PROCEEDINGS OF SPIE. Characterization of the HEFT CdZnTe pixel detectors

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PROCEEDINGS OF SPIE

SPIEDigitalLibrary.org/conference-proceedings-of-spie

Characterization of the HEFT CdZnTe

pixel detectors

C. M. Hubert Chen, Walter R Cook, Fiona A Harrison, Jiao

Y Y Lin, Peter H Mao, et al.

C. M. Hubert Chen, Walter R Cook, Fiona A Harrison, Jiao Y Y Lin, Peter H

Mao, Stephen M Schindler, "Characterization of the HEFT CdZnTe pixel

detectors," Proc. SPIE 5198, Hard X-Ray and Gamma-Ray Detector Physics

V, (20 January 2004); doi: 10.1117/12.506075

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C. M. Hubert Chen a , Walter R. Cook a , FionaA. Harrison a , JiaoY. Y. Lin b ,Peter H. Mao a and Stephen M. Schindler

a a

Department of Physics, MailCode 220-47,CaliforniaInstitute of Technology, Pasadena, CA 91125,USA

b

Department of MaterialsScience, MailCode 138-78,California Instituteof Technology, Pasadena, CA 91125, USA

ABSTRACT

WehavedevelopedlargeformatCdZnTepixeldetectorsoptimizedforastrophysicalapplications. Thedetectors, designed for the High Energy Focusing Telescope (HEFT) balloon experiment, each consists of an array of 2444 pixels, on a498 m pitch. Each of the anode segmentson a CdZnTe sensor is bonded to a custom, low-noise application-specic integrated circuit (ASIC) optimized to achieve low threshold and good energy resolution. We havestudied detectors fabricated by two dierent bonding methods and corresponding anode plane designs|therst detectorhasasteeringelectrodegrid,and isbondedto theASICwithindiumbumps; the second detector hasno gridbut anarrowergap betweenanode contacts, and isbonded to theASICwith conductiveepoxy bumps andgoldstud bumps in series. Inthis paper,we presentresultsfrom detailed X-ray testing oftheHEFTpixel detectors. Thisincludes measurementsof theenergyresolutionforbothsingle-pixel and split-pixel events, andcharacterizationof theeects of charge trapping,electrode biasesandtemperature onthespectralperformance. Detectorsfromthetwobondingmethodsarecontrasted.

Keywords: CdZnTeradiation detectors, conductiveepoxy, ip-chipdevices, indiumbump-bonding, radiation detector circuits,sampleandholdcircuits,semiconductordevicebonding,X-rayastronomydetectors

1.INTRODUCTION

We have developed large formatCdZnTe hybridpixel detectors optimized for use as focal plane detectors for hard X-ray telescopes. The detectors are designed for the High Energy Focusing Telescope (HEFT)

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balloon experiment, and are optimized to achieve good spectral resolution with low power over the energy range 5{ 100 keV. EachHEFT detector consistsof anarrayof 2444pixels on a498-mpitch. Eachpixel is bonded to its own amplier circuitry on a custom, low-noise application-specic integrated circuit (ASIC). We have investigatedbothindiumbump bondingaswellasgoldstud-epoxybonds,andwecomparedetectorsfabricated using each.

In this paper, we rst describe the detector geometry for two dierent sensor architectures that we have investigated. Next, wediscuss thedata processing stepswe take to obtainspectra outofthe signalsfrom the detector hardware. Finally,wecharacterizethe performanceof thetwodierentdetectorachitecturesin terms of itsspectralresolution,eÆciencyandotherproperties.

2.SYSTEM CONFIGURATION AND DETECTOR GEOMETRY

The HEFTdetector systemconsists of aCdZnTe-ASIC hybriddetector, ananalog-to-digitalconverter(ADC) fortheASICoutput,amicroprocessortooperatetheASIC,andsupportelectronics. Figure1showsadiagram of thedetectorsystem. Wedescribetherelevantcomponentsindetails inthissection.

