The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events

15.7 MB·English

by  J. Alme

The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high
multiplicity events
J.Almea,Y.Andresd,H.Appelsha¨userg,S.Babloka,N.Bialasg,R.Bolgena,U.Bonnesf,R.Brammh,P.Braun-Munzingerf,h,m,n,
R.Campagnolod,P.Christiansenj,A.Dobrinj,C.Engsterd,D.Fehlkera,P.Fokah,U.Frankenfeldh,J.J.Gaardhøjee,C.Garabatosh,
P.Gla¨sseli,C.GonzalezGutierrezd,P.Grosj,H.-A.Gustafssonj,H.Helstrupb,M.Hochd,M.Ivanovh,R.Janikc,A.Juniqued,
A.Kalweitf,R.Keidelo,S.Kniegeg,M.Kowalskik,D.T.Larsena,Y.Lesenechald,P.Lenoird,N.Lindegaarde,C.Lippmannd,
M.Magerd,M.Mastd,A.Matyjak,M.Munkejorda,L.Musad,B.S.Nielsene,V.Nikolicl,H.Oeschlerf,E.K.Olsene,
A.Oskarssonj,L.Ostermanj,M.Piknac,A.Rehmand,G.Renaulte,R.Renfordtg,S.Rosseggerd,D.Ro¨hricha,K.Røedb,
M.Richtera,G.Rueshmanng,A.Rybickik,H.Sann†h,H.-R.Schmidth,M.Siskac,B.Sita´rc,C.Soegaarde,H.-K.Soltveiti,
D.Soykh,J.Stacheli,H.Stelzerh,E.Stenlundj,R.Stockg,P.Strmenˇc,I.Szarkac,K.Ullalanda,D.Vranich,R.Veenhofd,
J.Westergaarde,J.Wiechulai,B.Windelbandi
aDepartmentofPhysics,UniversityofBergen,Bergen,Norway
bFacultyofEngineering,BergenUniversityCollege,Bergen,Norway
cFacultyofMathematics,PhysicsandInformatics,ComeniusUniversity,Bratislava,Slovakia
dEuropeanOrganizationforNuclearResearch(CERN),Geneva
eNielsBohrInstitute,UniversityofCopenhagen,Copenhagen,Denmark
fInstitutfu¨rKernphysik,TechnischeUniversita¨tDarmstadt,Darmstadt,Germany
gInstitutfu¨rKernphysik,Johann-Wolfgang-GoetheUniversita¨tFrankfurt,Frankfurt,Germany
hGSIHelmholtzzentrumfu¨rSchwerionenforschungGmbH,Darmstadt,Germany
iPhysikalischesInstitut,Ruprecht-Karls-Universita¨tHeidelberg,Heidelberg,Germany
jDivisionofExperimentalHighEnergyPhysics,UniversityofLund,Lund,Sweden
kTheHenrykNiewodniczanskiInstituteofNuclearPhysics,PolishAcademyofSciences,Cracow,Poland
lRudjerBosˇkovic´Institute,Zagreb,Croatia
mExtreMeMatterInstitute,EMMI,GSI,Darmstadt,Germany
nFrankfurtInstituteforAdvancedStudies,J.W.GoetheUniversity,Frankfurt,Germany
oZentrumfu¨rTechnologietransferundTelekommunikation(ZTT),FachhochschuleWorms,Worms,Germany
Abstract
The design, construction, and commissioning of the ALICE Time-Projection Chamber (TPC) is described. It is the main device
forpatternrecognition,tracking,andidentificationofchargedparticlesintheALICEexperimentattheCERNLHC.TheTPCis
cylindricalinshapewithavolumecloseto90m3andisoperatedina0.5Tsolenoidalmagneticfieldparalleltoitsaxis.
Inthispaperwedescribeindetailthedesignconsiderationsforthisdetectorforoperationintheextrememultiplicityenvironment
of central Pb–Pb collisions at LHC energy. The implementation of the resulting requirements into hardware (field cage, read-
out chambers, electronics), infrastructure (gas and cooling system, laser-calibration system), and software led to many technical
innovationswhicharedescribedalongwithapresentationofallthemajorcomponentsofthedetector, ascurrentlyrealized. We
alsoreportontheperformanceachievedaftercompletionofthefirstroundofstand-alonecalibrationrunsanddemonstrateresults
closetothosespecifiedintheTPCTechnicalDesignReport.
Keywords: ALICE,TimeProjectionChamber
PACS:07.77.-n,07.77.Gx,07.77.ka,29.40.Gx
1. Introduction Furthermore, a comprehensive experiment needs to cover full
azimuthandprovideasignificantacceptanceinpseudo-rapidity
The ALICE [1, 2] Time-Projection Chamber (TPC) [3] is η=−lntanθ/2withθthepolarangle. Inaddition,thedetector
themaindevice, intheALICE‘centralbarrel’, fortrackingof shouldprovideexcellentmomentumandenergy-lossresolution
chargedparticlesandparticleidentification. and run at extremely high rates (>300Hz for Pb–Pb central
ThemaingoaloftheALICEexperimentattheCERNLarge collisions,>1.4kHzforproton–protoncollisions).
