Fast Timing for High-Rate Environments with Micromegas

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by  Thomas Papaevangelou

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Fast Timing for High-Rate Environments with Micromegas
ThomasPapaevangelou1,a, DanielDesforge1, EstherFerrer-Ribas1, IoannisGiomataris1, Cyprien Godinot1, Diego
Gonzalez Diaz2, Thomas Gustavsson3, Mariam Kebbiri1, Eraldo Oliveri2, Filippo Resnati2, Leszek Ropelewski2,
6
1 GeorgiosTsiledakis1,RobVeenhof2,andSebastianWhite2,4
0
1CEA,IRFU,Centred’EtudesNucléairesdeSaclay,Gif-sur-Yvette91191,France
2
2EuropeanOrganizationforNuclearResearch(CERN),Genève1211,Switzerland

n 3LaboratoireInteractions,DynamiquesetLasers(LIDyL)CEA,CNRS,UniversitéParis-Saclay,GifsurYvette91191,France
a 4PrincetonUniversity,Princeton,NewJersey08544,USA
J

2
1 Abstract.Thecurrentstateoftheartinfasttimingresolutionforexistingexperimentsisoftheorderof100ps
onthetimeofarrivalofbothchargedparticlesandelectromagneticshowers.CurrentR&Donchargedparticle

] timingisapproachingthelevelof10psbutisnotprimarilydirectedatsustainedperformanceathighratesand
t
e underhighradiation(aswouldbeneededforHL-LHCpileupmitigation).WedemonstrateaMicromegasbased
d solutiontoreachthislevelofperformance. TheMicromegasactsasaphotomultipliercoupledtoaCerenkov-
-
s radiatorfrontwindow,whichproducessufficientUVphotonstoconvertthe∼100pssingle-photoelectronjitter
n intoa timingresponse of the order of 10-20 ps per incident charged particle. A prototype has been built in
i ordertodemonstratethisperformance. Thefirstlaboratorytestswithapico-second laserhaveshownatime
.
s resolutionoftheorderof27psfor∼50primaryphotoelectrons,usingabulkMicromegasreadout.
c
i
s
y
h 1 Introduction also the advantagesof being low-cost, simple to operate,
p
andarepotentiallyinsensitivetoirradiationeffects.
[
From2025onward,itisplannedtooperatetheLHCwith Modernphoto-lithographictechnologyhasenabledthe

2 typically 140 collisions per proton bunch crossing (HL- development of a series of novel Micro Pattern Gaseous
v LHC phase), which will greatly complicate the analysis, Detectors (MPGD): Micro-Strip Gas Chamber (MSGC),
3
particularly in the forward regions, where it will be very Gas Electron Multiplier (GEM), Micro-mesh Gaseous
2
difficulttolinkthetrackswiththeprimaryvertex(associ- Structure(Micromegas),andmanyothersrevolutionizing
1
0 atedwiththeonlyinterestingcollision)andthustoprevent cellsizelimitsformanygasdetectorapplications.TheMi-
0 theformationoffakejetscreatedbytherandomstacking cromegasdetectorfulfilstheneedsofhigh-luminositypar-
1. or pile-up of tracks from several secondary vertices. A ticle physics experiments, and compared to classical gas
0 solution proposedto fight against this pile-up effect is to counters, it offers intrinsic high rate capability, excellent
6 measure very accurately the time of arrival of the parti- spatialresolutionandexcellenttimeresolution [3,4].