Furtherauthorinformation: (SendcorrespondencetoC.M.H.C.) C.M.H.C.: E-mail: hubert@caltech.edu,Telephone: 16263956630

Hard X-Ray and Gamma-Ray Detector Physics V, edited by Larry A. Franks, Arnold Burger,

Ralph B. James, Paul L. Hink, Proceedings of SPIE Vol. 5198 (SPIE, Bellingham, WA, 2004)

0277-786X/04/$15 · doi: 10.1117/12.506075

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3x128kB SRAM

Clk

12−bit ADC

MISC

CdZnTe

ASIC

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Level Shifter

EIA−422 serial line

CdZnTe

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Figure1. Diagramandowchartofthedetectorsystem.

300 m pitch for pixels along mating edge

µ

14 m wide grid

µ

µ

50 m gap from anode to grid

498 m pitch pixels

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Gridded detector:

1 mm guard ring on 3 sides

0.1 mm guard ring on mating edge

300 m pitch for pixels along mating edge

µ

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30 m gap from anode to anode

498 m pitch pixels

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1 mm guard ring on 3 sides

0.1 mm guard ring on mating edge

Detector with no grid:

Figure 2. Anodeplanepatterns. Thegriddedpatternatthetopisdesignedfor indiumbumpbondingtominimizethe anode contactsize and inputcapacitance. Theno-gridpatternat thebottomis designed for bondingwithconductive epoxyandgoldstudbumps,whichare4{5timestallerthanindiumbumps. Bothpatternscontainamatingedge,where thelastrowofpixelsandtheguardringarecontractedforaseconddetectortobeplacedsidebyside.

2.1. CdZnTe

We purchase CdZnTecrystals with platinumelectrodesfrom eVProducts. The size of thesensor is 23.6 mm by 12.9 mmby 2mm thick. This was the maximumsize available given specication of single-crystal,highly uniformmaterialatthetimeofourrstdesign. Thecathodeisamonolithicplatinumcontact,whiletheanode planeispatternedintoa2444pixelarrayof498-mpitch,surroundedbyaguardringthatis1mmonthree sides,and0.1mmonthefourth(sothattwodetectorscanbeplacedsidebysidetoformaroughlysquaresensor area withminimal dead areain between). Figure 2showstwoanodeplane patterns wehave usedin dierent hybrids.

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contacts, 2

and onewithlarger pixel contacts,and a30 m gap in between. These twoanode planepatterns accommodatethetwobondingmethodsweusetoconnecttheCdZnTetotheASIC.TheCdZnTesensorswith the gridded anode pattern are indium bump-bonded to ASICs. These indium bumps are only 8{10 m tall, requiringustoreduce theanodecontactsize tominimizeinputcapacitance. Becauseofthelargergapbetween contacts,weaddagrid,heldatapotentialintermediatebetweenthecathodeandanodetosteerchargehitting the surfacebetweenpixelstowardtheanode,minimizingchargelossin thegap. Adierentbondingapproach, whichwehaveadoptedforallfuturesensors,aordsalargerCdZnTetoASICseparation,enablingustoeliminate the grid, use largercontacts, yet maintainlowinput capacitance. Thebonds consist of aseries connection of conductiveepoxybumps(ontheCdZnTe)and goldstud bumps(ontheASIC).

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These connectionsareabout 40m tall. Inthispaper,wepresentresultsfromonehybridofeachof thesearchitectures.

2.2. ASIC readout

WedevelopedthecustomASICatCaltechaspartoftheHEFTprogram. Itconsistsofa2444pixelarray,also of 498-mpitch, withthepixel patternmatchingthat ontheCdZnTeanodeplane. Eachpixel isimplemented with itsown preamplier, shapingamplier,discriminator,andsampling and pulsingcircuits. All pixelsshare aserialreadoutline. WedesignedtheASICforlownoiseandpower;itconsumesabout50mWofpowerunder normaloperation. Weprovidefurtherdetails ontheASICcircuitryinSection 3.

2.3. Digital circuitry

AP24microprocessor|a24-bitMinimalInstructionSetComputer(MISC)implementedonanActelA54SX72A FPGAtorunForth|controlstheASIC.TheMISCrunsona7.3728MHzclockcycle,drivenbya14.7456MHz oscillator chip. Three 128kBSRAMsprovideitwith128kof24-bitmemory. Theoutput oftheASICreadout line is digitized byan80 mW, 12-bit ADC12062. The MISC then pipesthedigitized data outto anEIA-422 serial lineviaalevelshifter. Theentiredetectorsystemconsumesabout700mWofpower.

