A Leakage Current-based Measurement of the Radiation Damage in the ATLAS Pixel Detector

1.7 MB·English

by  Igor V. Gorelov

PreprinttypesetinJINSTstyle-HYPERVERSION
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2 A Leakage Current-based Measurement of the

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e Radiation Damage in the ATLAS Pixel Detector
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- IgorV. Gorelovonbehalfofthe ATLASCollaboration
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aDepartmentofPhysicsandAstronomy,UniversityofNewMexico,
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s. Albuquerque,NM87131,USA.
c E-mail: [email protected]
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y
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p ABSTRACT: Ameasurementhasbeenmadeoftheradiation damageincurred bytheATLASPixel
[
Detector barrel silicon modules from the beginning of operations through the end of 2012. This

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translatestohadronicfluencereceivedoverthefullperiodofoperationatenergiesuptoandinclud-
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2 ing 8TeV. Themeasurement isbased ona per-module measurement ofthe silicon sensor leakage
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current. The results are presented as a function of integrated luminosity and compared to predic-
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7 tions bytheHamburg Model. Thisinformation canbeused topredict limitson thelifetime ofthe
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PixelDetectorduetocurrent, forvariousoperating scenarios.
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5 KEYWORDS: Simicrostrip andpaddetectors; Pixelateddetectors; Radiation-hard detectors.
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Contents
1. Introduction 1
2. MeasuredQuantities 2
3. TheATLASCurrentMeasurementSubsystem 3
4. CurrentMeasurements, 2011-2012 4
5. Precision andSystematicUncertainties 5
6. LifetimeEstimate 6
7. SummaryandProspects 7
1. Introduction
TheATLASPixelDetector[1]isconstructedfromplanarsiliconpixelmodulesarrangedinabarrel
and disk geometry. It is centered on the interaction point at the heart of the ATLAS Detector [2]
attheLargeHadronCollider(LHC).Alinearrelationship betweenleakage currentandahadronic
particle fluenceisexpected toapplytothepixelsiliconsensorsandisgivenby
D I=a F V, (1.1)
eq
· ·
whereD IisthedifferenceinleakagecurrentatfluenceF relativetothevaluebeforeirradiationof
eq
thephysicalvolumeV,anda isthecurrent-relateddamagecoefficient[3]. Thegoalofthisstudyis
tomeasuretheleakagecurrentsofarepresentative sampleofsensorsintheATLASPixelDetector
inordertomonitorandunderstandthesensordamageresultingfromincreasingradiationdose. The
measured results are compared to predictions based on the Hamburg Model [4] in which reverse-
bias current is generated by the introduction of defect levels in the band gap due to non-ionizing
energyloss(NIEL)ofthrough-going particles. Thedefectsmigrateandrecombinethroughseveral
processes that result inboth short-term beneficial annealing (which reduces the current) andlong-
term anti-annealing (which increases the current even in the absence of further radiation). The
modelcanbecalibratedtothedatatopredictthelifetimeofthesensorsandthePixelDetectorasa
wholeforvariousoperating scenarios.
The detector is instrumented as three barrels, specifically the Layer-0 at r = 50.5mm, the
Layer-1 atr=88.5mm,andtheLayer-2 atr=122.5mm,comprising intotal 1456modules. The
endcap areas are instrumented with three disks located at z= 495mm, 580mm and 650mm
± ± ±
in each of the forward and backward regions, comprising an additional 288 modules. The Pixel
–1–
Detector and the other elements of the ATLAS Inner Detector span a pseudorapidity range h <
| |
2.51. Thepixelsensormodules aremountedonmechanical/cooling supports, calledstaves, inthe
barrel region. Thirteen modules are mounted on a stave with identical layouts for all layers. The
temperature is maintained stable by an evaporative cooling system. The entire 1.7m2 sensitive
∼
area of the ATLAS pixel detector is covered with 1744 identical modules. Each module has an
activesurfaceof6.08 1.64cm2. Thedetailsofthepixelsensorgeometryandlayoutcanbefound
×
in[1].
Theradiation fieldin the region of the detector has been predicted with the aid of a FLUKA
software package [5]. The FLUKA transport code to cascade the particles through the ATLAS
detectormaterialhasbeenusedtogetherwithPYTHIA 6.2 orPHOJETgeneratingproton-proton
minimum-bias eventsasaninputtoFLUKA [1].