1 cles [1]. A time resolution of a few tens of picoseconds
:
v withaspatialgranularitybetterthan10mm(dependingon
2 Micromegas for fasttiming
i pseudo-rapidity)willbeneededtoobtainafakejetrejec-
X
tionratethatisacceptableforphysicsanalyses. Suchper- 2.1 Detectionprinciple
r
a formances should be obtained with a detector capable of
Theconceptof aMicromegasdetectorcoupledto a front
withstandingtheveryimportantradiationlevelsexpected
window that acts as Cerenkov-radiator, equipped with a
intheHL-LHCera.
photocathode, is examined in order to address its timing
The ongoing R&D on timing for charged particles is
performance. The radiator should produce sufficient UV
approaching a level of 10 ps [2] but is not primarily di-
photonsandshouldbeequippedwithanappropriatepho-
rected at sustained performance at high rates and under
tocathodeinordertocreatethenecessarynumberofelec-
high radiation (as would be needed for HL-LHC pileup
tronstoconverttheexpected∼100pssingle-photoelectron
mitigation).Severaltechnologiesarepresentlyundereval-
timejitterintoatimingresponseof10-20psperincident
uation within the LHC collaborations: Silicon detectors
particle. The photocathode has to be spark-resistant and
with some built-in amplification, Silicon detectors with-
musthavegoodagingproperties,appropriateforthehigh
out amplification and gaseous detectors. Gaseous detec-
particlefluxenvironmentofHL-LHC.
tors have the potential of very good timing performance,
Ingaseousdetectorsthetimeresolutionistypicallyof
providedtheamplificationgapissmallenough.Theyhave
the order of several ns because of the time jitter of pri-
ae-mail:[email protected] mary ionization (for high energy MIPs) in the drift gap.
EPJWebofConferences
Ourstrategytoimprovethetimeresponseistominimize photons. IfListhethicknessincm,T(E)isthetransmis-
thisjitterbycreatingsimultaneouslyallprimaryelectrons sion of the crystal and QE(E) the quantum efficiency of
at about the same distance from the amplification region thephotocathode,thenumberofproducedphotoelectrons
andbyminimizingthedriftspace. Theprimaryelectrons willbe:
arecreatedbytheCerenkovlightwhichisemittedbythe
chargedparticle when it crosses a 2-3 mm MgF crystal. 1
2 N ≈370L T(E)QE(E) 1− dE
A thin CsI photocathode is deposited on the micromesh p.e. Z n2(E)!
(reflective mode) or on the bottom of the crystal (semi-
transparent mode) as shown in figure 1, and it is used to For a MgF2 crystal and a CsI photocathode with
convertUV photonsto electrons. The time spread of the QE(E) takenfrom[7], T(E)from[8] andn(E) from[9],
Cerenkovlight(∼10ps)istoosmalltolimittheprojected we expect 370 × 0.89 = 330 p.e./cm (figure 2). How-
timeaccuracy. ever, this QE concerns operation in reflective mode. In
thesemitransparentmodeitisexpectedtobesignificantly
lower(factor2-3),so,fora3mmMgF crystalwewould
2
expect30-50photoelectrons.
1 MgF2 Transmission
E CsI Q.E. (reflective)
Q
n or 0.8 1T -× 1 Q/nE2( E× ) 1(o -) 1/n2(E)
sio ∑T×QE 1 - 1 ∆E = 0.89
s n2(E)
mi 0.6
s
n
a
Tr 0.4
0.2
Figure1.Schematicviewofthechargedparticledetectionprin-
ciple, in the two modes; Reflective mode: a thin CsI layer is
5 6 7 8 9 10 11 12
depositedontopofthemicromesh;Semitransparentmode: CsI E [eV]
isdepositedonthebottomsurfaceoftheUVcrystal,defininga
driftgapwiththemicromesh.
Figure2. Transmission,refractiveindexofMgF andquantum
2
efficiencyofCsIinreflectivemodewhichwereusedforthecal-
culationofthenumberofphotoelectrons.
