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Study of charge-sharing in MEDIPIX3 using amicro-focused synchrotron beamTo cite this article E N Gimenez et al 2011 JINST 6 C01031

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2011 JINST 6 C01031

PUBLISHED BY IOP PUBLISHING FOR SISSARECEIVED October 27 2010

ACCEPTED November 7 2010PUBLISHED January 11 2011

12th INTERNATIONAL WORKSHOP ON RADIATION IMAGING DETECTORSJULY 11thndash15th 2010ROBINSON COLLEGE CAMBRIDGE UK

Study of charge-sharing in MEDIPIX3 using amicro-focused synchrotron beam

EN Gimeneza1 R Ballabrigab M Campbellb I Horswella X Llopartb J Marchala

KJS Sawhneya N Tartonia and D Turecekb

aDiamond Light Source Ltd Harwell Science and Innovation CampusDidcotOX11 ODE Oxfordshire UK

bEuropean organization for Nuclear Research CERNCH-1211 Geneve 23 Switzerland

E-mail EvaGimenezdiamondacuk

ABSTRACT X-ray photon-counting detectors consisting of a silicon pixel array sensor bump-bonded to a CMOS electronic readout chip offer several advantages over traditional X-ray detectiontechnologies used for synchrotron applications They offer high frame rate dynamic range countrate capability and signal-to-noise ratio A survey of the requirements for future synchrotron detec-tors carried out at the Diamond Light Source synchrotron highlighted the needs for detectors witha pixel size of the order of 50microm Reducing the pixel size leads to an increase of charge-sharingevents between adjacent pixels and therefore to a degradation of the energy resolution and imagequality of the detector This effect was observed with MEDIPIX2 a photon-counting readout chipwith a pixel size of 55microm The lastest generation of the MEDIPIX family MEDIPIX3 is designedto overcome this charge-sharing effect in an implemented readout operating mode referred to asCharge Summing Mode MEDIPIX3 has the same pixel size as MEDIPIX2 but it is implementedin an 8-metal 013microm CMOS technology which enables increased functionality per pixel Thepresent work focuses on the study of the charge-sharing effect when the MEDIPIX3 is operatedin Charge Summing Mode compared to the conventional readout mode referred to as Single PixelMode Tests of a standard silicon photodiode array bump-bonded to MEDIPIX3 were performed inbeamline B16 at the Diamond Light Source synchrotron A monochromatic micro-focused beamof 29microm x 22microm size at 15keV was used to scan a cluster of nine pixels in order to study thecharge collection and X-ray count allocation process for each readout mode Single Pixel Modeand Charge Summing Mode The study showed that charge-shared events were eliminated whenMedipix3 was operated in Charge Summing Mode

KEYWORDS X-ray detectors Si microstrip and pad detectors Hybrid detectors X-ray diffractiondetectors

1Corresponding author

ccopy 2011 IOP Publishing Ltd and SISSA doi1010881748-0221601C01031

2011 JINST 6 C01031

Contents

1 Introduction 1

2 Charge-sharing effect in small pixel detectors 221 Charge-sharing effect on the detector imaging performance 222 Charge-sharing reduction in the sensor 323 Charge-sharing effect suppression in the readout chip 3

3 Experimental set-up 331 MEDIPIX3 332 Synchrotron beamline 4

4 Charge-sharing effect between neighboring pixels 541 Charge-sharing effect at pixel edges 542 Charge-sharing effect at pixel corners 5

5 Single pixel mode vs charge summing mode 651 Differential spectra 652 Pixel sensitivity scan 7

6 Conclusion 8

1 Introduction

Pixel Array Detectors (PADs) consisting of silicon pixel array sensors bump-bonded to Applica-tion Specific Integrated Chips (ASICs) are becoming a standard component of many synchrotronbeamlines around the world The readout of PADs can be of two types single X-ray photon count-ing or X-ray energy integrating The PILATUS detectors commercialized by DECTRIS [1 2] theXPAD detectors commercialized by ImXPAD [3 4] and the MEDIPIX2-based MAXIPIX detec-tor developed by ESRF [5 6] are X-ray photon-counting PADs used for synchrotron experimentsAn example of X-ray energy-integrating PAD also suitable for synchrotron X-ray analysis is theMMPAD detector developed by CHESS and ADSC [7 8] A number of PILATUS PADs havebeen commissioned at the Diamond Light Source [9] They are used for Macromolecular Crystal-lography Small-Angle X-ray Scattering Surface and Interface Diffraction and X-ray reflectivityexperiments When compared with more traditional X-ray detector technologies PADs offer theadvantage of high dynamic range image noise determined only by X-ray statistics and fast framerate with small readout time of several milliseconds Gas-filled X-ray counting detectors such asthe RAPID2 detector system installed on the SAXS beamline at the Diamond Light Source (DLS)[10] offer faster frame rates than PADS but at the expense of a lower global count rate due to thecoincidence readout mode of operation

ndash 1 ndash

2011 JINST 6 C01031

A drawback of current PAD detector designs is the dead-space of several millimetres betweenmodules required for wire-bonds at the edge of the ASICs In some applications such as X-raycoherent diffraction and X-ray photon-correlation spectroscopy where features of around 50micromin size need to be imaged a pixel size of the order of 50microm is required as highlighted in a sur-vey by DLS beamline scientists Other applications would also benefit from a high frame-ratePAD with small pixel size Only the MEDIPIX range of detectors with their 55microm pixel sizeachieves this kind of spatial resolution at the moment However reducing the pixel size leads toan increase of charge-sharing events between adjacent pixels and therefore to a degradation ofthe energy resolution and image quality of the detector This effect was observed in the previousgeneration of MEDIPIX based detectors MEDIPIX2 which was implemented in 025microm CMOStechnology The last generation of the MEDIPIX readout chip MEDIPIX3 was designed to over-come the charge-sharing effect The fact that MEDIPIX3 was implemented in an 8-metal 013micromCMOS technology enabled increased functionality per pixel and implementation of an operatingreadout mode referred to as Charge Summing Mode which eliminates the charge-sharing effectbetween pixels

