Diamond Detectors for the TOTEM Timing Upgrade

3.6 MB·

by  TOTEM Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
TOTEM2016–05 CERN-PH-EP-2016-317
17December2016 17December2016
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Diamond Detectors for the TOTEM Timing Upgrade
0
2

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a TheTOTEMCollaboration
J

8
1 G.Antcheva),P.Aspell9),I.Atanassova),V.Avati8),J.Baechler9),V.Berardi5b,5a),
M.Berretti3a,9),E.Bossini7b),U.Bottigli7b),M.Bozzo6a),P.Broul´ım1a),A.Buzzo6a),

t] F.S.Cafagna5a),M.G.Catanesi5a),M.Csana´d4a,b),T.Cso¨rgo˝4a,4b),M.Deile9),
e
d F.DeLeonardis5c,5a),A.D’Orazio5c,5a),M.Doubek1c),K.Eggert10),V.Eremind),F.Ferro6a),A.
- Fiergolski9),F.Garcia3a),V.Georgiev1a),S.Giani9),L.Grzanka8,c),C.Guaragnella5c,5a),
s
n J.Hammerbauer1a),J.Heino3a),A.Kareve) J.Kasˇpar1b,9),J.Kopal1b),V.Kundra´t1b),S.Lami7a),
.i G.Latino7b),R.Lauhakangas3a),R.Linhart1a),M.V.Lokaj´ıcˇek1b),L.Losurdo7b),
s
c M.LoVetere6b,6a),F.LucasRodr´ıguez9),D.Lucsa´nyi9),M.Macr´ı6a),A.Mercadante5a),
si N.Minafra9,5b),S.Minutoli6a),T.Naaranoja3a,3b) F.Nemes4a,b),H.Niewiadomski10),
y T.Nova´k4a,4b),E.Oliveri9),F.Oljemark3a,3b),M.Oriunnof),K.O¨sterberg3a,3b),P.Palazzi9),
h
p L.Palocˇko1a),V.Passaro5c,5a),Z.Peroutka1a),V.Petruzzelli5c,5a),T.Politi5c,5a),J.Procha´zka1b),
[ F.Prudenzano5c,5a),M.Quinto9),E.Radermacher9),E.Radicioni5a),F.Ravotti9),E.Robutti6a),

1 C.Royong),G.Ruggiero9),H.Saarikko3a,3b),A.Scribano7a),J.Smajek9),W.Snoeys9),
v J.Sziklai4a),C.Taylor10),N.Turini7b),V.Vacek1c),J.Welti3a,3b),P.Wyszkowski8),K.Zielinski8)
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2 1aUniversityofWestBohemia,Pilsen,CzechRepublic.
2
5 1bInstituteofPhysicsoftheAcademyofSciencesoftheCzechRepublic,Praha,CzechRepublic.
0 1cCzechTechnicalUniversity,Praha,CzechRepublic.
.
1 2NationalInstituteofChemicalPhysicsandBiophysicsNICPB,Tallinn,Estonia.
0 3aHelsinkiInstituteofPhysics,Helsinki,Finland.
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1 3bDepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland.
: 4aWignerResearchCentreforPhysics,RMKI,Budapest,Hungary.
v
i 4bEKEKRC,Gyo¨ngyo¨s,Hungary.
X
5aINFNSezionediBari,Bari,Italy.
r
a 5bDipartimentoInterateneodiFisicadiBari,Bari,Italy.
5cDipartimentodiIngegneriaElettricaedell’Informazione-PolitecnicodiBari,Bari,Italy.
6aINFNSezionediGenova,Genova,Italy.
6bUniversita` degliStudidiGenova,Italy.
7aINFNSezionediPisa,Pisa,Italy.
7bUniversita` degliStudidiSienaandGruppoCollegatoINFNdiSiena,Siena,Italy.
8AGHUniversityofScienceandTechnology,Krakow,Poland.
9CERN,Geneva,Switzerland.
10CaseWesternReserveUniversity,Dept. ofPhysics,Cleveland,OH,USA.
