Studies on a silicon-photomultiplier-based camera for Imaging Atmospheric Cherenkov Telescopes

0.38 MB·

by  C. Arcaro

Studies on a silicon-photomultiplier-based camera for Imaging Atmospheric Cherenkov
Telescopes
C.Arcaro*,a,b,D.Cortib,A.DeAngelisb,c,d,M.Doroa,b,C.Maneae,M.Mariottia,b,R.Randoa,b,I.Reichardtb,D.Tescarob
aUniversita`diPadova,I-35131Padova,Italy
bINFN,I-35131Padova,Italy
cINAFPadova,Italy
dLIP/ISTLisboa,Portugal
eINFNTIFPA,I-38123Povo,Italy
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0
2Abstract

nImaging Atmospheric Cherenkov Telescopes (IACTs) represent a class of instruments which are dedicated to the ground-based
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observationofcosmicVHEgammarayemissionbasedonthedetectionoftheCherenkovradiationproducedintheinteractionof
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gammarayswiththeEarthatmosphere.Oneofthekeyelementsofsuchinstrumentsisapixelizedfocal-planecameraconsistingof
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photodetectors. Todate,photomultipliertubes(PMTs)havebeenthecommonchoicegiventheirhighphotondetectionefficiency
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(PDE) and fast time response. Recently, silicon photomultipliers(SiPMs) are emergingas an alternative. This rapidly evolving

]technology has strong potential to become superior to that based on PMTs in terms of PDE, which would further improve the
M
sensitivity of IACTs, andsee a price reductionpersquaremillimeterof detectorarea. We are workingto developa SiPM-based
Imoduleforthefocal-planecamerasoftheMAGICtelescopestoprobethistechnologyforIACTswithlargefocalplanecamerasof
.
hanareaoffewsquaremeters. Wewilldescribethesolutionsweareexploringinordertobalanceacompetitiveperformancewith
paminimalimpactontheoverallMAGICcameradesignusingraytracingsimulations. Wefurtherpresentacomparativestudyof
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otheoveralllightthroughputbasedonMonteCarlosimulationsandconsideringthepropertiesofthemajorhardwareelementsofan
rIACT.
t
s
Keywords: Siliconphotomultiplier,Photodetectors,Cherenkovdetector,Imagingdetectorsandsensors
a
[PACS:29.40.Mc,85.60.Gz,29.40.Ka,42.79.Pw