Weassumethatthedominantradiation damagetypeisdisplacement defectsinthebulkofthe
pixel sensor, initiated by hadronic species and caused by NIEL.Surface ionization isneglected in
this treatment. Charged pions are expected to dominate the bulk damage for the radii covered by
thePixelDetector. Albedoneutrons originating intheouterATLASdetectors alsocontribute. The
resulting displacement defects increase the reverse leakage current, degrade the charge collection
efficiency, and change the effective doping concentration which directly determines the depletion
voltage. WedefinetheeffectivefluenceF asthenumberofparticles causing damageequivalent
eq
to that of 1MeV neutrons traversing 1cm2 of asensor’s surface. The fluence F accumulated by
eq
ATLASpixeldetector, andmeasuredinunitsof cm 2,isexpected tobeproportional tointegrated
−
luminosity L dt,measuredin pb 1.
−
R
2. Measured Quantities
Theparametera inEquation1.1hasbeenmeasuredunderavarietyofconditions,forexample[3].
The linear form applies to the leakage currents drawn by sensors past their beneficial annealing
periods. Atthebeginning ofdata-taking andduring beneficial annealing periods, thesensors draw
currents at levels comparable to dark currents; thus any hardware implementations for leakage
current measurementshouldprovideaccesstothelowestcurrent rangepossible.
The reverse bulk generation current depends on the sensor temperature T. The current mea-
surements I(T)arenormalized tothereference temperature T (0 C)with
R ◦
E
I(T)=I(T )/R(T), whereR(T)=(T /T)2 exp g (1/T 1/T) , (2.1)
R R R
· (cid:18)−2k − (cid:19)
B
whereE =1.21eVistheenergyofthesiliconbandgap,k istheBoltzmannconstant,andTandT
g B R
aremeasuredinK[6].
Themeanpixel module temperature for allbarrel pixel modules isshown versus timein Fig-
ure 1 [7]. There were a few exceptions when the temperature rose to ambient due to cooling
interruptions and calibration scans. Temperature is read out from every module continuously and
1ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalinteractionpoint(IP)inthecentreof
thedetectorandthez-axisalongthebeampipe. Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe
y-axispointsupward.Cylindricalcoordinates(r,f )areusedinthetransverseplane,f beingtheazimuthalanglearound
thebeampipe.Thepseudorapidityisdefinedintermsofthepolarangleq ash = lntan(q /2).
−
–2–
averaged over30-minute intervals. Temperature data(aswellasother slowcontrol data) arecom-
mittedintotheATLASDetectorControlSystem(DCS)database.
25
ATLAS Preliminary
20
15
C
(cid:176)e, 10
ur 5
at
er 0
p
m -5
e
T
-10
-15 Mean Pixel Module Temperature: Layer-0 / Layer-1 / Layer-2
-20
02/07/11 01/01/12 01/07/12 31/12/12
Date, dd/mm/yy
Figure1: Themeantemperature( C)ofallbarrelpixelmodules,versustime(days)[7].
◦
3. The ATLAS Current Measurement Subsystem
The ATLAS High Voltage Patch Panel 4 (HVPP4) serves as a fan-out point for the bias voltages
delivered by Type II boards from Iseg [8] high voltage power supplies to the pixel modules. The
leakagecurrentismonitoredatthepixelmodulegranularitylevelbyaCurrentMeasurementBoard
(CMB)system. ACMBismountedoneveryTypeIIfan-outboard. Thebiasvoltagetothesensors
isprovidedbytheIsegchannels.
Inthepresentdata-taking era,whentheradiation damageofthesensorsisrelatively low,6or
7 pixel modules are fed by one Iseg power supply channel. At some point after inversion of the
sensors, before the current drawn by 6 or 7 pixel modules reaches the Iseg limit, power supplies
can be added until the system provides one Iseg power supply channel per pair of pixel modules.
TheCMBsystem allows simultaneous measurement ofthe leakage currents in4pixel modules in
the current range from 0.04m A to 2mA with a precision of better than 20% per module. ATLAS
uses21CMBsinLayer-0, 16inLayer-1,and16inLayer-2. Theanalogcurrentmeasurementsare
digitized by the 64-channel ATLASEmbedded Local Monitoring (ELMB)board and sent via the
ControllerAreaNetwork(CAN)bustotheDCSdatabase(see[1]fordetailsontheTypeIIfan-out
board, ELMB, CAN, and DCS). The building block of the CMB circuit is a current to frequency
converter optically coupled to a frequency to voltage converter as shown in Figure 2. Each CMB
holds4 2currentmeasurementcircuitsandprovidesthecurrentmeasurementfor4pixelmodules.
×
Twosimilarcircuits withdifferent gainsareallocated perCMBchannel serving thesamemodule.
