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CCP4BB  July 2017

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Subject:

Re: Fine Phi Slicing

From:

Gerd Rosenbaum <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Thu, 13 Jul 2017 17:02:06 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (47 lines)

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
> *******************************************************************************
> Fred Dyda, Ph.D.                       Phone:301-402-4496
> Laboratory of Molecular Biology        Fax: 301-496-0201
> DHHS/NIH/NIDDK                         e-mail:[log in to unmask]
> Bldg. 5. Room 303
> Bethesda, MD 20892-0560      URGENT message e-mail: [log in to unmask]
> Google maps coords: 39.000597, -77.102102
> http://www2.niddk.nih.gov/NIDDKLabs/IntramuralFaculty/DydaFred
> *******************************************************************************

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