In the semitransparent mode the photocathode is de-
positedatthebottomfaceoftheMgF crystalatadistance
2
of 100-300μm from the micromesh to limit longitudinal
diffusion. A strong field can be applied in the drift gap
allowing some pre-amplification. The advantagesof this 2.3 Diffusioninthedriftregion
configuration are the high single-electron sensitivity due
tohighertotalgain,thepossibilitytousewovenmeshMi-
Since the time jitter in photoelectron creation from the
cromegas (bulk technology [5]) and the fabrication sim-
Cerenkovlightissmall(<10ps),thedominanteffectlim-
plicity. However,thepartialexposureofthephotocathode
itingthetimeresolutionwillbethediffusionofthephoto-
totheionbackflowmaycauseagingandrobustnessissues.
electronsinthedriftregion.
Inthereflectivemodethephotocathodeisdepositedonthe
Inthecaseofthereflectivephotocathode,thejitter in
Micromegasmeshandnostrongdriftfieldisneeded. The
thedriftpathwillreflectthedifferenceinthepathdepend-
main advantagesofthis methodis the veryeffectivepro-
ing on the creation point on the mesh and its distance to
tectionofthephotocathodefromtheionbackflow(better
theamplificationgap,sofinepitchmeshesarepreferable.
aging properties, robustness) and the complete suppres-
Simulationswerecarriedouttoinvestigatethesemitrans-
sion of gas ionization in the drift region. However, this
parent mode. Magboltz simulation results are shown in
mode is limited to flat mesh detectors (Microbulk tech-
figure3wherethetimespreadduetoelectrondiffusionin
nologyisthemoreappropriate[6]),whiletheinternalre-
a200μmdriftgapisplottedasfunctionofthepreampli-
flectionofthe Cerenkovlightonthecrystal-gasinterface
ficationforseveralgasmixtures. Afirstassessmentbased
mightlimittheefficiency.
onMagboltzthusindicatesthatasinglephotoelectrontime
resolutionof100-200pscouldbereachableforrelatively
2.2 Photoelectronproduction small(orevenintheabsenceof)pre-amplification.
Compared to noble-gas admixtures, pure quenchers
AMIPpassingthroughacrystalwillemit: show potentially a smaller time spread, but demand
stronger electric fields in order to achieve the same gain.
d2N α2z2 370 1 Theoptimumselectionwillbeatradeoffbetweenthemin-
= sin2θ ≈ 1−
dEdx r m c2 c eVcm n2(E)! imumtimejitterandthemaximumsignaltonoiseratio.
e e
MPGD2015
Magboltz simulations theUV laser, ismountedonthetopsurfaceofthe cham-
1400 ber. Thechamberandthecomponentswereproperlycho-
Ne/ethane (95/5)
1300
ps] 1200 Npuer/ee tehtahnaen e (90/10) sen andspecialprecautionwastakenwhile mountingthe
m [ 1100 pure CF detector in order to be appropriate for operating without
µ200 1900000 pure CO42 agnadscpiarcrtuiclaletiobne,atmoffaacciilliittaietes.thusthemeasurementsatlaser
d in 800
ea 700
e spr 600 2.5 Performanceoftheprototype
m 500
n ti 400 The gas mixture was chosen to be 90% Ne plus 10%
o
ctr 300 Ethane, a mixture that gives high gain with good energy
ele 200 resolutionandhasshowngoodbehaviorfor sealed mode
100
0 operation [10], though it is not optimal for timing. We
1 10 100
pre−amplification in 200µm usedapulseddeuteriumlampinordertomonitorthegain
stabilitywithoutgascirculation.Dataweretakenoverone
Figure3.Electrontimespreadduetodiffusionforseveralgases weekshowingonlyrandomfluctuations(arisingfromthe
asafunctionoftheachievedpre-amplificationina200μmgap. lampintensity)andnosignificantdropofthegain,demon-
strating the possibility to perform measurementswithout
renewingthegas.
2.4 TheMicromegasprototype
s
nt candle @ 20 cm
Aprototypedetectorhasbeenconstructed(figure 4)inor- ou250 candle @ 30 cm
C
dertostudythetimingpropertiesoftheproposedscheme.
200
It has been designed to be compatible with operation in
bothsemitransparentandreflectivemode.