The present work focuses on the study of the charge-sharing effect when the MEDIPIX3 isoperated in Charge Summing Mode (CSM) compared to the conventional readout mode referredas Single Pixel Mode (SPM)

2 Charge-sharing effect in small pixel detectors

21 Charge-sharing effect on the detector imaging performance

The charge-sharing effect is the process by which the charge cloud generated by an impinging eventnear the pixel boundaries causes an electrical signal in several pixels of the PAD This signal resultsin count-loss or multiple counts at the pixels borders depending on the detection threshold settingwith respect to the impinging X-ray energy The charge-sharing effect can thus be represented by areduction or an increase of the effective pixel size This threshold- and energy-dependent effectivepixel size affects the gain of the detector its spatial resolution and its image noise characteristicsThe probability of charge-shared events occurring worsens when reducing the pixel size Theeffect on the Detective Quantum Efficiency of a detector can be modelled using the theoreticalframe work presented in [11] In one-dimensional detectors (strip detectors) the charge-sharingeffect is suppressed by setting a detector threshold at half the impinging X-ray energy which isonly possible with monochromatic X-rays This is mostly the case in synchrotron experimentsbut fluorescence X-rays emitted from the sample at a different energy can also be present in thebackground of diffracted images In two-dimensional detectors (PADs) a loss of counts related tothe charge-sharing effect will take place at the pixel corners even with a threshold set at half themonochromatic X-ray energy This leads to a reduction in detection efficiency and the presence offixed-pattern noise in the image This fixed-pattern noise is related to the fact that count-loss is notconstant from one pixel to another due to local pixel-to-pixel threshold mismatch remaining afterpixel threshold equalization This fixed-pattern noise can be removed by performing an appropriateflat-field correction But the flat-field image will depend on the X-ray energy and on the thresholdsetting which makes flat-field corrections tedious when high image uniformity is required

ndash 2 ndash

2011 JINST 6 C01031

22 Charge-sharing reduction in the sensor

The charge-sharing effect in PADs can be reduced by modifying the electrode structure of siliconsensors Whereas a traditional planar sensor consists of electrodes implanted in the top and thebottom surfaces of the wafer a 3D-sensor consists of an array of p and n electrode columns thatpenetrate into the detector bulk perpendicular to the surface [12] An advantage of these 3D-sensor structures is an improved radiation tolerance as a result of the shorter charge drift distancebetween the column electrodes This electrode column structure has also the advantage of self-shielding pixels against charge-sharing Such 3D sensor structures have been developed at CNMBarcelona and bump-bonded to a MEDIPIX2 chip [13] A charge-sharing reduction using 3D-sensor PADs was demonstrated by measuring integral spectra of 15keV monochromatic X-rays onB16 beamline at the Diamond Light Source and comparing them to spectra obtained with planardetectors [14] However a serious drawback of 3D sensors when employed for X-ray detectionis the loss of sensitivity inside the sim10microm diameter electrodes Effects of inactive volumes inthe sensor on detection efficiency have been investigated in [15] on 3D-sensors bump-bonded toMEDIPIX2 chips

23 Charge-sharing effect suppression in the readout chip

Another method to suppress charge-sharing effect in small pixel detectors consists of integratingan inter-pixel communication functionality in the readout ASIC This implementation was possiblein the design of the MEDIPIX3 readout chip as a result of the 013microm CMOS technology usedwhich enabled increased functionality associated with each pixel In Charge Summing Mode ofoperation the X-ray generated charge that is collected in each pixel is summed in clusters of 4pixels An arbitration logic algorithm implemented at the pixel-level ensures that a single hit isallocated to the cluster with the highest summed signal [16] Such readout scheme provides adetector free of charge-sharing effect and with a spatial resolution still defined by the pixel size(55microm)

3 Experimental set-up

31 MEDIPIX3

The detector characterization was performed with a PAD consisting of a 300 microm thick planarsilicon sensor bump-bonded to a MEDIPIX3 readout chip shown in figure 1 A description of theMEDIPIX3 chip can be found in [16] Solder bump-bonds were used to connect every 55microm widepixel of the silicon sensor to the corresponding pixel of the readout chip The single-chip PADassembly was glued to a chip motherboard allowing low voltage differential signals and sensorbias voltage to be routed to a very high density 68-way connector The system was operated with a90 V bias voltage resulting in full depletion of the sensor The control and readout of this PAD wasperformed using a MEDIPIX3 control interface and control software developed by IEAP [17 18]The MEDIPIX3 chip was operated in high gain operation mode suitable for detection of X-raysin the 6keV to 20keV energy range Two readout modes of operation were tested Single PixelMode (SPM) and Charge Summing Mode (CSM) In both readout modes detection thresholdswere equalized by means of 5 equalization bits as explained in [19]

ndash 3 ndash

2011 JINST 6 C01031

Figure 1 Photograph of the MEDIPIX3-based PAD used during the test

Figure 2 Set-up used for probing detector MEDIPIX3-based PAD response to a micro-focused X-ray beam

32 Synchrotron beamline

The synchrotron X-ray beam produced at B16 test beamline [20] was used to characterize thecharge-sharing effect in a MEDIPIX3-based PAD The global detector response was investigatedwith a wide monochromatic illumination of the detector achieved using an unfocused beam Forexperiments requiring small size beam (ie local charge-sharing effects between adjacent pixels)a stack of beryllium compound refractive lenses was used to focus the 15keV synchrotron X-raybeam down to 22microm x 29microm FWHM The detector assembly was mounted on high-precisiontranslation and rotation stages required to align the detector plane perpendicular to the beam fol-lowing the method explained in [19] Figure 2 shows the set-up of the experiment

ndash 4 ndash

2011 JINST 6 C01031

Figure 3 Schematic of scan locations in the pixel boundaries with the micro-focussed beam (a) Charge-sharing effect study at pixel edges (b) Charge-sharing effect study at pixel corners