HadronCollider(LHC)istheinvestigationofPb–Pbcollisions
atacenter-of-massenergyof5.5TeVpernucleonpair. Track- The resulting detector choice was a large-volume TPC with
ing of charged particles in such an environment can only be overall ‘conventional’ lay-out but with nearly all other design
performedwithadetectorwhichcancopewithunprecedented parameters beyond the state of the art. This manuscript de-
densitiesofchargedparticles: themaximumexpectedrapidity scribesindetailtheresultingdetectorandoutlinesthepathfrom
density in Pb–Pb collisions at LHC energy is about 3000 [4]. designconsiderationstoconstructionandcommissioning.
PreprintsubmittedtoNuclearInstrumentsandMethods January12,2010
Figure1: ALICEschematiclayout[2].
2
In outline the ALICE TPC consists of a hollow cylinder Thefirstmajorchallengewasthedesignandconstructionof
whoseaxisisalignedwiththebeamsfromtheLHCandispar- the field cage, whose overall thickness should not exceed 5%
allel to the ALICE detector’s solenoidal magnetic field. The ofaradiationlengthwhileproviding, overavolumeofnearly
activevolumehasaninnerradiusofabout85cm,anouterra- 90m3, an axial electric field of 400V/cm with distortions in
dius of about 250cm, and an overall length along the beam the 10−4 range. The realization of this device is described in
directionof500cm.Aconductingelectrodeatthecenterofthe Sec.2.
cylinder, charged to 100kV, provides, together witha voltage Thereadoutchambersareinstalledatthetwoendplatesofthe
dividingnetworkatthesurfaceoftheouterandinnercylinder, cylinder. TheirdesignisbasedontheMulti-WireProportional
apreciseaxialelectricfieldof400V/cm. Thedetectorisfilled Chamber(MWPC)techniquewithpadreadout. Toensurelow
withacountinggasconsistingofaNe–CO –N mixtureatat- diffusion of the drifting electrons and a large ion mobility, Ne
2 2
mospheric pressure. Charged particles traversing the detector was chosen as the main component of the counting gas. Fur-
ionize the gas. The ionization electrons drift, under the influ- thermore, the size of the readout pads had to be adapted to
enceoftheelectricfield,totheendplatesofthecylinder,where the expected large multiplicities, implying pad sizes as small
their arrival point in the cylinder plane is precisely measured. as4×7.5mm2 intheinnermostregion. Asaconsequence,the
Togetherwithanaccuratemeasurementofthearrivaltime(rel- readoutchambershavetobeoperatedsafelyatgainsnear104.
ative to some external reference such as the collision time of In Sec. 3 we describe the technical implementation and report
the beams from the LHC) the complete trajectory in space of onthefirstoperatingexperienceofthesedetectors.
allchargedparticlestraversingtheTPCcanbedeterminedwith In Sec. 4 we discuss the design and implementation of the
precision. electronics chain. Because of the high granularity (557568
The ALICE set-up is shown in Fig. 1. The TPC surrounds readout channels) special emphasis was placed on very low
theInnerTrackingSystem(ITS)whichisoptimizedforthede- power consumption. To cope with the large dynamic range
terminationoftheprimaryandsecondaryverticesandprecision needed to track particles from very low to high momenta,
trackingoflow-momentumparticles.OntheoutsidetheTransi- andtoprovidelownoiseperformancecombinedwithefficient
tion Radiation Detector (TRD) is designed for electron identi- baselinerestorationandzero-suppression,thesignalsfromthe
fication. The outermost Time-Of-Flight (TOF) array provides preamplifier/shaperchipwerefedintoa10-bit,10MSPSADC
pion, kaon, and proton identification. In addition, there are integratedintoadigitalchip. Wereportontheimplementation
three single-arm detectors: the Photon Spectrometer (PHOS), and running experience of the electronics chain as realized in
the Electro-Magnetic CALorimeter (EMCAL) and an array of theALICETPC.
RICH counters optimized for High-Momentum Particle IDen- SuccessfuloperationofaverylargedetectorliketheALICE
tification(HMPID). TPC depends on a considerable amount of infrastructure and
The0.5Tmagneticfieldinthecentralbarrelisprovidedby services,alongwithsophisticatedgasandcoolingsystems.One
theL3solenoidalmagnetpreviouslyusedbytheL3experiment. ofthemajorchallengesinthiscontextwastoprovideatemper-
TheALICETPCwasdesignedtocopewiththehighestcon- ature stability of less than 0.1 K across the full volume of the
ceivable charged particle multiplicities predicted, at the time TPC. This requirement originates from the strong temperature
of the Technical Proposal (TP), for central Pb–Pb collisions dependenceofthedriftvelocityintheNe–CO –N mixtureat
2 2
at LHC energy [1, 5, 6], i.e. rapidity densities approaching realisticallyaccessibleelectricfields. Furthermore,itisessen-
dN /dy = 8000atcenter-of-massenergyof5.5TeV.1 Itsac- tialtocontroltheO contentofthecountinggasbelowalevel
ch 2
ceptance covers 2π in azimuthal angle and a pseudo-rapidity of5ppmtokeeptoaminimumtheabsorptionofelectronsover
interval |η| < 0.9. Including secondaries, the above charged the long drift length. The approach to solve these and many
particle rapidity density could amount to 20000 tracks in one other technological challenges is described in the sections on
interactionintheTPCacceptance. cooling, gassystem, infrastructureandservices, andDetector-
Furthermore, the design of the readout chambers, electron- ControlSystem(DCS).