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v
1. Introduction thecorrespondingreadoutelectronicsandalightconcentrator,
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2 usuallyaWinstonconemadeofmaterialwithhighreflectivity
In recent years ground-based very high energy (VHE, E ≥
6 in the wavelength range of Cherenkov radiation, to guide the
3100GeV) gamma-ray astronomy has experienced a major lightreflected fromthe mirrordish to the effective area of the
0breakthrough, as demonstrated by the impressive astrophys-
photodetectorandtoreducetheacceptanceofstraylight.
.ical results obtained with Imaging Atmospheric Cherenkov
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0Telescopes (IACTs) like H.E.S.S., MAGIC or VERITAS [1]. Silicon photomultipliers (SiPMs), a rapidly evolving class
7IACTs,withalargesinglemirrordishoftheorderoffewtens of solid-statephotonsensors, are verypromisingdetectorsfor
1ofmetersindiameterandacameraareaoftheorderofsquare Cherenkovapplications,thankstohighPDEandhighgain,low
v:meters, as MAGIC, are suited to cover the lower end of the operating voltage, tolerance to high illumination levels, good
iVHE energy range. They are designed to collect the maxi- single-photonsensitivity, fast response, andlow-amplitudeaf-
X
mum number of Cherenkov photons possible by means of a terpulses. Drawbacksofsuchphotosensorsareahighercapac-
arlarge mirror dish of high reflectance in the wavelength range itanceandhighercrosstalk.
where Cherenkov light is emitted. Such light is generated by
ThefeasibilityofSiPMs forIACT applicationsisbeingex-
theinteractionofVHEgammarayswiththeEarthatmosphere
ploredfortelescopeswithasmallfocalplanecamera,i.e.,with
andemittedaslightflashesoffewnanoseconds. Tomaximize anareaoftheorderoffewhundredsofsquarecentimetersanda
the benefit of the large collecting area, the efficiency of the
pixelsizeoffewmillimeters,andtelescopeswithdouble-mirror
photodetectorshas thus to be as highas possible in the wave-
Schwarzschild-Couderlayout[2].TheFirstG-APDCherenkov
length range of interest. The baseline design of a large IACT
Telescope(FACT;[3])hasshownthatSiPMsshowastableand
includescommonlyafocal-planecamerabasedonphotomulti-
reliablelong-termperformance[4]. However,largesizeSiPM-
pliertubes(PMTs)givenitsphotondetectionefficiency(PDE) basedIACTcameras,withanareaoffewsquaremetersanda
andfasttimeresponse.EachpixelusuallyincorporatesaPMT,
pixelsizeoftheorderofseveralcentimeters,havenotyetbeen
built. The problems encountered in the construction of large,
monolithic photon-sensitivesilicon devices are mainly related
*Correspondingauthorat: Universita` diPadova, I-35131Padova, Italy;
INFN,I-35131Padova,Italy;[email protected] tothelargedetectorcapacity,whichsignificantlyincreasesthe
PreprintsubmittedtoNuclearInstrumentsandMethodsA January16,2017
Figure2: Addercircuitbasedonatwostageoperationalamplifierdesign. In
thecurrentprototype,thesignalsfrom96×6mm2SiPMsarefiltered(high-
pass)andsummedingroupsofthreeintothefirstsumstage; 3ofsuchfirst-
stagesumsarethensummedintoonefinaloutputinthesecond-stageadder.A
furtheramplificationstageispossibletosplittheoutputtohavetwogainlevels.
Figure1:SiPM-basedprototypepixelcluster(topleft)designedfortheMAGIC Theaddercircuit,whichisrealizedonaprintedcircuitboard
focal-planecamerawithsevenpixelseachconsistingofnine6×6mm2sensors, (PCB),isoptimizedtokeeptheshapeoftheindividualSiPMs
which areglued ontothePCB(bottom left), andequipped withtheMAGIC
signalsnarrow,whichisimportantforsignal-to-noiseratio,and
Winstoncone(topright). AfusedsilicaPCL(bottomright)isplacedonall
pixelsexceptthecentralone. foralowpowerconsumption.Afirstsumstagebasedonafast,
low-noise operational amplifier collects and sums the signals
fromthreeSiPMsensors,withsomegain. Asecondstagecol-
noise level of the sensor, and to the low device yield due to
lectsandsumsthesignalsfromthefirstsumsignals, togivea
thenaturaloccurrenceofdefectsonthesiliconwafers. Agood
single output of the nine sensors; some additional gain is ap-
compromiseisthereforetosegmentthedetectorintoelements
pliedhere.Finally,onelastadditionalgainstagecanbeusedto
ofsmallsize,andsummingtheoutputsignalelectronically,e.g.,
implementa two-gainoutputdesign(Fig. 2). The powercon-
throughananaloghighspeedadder. sumption for the 9-channelversion is ∼400mW, SiPM power
ThegoalofourresearchistodevelopaSiPM-basedmodule excluded,whilethepulsewidthis∼4ns.
for the focal-plane camera of large-dish IACTs. We thus de-
Thegoalofthecameraopticsistomapthefocalplaneinto
velopeda first prototypefor the MAGIC camera based on the
the pixelized camera avoiding dead areas between pixels and
conceptto balancea competitiveperformancewith a minimal
to compressthe hexagonalentrance pupilarea into the sensor
impactontheoverallcameradesign.
area. Theareacompressionratioislimitedbytheopticalinput
Oneadditionalissue thatneedstobeaddressedistheSiPM
acceptanceandtheangularacceptanceofSiPMs. Tooptimize
sensitivity to red photons: for IACT applications, sensitivity
theperformanceoftheopticalelementsofthefocal-planecam-
beyondtherangeoftheCherenkovspectrumlargelyincreases
era pixelsin terms of efficiencyin the area compressionratio,
the backgrounddueto theabundanceofatmosphericphotons.
detailedraytracingsimulationhavebeenperformedanddiffer-
WethuscomparetheperformanceamongthePMTandSiPM-
ent possible solutions are under investigation for the MAGIC
basedpixeldesignusingraytracingsimulationsandstudythe
telescopes.
overallperformanceoftheIACT,includingadiscussiononthe
ThedesignoftheWinstonconesisusuallyoptimizedtocom-
effectoftheNightSkyBackground(NSB).
presstheentranceareatothesurfaceofthePMTs,whichisusu-
ally curved(typicallyhemispheric). SiPM sensorsare usually
flatandtheirPDEdecreaseswithincreasingincidentangle.As
2. SiPM-basedmodule
the compression from the entrance of the Winston cone to its
exit apertureresults in photonshitting the sensor surface with
The approach described here involves covering the active
large incident angle, these Winston cones are not optimal for
area of the standardMAGICpixelwith severalSiPM sensors,
thistypeofphotosensors.
andsummingtheirindividualanalogsignalsintoonesingleout-
put, to beprocessedbytheexistingPMTreadoutchain. Each AsimplesolutiontoadapttheWinstoncone,whichhasbeen
segmentedsensorcovers0.1◦ andmatchestheexitwindowof exploredforthe realizationofthe MAGICSiPM-basedproto-
typepixel,istoaddafusedsilicaplane-convexlens(PCL)on
the existing light concentrator with hexagonal entrance pupil,
top of the silicon sensor, creating a curved surface similar to
with each SiPM segment covering a total active area of few
theentrancewindowofPMTs(Fig.1).Raytracingsimulations
squarecentimeters.
usingtheTraceProsoftwareprovidedbytheLambdaResearch
A first cluster was designedand assembled at INFN Padua.
Corporationwereusedtodeterminetheprofileofthelensthat
The design is based on nine SiPM sensors from FBK (Fon-
dazione Bruno Kessler1) of 6 × 6mm2 size for each channel maximizestheefficiency.
(seeFig.1). Other solutions consider alternative Winston cone designs
as well as solid concentrators of adequate material, that is
lightweight and transparent in the near ultraviolet (UV), with
1http://www.fbk.eu/ layouts adapted to the possible dimension of a SiPM-based
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-1m 18 60 GeV -1m140 400 GeV -1m600 1500 GeV
photons n 1146 46555°°° photons n120 46555°°° photons n500 46555°°°
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10 80
300
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6 200
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20 100
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0300 400 500 600 700 800 900 0300 400 500 600 700 800 900 0300 400 500 600 700 800 900
λ [nm] λ [nm] λ [nm]