3. READOUT CIRCUITRY|DESIGN DETAILS

3.1. Basic concepts

The designoftheASICcircuitry isthekeytoachievinglowpower,lownoiseandgoodspectralresolution. In order to achievelowpower,wehavechosenadesignthatis dierentfrom theconventional amplier chain. In our design, thesignal shaping and peak detection stages of the conventional chain are replaced by abank of 16switchcapacitorsarrangedtocontinuouslycapturesuccessivesamplesofthepreamplieroutput. Theresult isalargereductioninpowerdissipation|from250Wto50Wperpixel|whileallowingo-chipdigitalsignal processingtoextractnearoptimalenergyresolution.

3.2. Implementation

The implementation of the sample and store mechanism with abank of 16 capacitors is illustrated in Fig. 3. Thepreampliferoutputisconvertedtoacurrentandisintegratedbythecapacitors,cyclicallyonebyone,with a1sintegrationtime. Thisprocess givesus arecordofthecurrentlevelduring theprevious15{16satany giventime. Whenatriggeris detected,sampling continuesfor 8moresamples, after which thecircuit freezes while thesamplesarereadout.

Inadditiontoreadingoutthe16samplesfromeachtriggeredpixel,wealsoreadoutsamplesfromacollection of other pixels for additionalinformation to help with thepulse heightrecoveryprocess. These pixels include all the ones neighbouring anytriggering pixel (ie, those sharingan edge ora corner with a triggeringpixel), and a 33 array of reference pixels remote from the triggered pixels. Samples at the neighbouring pixels containanysystematicnoisethatiscommoninthevicinityofthetriggeringpixel,whilesamplesatthereference pixelscontainnoisethat iscommonto theentirechip. Theneighbouringpixelsmayalsohavecollectedasmall fraction ofthe charge inducedfrom the X-ray event, ifthe event hasoccurred near the edge of thetriggering pixel,andthischargemaybetoosmalltohavetriggeredtheneighbouringpixel. Withtheseadditionalsamples, weimplementaseconddiscriminatorinsoftwarewithamuchlowerthreshold,andalsoremovesystematicnoise from thetriggeredpixels.

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i

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t

s(t)

t

s[t]

s(t)

t

MUX

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x 16

Shaping amplifier

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i(t)

Figure3. Illustrationofthesampleandstoremechanismwithabankof16capacitors.

Withthisscheme,aneventtriggeringasinglepixelinvolvesreadingout16samplesfrom3 3 + 3 3=18pixels (1816 =28812-bitnumbers). An event where twoadjacentpixels trigger(which we termacharge-sharing event) requires readingout 16samples from 43+33= 21pixels (2116 =336 12-bit numbers). With additional information(such aspixel coordinates, timeinformation, etc), eacheventproduces about0.5kbyte of information. Thereadoutprocesstakesaboutabout30 ms. AsimplementedforHEFT, withone ADC for twohybridsensors,thefocal planecan toleratecountratesof upto100counts/sbeforesaturating.

4.SOFTWARE PROCESSING

Withmuchsignalprocessingdelegatedo-chip,softwareprocessingbecomesanimportantprocedurethat deter-mines thequalityofpulseheightrecovery. Thesoftwareprocessingsequencestartswithapreliminaryscreening oftheeventrecord,whichscreensoutabnormalevents. Inmostoftheseabnormalevents,thereareeithermore thantwotriggeringpixels,ortwotriggeringpixelsthatdonotshareacommonedge. Noiseisthemostcommon cause for theseevents. According to ourpreviousdetector modelling results,

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weexpect no morethan3% of all eventsto havechargesharedamongstthree ormorepixels. Therateofhavingtwophotoneventsoccuring within asingle1s integrationperiod isalso low. Other eventsthat arescreened outbythis processincludes theoccasional(afewtimesinamillion)occurrenceofbadencodings,andeventsoccuringtoosoonafteracircuit reset, whenthequiescentsignallevelhasnotbeenstabilizedyet.