4 Charge-sharing effect between neighboring pixels

The local charge-sharing effect between adjacent pixels of a MEDIPIX3-based PAD was inves-tigated by scanning across the boundaries of adjacent pixels in a cluster with the micro-focusedbeam as it is schematized in figure 3 For each of the micro-focused beam positions a thresholdscan was performed operating the chip in SPM mode The count-rate for each pixel in the clusterwas recorded and plotted as a function of the detector threshold energy resulting in an integralspectrum referred to as S-curve Differentiating the S-curve spectra leads to the differential X-rayenergy spectra for each pixel as a function of X-ray interaction location The set of differentialspectra provides information about how the X-ray generated charge cloud after drift and diffusionin the silicon sensor is shared between adjacent pixels as explained in subsections 41 and 42

41 Charge-sharing effect at pixel edges

The charge-sharing effect at the pixel edges was studied by scanning a pixel across a central lineperpendicular to the pixel edges as plotted in figure 3(a) A 15keV micro-focused beam was usedto perform the scans every 5microm Figure 4 shows the differential spectra recorded at each beamposition for the three adjacent pixels When the incident X-ray beam impinged at the edge betweentwo pixels (ie at 0microm and 55microm positions) the charge cloud generated by an event was clearlyshared equally between the two pixels resulting also in a split of the incoming X-ray energy As thebeam moved close to the centre of a pixel the charge-sharing effect disappeared and the full X-raygenerated charge cloud was collected in a single pixel and thus the total energy of the event Thespectra show that at 10microm from the edge almost the whole charge was collected in a single pixel

42 Charge-sharing effect at pixel corners

The study of the charge-sharing effect at the pixel corners was performed by positioning the 15keVmicro-focused X-ray beam at different locations along a diagonal line across a pixel corner fromthe centre of pixel (118109) to the centre of pixel (119108) in steps of 7microm (see figure 3(b))The response of each of the four adjacent pixels for each of the scan positions is shown in thedifferential spectra of figure 5 When the beam was close to a pixel corner (eg position 42microm)the X-ray generated charge cloud and thus the impinging X-ray energy was split between the

ndash 5 ndash

2011 JINST 6 C01031

Figure 4 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween two adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

four adjacent pixels Spectra also show that the 15 keV X-ray peak position differed by 35 keV(10 DAC units) from one pixel to another in spite of the threshold equalization performed on thedetector at the start of the experiment This was an indication that threshold-to-threshold dispersionwas larger than expected as discussed in the next sections

5 Single pixel mode vs charge summing mode

The ability of the CSM operating mode of MEDIPIX3 chip to eliminate the charge-sharing effectbetween adjacent pixels was demonstrated by comparing the behaviour of the detector operated inthis mode to its response in standard SPM readout mode

51 Differential spectra

Differential spectra were acquired for both modes of operation (SPM and CSM) while illuminatinga large area of the detector with an unfocused 15keV X-ray beam Spectra shown in figure 6 wereobtained by differentiating the S-curve produced by plotting the mean detected count rate as afunction of detector threshold The low-energy tail visible on the SPM spectrum of figure 6(a)corresponds to the counting of X-ray events where the charge is shared between pixels (see section4) This proportion of charge-shared events is relatively high due to the small size of the pixel(55microm) compared to the width of the X-ray generated charge cloud

ndash 6 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

A drawback of current PAD detector designs is the dead-space of several millimetres betweenmodules required for wire-bonds at the edge of the ASICs In some applications such as X-raycoherent diffraction and X-ray photon-correlation spectroscopy where features of around 50micromin size need to be imaged a pixel size of the order of 50microm is required as highlighted in a sur-vey by DLS beamline scientists Other applications would also benefit from a high frame-ratePAD with small pixel size Only the MEDIPIX range of detectors with their 55microm pixel sizeachieves this kind of spatial resolution at the moment However reducing the pixel size leads toan increase of charge-sharing events between adjacent pixels and therefore to a degradation ofthe energy resolution and image quality of the detector This effect was observed in the previousgeneration of MEDIPIX based detectors MEDIPIX2 which was implemented in 025microm CMOStechnology The last generation of the MEDIPIX readout chip MEDIPIX3 was designed to over-come the charge-sharing effect The fact that MEDIPIX3 was implemented in an 8-metal 013micromCMOS technology enabled increased functionality per pixel and implementation of an operatingreadout mode referred to as Charge Summing Mode which eliminates the charge-sharing effectbetween pixels

The present work focuses on the study of the charge-sharing effect when the MEDIPIX3 isoperated in Charge Summing Mode (CSM) compared to the conventional readout mode referredas Single Pixel Mode (SPM)

2 Charge-sharing effect in small pixel detectors

21 Charge-sharing effect on the detector imaging performance

The charge-sharing effect is the process by which the charge cloud generated by an impinging eventnear the pixel boundaries causes an electrical signal in several pixels of the PAD This signal resultsin count-loss or multiple counts at the pixels borders depending on the detection threshold settingwith respect to the impinging X-ray energy The charge-sharing effect can thus be represented by areduction or an increase of the effective pixel size This threshold- and energy-dependent effectivepixel size affects the gain of the detector its spatial resolution and its image noise characteristicsThe probability of charge-shared events occurring worsens when reducing the pixel size Theeffect on the Detective Quantum Efficiency of a detector can be modelled using the theoreticalframe work presented in [11] In one-dimensional detectors (strip detectors) the charge-sharingeffect is suppressed by setting a detector threshold at half the impinging X-ray energy which isonly possible with monochromatic X-rays This is mostly the case in synchrotron experimentsbut fluorescence X-rays emitted from the sample at a different energy can also be present in thebackground of diffracted images In two-dimensional detectors (PADs) a loss of counts related tothe charge-sharing effect will take place at the pixel corners even with a threshold set at half themonochromatic X-ray energy This leads to a reduction in detection efficiency and the presence offixed-pattern noise in the image This fixed-pattern noise is related to the fact that count-loss is notconstant from one pixel to another due to local pixel-to-pixel threshold mismatch remaining afterpixel threshold equalization This fixed-pattern noise can be removed by performing an appropriateflat-field correction But the flat-field image will depend on the X-ray energy and on the thresholdsetting which makes flat-field corrections tedious when high image uniformity is required

