cspad faq - slacportalwelcomepage

CSPAD FAQ

Philip Hart, LCLS Users’ Workshop, Detector session, 2 Oct 2013

And starting point for discussion

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Outline

LCLS Users’ Meeting, Detector Workshop, FAQ

Planning CSpad-based experiments •  getting latest specs •  MC-based experimental design studies •  setting up analysis tools •  metrology •  detector configuration for your science Getting the best detector response while running •  generating latest calibration/metrology •  monitoring •  fast feedback •  offline analysis in almost realtime Useful info for analysis •  handling noise •  gain correction •  xtalk correction •  useful sanity checks •  corner cases to watch out for Analyzing older data

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Planning CSpad-based Experiments (Detector View)


Start by talking to the Point Of Contact (POC) at your prospective hutch Topics to investigate: •  What detectors are available, and what are their current specs?

•  Special detector configuration? •  What new analysis tools are available?

•  Online, prompt, offline •  Introductions, examples, test data sets described in data and

analysis workshop •  We can generate MC data sets if you’re interested

•  What calibration data sets will I need? •  Metrology (at CXI)

•  May need to plan to take ring data •  gain (science dictates if a correction is needed)

•  For 140ks can be determined ex-situ •  Best calibration comes from in-situ control samples similar to data

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Monte Carlo can help understand what’s going on


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Your POC may point you to us or to detector info online


These talks Jargon file Links to detector papers Offline resources and sample data sets …

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Some (in part SLAC-oriented) detector jargon


ADU - Analog to Digital Unit. The digital value measured for a channel is quoted in counts or ADU. Pedestal - the dark level of a channel, usually the average of multiple measurements Pedestals - the set of all pedestals for a camera Gain - the conversion factor used to relate signal (photons or charge etc) to the quantity reported by the electronics. Gain (correction) file – correction factors to make a uniform response in an analysis after dark correction. Gain map - a list of gain settings for a camera. For the CSPAD, in which each pixel can be high or low gain, this is a matrix of 0s and 1s. Damage - either "damaged event"", daq lingo for an event with communications problems (such events must be dropped from analysis if you use the detector which complained) or, in the detector context, typically radiation damage. Frame noise, common-mode - detectors exhibit fixed pattern noise, and for the CSPAD this is to first order a constant ("common") change in pedestal level across a 2x1 Etc…

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Getting the best camera response while running


Taking metrology (at CXI), gain calibration may take some hours •  Should be “push a button” to install results this run •  Online display assumes right angle tiling, offline has full info POC will take and mark dark runs, produce dark and noise maps •  more or fewer depending on science, stability, damage rate, … Online monitoring provides many tools to evaluate data utility •  Single photon counting based on ADU and noise thresholds

•  Spectra, hit maps, projections, … •  Analysis by region •  Masking •  … •  Utility depends on science

•  Plug-in modules provide more specific monitoring

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… while running, cont.


Data is available for offline analysis promptly •  If your offline jobs are set up to read xtc, you can start

looking at data while a run is in progress •  exp=cxi12345:run=123:live:dir=/reg/d/ffb/cxi/cxi12345/xtc

•  Or you can use offline_ami (soon to be psami) to replay the data and use the online tools

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Most analyses can be done fine with SLAC software


(up to the science part) See POC, offline group Some comments on what’s going on under the hood follow for those who want to have a deeper understanding or to write their own code

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Analysis approaches


Each pixel may have slightly different behavior from its neighbors, so along with the frame data we use •  Dark map •  Gain map •  Noise map •  Camera tiling

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Darks


Dark levels (aka pedestals) are stable (ignoring occasional local radiation damage) How well you need to follow them depends on science •  E.g., important if

•  you’re looking at single photons •  you need uniform full well capacity •  you want to mask the occasional bad pixel (too low or too

high pedestal) •  you need to do a sophisticated crosstalk correction

•  Readout level and readout time are coupled in the CSpad, which affects crosstalk – see papers

•  A pixel’s dark level affects the crosstalk it sees

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Gains


Gain variation from pixel to pixel (e.g., an 8 keV photon results in say 24 or 26 ADU on average) at low intensity affects single photon counting •  Was 20-30% in first cameras •  About 12% in v1.5 •  About 6% in v.1.6 •  Can be corrected using low-intensity flat field •  Gain variation at higher intensity (~>5-10 photons in a

pixel) should be 1-2%

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Noise


Varies as the gain (they’re correlated) with camera version Dominated by ASIC-level frame common mode noise Signal/noise is ~6-7 in high gain at 8keV so noise matters if you want to count single photons •  Charge sharing (~30% of photons) also affects counting •  Noise maps from dark make better thresholds in online

monitoring than a hard cut (e.g., 3 sigma*noise vs 20ADU)

Should be calculated after gain correction (if any)

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Frame common mode noise


Fluctuates on the ASIC level (2x1 for low illumination) Corrected by finding pixels very near the dark level and adjusting the overall module by the difference Is typically about 3-4 ADU •  Correction is useful when counting single photons

(~24ADU at 8keV) •  Doesn’t matter if your science is at higher energy

Can function as a 0th order crosstalk correction

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Sanity checks (not just CSpad/detector issues)


Photon hit maps (spatial distribution of signal) •  Is there a blip or bump somewhere for no reason? Hits vs noise •  Are noisy pixels contributing more than they should? Hits vs time •  Are changes in signal/background rates reasonable given

running conditions? Sensitivity to beam intensity •  Do low-, mid-, high-intensity shots yield similar results (within

statistics?) assuming your science isn’t affected? If you’ve made a profile plot, look at the scatter plot too Print the # of entries on every histogram

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Corner cases in OLDER DATA


Things it (was) hard for the CSpad to do •  (Besides be the perfect detector for every application) •  Be perfectly homogeneous in gain/noise/… •  Maintain uniform small-signal response on a module that

has significant high-signal illumination •  Saturate monotonically at extremely high signal in a pixel Regions of simple behavior •  At low illumination •  At broadly high illumination •  Energy scale of effects is roughly the full width of the dark

pixel distribution (maybe 150 ADU)

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Some texts


Detectors Radiation Detection and Measurement, Glenn Knoll Radiation Detectors and Signal Processing, Helmuth Spieler Lutz Janesick Analysis Statistics for Nuclear and Particle Physicists, Louis Lyons Statistics: A Guide to the Use of Statistical Methods in the Physical Sciences, R. J. Barlow Statistical Data Analysis, Glen Cowan

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Reference plots: two pixels, V1.5 and V1.6 spectra


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Reference plots: dark spectrum


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Crosstalk dependence on pedestal (dark level)


Shift in unilluminated area when saturating other part of asic

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