ics, and data handling allows inspection of up to several hun- Calibration and commissioning of the ALICE TPC relied,
dred such events per second with a maximum interaction rate beforetheavailabilityofanycollisionsfromtheLHC,onthree
of8kHzforPb–Pbcollisions,implyingspecialprecautionsto different methods: a set of external UV laser beams was used
minimize the effects of space-charge built-up in the drift vol- tocharacterizefielddistortionsandtodeterminethemagnitude
umeoftheTPConthetrackreconstruction. of the correction from E × B effects on the drifting electrons
Torealizeadetectorwhichperformsefficientlyinsuchanen- originatingfromtheresidualnon-parallelismoftheelectricand
vironmentrequiredthedevelopmentofmanynewcomponents magnetic field inside the drift volume. Furthermore, radioac-
and procedures. A summary of the design parameters is pre- tive krypton was inserted through the gas system into the de-
sented in Tabs. 1–3. A summary and system overview can be tector to provide efficient and precise amplitude calibration of
foundin[2]. all557568readoutchannels. Finally,extensivemeasurements
Inthispaperwedescribethemajorcomponentsofthedetec- with cosmic rays were performed to determine tracking effi-
torascurrentlyrealizedandreportontheperformanceachieved ciencies,energyloss,andmomentumresolutionofthedetector.
aftercompletionofthefirstroundofcalibrationruns. Themethodsusedandresultsobtainedduringthesecalibrations
are described in detail in Secs. 10 and 11. They demonstrate
1Morerecentestimates[4]putthisnumberatdNch/dy<3000. thatdetectorperformanceisclosetothatspecifiedintheorigi-
3
Table1: GeneralparametersoftheALICETPC.
Pseudo-rapiditycoverage −0.9<η<0.9forfullradialtracklength
−1.5<η<1.5for1/3radialtracklength
Azimuthalcoverage 360◦
Radialposition(activevolume) 848<r<2466mm
Radialsizeofvessel(outerdimensions) 610<r<2780mm
Radialsizeofvessel(gasvolume) 788<r<2580mm
Length(activevolume) 2×2497mm
Segmentationinϕ 20◦
Segmentationinr 2chamberspersector
Totalnumberofreadoutchambers 2×2×18=72
Innerreadoutchambergeometry trapezoidal,848<r<1321mmactivearea
padsize 4×7.5mm2 (rϕ×r)
padrows 63
totalpads 5504
Outerreadoutchambergeometry trapezoidal,1346<r<2466mmactivearea
padsize 6×10and6×15mm2 (rϕ×r)
padrows 64+ 32= 96 (smallandlargepads)
totalpads 5952+4032=9984 (smallandlargepads)
Detectorgas Ne–CO –N [85.7–9.5–4.8]
2 2
Gasvolume 90m3
Driftvoltage 100kV
Anodevoltage(nominal) 1350V(IROC)
1570V(OROC)
Gain(nominal) 7000−8000
Driftfield 400V/cm
Driftvelocity(NTP) 2.65cm/µs
Drifttime(NTP) 94µs
√
Diffusion(longitudinalandtransversal) 220µm/ cm
Materialbudget(includingcountinggas) X/X =3.5%nearη=0
0
Table2: ALICETPCelectronicsparameters.
Front-EndCards(FECs) 121persector×36=4356
Readoutpartitions 6persector,18to25FECseach
Totalreadoutcontrolunits 216
Totalpads—readoutchannels 557568
Padoccupancy(fordN/dy=8000) 40to15% inner/outerradius
Padoccupancy(forpp) 5to2×10−4 inner/outerradius
Eventsize (fordN/dy=8000) ≈70MByte
Eventsize (forpp) 0.1−0.2MByte
Totalbandwidth 35GByte/s
Maximumtriggerrate 300HzPb–Pbcentralevents
1.4kHzproton–protonevents
ADC 10bit
samplingfrequency 5−10MHz
timesamples 500−1000
Conversiongain 6ADCcounts/fC
4
Table3: Expectedresolutionparameters.
Positionresolution(σ)inrϕ 1100to800µm inner/outerradii
inz 1250to1100µm
dE/dxresolution,isolatedtracks 5.0%
dN/dy=8000 6.8%
naltechnicaldesignreport[3]. isolationofthehighvoltagefieldcagefromtherestoftheex-
periment is ensured by using CO filled gas gaps between the
2
containmentvesselsandthefieldcagevessels;seeFig.3.
Figure 3: Detail view of the outer field cage near the central
electrode.
Figure 2: 3D view of the TPC field cage. The high voltage
electrodeislocatedatthecenterofthedriftvolume. Theend-
ThedesignoftheALICEfieldcageissimilartothedesignof
plateswith18sectorsand36readoutchambersoneachendare
thefieldcageusedintheNA49experiment[7]. Animportant
shown.
partofthedesignistherequirementtopreventchargebuild-up,
andpossiblebreakdown,onsolidinsulatorsurfacesbetweenthe
field-defining strips and so the use of these insulators is mini-
2. Fieldcage
mizedorcompletelyavoided.