150
100
50
0
0 100 200 300 400 500
Amplitude [mV]
Figure5.Pulseamplitudespectraforacandleplacedat∼20cm
and∼30cmdistancefromthedetectorwithachargepreampli-
Figure4.Schematicviewofthedetectorchamberandexpanded
fier. Theshapeofthespectrumdoesnotchange,aclearindica-
viewoftheMicromegas-spacers-crystalassembly.
tionforsingleelectronpulses.
TheMicromegaswasconstructedusingbothBulkand
The sensitivity of the detector to single photons was
Microbulk technologies[5, 6]. It was a single-anodede-
verified using a candle flame, which is emitting continu-
tector of 1 cm diameter (bigger than the maximum pixel
ously light from the IR down to the VUV, the spectrum
size of a possible tracker). The amplification gap was
stoppingat∼200nmduetotheabsorptionoftheair[10].
50 μm forthe Microbulkand 128μm for the bulk. Kap-
The quantum efficiency of the aluminium is sufficient to
ton rings, 50 μm thick, were used in orderto supportthe
haveseveralphotoelectronspersecondwhenthedistance
MgF orQuartzcrystalonwhichthephotocathodeisde-
2 to the flame isnotverylong. Two spectra takenata dis-
posited and define the driftgap of 200 μm. The top ring
tance of 20 and 30 cm for the same time duration are
was metalized on the top surface, in order to apply the
shown in figure 5. The amplitude distribution does not
biasvoltagetothephotocathode. Athin(10nm)Allayer
changewiththedistance,whiletheratedoes,provingthat
wasdepositedonaquartzcrystalandservedasphotocath-
thepulsescorrespondtosinglephotons.Theresultingvery
ode. This simplified configuration is sufficient to study
goodenergyresolutionis dueto the single stage amplifi-
thetimingpropertiesofthedetector,usingaveryfastUV
cationintheMicromegasgap.
laser. Forchargedparticledetection,thequartzhastobe
replacedwithMgF ,whilethephotocathodewillbeaCsI
2
layer (100-300 nm) deposited on a metallic substrate (5- 3 Time resolutionmeasurements
50nm).Theexactconfigurationwillbeoptimizedinterms
ofagingpropertiesandofthemaximizationofthenumber The time response of the bulk Micromegas detector in
ofproducedphotoelectrons. semitransparent mode was studied at LIDyL laboratory
The Micromegas-crystal assembly is encased in a (CEA/Saclay). An Optical Parametric Oscillator (Coher-
stainless-steel chamber. A quartz window, transparentto entMIRA-OPO),pumpedbyaTi:sapphirelaser(Coherent
EPJWebofConferences
MIRA900)provided120fspulsesat550nm. Therepeti-
taioPnurlsaete-Pwicaksevra(rCieodhbeertewnete9n2900k)H. zTahneds4e.c7o5nMdhHazrmbyonuiscinagt σ [ps]t dddaaatttaaa ssseeerrriiieeesss 123,,, <<<NNNpp..ee..>>> === 5513130..40
× p.e.
2o7u5tpnumtewnearsggyewnearsa0te.1d5i-n1ap5JmfomraBbBeOamcrdyisatmale.tTerh<et1ypmicmal. 0.5 N>p.e.1000 dspartae asde rfireosm 4 ,d i<ffNups.ieo.>n (=M 1a.g2b6oltz)
< 1D avalanche simulation
ThoughtheextractionefficiencyfortheAlphotocath- on,
uti
odeatthosewavelengthsissmall,theintensityofthelaser ol
(up to 106 photons/pulse) allows for the production of a me res
sufficientnumberofphotoelectrons(upto1000p.e./pulse n ti
o
for10nmAl,dependingalsoonthebeamtuningandthe phot 100
−
appliedMicromegasdriftfield).Thenumberofphotoelec- gle
n
trons per laser pulse was controlled by using a series of si

2 3 4 5 6 7 8 9 10 11 12
light attenuators: fine electroformednickelmeshes (100- E [kV/cm]
drift
2000LPI)withopticaltransmissionvaryingbetween10%
-25%,givingattenuationfactorsof4,5,10andtheircom- Figure7. Dependenceofthemeasuredtimeresolutionwiththe
binations. Thepitchofthemesheswasvariedinorderto driftfield,scaledtothesinglephotoelectroncase.Thedifference
avoidsystematiceffectswhencombiningseveralofthem. betweenthethinandthicklinesindicatestheimprovementdueto
pre-amplification,accordingtoastochastic1Davalanchemodel.