ndash 2 ndash

2011 JINST 6 C01031

22 Charge-sharing reduction in the sensor

The charge-sharing effect in PADs can be reduced by modifying the electrode structure of siliconsensors Whereas a traditional planar sensor consists of electrodes implanted in the top and thebottom surfaces of the wafer a 3D-sensor consists of an array of p and n electrode columns thatpenetrate into the detector bulk perpendicular to the surface [12] An advantage of these 3D-sensor structures is an improved radiation tolerance as a result of the shorter charge drift distancebetween the column electrodes This electrode column structure has also the advantage of self-shielding pixels against charge-sharing Such 3D sensor structures have been developed at CNMBarcelona and bump-bonded to a MEDIPIX2 chip [13] A charge-sharing reduction using 3D-sensor PADs was demonstrated by measuring integral spectra of 15keV monochromatic X-rays onB16 beamline at the Diamond Light Source and comparing them to spectra obtained with planardetectors [14] However a serious drawback of 3D sensors when employed for X-ray detectionis the loss of sensitivity inside the sim10microm diameter electrodes Effects of inactive volumes inthe sensor on detection efficiency have been investigated in [15] on 3D-sensors bump-bonded toMEDIPIX2 chips

23 Charge-sharing effect suppression in the readout chip

Another method to suppress charge-sharing effect in small pixel detectors consists of integratingan inter-pixel communication functionality in the readout ASIC This implementation was possiblein the design of the MEDIPIX3 readout chip as a result of the 013microm CMOS technology usedwhich enabled increased functionality associated with each pixel In Charge Summing Mode ofoperation the X-ray generated charge that is collected in each pixel is summed in clusters of 4pixels An arbitration logic algorithm implemented at the pixel-level ensures that a single hit isallocated to the cluster with the highest summed signal [16] Such readout scheme provides adetector free of charge-sharing effect and with a spatial resolution still defined by the pixel size(55microm)

3 Experimental set-up

31 MEDIPIX3

The detector characterization was performed with a PAD consisting of a 300 microm thick planarsilicon sensor bump-bonded to a MEDIPIX3 readout chip shown in figure 1 A description of theMEDIPIX3 chip can be found in [16] Solder bump-bonds were used to connect every 55microm widepixel of the silicon sensor to the corresponding pixel of the readout chip The single-chip PADassembly was glued to a chip motherboard allowing low voltage differential signals and sensorbias voltage to be routed to a very high density 68-way connector The system was operated with a90 V bias voltage resulting in full depletion of the sensor The control and readout of this PAD wasperformed using a MEDIPIX3 control interface and control software developed by IEAP [17 18]The MEDIPIX3 chip was operated in high gain operation mode suitable for detection of X-raysin the 6keV to 20keV energy range Two readout modes of operation were tested Single PixelMode (SPM) and Charge Summing Mode (CSM) In both readout modes detection thresholdswere equalized by means of 5 equalization bits as explained in [19]

ndash 3 ndash

2011 JINST 6 C01031

Figure 1 Photograph of the MEDIPIX3-based PAD used during the test

Figure 2 Set-up used for probing detector MEDIPIX3-based PAD response to a micro-focused X-ray beam

32 Synchrotron beamline

The synchrotron X-ray beam produced at B16 test beamline [20] was used to characterize thecharge-sharing effect in a MEDIPIX3-based PAD The global detector response was investigatedwith a wide monochromatic illumination of the detector achieved using an unfocused beam Forexperiments requiring small size beam (ie local charge-sharing effects between adjacent pixels)a stack of beryllium compound refractive lenses was used to focus the 15keV synchrotron X-raybeam down to 22microm x 29microm FWHM The detector assembly was mounted on high-precisiontranslation and rotation stages required to align the detector plane perpendicular to the beam fol-lowing the method explained in [19] Figure 2 shows the set-up of the experiment

ndash 4 ndash

2011 JINST 6 C01031

Figure 3 Schematic of scan locations in the pixel boundaries with the micro-focussed beam (a) Charge-sharing effect study at pixel edges (b) Charge-sharing effect study at pixel corners

4 Charge-sharing effect between neighboring pixels

The local charge-sharing effect between adjacent pixels of a MEDIPIX3-based PAD was inves-tigated by scanning across the boundaries of adjacent pixels in a cluster with the micro-focusedbeam as it is schematized in figure 3 For each of the micro-focused beam positions a thresholdscan was performed operating the chip in SPM mode The count-rate for each pixel in the clusterwas recorded and plotted as a function of the detector threshold energy resulting in an integralspectrum referred to as S-curve Differentiating the S-curve spectra leads to the differential X-rayenergy spectra for each pixel as a function of X-ray interaction location The set of differentialspectra provides information about how the X-ray generated charge cloud after drift and diffusionin the silicon sensor is shared between adjacent pixels as explained in subsections 41 and 42

41 Charge-sharing effect at pixel edges

The charge-sharing effect at the pixel edges was studied by scanning a pixel across a central lineperpendicular to the pixel edges as plotted in figure 3(a) A 15keV micro-focused beam was usedto perform the scans every 5microm Figure 4 shows the differential spectra recorded at each beamposition for the three adjacent pixels When the incident X-ray beam impinged at the edge betweentwo pixels (ie at 0microm and 55microm positions) the charge cloud generated by an event was clearlyshared equally between the two pixels resulting also in a split of the incoming X-ray energy As thebeam moved close to the centre of a pixel the charge-sharing effect disappeared and the full X-raygenerated charge cloud was collected in a single pixel and thus the total energy of the event Thespectra show that at 10microm from the edge almost the whole charge was collected in a single pixel

42 Charge-sharing effect at pixel corners

The study of the charge-sharing effect at the pixel corners was performed by positioning the 15keVmicro-focused X-ray beam at different locations along a diagonal line across a pixel corner fromthe centre of pixel (118109) to the centre of pixel (119108) in steps of 7microm (see figure 3(b))The response of each of the four adjacent pixels for each of the scan positions is shown in thedifferential spectra of figure 5 When the beam was close to a pixel corner (eg position 42microm)the X-ray generated charge cloud and thus the impinging X-ray energy was split between the

ndash 5 ndash

2011 JINST 6 C01031

Figure 4 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween two adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

four adjacent pixels Spectra also show that the 15 keV X-ray peak position differed by 35 keV(10 DAC units) from one pixel to another in spite of the threshold equalization performed on thedetector at the start of the experiment This was an indication that threshold-to-threshold dispersionwas larger than expected as discussed in the next sections

