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Synchrotron beam test with a photon-counting pixel detector

Synchrotron beam test with a photon-counting pixel detector

301J.

301J. Synchrotron Rad. (2000). 7, 301±306Synchrotron beam test with a photon-counting pixel detectorCh. BroÈ nnimann, a S. Florin, b M. Lindner, b * B. Schmitt a and C. Schulze-Briese aa Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland, andb Physikalisches Institut der UniversitaÈt Bonn, Nussallee 12, D-53115 Bonn, Germany.E-mail: lindner@physik.uni-bonn.de(Received 27 March 2000; accepted 20 June 2000)Synchrotron beam measurements were performed with a single-photon-counting pixel detector toinvestigate the in¯uence of threshold settings on charge sharing. Improvement of image homogeneityby adjusting the threshold of each pixel individually was demonstrated. With a ¯at-®eld correction,the homogeneity could be improved. A measurement of the point spread function is reported.Keywords: pixels; charge sharing; ¯at-®eld corrections; photon counting.1. IntroductionProtein crystallography is an important application ofsynchrotron radiation that takes advantage of the newdevelopments in radiation sources such as the Swiss LightSource (SLS) (Bengtsson et al., 1997; Joho et al., 1994). Atthe SLS protein crystallography beamline a minigap invacuumundulator will produce high-brightness synchrotronradiation. The high brightness leads to the need fornew detector systems providing high readout speed forshort dead times, to minimize radiation damage of sensitivebiological crystals.The most promising detector type for this application aresingle-photon-counting pixel detectors (BroÈ nnimann et al.,2000). They provide a large dynamic range, high ratecapability, low noise performance, very fast data readout,suf®cient spatial resolution and can be assembled to largearea detectors (Barna et al., 1995; Beauville et al., 1997;Fischer et al., 1999). A 2k 2k system with 200 mm 200 mm pixels is under development at the SLS.However, there has been concern regarding chargesharingeffects in pixel detector systems, leading toproblems with their suitability as detectors for proteincrystallography. In this application a very homogeneousresponse of the detector to the incident radiation isnecessary in order to determine the relative intensities ofre¯ections with high accuracy. Areas between the pixelswith a higher or lower ef®ciency owing to charge sharingwould lead to problems.Therefore the in¯uence of basic detector parameters oncharge sharing between pixels and on the overall detectorhomogeneity has been investigated.2.1. BeamlineESRF bending magnets provide a beam of divergence6 mrad 0.2 mrad and a critical energy of 20.4 keV. Thebeam size at BM5 is 60 mm 5 mm. Energy is selectablewith a monochromator in the range 8±25 keV. The beamwas collimated with motorized slits to 10 mm 10 mm. Asecond slit system was positioned at a distance of 1 mfrom the ®rst one, directly in front of the detector, to blockslit scattering. The settings of the slits were monitored withan NaI scintillator, which was also used to determine thebeam pro®le from a slit scan (Fig. 1).2.2. DetectorSince a bump-bonded SLS pixel chip (BroÈ nnimann et al.,2000) is not yet available, the MPEC chip (Fischer et al.,1998) was used for the present measurements. It is wellcharacterized (Fischer et al., 1999) and provides the samefunctionality as the SLS chip, except for radiation hardnessand pixel geometry.The MPEC chip is divided into 12 columns and 63 rowsof identical pixels of size 433.4 mm 50 mm. Each pixelcontains the complete pixel readout electronics including2. Experimental set-upAll measurements were performed at the optics beamline(BM5) at the European Synchrotron Radiation Facility(ESRF) in Grenoble, France.Figure 1Beam pro®le at 12 keV obtained with a slit scan.# 2000 International Union of Crystallography Journal of Synchrotron RadiationPrinted in Great Britain ± all rights reserved ISSN 0909-0495 # 2000electronic reprint

302 Synchrotron beam testan ampli®er for positive input polarity, a discriminatorwhich detects an event above threshold, a 15-bit counter,and test and masking capabilities.The discriminator is controlled by two thresholdvoltages: a global threshold for all pixels and a correctionvoltage, stored on a capacitor in each pixel to compensatethreshold variations on the chip. This feature is extremelyimportant for a homogeneous response of the detector, aswill be shown in x3.The counter is realized as a linear feedback shift register(Fischer et al., 1996) with dynamic ¯ip-¯ops and can be readout serially by connecting all counters of a column. All 12columns are then read out in parallel.The large dynamic range of 2 15 1 is given by thecounter size and can be increased by multiple readout ofthe detector during data acquisition.The sensor bump-bonded onto the electronic chip is a300 mm-thick silicon p + n diode array, which has adequatequantum ef®ciency in the low energy range used here (99%at 8 keV and 52% at 15 keV).The chip behaviour with sensor has been studiedpreviously (Fischer et al., 1999). The noise of 135 41 eallowed threshold settings of below 2000 e . After adjustment,the threshold variation over the complete chipmeasured with a threshold scan (Fischer et al., 1999) was38 e .2.3. Readout systemThe chip was wire-bonded onto a printed circuit boardconnected to a Blue Board test system (Silicon SolutionsGbR, Rosenstrasse 7±9, D-50678 KoÈ ln, Germany)providing all voltages and currents for the chip and transferringthe digital signals to the PC. The refresh of thedynamic counters is performed by a free programmablegate array on the Blue Board; the refresh of the thresholdadjustment is performed by the PC in a background taskrunning independently from the measurement program.3. Results3.1. HomogeneityThe detector homogeneity is one of the most importantquality criteria for protein crystallography as well as formedical imaging.A homogeneity measurement was performed, irradiatingthe detector uniformly. For this, a ¯uorescent crystalconsisting of a glass plate with an amorphous Ge-dopingwas put into the beam, producing a homogeneous ®eld ofenergy 12 keV.The measurement was performed with 120 V sensor biasvoltage and thresholds set to 2000 e . To investigate thein¯uence of the threshold adjustment, the measurementwas performed with and without adjustment. The totalexposure time was 33 min.For the homogeneity studies, the border pixels wereexcluded in order to eliminate the effects of a sensorproblem reported by Fischer et al. (1999).In Fig. 2 the histogram of the count rates is shown withand without threshold adjustment. With adjustment, theaverage counts per pixel was 5640 with a standard deviationof 155 (Fig. 2b). This should be compared with Fig. 2(a)showing the same measurement without threshold adjustment.The gain in homogeneity owing to the thresholdadjustment is clear.To further investigate the in¯uence of the threshold onthe count rates in the pixels, the count rate is plotted versusthe threshold in Fig. 3. As expected, there is a strongnegative correlation visible, i.e. pixels with a higherthreshold have lower count rates.To further improve the homogeneity a ¯at-®eld correctionwas performed. The ef®ciency map of the detector hasbeen calculated from the ®rst half of the data set and thenapplied to the second half. The result after applying thecorrection map is compared with the homogeneity withoutef®ciency correction in Fig. 4. The ¯at-®eld correctionreduced the standard deviation in sensitivity from 2.77% to1.89%.Figure 2Count-rate histograms (a) without and (b) with thresholdadjustment (events = 1±500; = 156 counts). The improvementby the adjustment is obvious.Figure 3Count rate versus threshold, without adjustment. The negativecorrelation is obvious.electronic reprint

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