have to watch for exceeding either
the count rate limit or the total count limit. The former is not an easy
task because you will see only the count per exposure. Divide that by
the exposure time you get the AVERAGE rate, not the peak rate.
The count rate limit, very short exposure time (using high flux) and the
1-pixel point spread function work against each other. Exposure time =
0.01 s, count rate limit 1e6 /sec (Pilatus2), 1-2 pixel per spot =>
10-20k counts per spot maximum for the strongest peak.
Dectris has come up with an ingenious hardware and software in the
Pilatus3 pushing the rate for reasonable dead time correction to over
1e7 counts/sec so even with 10 ms exposures weak reflections can be well
recorded besides strong reflections.
Gerd
On 13.07.2017 15:34, Dyda wrote:
I could be wrong here, but isn't the case that fine slicing is an option
with a CCD and a necessity on a PAD b/c of dead time and/or counter dynamic
range
issues?
(no current and/or former financial ties to any manufacturer)
Fred
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