5 Single pixel mode vs charge summing mode

The ability of the CSM operating mode of MEDIPIX3 chip to eliminate the charge-sharing effectbetween adjacent pixels was demonstrated by comparing the behaviour of the detector operated inthis mode to its response in standard SPM readout mode

51 Differential spectra

Differential spectra were acquired for both modes of operation (SPM and CSM) while illuminatinga large area of the detector with an unfocused 15keV X-ray beam Spectra shown in figure 6 wereobtained by differentiating the S-curve produced by plotting the mean detected count rate as afunction of detector threshold The low-energy tail visible on the SPM spectrum of figure 6(a)corresponds to the counting of X-ray events where the charge is shared between pixels (see section4) This proportion of charge-shared events is relatively high due to the small size of the pixel(55microm) compared to the width of the X-ray generated charge cloud

ndash 6 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

22 Charge-sharing reduction in the sensor

The charge-sharing effect in PADs can be reduced by modifying the electrode structure of siliconsensors Whereas a traditional planar sensor consists of electrodes implanted in the top and thebottom surfaces of the wafer a 3D-sensor consists of an array of p and n electrode columns thatpenetrate into the detector bulk perpendicular to the surface [12] An advantage of these 3D-sensor structures is an improved radiation tolerance as a result of the shorter charge drift distancebetween the column electrodes This electrode column structure has also the advantage of self-shielding pixels against charge-sharing Such 3D sensor structures have been developed at CNMBarcelona and bump-bonded to a MEDIPIX2 chip [13] A charge-sharing reduction using 3D-sensor PADs was demonstrated by measuring integral spectra of 15keV monochromatic X-rays onB16 beamline at the Diamond Light Source and comparing them to spectra obtained with planardetectors [14] However a serious drawback of 3D sensors when employed for X-ray detectionis the loss of sensitivity inside the sim10microm diameter electrodes Effects of inactive volumes inthe sensor on detection efficiency have been investigated in [15] on 3D-sensors bump-bonded toMEDIPIX2 chips

23 Charge-sharing effect suppression in the readout chip

Another method to suppress charge-sharing effect in small pixel detectors consists of integratingan inter-pixel communication functionality in the readout ASIC This implementation was possiblein the design of the MEDIPIX3 readout chip as a result of the 013microm CMOS technology usedwhich enabled increased functionality associated with each pixel In Charge Summing Mode ofoperation the X-ray generated charge that is collected in each pixel is summed in clusters of 4pixels An arbitration logic algorithm implemented at the pixel-level ensures that a single hit isallocated to the cluster with the highest summed signal [16] Such readout scheme provides adetector free of charge-sharing effect and with a spatial resolution still defined by the pixel size(55microm)

3 Experimental set-up

31 MEDIPIX3

The detector characterization was performed with a PAD consisting of a 300 microm thick planarsilicon sensor bump-bonded to a MEDIPIX3 readout chip shown in figure 1 A description of theMEDIPIX3 chip can be found in [16] Solder bump-bonds were used to connect every 55microm widepixel of the silicon sensor to the corresponding pixel of the readout chip The single-chip PADassembly was glued to a chip motherboard allowing low voltage differential signals and sensorbias voltage to be routed to a very high density 68-way connector The system was operated with a90 V bias voltage resulting in full depletion of the sensor The control and readout of this PAD wasperformed using a MEDIPIX3 control interface and control software developed by IEAP [17 18]The MEDIPIX3 chip was operated in high gain operation mode suitable for detection of X-raysin the 6keV to 20keV energy range Two readout modes of operation were tested Single PixelMode (SPM) and Charge Summing Mode (CSM) In both readout modes detection thresholdswere equalized by means of 5 equalization bits as explained in [19]

ndash 3 ndash

2011 JINST 6 C01031

Figure 1 Photograph of the MEDIPIX3-based PAD used during the test

Figure 2 Set-up used for probing detector MEDIPIX3-based PAD response to a micro-focused X-ray beam

32 Synchrotron beamline

The synchrotron X-ray beam produced at B16 test beamline [20] was used to characterize thecharge-sharing effect in a MEDIPIX3-based PAD The global detector response was investigatedwith a wide monochromatic illumination of the detector achieved using an unfocused beam Forexperiments requiring small size beam (ie local charge-sharing effects between adjacent pixels)a stack of beryllium compound refractive lenses was used to focus the 15keV synchrotron X-raybeam down to 22microm x 29microm FWHM The detector assembly was mounted on high-precisiontranslation and rotation stages required to align the detector plane perpendicular to the beam fol-lowing the method explained in [19] Figure 2 shows the set-up of the experiment

ndash 4 ndash

2011 JINST 6 C01031

Figure 3 Schematic of scan locations in the pixel boundaries with the micro-focussed beam (a) Charge-sharing effect study at pixel edges (b) Charge-sharing effect study at pixel corners

4 Charge-sharing effect between neighboring pixels

The local charge-sharing effect between adjacent pixels of a MEDIPIX3-based PAD was inves-tigated by scanning across the boundaries of adjacent pixels in a cluster with the micro-focusedbeam as it is schematized in figure 3 For each of the micro-focused beam positions a thresholdscan was performed operating the chip in SPM mode The count-rate for each pixel in the clusterwas recorded and plotted as a function of the detector threshold energy resulting in an integralspectrum referred to as S-curve Differentiating the S-curve spectra leads to the differential X-rayenergy spectra for each pixel as a function of X-ray interaction location The set of differentialspectra provides information about how the X-ray generated charge cloud after drift and diffusionin the silicon sensor is shared between adjacent pixels as explained in subsections 41 and 42