Thenextstage of processinginvolvestheremovalof systematicnoisecommonto allpixels. Weobtainthe noise level as the average of samples measured at the nine reference pixels. For each of the 16 integration periods in anevent record, the average noiselevel is subtracted from the samples measuredat the triggering and neighbouring pixels. We then calculate a pulse height from each pixel that has anoise-corrected sample sequence, as thedierence betweentheaverageof thelast sixsamplesand theaverageof therstsix samples in thesequence. Thepulseheightsarethenpassedthroughasoftwarediscriminator,in searchofcharge-sharing events that arehidden from thehardwarediscriminatorcircuit by noise. Because thecommon noisehasbeen removed, wearenowabletoset thethresholdbelow1keV,asopposedto thehardware thresholdnear8keV. Eventswithfalsetriggersarealsoeliminatedatthis stage.

Forthesingle-pixelandtwo-pixel(charge-sharing)eventsthatremain,werecalculatetheirpulseheightswith a moresophisticatedformula that takesthevaluesof all16 noise-correctedsamplesinto account. Finally, the pulse heightsareshiftedandscaledto compensatefordierencesin ampliergains andcapacitorosetsacross pixels. Thepulseheightpairsincharge-sharingeventsaresummed,andalleventsarebinnedtoproducespectra and otherrelatedinformation,whichwepresentin thenextsection.

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temperaturecombinations. Theinset mapshowsthespatialdistributionofthesamedata,withbrightnessproportional tothenoisemagnitude.

5.DETECTOR CHARACTERIZATION

In thissection, wepresentthe detectorperformance underarange oftemperaturesand biasvoltages forboth architechtures: thegrideddetectorwithindiumbumps,andthedetectorwithnogrid,bondedwithepoxystuds. For practical reasons (to collect suÆcient statistics in a reasonable time) we present detailed results from a contiguousareaof1011pixels.

5.1. Electronic noise

The intrinsic energy resolution of the detector hybridat low X-ray energies is predominantly determined by the electronic noisein theASICcircuitry. We measurethe electronicnoiseasthe fullwidth at half maximum (FWHM) ofaGaussianspectralline producedbyelectronicpulseswithenergiesequivalentto75keVphotons. Thedistribution ofelectronicnoiseforthegriddeddetectorisshowninFig.4.

5.1.1.Temperature dependence

The dotted and dashed lines in Fig. 4 show the distributions of the electronic noise for 110 pixels, at room temperature ( 22

o

C) and at 0 o

C, respectively, when all electrode biases are set to zero. Zero bias ensures that wearemeasuringthenoisecomponentfrom theelectronicsitself,ratherthanshotnoisecausedbyleakage currentthroughtheCdZnTecrystalthatis channelled intothepreamplier inputs. Atroomtemperature,our ASICdesignhasanenergyresolutionof(62429)eVFWHMat75keV,dueto thermalnoiseinthecircuitry; at 0

o

C, jitter in the circuitry decreases, and the resolution is improved slightly to (54234) eV FWHM at 75 keV. Ifpulses atthese pixelsare allsummedtogether, theresulting75-keVline hasaFWHM of623eVat roomtemperatureand540.eVat0

o C. 5.1.2.Leakage current contribution

Forthegrideddetector, leakagecurrentisintroducedby surfaceleakagebetweenthegridand contact,as well asbybulkleakage. Themagnitudeofthecontributiondependsonthesurfaceandbulkresistivities,whichvary fromdetectortodetector,andtheoperatingbiasvoltage. Forthegridedhybridevaluatedhere,whenthebiases are set to nominal valuesof 300V at the cathode and 4V at the steering electrode grid, bothrelativeto theanodes,noiseisintroducedbysurfaceleakagecurrent,andtheresolutiondegradesto(79199)eVatroom temperatureand(75736)eVat 0

o

C. Theresolutionsin thesummedspectraare779eVand756eVFWHM,

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Detector without grid

Detector with grid

Figure 5.Distributionoflinewidthsfromthe59.54-keVlineof 241

Am,at0 o

C,asmeasuredat1110pixelsofeachof the two detectordesigns. Thehistogramontheleft shows aslightlybetter performance ofthedetector withoutagrid (solid) thanthegriddeddetector. Theintensitymapsontherightshowthespatialdistributionofthelinewidths. Both mapsaredrawnwiththesamegreyscale,withbrightnessproportionaltotheline width. Notethatpixelsatthecorners are shieldedfromthesourcebythecircularcollimator opening. Thesepixelsarenotincludedinthehistogram.

respectively. Thesolidanddash-dotlinesinFig.4showtheFWHMdistributionatthesame110pixelsatthese biases.