Abstract
Thispaperdescribesthedesignandtheperformanceofthetimingdetectordevelopedbythe
TOTEMCollaborationfortheRomanPots(RPs)tomeasuretheTime-Of-Flight(TOF)ofthe
protonsproducedincentraldiffractiveinteractionsattheLHC.ThemeasurementoftheTOF
of the protons allows the determination of the longitudinal position of the proton interaction
vertex and its association with one of the vertices reconstructed by the CMS detectors. The
TOFdetectorisbasedonsinglecrystalChemicalVaporDeposition(scCVD)diamondplates
and is designed to measure the protons TOF with about 50 ps time precision. This upgrade
to the TOTEM apparatus will be used in the LHC run 2 and will tag the central diffractive
eventsuptoaninteractionpileupofabout1. Adedicatedfastandlownoiseelectronicsforthe
signalamplificationhasbeendeveloped. Thedigitizationofthediamondsignalisperformed
bysamplingthewaveform. Afterintroducingthephysicsstudiesthatwillmostprofitfromthe
additionofthesenewdetectors,wediscussindetailtheoptimizationandtheperformanceof
thefirstTOFdetectorinstalledintheLHCinNovember2015.
Keywords: TOF,Timingdetectors,Diamonddetectors,LHC
PACS:29.40.Wk,29.40.Gx
aINRNE-BAS,InstituteforNuclearResearchandNuclearEnergy,BulgarianAcademyofSciences,Sofia,Bulgaria.
bDepartmentofAtomicPhysics,ELTEUniversity-Budapest,Hungary
cInstituteofNuclearPhysics,PolishAcademyofScience,Krakow,Poland.
dIoffePhysical-TechnicalInstituteofRussianAcademyofSciences,St.Petersburg,RussianFederation
eJINR,Dubna,Russia.
fSLACNationalAcceleratorLaboratory,StanfordCA,USA.
gUniversityofKansas,Lawrence,KS,USA.
TOTEMtimingdetectors 1
Contents
1 Introduction 3
2 TheTOTEMApparatus 3
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 TheExistingRomanPotTrackingDetectors . . . . . . . . . . . . . . . . . . . . . 4
2.3 TOFMeasurementinTOTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 TheTimingDetectors 5
3.1 RequirementsandDesignPrinciples . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 TheDiamondSensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 DiamondCrystalCharacterization . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4.1 TheAmplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.2 TheHybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.4.3 MechanicalChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.5 Digitization: theSAMPICChip . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.6 ClockDistributionandDAQBoard . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 TestsandDetectorPerformance. 16
4.1 EfficiencyoftheSegmentedDiamondDetector. . . . . . . . . . . . . . . . . . . . 17
4.2 TimePrecision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5 DetectorPackageIntegrationandInstallation 21
5.1 TheHVdistribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 TheCoating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.3 VacuumOperationandCooling. . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.4 CommissioningandOperationintheLHCTunnel . . . . . . . . . . . . . . . . . . 22
2 TheTOTEMCollaboration(G.Antchevetal.)
6 Conclusions 23
TOTEMtimingdetectors 3
1 Introduction
TheTOTEMexperimentatCERN’sLargeHadronCollider(LHC)wasoptimizedforthemeasure-
mentoftheelasticppscatteringoverafour-momentumtransfersquared|t|rangingultimatelyfrom
≤10−3GeV2 to∼10GeV2 andhasmeasuredthetotalppcross-sectionindedicatedspecial-optics
runs[1–4]andstudiedtheinelasticrate[2,5].
Diffractivescatteringrepresentsauniquetoolforinvestigatingthedynamicsofstronginteractions
andprotonstructure. Theseeventsaredominatedbysoftprocesseswhichcannotbecalculatedwith
perturbative QCD. Various model calculations predict diffractive cross-sections that are markedly
different at the LHC energies [6–8]. A special optics configuration of the accelerator (large β∗)
permitstoseeallmassesincentraldiffractivetopologies.