TheALICEfieldcageconsistsoftwoparts;afieldcageves-
Thepurposeofthefieldcageistodefineauniformelectro- sel with a set of coarsely segmented guard rings and a finely
static field in the gas volume in order to transport ionization segmentedfieldcagewhichislocatedinsidethefieldcageves-
electrons from their point of creation to the readout chambers sel. Theguardringsonthefieldcagevesselhelptoavoidlarge
ontheendplateswithoutsignificantdistortions. Thefieldcage electricfieldsduetochargebuild-uponthesurfaceoftheves-
provides a stable mechanical structure for precise positioning sel. Theringshavea92mmgapbetweenthemandthiscorre-
ofthechambersandotherdetectorelementswhilebeingasthin sponds to a relatively low field gradient of 46.7V/mm on the
aspossibleintermsofradiationlengthspresentedtothetracks insulatingsurfacebetweentherings. Theguardringsaremade
enteringtheTPC(seeFig.2). Inaddition,thewallsofthefield of13mmwidestripsofaluminumtapeandtheyareplacedon
cageprovideagas-tightenvelopeandensureappropriateelec- both sides of the containment vessel with a pitch of 105mm.
tricalisolationofthefieldcagefromtherestoftheexperiment. Smallholesweredrilledthroughthewallsofthevesseltoallow
It is a classical TPC field cage with the high voltage elec- for electrical contact between corresponding rings and filled
trodeinthemiddleofthedetector. Electronsdrifttobothend with Al foil feed-throughs and sealed with epoxy. The poten-
platesinauniformelectricfieldthatrunsparalleltotheaxisof tialsfortheguardringsaredefinedbyanindependentchainof
thecylinder. TheTPCisfilledwithamixtureofneon,carbon 24×500MΩresistors(perend). Thefirstoftheseresistorsis
dioxide, andnitrogenbecausethemultiplecoulombscattering connectedtotherimofthehigh-voltageelectrode. Thelastone
inthisgasmixtureisrelativelylow,ithasgooddiffusionchar- isconnectedtogroundthrougha100kΩresistor,acrosswhich
acteristics,andithasahighpositiveionmobilitythathelpsto thevoltagedropismeasuredformonitoringpurposes.Thefield
clearpositiveionsoutofthedriftvolumeinashortamountof gradientbetweentheguardringsmatchesthefieldgradienton
time(seeSec.6). However,toalsohavefastelectrondriftve- thefinelysegmentedfieldcagewhichliesinsidetheguardring
locities requires putting 100kV on the central electrode. The vessel.
5
The finely segmented field cage is made of 165 free stand-
ing mylar strips. In principle, there is space for 166 strips but
mechanical considerations near the central electrode prevents
the installation of the first strip and so it is left out. (The re-
sistor chain for the field cage includes 166 resistors and so in
this way the missing strip is included, see below.) The mylar
stripsdonotcomeintocontactwiththefieldcagevesselorthe
gas containment vessel but, instead, are wrapped around a set
of18Macrolon® rodsthatareregularlyspacedaroundthecir-
cumference of the TPC. The Macrolon® rods are located at a
radius of 2542.5mm on the outer field cage and 815mm on
the inner field cage. The field cage strips are made of 13mm
widealuminizedmylar. TheyarestretchedovertheMacrolon®
rodswithapitchof15mm. Thisleavesa2mminsulationgap
between each pair of strips and creates a voltage gradient of
300V/mm across the gap. The neon gas mixture is the only
insulatorthatseparatesthestripsofthefieldcageexceptforthe
region where the strips touch the rods. The resistor chains for
thefieldcagesarelocatedinsideoneoftheMacrolon®rodsfor
the outer field cage, one on each end, and also inside one rod
Figure 4: Cross-sectional side view of the TPC with relevant
fortheinnerfieldcage,oneoneachend.
dimensions(inmm).Theservicesupportwheelsandoneofthe
The combination of the fine segmentation of the field cage
I-barsarealsoshown.
strips and the coarsely segmented guard rings is a robust and
stable electrostatic design when 100kV is applied to the cen-
tral electrode. This design minimizes the electric field distor- parts:acentraldrumwhichsurroundstheinnertrackingsystem
tionsthatwouldoccurinsidethedriftvolumeoftheTPCifthe (ITS)andtwoconesthatsupportthedrum;seeFig.4. Thecen-
electricfieldlinesweretogodirectlytogroundfromthefield traldrumiscylindricalinshape(1420mmlongand610mmin
cage. Electrostaticcalculationsdemonstratethatthefieldshape radius), and has a Nomex® core that is 5mm thick. The sup-
distortionsinsidethedriftvolumearebelow10−4 atadistance portconesaremadeof3mmthickaluminumandtheyspanthe
of15mmfromthestrips[8]. distancefromthecentraldrumtoeachendplate. Theyprovide
support for the ITS while leaving room for a muon absorber
2.1. Vessels on one end and services for the inner tracking system on the
Fourcylindersarerequiredtomakethecompletefieldcage other. Theattachmentbetweenthecentraldrumandthecones
structure; two field-cage vessels (one inner and one outer) issealedwitha2mmthickflatneoprenerubberring. TheCO
2
and two containment vessels (inner and outer). The cylin- gap between the inner containment vessel and the inner field
dersarecompositestructuresmadewithaNomex®honeycomb cage vessel is 156mm thick at the centerline of the detector
coresandwichedbetweenprepregsheets(epoxyfiberglass)and anddecreasesto80mmneartheendplates.
Tedlar®foils,toprovidealight,rigid,andgas-tightstructure.A The thickness of the critical components of the field cage
cross-sectionalviewoftheTPCwithsomerelevantdimensions vesselsarelistedinTab.4inunitsofradiationlength.
isshowninFig.4.