The channels are isolated from each other and from the pixel module readout system. For each
pixel module, the leakage current range [10 8 10 5]A is covered by the high-gain channel, and
− −
−
therange[10 6 2 10 3]Aiscoveredbythelow-gainchannel. TheoutputCMBvoltagerangeis
− −
− ·
definedbyoneoffiveavailable ELMBinputvoltage ranges, whichare[0 25]mV,[0 100]mV,
− −
[0 1]V, [0 2.5]V, and [0 5]V. The output range [0 5]V is necessary to comply with the
− − − −
specifications of the digital ELMB board. The 16-bit ADC of the ELMB provides a resolution
of (ELMB range)/(216 1). The range [0 1]V was used for data taken through 2011; the range
− −
–3–
Figure2: TheCurrentMeasurementBoardcircuit
.
was changed to [0 5]V in 2012 as the high-gain channels reached saturation, corresponding to
−
pixelleakagecurrentsgreaterthan10 5A. AfterLHCshutdownof2013-2014,thedata-takingwill
−
resume with the low-gain channels. Before installation, each CMB is calibrated offline assuming
a linear model. The calibrated gains of the channels are stored in the database. The pedestals are
re-calibrated insitu.
4. Current Measurements, 2011-2012
Figure 3 shows the leakage current recorded by the CMB system from the beginning of ATLAS
data-takingthroughtheendof2012,asafunctionofintegratedluminosity, formodulesinallthree
PixelDetectorbarrellayers[7]. InstallationoftheCMBsystemproceededconcurrently withearly
data-taking, and this is the reason that some Layer-2 data are missing from the early part of the
run. Itisforeseen thatthedisks willbeinstrumented withcurrent monitorboards priortothestart
of data-taking in 2015 (Run 2). The points at which LHC and cooling were off, hence beneficial
annealingofthesiliconwasunsuppressed,areapparentasdiscontinuitiesinthisfigure. Thiseffect,
producing thesawtoothfeatures, isduetotheincrease oftheeffectivedoping concentration ofthe
silicon duetoannealing ofacceptors [3,4].
–4–
Figure3: ATLAS Pixel module leakage current
versus integrated LHC luminosity [7]. The cur-
rents are averaged over their layer for all mod-
ulesequipped withCurrentMeasurement Boards
within the layer. The current is continuously
monitored by the ATLAS Detector Control Sys-
tem. A prediction based on the Hamburg Model
is included. Discontinuities are due to annealing
duringLHCandcooling stops.
Figure3alsoincludesaprediction[4]oftheleakagecurrentforthedetailedATLASgeometry
and LHC luminosity profile based on the Hamburg Model. The predictions of the fluence F
eq
accordingtotheluminosityprofilearemadeusingtheFLUKAtransportcode[5]. Thepredictions
are uniform w.r.t. the azimuthal angle f and suppose a forward-backward symmetry w.r.t. the
polar angle q , i.e. F (h )=F ( h ). At the end of Run 1 with L dt =30fb 1, the Layer-0
eq eq −
−
moduleshavereceivedatotalfluenceF =7.0 1013cm 2,whileLRayer-1andLayer-2havebeen
eq −
·
irradiated to F =3.0 1013cm 2 and F ∼=1.8 1013cm 2 respectively. The uncertainty of the
eq − eq −
· ·
leakage current p∼redictions comprises the u∼ncertainty of the calculations of the hadronic particle
spectra initiated by pp interactions in the ATLAS detector and the uncertainty of the Hamburg
modelindescribing radiation damage effectsinflicted byNIELofhadrons crossing thevolumeof
the silicon pixel sensors. Figure 4a shows the distribution in azimuthal angle of modules whose
currentissampledbyCMBsintheATLASPixelDetector. Figure4bshowsthedataforeachoffour
quadrants inthe azimuthal angle astheyaredefined byFigure 4a[7]. Nosystematic difference in
a module leakage current is observed w.r.t. to the quadrant where the module islocated. Figure 5
shows the instrumented module distribution, segmented into five sectors in pseudorapidity. The
pseudorapidity acceptance of each sector of each layer is indicated. Plots in Figure 6a through
Figure 6e, show the leakage current data for each of the 5 ranges in pseudorapidity [7]. Figure 6f
shows the current data versus pseudorapidity for the sectors defined by Figure 5, for integrated
luminosity 25fb 1 [7]. Thepossible slight asymmetry ofthedatapoints inFigure6fisthought to
−
beduetotheasymmetricbeamprofile. Yetthedistribution isinagreementwithintheuncertainties
withtheleakagecurrentpredictionswhicharesymmetricalaccordingtothefluencesimulation[5].
5. Precisionand SystematicUncertainties
Thecontributions totheuncertainty onthecurrentinclude
theCMBandELMBprecision oncurrentmeasurements: 12%forbothranges.
•
thenumberofcurrentmeasurements (twoperhour).
•
theuncertainty ontheluminosity, 1.8%in2011and2.8%in2012[9].
•
–5–
(a)Adiagrammofmodules. (b)ModuleleakagecurrentversusintegratedLHCluminosity.