41 Charge-sharing effect at pixel edges

The charge-sharing effect at the pixel edges was studied by scanning a pixel across a central lineperpendicular to the pixel edges as plotted in figure 3(a) A 15keV micro-focused beam was usedto perform the scans every 5microm Figure 4 shows the differential spectra recorded at each beamposition for the three adjacent pixels When the incident X-ray beam impinged at the edge betweentwo pixels (ie at 0microm and 55microm positions) the charge cloud generated by an event was clearlyshared equally between the two pixels resulting also in a split of the incoming X-ray energy As thebeam moved close to the centre of a pixel the charge-sharing effect disappeared and the full X-raygenerated charge cloud was collected in a single pixel and thus the total energy of the event Thespectra show that at 10microm from the edge almost the whole charge was collected in a single pixel

42 Charge-sharing effect at pixel corners

The study of the charge-sharing effect at the pixel corners was performed by positioning the 15keVmicro-focused X-ray beam at different locations along a diagonal line across a pixel corner fromthe centre of pixel (118109) to the centre of pixel (119108) in steps of 7microm (see figure 3(b))The response of each of the four adjacent pixels for each of the scan positions is shown in thedifferential spectra of figure 5 When the beam was close to a pixel corner (eg position 42microm)the X-ray generated charge cloud and thus the impinging X-ray energy was split between the

ndash 5 ndash

2011 JINST 6 C01031

Figure 4 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween two adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

four adjacent pixels Spectra also show that the 15 keV X-ray peak position differed by 35 keV(10 DAC units) from one pixel to another in spite of the threshold equalization performed on thedetector at the start of the experiment This was an indication that threshold-to-threshold dispersionwas larger than expected as discussed in the next sections

5 Single pixel mode vs charge summing mode

The ability of the CSM operating mode of MEDIPIX3 chip to eliminate the charge-sharing effectbetween adjacent pixels was demonstrated by comparing the behaviour of the detector operated inthis mode to its response in standard SPM readout mode

51 Differential spectra

Differential spectra were acquired for both modes of operation (SPM and CSM) while illuminatinga large area of the detector with an unfocused 15keV X-ray beam Spectra shown in figure 6 wereobtained by differentiating the S-curve produced by plotting the mean detected count rate as afunction of detector threshold The low-energy tail visible on the SPM spectrum of figure 6(a)corresponds to the counting of X-ray events where the charge is shared between pixels (see section4) This proportion of charge-shared events is relatively high due to the small size of the pixel(55microm) compared to the width of the X-ray generated charge cloud

ndash 6 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

Figure 1 Photograph of the MEDIPIX3-based PAD used during the test

Figure 2 Set-up used for probing detector MEDIPIX3-based PAD response to a micro-focused X-ray beam

32 Synchrotron beamline

The synchrotron X-ray beam produced at B16 test beamline [20] was used to characterize thecharge-sharing effect in a MEDIPIX3-based PAD The global detector response was investigatedwith a wide monochromatic illumination of the detector achieved using an unfocused beam Forexperiments requiring small size beam (ie local charge-sharing effects between adjacent pixels)a stack of beryllium compound refractive lenses was used to focus the 15keV synchrotron X-raybeam down to 22microm x 29microm FWHM The detector assembly was mounted on high-precisiontranslation and rotation stages required to align the detector plane perpendicular to the beam fol-lowing the method explained in [19] Figure 2 shows the set-up of the experiment

ndash 4 ndash

2011 JINST 6 C01031

Figure 3 Schematic of scan locations in the pixel boundaries with the micro-focussed beam (a) Charge-sharing effect study at pixel edges (b) Charge-sharing effect study at pixel corners

4 Charge-sharing effect between neighboring pixels

The local charge-sharing effect between adjacent pixels of a MEDIPIX3-based PAD was inves-tigated by scanning across the boundaries of adjacent pixels in a cluster with the micro-focusedbeam as it is schematized in figure 3 For each of the micro-focused beam positions a thresholdscan was performed operating the chip in SPM mode The count-rate for each pixel in the clusterwas recorded and plotted as a function of the detector threshold energy resulting in an integralspectrum referred to as S-curve Differentiating the S-curve spectra leads to the differential X-rayenergy spectra for each pixel as a function of X-ray interaction location The set of differentialspectra provides information about how the X-ray generated charge cloud after drift and diffusionin the silicon sensor is shared between adjacent pixels as explained in subsections 41 and 42

41 Charge-sharing effect at pixel edges

The charge-sharing effect at the pixel edges was studied by scanning a pixel across a central lineperpendicular to the pixel edges as plotted in figure 3(a) A 15keV micro-focused beam was usedto perform the scans every 5microm Figure 4 shows the differential spectra recorded at each beamposition for the three adjacent pixels When the incident X-ray beam impinged at the edge betweentwo pixels (ie at 0microm and 55microm positions) the charge cloud generated by an event was clearlyshared equally between the two pixels resulting also in a split of the incoming X-ray energy As thebeam moved close to the centre of a pixel the charge-sharing effect disappeared and the full X-raygenerated charge cloud was collected in a single pixel and thus the total energy of the event Thespectra show that at 10microm from the edge almost the whole charge was collected in a single pixel

42 Charge-sharing effect at pixel corners

The study of the charge-sharing effect at the pixel corners was performed by positioning the 15keVmicro-focused X-ray beam at different locations along a diagonal line across a pixel corner fromthe centre of pixel (118109) to the centre of pixel (119108) in steps of 7microm (see figure 3(b))The response of each of the four adjacent pixels for each of the scan positions is shown in thedifferential spectra of figure 5 When the beam was close to a pixel corner (eg position 42microm)the X-ray generated charge cloud and thus the impinging X-ray energy was split between the

ndash 5 ndash

2011 JINST 6 C01031

Figure 4 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween two adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

four adjacent pixels Spectra also show that the 15 keV X-ray peak position differed by 35 keV(10 DAC units) from one pixel to another in spite of the threshold equalization performed on thedetector at the start of the experiment This was an indication that threshold-to-threshold dispersionwas larger than expected as discussed in the next sections