Theinnerandouterfield-cagevessels(seeFig.4)definethe 2.2. Centralelectrode
gasvolumeoftheTPC.Theyhaveradiiof788and2580mm,
respectively. ThethicknessoftheNomex® honeycombcoreis Thecentralelectrodeismadeofastretched23µmthickmy-
lar foil which is aluminized on both sides and held flat by an
20mm for both vessels. Clamps are glued to the inside walls
innerandouteraluminumrim. Threefoilsweregluedtogether
ofthecylinderstoprovidesupportfortheouterrimofthehigh
bylayingthemsidebysideandgluing50mmwidealuminized
voltageelectrodeandalltherods.
mylarbandsoverthejunctions. Theresulting6×6m2foilwas
Thecontainmentvesselssurroundthefieldcagevesselsand
stretchedwithpneumaticjacksandgluedontoasetofinnerand
theyprovidegastightandgroundedenclosuresattheinnerand
outerrims. Aftercuring,asecondsetofrimswasloweredinto
outer diameters of the TPC. To maintain a good ground, both
positionandgluedtothefoil.
walls of the cylindrical composite structures are covered with
50µmthickaluminumfoil. Thecontainmentvesselsaresepa-
2.3. Rods
ratedfromthefieldcagevesselsbyaninsulatinggapandthese
gapsarecontinuouslyflushedwithCO toisolatethefieldcage Atotalof72rodsarepositionedaxiallyontheinternalwalls
2
voltagefromthegroundedcontainmentwalls. Thedistancebe- oftheinnerandouterfield-cagevesselsandinthecornersbe-
tweentheouterfieldcagevesselandtheoutercontainmentves- tweenthereadoutchambers. Theirmainroleistoholdthefield
sel is 147mm. The Nomex® core for the containment vessel cage strips for the inner and outer field cages. The rods are
is30mmthick. Theinnercontainmentvesselismadeofthree made of several 178 and 209mm long pieces of Macrolon®
6
copper plates are connected to these ceramic pipes with ther-
Table 4: The thickness of the inner and outer field cage com-
mally conductive glue in an alternating pattern. The pipes are
ponentsarelistedinradiationlengths. Thetotalthicknesspre-
bridgedtogetheratthehighvoltageendbyastainlesssteeltube
sentedtoaparticleenteringtheTPCatη=0isabout1%X .
0 sothatde-ionizedwaterflowsintooneoftheceramicpipesand
Part X/X0[%] returnsthroughtheotherpipe. Inthismanner,thepowerdissi-
Centraldrum 0.470 patedbythevoltagedivideriscoupledtothecopperplatesand
InnerCO gap 0.085 isremovedbythecoolingwater.
2
Innerfieldcagevessel 0.401
Innerfieldcagestrip 0.012
Innerfieldcagetotal 0.968
Driftgas 0.607
Outerfieldcagestrip 0.012
Outerfieldcagevessel 0.401
OuterCO gap 0.081
2
Outercontainmentvessel 1.330
Outerfieldcagetotal 1.824
tube (a special sort of Plexiglas) which have been glued to-
gether. Thefinalrodassemblieshaveanouter(inner)diameter
of 44mm (36mm). Their outer surfaces were machined with Figure 5: Detailed view of the high-voltage end of the resis-
2mmwideand2.5mmdeepgrooves, atapitchof15mm, to torrod,showingthecoolingpipes,thecentralPEEKpipe,the
increase the distance along the insulator surface between the heat-dissipatingcopperplates,thecontactstothestrips,andthe
strips. An aluminum ring at each glue junction helps to min- high-voltagecontact,whichmatchesthecontactatthehousing
imize and redistribute the accumulation of charge along the rod.
rods. The gluing operation was performed on a precision jig
in order to achieve a uniform spacing of the strips to within
100µm. The rods for the outer field cage, except for their
grooves, are coated with copper to avoid charge accumulation
on their exposed surface. The rods are held in position with
holdingclampswhicharegluedtothewallsofthefieldcages
witha500mmspacingbetweentheclamps.
2.3.1. Resistorrods
The voltage dividers are integrated with the so-called resis-
torrods,andtheyareinsertedintofouroftherodsofthefield
cage: inner,outer,andonbothsides. Theresistorrodscontain
Figure 6: Schematic of the mechanical and electrical arrange-
a chain of resistors which define the potential on each strip of
mentofthegroundsideoftheresistorrod.Thecurrentsthrough
the field cage. The innovative design of these rods allows for
the resistor chain and through the cooling water are measured
water cooling and serviceability. The power dissipated by the
independently.
resistors is removed by a water-cooling circuit that runs back
and forth through the rods. A set of contacts ensures a good
connectionbetweentheresistorsinthechainandtoeachstrip. A 7.5MΩ resistor is connected between each of the Cu
Provisions are made for insertion, contacting, locking, and re- plates. Thefirstresistor,fromthecentralmembranetothefirst
movaloftherodsforservice. Detailsofbothendsofaresistor Cuplate,hasavalueof15MΩtocompensateforthemissing
rodcanbeseeninFigs.5and6. first strips in the field cage. In all cases, the resistors are sol-
For each resistor rod, a set of 165 copper plates, 0.5mm dered to washers which are then screwed to the plates. This
thick, are held together by short sections of PEEK® (poly- results in a resistor chain with a total resistance of 1245MΩ
acryletheretherketone,athermoplastic)tubingwhichareglued insidetheTPCdriftvolumeplusanadditional4.286MΩatthe
together, thus defining the 15mm pitch for the strips. The re- endofthechaintoallowforpreciselytuningthevoltageonthe
sulting tube is 2.5m long and the central hole is used to flush laststripofthefieldcage;seeFig.6. AsmallpieceofPEEK®
thesystemwithdriftgassincetheMacrolon®rodisnotneces- materialisscrewedtoeachcopperplate,ontowhichaflexible,
sarilygas-tight. Inadditiontothecentralhole,twomoreholes gold-plated stainless steel electrical contact is again screwed.