Figure 4: (a) Sampling of 4 quadrants in azimuthal angle with modules instrumented by
CMBs. (b) The module leakage current in each of the 4 quadrants in azimuthal angle [7]. The
currents areaveraged overeachlayerforallmodules equipped withCurrentMeasurement Boards
withintheparticular quadrant ofthelayer. Predictions basedontheHamburgModelareincluded.
Figure 5: A diagram of the dis-
tribution of instrumented mod-
ulesineachoffivepseudorapid-
ityranges.
the temperature uncertainty, which, at less than 0.3 C, contributes to the uncertainty on the
◦
•
current with3.4%.
6. Lifetime Estimate
Thetemperature-correctedcurrentreadingspermodulecanbeextrapolatedtopredicttheamountof
currentthepixelmoduleswilldrawafteracertainintegratedluminosityhasbeencollectedwiththe
ATLASpixeldetector. Asleakagecurrentcanleadtoexcessivepowerandthermalrunaway,limits
on the module lifetime due to current can be determined from the limit on the bias voltage that
canbeapplied. AsingleIsegpowersupplychannelcansustainamaximumcurrentof <4000m A
fortwomodules. Thereareotherconsequences ofradiation-induced leakage current up∼onmodule
–6–
(a) (b) (c)
(d) (e) (f)
Figure6: (a)through(e): ATLASPixelmoduleleakagecurrentineachof5pseudorapiditysectors,
versus integrated LHC luminosity [7]. The currents are averaged over their layer for all modules
equipped with Current Measurement Boards within the layer. The current is continuously mon-
itored by the ATLAS Detector Control System. Predictions based on the Hamburg Model are
included. Discontinuities are due to annealing during LHC and cooling stops. (f): the module
leakagecurrentindefined5pseudorapidity bins,fortheintegratedluminosityequalto25fb 1[7].
−
lifetime, including the noise, bias, and threshold limitations of the front end chips as well as the
finitecapacity ofthecoolingsystem;whileimportant, thesearenottreatedhere.
Weextrapolate the present rate ofcurrent increase withintegrated luminosity, assuming opti-
misticallythatthemoduleswillbeexposedfor10daysto20 Candotherwisemaintainedat 13 C
◦ ◦
−
in future years. The model assumes that the bias voltage is raised throughout LHC operation as
neededtokeepthebulkfullydepleted,untilamaximumallowedvoltageof600Visreached. Only
the effects of proton runs are considered. Figure 7 shows the implied leakage current per module
versus date and versus integrated luminosity for Layer-0 in this scenario [7]. This extrapolation
predicts a leakage current of about 500m A per Layer-0 module (of thickness 250m m) after the
accumulation ofabout450fb 1 ofdeliveredluminosity, whichcorresponds toJanuary2024inthe
−
scenario proposed here.
7. Summary andProspects
Wehave described the principles ofradiation damage monitoring using the current measurements
provided by the circuits of the ATLAS Pixel high voltage delivery system. We have observed
radiation damage in the ATLAS Pixel Detector with characteristics in agreement with the Ham-
burg Model. The dependence of the leakage current upon the integrated luminosity has been pre-
–7–
0.6
e0.5 ATLAS Preliminary
ul (cid:176)
od0.4 Leakage Current per Module at -13C : Layer-0
m
per 0.3
A] Figure 7: Predicted leakage
m0.2
[Ileak0.1 current per Layer-0 module at
13 C, versus date (upper) and
◦
0 −
31/12/12 31/12/14 31/12/16 31/12/18 31/12/20 31/12/22
integratedluminosity(lower)for
Date [dd/mm/yy]
the scenario in which modules
0.6
will be exposed for 10 days to
e0.5 ATLAS Preliminary
odul0.4 Leakage Current per Module at -13(cid:176)C: Layer-0 20◦C and otherwise maintained
per m0.3 at−13◦C[7].
A]
m0.2
[Ileak0.1
0
0 50 100 150 200 250 300 350 400 450
Integrated L [fb-1]
sented. Alinearbehavior oftheresponse hasbeenobserved. Thecurrentdistribution inazimuthal
quadrants is symmetric as expected. The shape of the current distribution in pseudorapidity is
in agreement with a fluence calculation based on the FLUKA transport code within the assigned
systematic uncertainties. Extrapolation of leakage current with time predicts survival of Layer-0
up to 450fb 1 of data. This current monitoring system will be expanded to encompass pixel disk
−
modulesinRun2andbeyond.
Acknowledgments
Theauthorexpresseshisgrateful appreciation tothecolleagues fromATLASPixelDetector
groupforfriendly cooperation andsupport throughallphasesoftheproject onpixelsensor
leakage currentmonitoring andmeasurement.
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