CSPAD FAQ And starting point for discussion Philip Hart, LCLS Users Workshop, Detector session, 2 Oct 2013
Outline 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 2
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 3
Monte Carlo can help understand what s going on 4
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 5
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 6
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 7
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 8
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 9
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 10
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 11
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% 12
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) 13
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 0 th order crosstalk correction 14
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 15
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) 16
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 17
Reference plots: two pixels, V1.5 and V1.6 spectra 18
Reference plots: dark spectrum 19
Crosstalk dependence on pedestal (dark level) Shift in unilluminated area when saturating other part of asic 20