5 Single pixel mode vs charge summing mode

The ability of the CSM operating mode of MEDIPIX3 chip to eliminate the charge-sharing effectbetween adjacent pixels was demonstrated by comparing the behaviour of the detector operated inthis mode to its response in standard SPM readout mode

51 Differential spectra

Differential spectra were acquired for both modes of operation (SPM and CSM) while illuminatinga large area of the detector with an unfocused 15keV X-ray beam Spectra shown in figure 6 wereobtained by differentiating the S-curve produced by plotting the mean detected count rate as afunction of detector threshold The low-energy tail visible on the SPM spectrum of figure 6(a)corresponds to the counting of X-ray events where the charge is shared between pixels (see section4) This proportion of charge-shared events is relatively high due to the small size of the pixel(55microm) compared to the width of the X-ray generated charge cloud

ndash 6 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

Figure 3 Schematic of scan locations in the pixel boundaries with the micro-focussed beam (a) Charge-sharing effect study at pixel edges (b) Charge-sharing effect study at pixel corners

4 Charge-sharing effect between neighboring pixels

The local charge-sharing effect between adjacent pixels of a MEDIPIX3-based PAD was inves-tigated by scanning across the boundaries of adjacent pixels in a cluster with the micro-focusedbeam as it is schematized in figure 3 For each of the micro-focused beam positions a thresholdscan was performed operating the chip in SPM mode The count-rate for each pixel in the clusterwas recorded and plotted as a function of the detector threshold energy resulting in an integralspectrum referred to as S-curve Differentiating the S-curve spectra leads to the differential X-rayenergy spectra for each pixel as a function of X-ray interaction location The set of differentialspectra provides information about how the X-ray generated charge cloud after drift and diffusionin the silicon sensor is shared between adjacent pixels as explained in subsections 41 and 42

41 Charge-sharing effect at pixel edges

The charge-sharing effect at the pixel edges was studied by scanning a pixel across a central lineperpendicular to the pixel edges as plotted in figure 3(a) A 15keV micro-focused beam was usedto perform the scans every 5microm Figure 4 shows the differential spectra recorded at each beamposition for the three adjacent pixels When the incident X-ray beam impinged at the edge betweentwo pixels (ie at 0microm and 55microm positions) the charge cloud generated by an event was clearlyshared equally between the two pixels resulting also in a split of the incoming X-ray energy As thebeam moved close to the centre of a pixel the charge-sharing effect disappeared and the full X-raygenerated charge cloud was collected in a single pixel and thus the total energy of the event Thespectra show that at 10microm from the edge almost the whole charge was collected in a single pixel

42 Charge-sharing effect at pixel corners

The study of the charge-sharing effect at the pixel corners was performed by positioning the 15keVmicro-focused X-ray beam at different locations along a diagonal line across a pixel corner fromthe centre of pixel (118109) to the centre of pixel (119108) in steps of 7microm (see figure 3(b))The response of each of the four adjacent pixels for each of the scan positions is shown in thedifferential spectra of figure 5 When the beam was close to a pixel corner (eg position 42microm)the X-ray generated charge cloud and thus the impinging X-ray energy was split between the

ndash 5 ndash

2011 JINST 6 C01031

Figure 4 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween two adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

four adjacent pixels Spectra also show that the 15 keV X-ray peak position differed by 35 keV(10 DAC units) from one pixel to another in spite of the threshold equalization performed on thedetector at the start of the experiment This was an indication that threshold-to-threshold dispersionwas larger than expected as discussed in the next sections

5 Single pixel mode vs charge summing mode

The ability of the CSM operating mode of MEDIPIX3 chip to eliminate the charge-sharing effectbetween adjacent pixels was demonstrated by comparing the behaviour of the detector operated inthis mode to its response in standard SPM readout mode

51 Differential spectra

Differential spectra were acquired for both modes of operation (SPM and CSM) while illuminatinga large area of the detector with an unfocused 15keV X-ray beam Spectra shown in figure 6 wereobtained by differentiating the S-curve produced by plotting the mean detected count rate as afunction of detector threshold The low-energy tail visible on the SPM spectrum of figure 6(a)corresponds to the counting of X-ray events where the charge is shared between pixels (see section4) This proportion of charge-shared events is relatively high due to the small size of the pixel(55microm) compared to the width of the X-ray generated charge cloud

ndash 6 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

Figure 4 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween two adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

four adjacent pixels Spectra also show that the 15 keV X-ray peak position differed by 35 keV(10 DAC units) from one pixel to another in spite of the threshold equalization performed on thedetector at the start of the experiment This was an indication that threshold-to-threshold dispersionwas larger than expected as discussed in the next sections

5 Single pixel mode vs charge summing mode

The ability of the CSM operating mode of MEDIPIX3 chip to eliminate the charge-sharing effectbetween adjacent pixels was demonstrated by comparing the behaviour of the detector operated inthis mode to its response in standard SPM readout mode

51 Differential spectra

Differential spectra were acquired for both modes of operation (SPM and CSM) while illuminatinga large area of the detector with an unfocused 15keV X-ray beam Spectra shown in figure 6 wereobtained by differentiating the S-curve produced by plotting the mean detected count rate as afunction of detector threshold The low-energy tail visible on the SPM spectrum of figure 6(a)corresponds to the counting of X-ray events where the charge is shared between pixels (see section4) This proportion of charge-shared events is relatively high due to the small size of the pixel(55microm) compared to the width of the X-ray generated charge cloud

ndash 6 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

Figure 5 X-ray spectra recorded in individual pixels as a function of beam position across the boundarybetween four adjacent pixels with MEDIPIX3-based PAD detector operated in Single Pixel Mode

In the differential spectrum obtained in CSM mode shown in figure 6(b) there is no low-energytail indicating that every X-ray is counted as a single event even when its charge is split betweenseveral pixels