Forthedetectorwithoutagrid,theleakagecurrentresultsfromthebulkleakagecomponentonly. 5.2. Spectral resolution for X-ray events

TocharacterizetheresponseofthedetectorstoX-rayevents,andthustheperformanceoftheCdZnTesensors, wetested thedetectors with anAm-241 sourcecollimated into acircular beamwith a10-to 11-pixel (5 mm) diameter.

5.2.1.Single-pixelevents

Figure 5 shows the distribution of the 59.54-keVline widths for the 110 pixels under the collimator at each detector. Themeasurementsweremadeat 0

o

C, thetargetedoperatingtemperatureforHEFT.Theplotshows only events triggeringone pixel. For the gridded detector with 10 m indium bumps, the energy resolution ranges from 0.8 to 1.4 keV FWHMat 59.54 keV,with themajorityof thepixels having1.0 keV FWHM. For thenon-griddeddetectorwith40mhighepoxyandstudbumps,theaverageenergyresolutionisimprovedby anaverageof0:1keV.

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Figure 6.Spectraof Am,fromallsingle-pixeleventssummedoveran1110pixelareaoftheHEFTdetectors. The Gaussianline widthsastted fromthespectra arelisted. Thenon-griddeddetectorin(b)measuresamoresymmetric 59.54-keVlinethanthegriddeddetectorin(a).

Figure 6 shows the summed spectra from all 110 pixels. Also indicated in the gure are the measured Gaussian line widths of the variousspectrallines of Am-241. Atlowenergies, theline widths are comparable to the electronic noise (the pulser line width), indicating the absence of systematic eects (e.g. incomplete chargecollection)in theCdZnTe. Forthegriddeddetector, the59.54-keVline showssomeresiduallow-energy tailing, with a FWHM of 931 eV. If oneignores the low-energytail and ts the line only down to the lower half-maximumpoint,then theFWHM becomes863eV.Incontrast,thenon-griddeddetectorproducesamore symmetric59.54-keVlinewithaFWHMof825eV(756eVifthelow-energytailisignored).

5.2.2.Charge-sharing events

Therecoveryofcharge-sharingeventsisimportantfordetectorswithpixelsofthissmallsize,sincetheseevents account for as much as 50% of the total.

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Figure 7 shows the spectra from the two detectors obtained from charge-sharingeventsonly. The 59.54-keVline measuresaFWHMof 1.83keV fromthegridded detector, and 1.18keVfrom thenon-griddeddetector. Ithascomeasasurprisetousthatthenon-griddeddetector, withits narrowerinterpixel gaps, doesbetterthan thegridded detector,with itssteeringelectrodes. Whensingle-pixel and charge-sharingeventsare bothsummed together, the spectra thus produced are shown in Figure 8. The FWHM measurementsare1.23keVand973eVforthegriddedandnon-griddeddetectors,respectively.

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Figure 7. Spectra of Am, fromall 2-pixel charge-sharing events summed overan1110pixel area of the HEFT detectors. Forcomparison,the spectrafromsingle-pixel eventsare scaledtothe samecountrateand displayedherein dottedlines.

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Figure 8. Spectraof Am,fromall eventssummedoveran1110pixelareaoftheHEFTdetectors. 5.3. Optimization of operating biases for the gridded detector

To minimize charge trapping in the CdZnTe crystal, electrode biases have to be tuned appropriately. We measured theenergyresolution ofthe 59.54-keVline at various biascombinationsat the cathodeand steering electrodeofthegriddeddetector. Foreachcongurationwemeasuredthewidth(FWHM) andskewnessofthe line. Figure9showstheirtrendasafunctionofthebiasvoltages. Weseethatwhilegreatergrid-to-anodebiases arealwaysdesirableupto 4V(producingasurfaceeldstrengthof4V =50m=80000V =m),cathodebiases that are toolargewill reduce therelativegrid bias(E

grid anode =E

cathode anode

), andthus reducethe steering eect ofthe gridand increasetailingof thespectralline. On theother hand,cathodebiasesthat are toolow decrease theelectronmeanfreepaththroughthecrystal,thus increasingchargetrappingandsubsequentlythe line width. Therefore, we need to nd a balance between the two eects; according to our measurements, a

450Vcathodebiasisappropriate(producingabulkeld strengthof450V =2mm=225000V =m). 6.CONCLUSION

TheHEFT detectorsareoptimizedto achievegoodenergyresolution withlowpower. Bycarefuldesignof the ASICcircuitryando-chipdigitaldataprocessing,wehaveachievedenergyresolutionsbelow1keVFWHMat hardX-rayenergies,foralarge-formatCdZnTe-ASIChybriddetector at0

o C.