Thelaserbeamwassplitintwo,partofitilluminating
our prototype and the other part an ultra fast Si detector
(ThorlabsDET10A/M).Bothsignalswererecordedbya2
GHzoscilloscopeoperatingat10GS/s. Thetimejitterof
thephotodiodewasoftheorderof13ps,beingappropriate
astriggerandasreferenceforthemeasurement. Thesig-
nal from the Micromegaswas filtered for high frequency ps]
noiseusinga movingaverage. Thephotocathodeandthe σ [t100
filtered Micromegassignal times were calculated by tak- on,
uti
ing a constant fraction of each signal. Best results were ol
es
obtainedfora30%fractionoftheamplitude(figure6). me r data (Edrift=10kV/cm)
ti spread from diffusion (Magboltz)
spread from diffusion × 0.65
1D avalanche simulation
400 <<NNpp..ee..>> == 5511..44 ,, σσCCFF 3300%% == 2266..99 ppss
<<<<NNNNpp..ee..>>>> ==== 11113.3.22..0606 ,,,, σσσσCCFF 3300%% ==== 51512727.4.488.. 5 5pp ppssss 101 10 100
pp..ee.. CCFF 3300%% number of photoelectrons, N
PPDD ((11..2266 pp..ee..)) ,, σσ == 1122..66 ppss p.e.
300 CCFF 3300%%
Figure8. Dependenceofthemeasuredtimeresolutionwiththe
meannumberofphotoelectrons,forfixedamplificationanddrift
200
fields.Aresolutionof200pspersinglephotoelectronand27ps
for 50photoelectronshasbeenachieved.
100
4 Conclusions
−7.5 −7 −6.5 −6
t [ns]
A Micromegas photodetector, equipped with a UV-
Figure6.Distributionofthetimeatthe30%fractionoftheam- transparentcrystalandametallicphotocathodedeposited
plitudefortheMicromegaspulses,fordifferentmeannumberof on the bottom of the crystal has been constructed in or-
photoelectrons.Thetimespreadofthephotodiodeisalsoshown. der to investigatethe potentialof MPGDs forfast timing
tracking of minimum ionizing particles. The aim of the
firsttestswastheinvestigationoftheintrinsictimecharac-
teristicsofthedetector. AveryfastUVlaser(timespread
Data were taken for different values of the drift field 120 fs) was used to producethe photoelectronsin a con-
in combinationwith the attenuators, in order to maintain trolledway.Withagasmixtureconsistingof90%Neand
the signal within a reasonable dynamic range. The time 10%Ethanewemeasuredatimeresolutionoftheorderof
resolutionimproveswiththedriftfieldintensityduetothe 200 ps for single photoelectronsand 27 ps for 50 photo-
decrease of the longitudinaldiffusion but also due to the electrons. Simulationsshowthatthisalreadysatisfactory
preamplificationathighfieldvalues. Theresultsaresum- resultcanbefurtherimprovedbyoptimizingthegasmix-
marizedinfigures7and8,incomparisonwithastochastic ture. A more exhaustive description of these results will
1D avalanchesimulation(thatwill be detailed in a forth- be publishedelsewhere. Tests on a particle beam, with a
comingpublication). MgF crystalandCsIphotocathodewillbemadein2016.
2
MPGD2015
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[6] S.Andriamonjeetal.,JINST5(2010)P02001
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