The SPM and CSM spectra also enable an estimation of the overall energy resolution of thedetector which is determined essentially by the electronic noise and the pixel-to-pixel dispersionremaining after threshold equalization [1] The differential spectra in figure 6 show a standarddeviation of the 15keV X-ray energy peak of 16keV in SPM and 14keV in CSM These relativelyhigh values are due to unexpectedly high transistor mismatch in the chip preventing an accurateequalization of the thresholds It should be noted that this energy resolution could be improved byoptimizing threshold equalization procedures (ie using X-rays rather than noise edge) [19]

52 Pixel sensitivity scan

The effect of charge-sharing on the detection efficiency of the detector was investigated by scanningthe 15keV micro-focused beam across a region slightly larger than a pixel The threshold of thedetector was set at 75keV half the X-ray energy in order to minimize charge-sharing effect inthe detector Figure 7 shows the number of counts summed over all the adjacent pixels in SPMand CSM modes of operation As expected in SPM mode (figure 7(a)) the corners of the pixelswere insensitive regions of the detector since the 15keV X-ray generated charge cloud was sharedbetween more than two pixels (see section 4) and did not result in a signal higher than a 75keVequivalent signal in any pixel However figure 7(b) shows that the detection efficiency was constant

ndash 7 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

Figure 6 Differential spectra obtained in SPM (a) or CSM (b) A wide illumination of the detector wasachieved using the unfocused 15keV monochromatic beam

Figure 7 Mapping of detection sensitivity in SPM (a) and CSM (b) modes obtained by scanning the 15keVmicro-focused beam across a region slightly larger than a pixel The detector threshold was set at 75keV(half the X-ray energy)

over the whole region scanned by the micro-focused beam when operating MEDIPIX3 in CSMmode This was a confirmation that every X-ray was detected as a single count independently ofthe interacting point including the region at the pixel corners

Micro-focused scans also highlighted an issue with the current implementation of the CSMmode of Medipix3 Mapping the sensitivity of individual pixels led to the observation that X-rayevents were sometimes allocated to wrong pixels This was a result of the unexpectedly high pixel-to-pixel threshold variation in the chip remaining after the pixel equalization In the presence oflarge threshold variations the algorithm implemented in CSM mode and performed over adjacentpixels led to a spatial distortion of the image as explained in detail in [19]

6 Conclusion

The charge-sharing effect in a MEDIPIX3-based Pixel Array Detector when exposed to X-rays wasinvestigated on B16 synchrotron beamline at the Diamond Light Source Micro-focused 15keV X-

ndash 8 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

ray beam scans across pixel boundaries showed that the charge-shared between adjacent pixelsis present in a 15microm wide region on each side of the pixel boundary Results demonstrated thatthe on-chip inter-pixel communication capability CSM mode of operation implemented in theMEDIPIX3 chip ensured that one and only one count per X-ray was register by the detector wher-ever the interaction point of the X-ray was (ie even at pixel edges or corners) This result wasindependently confirmed by inspecting the integral spectrum obtained over the whole detector il-luminated with a large 15keV X-ray beam The charge-sharing event related tail present in thespectrum acquired in SPM mode was completely removed by operating the detector in CSM modeHowever measurements also highlighted a degree of mismatch between chip components whichcontributed to a misallocation of counts when operating MEDIPIX3 in CSM mode This problemwill be addressed by modifying the pixel architecture of the next iteration of MEDIPIX3 design inorder to improve its robustness to component mismatch

Acknowledgments

The authors wish to acknowledge the support from the technical and scientific staff of B16 beamlineat the Diamond Light Source and to Brian Willis from the Detector Group for his contribution tothe experimental set-up

References

[1] P Kraft et al Performance of single-photon counting PILATUS detector modules J SynchotronRadiat 16 (2009) 368

[2] Dectris httpwwwdectriscom

[3] K Medjoubi et al Detective quantum efficiency modulation transfer function and energy resolutioncomparison between CdTe and silicon sensors bump-bonded to XPAD3S J Synchotron Radiat 17(2010) 486

[4] ImXPAD httpimxpadcom

[5] X Llopart et al First test measurements of a64k pixel readout chip working in single photoncounting mode Nucl Instrum Meth A 509 (2003) 157

[6] C Ponchut et al Photon-counting X-ray imaging at kilohertz frame rates Nucl Instrum Meth A576 (2007) 109

[7] E Ercan et al Analog pixel array detectors J Synchrotron Radiat 13 (2006) 110

[8] ADSC httpwwwadsc-xraycom

[9] J Marchal et al Synchrotron applications of pixel and strip detectors at Diamond Light SourceNucl Instrum Meth A 604 (2009) 123

[10] A Berry et al The Rapid2 X-ray detection system Nucl Instrum Meth A 513 (2003) 260

[11] J Marchal Theoretical analysis of the effect of charge-sharing on the Detective Quantum Efficiencyof single-photon counting segmented silicon detectors 2010 JINST 5 P01004

[12] SI Parker et al 3D mdash A proposed new architecture for solid-state radiation detectors NuclInstrum Meth A 395 (1997) 328

ndash 9 ndash

2011 JINST 6 C01031

[13] G Pellegrini et al First double-sided 3-D detectors fabricated at CNM-IMB Nucl Instrum Meth A592 (2008) 38

[14] D Pennicard et al Synchrotron tests of a 3D Medipix2 X-ray detector IEEE Trans Nucl Sci 57(2010) 387

[15] EN Gimenez et al 3D Medipix2 detector characterization with a micro-focused X-ray beam articlein press in Nucl Instrum Meth A (corrected proof)

[16] R Ballabriga et al The Medipix3 prototype a Pixel readout chip working in single photon countingmode with improved spectrometric performance IEEE Trans Nucl Sci 54 (2007) 1824

[17] Z Vykydal et al USB interface for Medipix2 pixel device enabling energy and position-sensitivedetection of heavy charged particles Nucl Instrum Meth A 563 (2006) 112

[18] Institute of experimental and applied physics Czech technical university in Praguehttpwwwutefcvutcz

[19] E N Gimenez et al Characterization of Medipix3 with synchrotron radiation accepted forpublication in IEEE Trans Nucl Sci (2010)

[20] KJS Sawhney et al A test beamline on Diamond Light Source AIP Conf Proc 1234 (2010) 387

ndash 10 ndash