A recently approved upgrade of the TOTEM detectors [9] will add the possibility to measure the
arrivaltimeoftheleadingprotonsintheRomanPots(RP).Amongmanyphysicschannelstheup-
graded system will provide an unprecedented sensitivity for low mass resonances (with particular
emphasisonglueballcandidates),exclusiveCentralDiffractive(CD)dijets,charmoniumstatesand
events with missing mass signatures [10]. Even at very low pileup, for inclusive CD events and
in particular for events with missing momentum the association of the protons to the CMS vertex
by using only the tracking variables is problematic. Here, as well as in all CD events measured
at moderate pileup, the proton Time-Of-Flight (TOF) measurement becomes crucial. As an ex-
ample,at µ =50%1,afive-foldenhancementoftheinclusiveCDpuritycanbeobtainedfromthe
installationoftheTOFdetectorintheRP,assumingatimeprecisionof50ps[11].
2 TheTOTEMApparatus
2.1 Overview
The TOTEM experiment [12] is composed of three sets of subdetectors placed symmetrically on
both sides of the interaction point: the Roman Pot detectors identify leading protons whereas the
T1andT2telescopesdetectchargedparticlesintheforwardregionfromprotondiffractivedisso-
ciation. T2consistsofGasElectronMultipliersthatdetectchargedparticleswith p >40MeV/c
T
at pseudo-rapidities of 5.3<|η|<6.5. The T1 telescope consists of Cathode Strip Chambers that
measure charged particles with p >100 MeV/c at 3.1< |η| <4.7. T1 and T2 can be used to tag
T
rapiditygapsinCDevents(p+p→p+X+p). Atpresent, todetectleadingprotonsscatteredat
verysmallangles,siliconsensorsareplacedinmovablebeam-pipeinsertions–so-called“Roman
Pots” (RP), special sections of the LHC vacuum chamber that can be positioned very close to the
circulating beam [13]. In order to maximize the experiment’s acceptance for elastically scattered
protons,theRPareabletoapproachthebeamcentretoatransversedistanceassmallas1mmor,
equivalently,toafewσ (thetransversebeamsize).
beam
EachRPstationiscomposedoftwounitsseparatedbyadistanceofabout5m. Aunitconsistsof
1µisdefinedastheaveragenumberofinelasticproton-protoninteractionsperbunchcrossing.
4 TheTOTEMCollaboration(G.Antchevetal.)
threeRPs,twoapproachingtheoutgoingbeamverticallyandonehorizontally. Thedetectorsinthe
horizontalpotcompletetheacceptancefordiffractivelyscatteredprotons. AllRPsarerigidlyfixed
withintheunit,togetherwithaBeamPositionMonitor(BPM).
2.2 TheExistingRomanPotTrackingDetectors
EachRPisequippedwithastackof10siliconstripdetectorsdesignedwiththespecificobjective
ofreducing toonlya fewtensofmicrometers [14]theinefficient area atthedetector edgeclosest
to the beam. The 512 strips with 66µm pitch of each detector are oriented at an angle of +45◦
(fiveplanes)and−45◦ (fiveplanes)withrespecttothedetectoredgefacingthebeam. Duringthe
measurementthedetectorsinthehorizontalRPsoverlapwiththeonesintheverticalRPs,enabling
aprecise(10µm)relativealignmentofthedetectorsinthethreeRPsofaunitbycorrelatingtheir
positions via common particle tracks. These detectors are described in detail in [13]. To provide
multi track separation capability TOTEM has added in 2014 in each arm one RP unit tilted in the
verticalplaneby8◦.
2.3 TOFMeasurementinTOTEM
In order to improve the experiment’s capability to explore and measure new physics in CD pro-
cesses TOTEM has proposed an upgrade program [9] which foresees adding timing capability
to the proton detectors installed in the very forward region. Common CMS-TOTEM data taking
is foreseen during the LHC Run 2, with a special LHC-optics configuration for which the proton
acceptance is optimal (all ξ =∆p/p for |t| >0.04 GeV2). The addition of proton Time-Of-Flight
(TOF)detectorsintheTOTEMforwardprotonarmsallowstoreconstructthelongitudinalinterac-
tionpositionandthustoassociatethistotheprotonvertexreconstructedbytheCMStracker.