are drilled into the copper plates through which two ceramic A50µmAu–Wwireisusedtomaketheconnectionbetween
pipes,3mminnerand9mmouterdiameter,areinserted. The the plate and the contact, thus minimizing the amount of heat
7
transmitted to the contacts and into the drift volume. At the
high-voltageend,aconnectorconsistingofacrownofflexible
contactsprovidestheelectricalconnectiontothecorresponding
partinthefield-cagerod;seeFig.5.
The ground end of the resistor rod, shown in Fig. 6, is
equippedwithvariousresistorsforproperlyterminatingtheas-
sembly to ground, tuning the potential of the last strip, mea-
suringthecurrentthroughtheresistorchainandmeasuringthe
current through the cooling water. The contacts on the field-
cagerodsaremadeofgold-platedbrassandaregluedintoholes
intherodwall. Thehookstowhichthestripsareattachedare
screwedontothesecontacts.
With100kVonthecentralelectrode,thetotalcurrentflow-
ing through one of the resistor chains is 91µA; this is a sum Figure7: Thefieldcagestripsconnecttotheresistorrodswith
of80µAflowingthroughthefieldcageresistorchain,8.4µA Cu-Behooks. Metallicscrewsholdthehooksontotherodand
through the guard ring chain and 2.5µA flowing through the providetheelectricalconnectiontocontactsinsidetherod.
coolingwaterfortherod.
2.3.2. High-voltagecablerod
Thecablethatprovideshighvoltagetothecentralelectrode
is inserted into one of the outer Macrolon® rods. The ground
shield for the cable has been removed over the entire length
oftherod(250cm)andreplacedbyasemi-conductivecarbon
loadedpolyethylenesleevethatprovidesasmoothvoltagegra-
dientinsidetherod. Thecontactforthecableissimilartothe
resistor-rodcontact,andagainaspecialcableconnectstherod’s
contact to the rim of the central electrode. A rod with a spare
contact is installed on the other side of the TPC. A flange in
the endplate ensures gas tightness of the rod and mechanical
supportforthecable.
2.3.3. Laserrods
Figure8: Aviewinsidethefieldcagewherethestripsandsup-
Six outer rods per side are devoted to the laser calibration
porting rods are visible. The central electrode reflects a view
system.Thelaserrodsarespaceduniformlyaroundtheperime-
ofthefieldcageandthereadoutchambers. Thesubdivisionof
teroftheTPC.Thecorrespondingflangesfortherodsinclude
thepadplanesoftheOROCsintofourboardscanbeseen. The
aquartzwindowforintroducingalaserbeamintotherod. The
skirtelectrodesaroundtheOROCsarealsovisible.
rod itself holds, in its interior, a set of mirrors which deflect
thelightintothedriftvolumeoftheTPCthroughopeningsma-
chinedintherodsforthispurpose.Thelasercalibrationsystem
and outer strips, respectively). A custom-made tool was then
isdescribedindetailinSec.7.
usedtofoldCu-Befoilaroundtheendofeachstriptoproduce
a hook. The strips were then strung around the rods and con-
2.3.4. Gasrods
nectedtosimilarhooksontheresistorrods,asshowninFig.7.
Ten rods from the outer field cage, and 17 rods for the in- Aphotographshowingtheinteriorofthefinalizedfieldcageis
ner field cage, are empty and so these rods plus the partially showninFig.8.
obscuredlaserrodsareusedtocirculategasthroughtheTPC.
Therodsaremachinedwithanarrayof1mmholeswhichhave 2.5. Skirts
a 15mm pitch. The inner rods are used for the gas inlet, and
Thestripsoftheinnerfieldcagerunclosetotheinneredge
the outer rods are used for the gas outlet and this is the only
ofthereadoutchambersthusenablingagoodmatchofthedrift
waythatgasgoesinandoutoftheTPC.Inthismanner,thegas
field with the potentials on the cover electrodes of the inner
flows radially through the system thus minimizing the forces
readoutchambers(seeSec.3). Thevoltagesonthecoverelec-
exertedonthecentralelectrode.
trodesaretunableandthishelpstoensureagoodmatch. How-
ever,thereisagapbetweentheouterreadoutchambersandthe
2.4. Strips
stripsoftheouterfieldcagewhichistoolargetobeleftunfilled.