With characterization completed, ourdetector is readyto beput into productionfor theupcoming HEFT balloon ights. The HEFT detectors have also found application in other elds of science. For instance, the materials science groupat Caltechis adopting ourdetector in anewgeneration ofMossbauerPowder Dirac-tometers,

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withtheexpectation ofimprovingthesignal-to-noiseratiofromabout1:1toabout10:1orbetter; this will be extremely benecial to the development of the novel technique of Mossbauer Diractometry, for

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Figure 9. IntensitymapsoftheFWHM(left)andskewness(right)ofthe59.54keVline of 241

Amatvariouselectrode bias pairs. Darkness isproportionalto theFWHMmagnitudeontheleft and totheskewnessmagnitudeontheright, sothatthemostdesirablecongurationsarethebrightestonesineachcase. Thereisapartialtrade-obetweenFWHM andskewness.

thedetermination ofmaterialstructures. Wearealsocontinuingtoexperimentwithnewhardwaredesignsand softwareanalysisapproachesto improvetheperformanceofourdetector system.

ACKNOWLEDGEMENTS

This researchwassupportedbytheNASASpaceScience \SupportingResearchandTechnology"(SR&T) pro-grammeunder GrantNumberNAG5-5398. FAHwasfurther supported by aPresidentialEarlyCareerAward, GrantNumberNAG5-5322. JLwassupportedbytheNationalScienceFoundationunder GrantNumber DMR-020-4920. We are grateful for their support. We also thank Aleksey E Bolotnikov for sharing his invaluable experiences with CdZnTe detector testing. Jill Burnham, Branislav Kecman and John Klemic have played crucial rolesinthefabricationandtestingofthedetectors.

REFERENCES

1. F.A. Harrison,S. E. Boggs,A. E. Bolotnikov,F.E. Christensen, W. R.Cook,W. W. Craig,C. J.Hailey, M. A. Jimenez-Garate, P. H. Mao, S. M. Schindler, and D. L. Windt, \Development of the High-Energy Focusing Telescope (HEFT) balloon experiment," in X-Ray Optics, Instruments, and Missions III, J. E. TruemperandB.Aschenbach,eds.,Proc. SPIE4012,pp.693{699,July2000.

2. A.E. Bolotnikov,S. E.Boggs, C.M.H.Chen,W. R.Cook,F.A.Harrison,andS. M.Schindler,\Optimal contactgeometryforCdZnTepixeldetectors,"inHardX-Ray,Gamma-Ray,andNeutronDetectorPhysicsII, R.B.JamesandR.C.Schirato,eds.,Proc. SPIE4141,pp.243{252,Nov.2000.

3. J.E.Clayton,\Veryhighpincountipchipassemblyusingconductivepolymeradhesives,"inIMAPS2003| 35thAnnualSymposiumonMicroelectronics,inprint.

4. C.M.H.Chen,S.E.Boggs,A.E.Bolotnikov,W.R.Cook,F.A.Harrison,andS.M.Schindler,\Numerical modelingofchargesharinginCdZnTepixeldetectors,"IEEE Trans.Nucl. Sci.49, pp.270{276,Feb.2002. 5. J.Y.Y.LinandB.Fultz,\Site-speciclong-rangeorderin

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AlmeasuredbyMossbauerdiractometry," Phil. Mag.83, pp.2621{2640,2003.

6. B.Fultzand J.Y. Y.Lin, \Mossbauerdiractometry,"inMaterial Research in AtomicScale byMossbauer Spectroscopy,M.Mashlan,M.Miglierini,andP.Schaaf,eds.,NATOScienceSeries: II:Mathematics,Physics andChemistry94, pp.285{295,2003.

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