√
The CD interactions measured by TOTEM at s=13 TeV are characterized by having two high
energy protons (with momentum greater than 5 TeV/c) scattered at less than 100 µrad from the
beamaxis. ThetrackingoftheprotonsisdonebysiliconstripdetectorsinstalledintheRPs. With
the addition of protonTOF detectors, it will be possible to measure the difference of the arrival
times, which is directly proportional to the longitudinal position of the vertex. In fact ift , t are
1 2
thearrivaltimesoftheprotonscomingfromaCDinteractioninthetwoRParms,thelongitudinal
vertexpositiongivenbyz=c(t −t )/2.
1 2
During the programmed joint CMS-TOTEM data taking when the two experiments share a com-
mon trigger, the reconstruction of the proton vertex position allows, even in presence of event
pileup, the association with one of the CD vertex reconstructed by CMS and therefore to com-
pletelyreconstructtheevent.
ThetimingdetectorwillbeinstalledinfourTOTEMverticalRomanPots(RP)locatedat∼220m
onbothsidesoftheinteractionpoint5(IP5)oftheLHC.
TOTEMtimingdetectors 5
3 TheTimingDetectors
3.1 RequirementsandDesignPrinciples
The physics program determines the specifications for the timing detector: in order to obtain a
precision on the longitudinal interaction vertex (Z = c∆t/2) of σ ∼1 cm, an overall time
vertex Z
precision σ(∆t) ∼50 ps for the measurement of the proton track timing is required. The track
distribution at the position of the timing detectors due to diffractive events and to overlapping
background is not uniform and has been measured from RP tracking data, see Figure 1. The
average number of particles per bunch crossing in one of the RP is about 0.2 (including the beam
background)atµ =0.5%[15].
Fig.1:ThedistributionoftheparticlestraversingtheRPplaneat220mwiththeTOFdetectortobeinstalled
intopandbottomRPforβ∗=90moptics. TheTOFdetectorswillbeinstalledintopandbottomRPs.
Thedetectorhastobesegmentedintopixelstominimizetheoccupancy2 ofeachcell.
Toguaranteeuniformoccupancyandasmallnumberofcellsinthedetectorapatternwithpixelsof
different sizeshas beensimulated [15] andoptimized inorder to minimize the inefficiencydue to
thesimultaneouspresenceoftwohitsinthesamecell. Astheoccupancyinthisscenarioisanyway
very low, this selection hardly affects the efficiency. The optimized detector geometry with eight
diamond plates obtained for a high-β∗ optics configuration at µ= 0.5 is shown in Figure 2. The
pixelsizeintheverticaldirectionyvariesbyalmostafactorsixfrom0.7mmto4.2mm.
2numberofparticlestraversingthedetectorcell/pixelinonebunchcrossing.
6 TheTOTEMCollaboration(G.Antchevetal.)
Atµ =50%,theinefficiencyintroducedbythepileupisapproximately0.6%.
Areducedgeometryhasbeenadoptedtobuildtheprototypedescribedhere: itcontainsonlyfour
diamondplatesandisshowninFigure3. Thefourdiamondsingle-pixelplatesthatarenotincluded
wouldcoveronlyanextra10%ofacceptance.
Fig. 2: Optimized layout of the detector pixels. On the right its position during physics data taking is
outlinedonthetrackdistributionofFigure1.
One detector package contains four identical planes, each equipped with four plates of 4.5×
4.5mm2 and 500µm thick scCVD diamond metalized with a pattern of one, two or four pixels.
Ifonecanbuildasinglesensorcapableofmeasuringwithatimeprecisionoftheorderof100ps,
thenwithtimemeasurementsfromfourdetectorsonereachestherequiredoveralltimeprecisionof
about50ps. Withsuchaprecisionforthemeasurementoftheprotontracktimingthelongitudinal
interactionvertexcanbedeterminedwithanuncertaintyof∼1cm.
Aftersomepreliminarymeasurementsinatestbeamwithadiamondsensorassembledwithelec-
tronicsofpreliminarydesign,wesawthatscCVDdiamonddetectorswithacustom-builtamplifier
wouldmatchalltherequirementsofthetimingdetectorneededfortheTOTEMupgrade.