The field-defining strips are made from aluminized mylar, Theelectricfieldwouldbedistortedifitwereleftexposedand
25µmthickand13mmwide. Underatensionof3.5N, they so a 38mm wide skirt is inserted into the gap. The skirts are
arecuttotherightlength(5.246mand16.018mfortheinner parallel to the endplate and are electrically interconnected so
8
theycanbesettoanappropriatepotentialtominimizethedis- 3.2. Mechanicalstructure
tortionsofthefield.Atemperaturesensor(PT1000)isgluedon
Theazimuthalsegmentationofthereadoutplaneiscommon
thebacksideofeachskirtsector,thusallowingfortemperature
with the subsequent ALICE detectors TRD and TOF, i.e. 18
measurementsinsidethevolumeoftheTPC.
trapezoidal sectors, each covering 20◦ in azimuth. The radial
dependenceofthetrackdensityleadstodifferentrequirements
2.6. Endplates
for the readout-chamber design as a function of radius. Con-
Thefunctionoftheendplatesistoalignthecylindersforthe sequently, there are two different types of readout chambers,
fieldcagevesselsandtoholdthereadoutchambersinposition. leadingtoaradialsegmentationofthereadoutplaneintoInner
The four cylinders are screwed to the flanges that connect the andOuterReadOutChamber(IROCandOROC,respectively).
field cage vessels and the containment vessels, and are made Inaddition,thissegmentationeasestheassemblyandhandling
gas-tightwithO-rings. Thealuminumstructureoftheendplate ofthechambersascomparedtoasinglelargeone,coveringthe
is 60mm thick and the spokes are 30mm wide. The cut-outs fullradialextensionoftheTPC.
for the readout chambers are equipped with provisions for the The dead space between neighboring readout chambers is
alignmentofthechambersrelativetothecentralelectrodeand minimized by a special mounting technique (described in
areindependentoftheendplateitself(seeSec.3).Gastightness Sec. 3.4) by which the readout chambers are attached to the
is achieved by a sealing foil and a double O-ring; one on the endplate from the inside of the drift volume. The dead space
chamber and one on the endplate. The endplates also provide between two adjacent chambers in the azimuthal direction is
feed-throughs and flanges for gas, laser and electrical connec- 27 mm. This includes the width of the wire frames of 12 mm
tions. oneachchamber(seeFig.9)andagapof3mmbetweentwo
chambers. The total active area of the ALICE TPC readout
2.7. I-bars chambers is 32.5 m2. The inner and outer chambers are ra-
TheTPCisinstalledatanangleof0.79degreeswithrespect dially aligned, again matching the acceptance of the external
tothehorizontalduetotheinclinationoftheLHCaccelerator detectors. Theeffectiveactiveradiallength(takingedgeeffects
at the ALICE collision hall. This puts a gravity load on the into account) varies from 84.1 cm to 132.1 cm (134.6 cm to
endplates and leads to a displacement of the inner field cage 246.6cm)fortheinner(outer)readoutchambers. Themechan-
withrespecttotheouterfieldcage. Theelasticdeformationof icalstructureofthereadoutchamberitselfconsistsoffourmain
theendplatesisremovedbypullingontheinnerfieldcagewith components: the wire planes, the pad plane, made of a multi-
apairofI-bars. InFig.4,theI-barsareshownattachedonthe layerPrintedCircuitBoard(PCB),anadditional3mmStesalit
righthandsideoftheTPCandweredesignedsothattheydonot insulationplate,andatrapezoidalaluminumframe.
obstructtheareaaroundthebeam-pipe. TheIbarsareattached
to the outer ring of the endplate and can push or pull on the 3.2.1. Wires
innerfieldcageringinordertore-alignthefieldcages. During
The wire length is given by the overall detector layout and
assemblyintheALICEdetector,itwasnecessarytopullonthe
varies from 27 cm to 44 cm in the inner chambers, and from
innerfieldcagewithaforceof3kNandanalignmentofabout
45cmto84cmintheouterchambers.
150µmwasactuallyachieved.
3. Readoutchambers
3.1. Designconsiderations
Large-scaleTPCshavebeenemployedandproventoworkin
GROUND
collider experiments before [9], but none of them had to cope
withtheparticledensitiesandratesanticipatedfortheALICE
experiment[5,6].
ForthedesignoftheRead-OutChambers(ROCs),thisleads
torequirementsthatgobeyondanoptimizationintermsofmo-
mentum and dE/dx resolution. In particular, the optimization
ofratecapabilityinahigh-trackdensityenvironmenthasbeen
thekeyinputforthedesignconsiderations.
Figure9:Crosssectionthroughareadoutchambershowingthe
The ALICE TPC has adopted MWPCs with cathode pad
padplane,thewireplanesandthecoverelectrode.
readout. In preparation of the TPC TDR [3] alternative
readout concepts had also been considered, such as Ring
Cathode Chambers (RCCs) [10] or Gas Electron Multipliers Atconstantpotential,thegasgainincreaseswithdecreasing
(GEMs) [11] as amplification structures. However, those con- anode-wirediameter.Thus,asmallanode-wirediameterispre-
cepts seemed, though conceptually convincing, not yet in an ferred.Owingtotheirsuperiorstrength,gold-platedtungstenis
R&D state to be readily adopted for a large detector project, preferabletocopper–beryllium(analloyof98%Cuand2%Be)
whichhadtoberealizedwithinarelativelyshorttimespan. forthethinanodewires. However,forthethickercathodeand
9
gatinggridwiresthisdensematerialwouldrequireunaffordable of thesignal. Theabsence offield wiresalso considerablyre-
tensionsonthethinwireledges. Therefore, copper–beryllium duces the mechanical forces on the wire frames. However, a
isused. chamber without field wires requires a somewhat higher volt-
However,electrostaticandgravitationalforcescausethean- age to achieve the required gas gain and a higher geometrical
ode wires to sag, leading to gas-gain variations along the precisioninthepositioningofthewires.
wire. Theelectrostaticsagisapproximatelyproportionaltothe
square of the length of the wire, and inversely proportional to 3.2.4. Cathode-wiregrid
thestretchingforce,whilethegravitationalsagdependsonthe The cathode-wire grid separates the drift volume from the
density of the wire material. Therefore, the wires need to be amplification region. A large number of the ions produced in
mechanically strong enough to withstand the required stretch- the amplification avalanche are collected at the cathode wires
ing forces. We have chosen for the anode wires a diameter of withoutcausinganoticeablereductioninelectrontransmission.