Withanappropriatemetalization, oneobtainsagoodcontacttothefaceofthediamondsensor; it
is fairly easy (fast and economic) then to implement a pattern with pixels of different sizes on a
diamondcrystalsurfacebymeansofasimplemetalization. Duetotheextremelyhighimpedance
of the diamond material, the difference in pixel size will barely affect the time response of the
signal. Diamond sensors show also a better radiation hardness [16] and shorter charge collection
timesthanothersemiconductorsensors[17,18].
TOTEMtimingdetectors 7
Fig.3: LayoutanddimensionsofthepixelsoftheTOTEMtimingdetectorsimplementedanddescribedin
thispaper.
Additional technical requirements were taken into account to design a detector system that could
beimplementedandproperlyfunctioninthehighradiationenvironmentoftheLHCtunnel. Inthe
followingsectionswedetailthestudiesperformedtoensurethatscCVDdiamonddetectorswillbe
abletomeetthetimingprecisionrequiredbythephysicsprogram.
TheTOFsystemmainblockswhichwillbedescribedindetailinthefollowingsectionsisshown
inFigure4.
3.2 TheDiamondSensors
The use of detector grade scCVD diamond for use in particle detectors has been proposed in the
past and extensively studied [18]. It is an ideal material for detecting particles due to its high
electronandholemobilityand>99%chargecollectionefficiency.
Thesedetectorsarenowwidelyusedinmanyspecificapplicationsandexperiments,albeitstillona
smallscale. TheavailabilityonthemarketofdetectorgradescCVDdiamonddetectorsofdifferent
sizesandthicknesseshasgreatlyimprovedinrecentyears. Severalcompanies3 offercustomsizes
andthicknesseswithreasonabledeliverytime.
3 ElementSixLtd., KingsRidePark, Ascot, BerkshireSL58BP,UK., Diam2Tek, Irma-Feldweg-Straße8, 75179
Pforzheim,Germany,amongfewothers.
8 TheTOTEMCollaboration(G.Antchevetal.)
Top Vertical
S haper x12 x12
Ampli fie r x12 48 channel
Pre-a mp
x12
Hyb rid Readout board Slow
x4 boar d control
SAMPIC (CCUM)
mezz. DAQ
One arm of the FPGA AND
p Beam TOTEM (SF2) CONTROL
timing system SAMPIC
m e z z . O p t i c a l
x4 bHoy abrrdid x3 (GliOnkH )
Prex-1a2
x12
Am plifie rx 12
Sh ape r
x1 2 48 channel
Bottom Vertical
Fig.4: TheTOFsystemfortheTOTEMVerticalRPupgrade.
3.3 DiamondCrystalCharacterization
Electrodes on both sides are needed to collect the charges and read out the electric signal. Thin
metal layers on both the top and bottom surface of the crystal generate the proper field and allow
forconnectiontotheamplifier. However, makingagoodohmicelectricalcontactwiththecrystal
structure is a delicate process. Metalization of the pixels on the surface of the diamond in some
instances led to poor collection of charge or to a particle-rate dependent reduction of the charge
collection [18]. The occurrence of ‘charging’ and ‘radiation’ damage of the diamond sensors has
alsobeenstudiedinhighfluenceHighEnergyPhysicsexperiments,seeforexample[19].
ThemetalizationareafortheTOTEMplatescovers4.2×4.2mm2 leavinganon-metalizedborder
regionof150µminordertoavoidenhancementofsurfaceleakagecurrentsanddischargesthrough
the diamond edge. The metalization mask is aligned with better than 100µm precision on the
diamond plate. When more than one pixel is present on one plate, the non metalized separation
betweenpixelsis100µm. Thediamondplates(fourareneededperhybrid)aregluedonthehybrid
PCBincontacttoeachotherandonemayconsidertheirmaximumdistancetobelessthan50µm.
Oftenthemetalizationprocessesarecompanyspecificandconfidential. Wehavetestedandcom-
pareddifferentmetalizationtechniquesforthediamondsensorsassuggestedandprovidedby:
– GSI4 (Cr-50nm+Au-150nm)
– AppliedDiamondInc. (USA)5 (Cr-50nm+Au-150nm)
4http://www.gsi.de
5http://usapplieddiamond.com/