20µmandastretchingforceof0.45N.Thecathodeandgating The cathode wire pitch is 2.5 mm. Electrostatic calculations
grid wires have a diameter of 75 µm and a stretching force of substantiatingtheabovelayoutnumericallyaredescribedinde-
0.6 and 1.2 N for inner and outer chamber, respectively. The tailinRef.[3].
wiretensionhasbeenmeasuredduringproductionforallwires
(seeSec.3.3.1). Themeasuredvaluesensureawiresagaround 3.2.5. Gating-wiregrid
50µmandthusarebelowthespecifiedlimitof70µm[3]. Thegatinggridislocatedabovethecathode-wiregrid,with
alternating wires connected together electrically. In the open
3.2.2. Wireplanes gatemode,allthegatinggridwiresareheldatthesamepoten-
The ALICE-TPC readout chambers employ a commonly tial V , admitting electrons from the drift volume to enter the
G
used scheme of wire planes, i.e. a grid of anode wires above amplificationregion. Intheabsenceofavalidtrigger,thegat-
thepadplane,acathode-wiregrid,andagatinggrid. Allwires ing grid is biased with a bipolar field V ±∆V (see Sec. 8.5),
G
runintheazimuthaldirection. Sincethedesignconstraintsare whichpreventselectronsfromthedriftvolumetogettotheam-
different for the inner and outer chambers (see below), their plificationregion.Thisconsiderablyreducestheintegralcharge
wire geometry is different, as shown in Fig. 10. The gap be- deposit on the anode wires. In addition, the closed gate stops
tween the anode-wire grid and the pad plane is 3mm for the ionscreatedintheavalancheprocessesofpreviouseventsfrom
outer chambers, and only 2mm for the inner chambers. The drifting back into the drift volume. This is important because
same is true for the distance between the anode-wire grid and escapingionsaccumulateinthedriftvolumeandcancausese-
the cathode-wire grid. The gating grid is located 3mm above veredistortionsofthedriftfield[12]. Thegoalisthereforeto
the cathode-wire grid in both types of chamber. The anode- avoid increasing the ion charge density above that created by
wire grid and the gating grid are staggered with respect to the primary ionization. The resulting requirement is that the ion
cathode-wiregrid.Henceforthweabbreviatethewiregeometry leakagefromtheamplificationregionhastobelessthan10−4.
oftheinnerchamberby(2-2-3),andthatoftheouterchamber Toachieveanelectrontransparencycloseto100%intheopen
by(3-3-3). modewhiletrappingionsandelectronsintheclosedmode,the
offset and bias potentials of the gating grid are carefully ad-
justed. On the other hand, any ionization produced by parti-
Outer Chamber Inner Chamber
cles traversing the gap between the gating grid and pad plane
1.25mm 1.25mm willunavoidablybeamplifiedattheanodewiresandthuscon-
Gating Grid
tribute to the integral charge accumulation. To minimize this
3mm 3mm
2.5mm 2.5mm effect,thegapbetweenthegatingandcathode-wiregridisonly
Cathode Wire Grid
2.5mm 2mm 3 mm, sufficient to trap the ions within a typical gate opening
3mm
2.5mm Anode Wire Grid time of 100 µs. To keep the alternating bias voltages low, the
2mm
pitchbetweenthegatinggridwiresis1.25mm.
3mm Pad Plane
3.2.6. Coverandedgegeometry
Figure10: Wiregeometriesoftheouter(left)andinner(right)
Thestandardwireconfiguration(seeFig.10)hasadisconti-
readoutchambers.
nuityatthetransitiontothenextchamberintheradialdirection.
Electrostaticsimulations,asshowninFig.11aforthestandard
3.2.3. Anode-wiregrid wireconfiguration,revealedasubstantialinefficiencyoftheion
Becauseoftheexpectedhighparticlemultiplicityandtherel- gate.
atively large gas gains required for the readout chambers (see Thedriftlinesofpositiveionsoriginatingfromtheamplifi-
below) a small anode-wire pitch was chosen for the ALICE cationzonearoundtheanodewireareshown.Asizablenumber
TPCtominimizetheaccumulatedchargeperunitlengthofthe ofpositiveionscouldleakbackintothedriftzoneforthispar-
anode wire and hence the risk of rate-induced gas-gain varia- ticularconfiguration. Inordertoimprovetheelectrostaticcon-
tions. This led to the choice of a 2.5 mm pitch for the anode figuration additional electrodes, i.e. ground and cover strips,
wires. Therearenofieldwiressincetheywouldreducethesig- were introduced (see Fig. 9). The voltage of the cover strip,
nal coupling to the pads, as they pick up a significant fraction whichframesthewholechamber,canbetunedtomaximizethe
10