1 1 1
60 GeV; Ratio to vertical incidence 400 GeV; Ratio to vertical incidence 1500 GeV; Ratio to vertical incidence
0.8 0.8 0.8
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0.4 0.4 0.4
0.2 0.2 0.2
0300 400 500 600 700 800 900 0300 400 500 600 700 800 900 0300 400 500 600 700 800 900
λ [nm] λ [nm] λ [nm]
Figure3: Cherenkovspectrumatgroundinfunctionoftheenergyanddirectionoftheprimarygamma. Toppanels: thetotalnumberofphotonsinastandard
telescopecameraforthreedifferentenergies: 60GeV(left),400GeV(middle)and1500GeV(right)andforthreezenithangles(5,45and65◦;solid,dashedand
dottedline,respectively). Themarkers(red)indicatethespectralpeaks. Bottompanels: thetotalnumberofphotonsoftheindividualspectraisnormalizedwith
respecttothe5◦case.
pixelfollowing[5]throughraytracingsimulationsandarecur- tober2015,withtheWinstonconebeingadaptedbyaPCLas
rentlyunderinvestigation. Thesimulationsaresteeredconsid- previouslydescribed(seeSec.2),totesttheperformanceofthe
ering the number of rays hitting the sensor, the area to which prototypeandtovalidatethepixellayoutinafunctionalIACT.
theyarecompressedbytheopticsundertestingandtheirinci- As the outputsignals of the SiPM cluster were adjusted to be
dent angle distribution. Among other elements considered in similar tothosefromthe PMTcluster, theobtaineddata, such
such simulations, we define a Cherenkov-light emitting light asthepixeltemperatureandthecharge,arerecordedbythereg-
sourceandpluginthepropertiesofthematerialconsideredfor ularMAGICreadout,allowingperformanceandlongtermtests
theopticalelementunderstudyandthesensorproperties,that tocompareindetailthePMTandSiPMclusters,whichwillbe
is its PDE and angular response. The spectral distribution of crucialforthedecisiononthenextstepsinSiPMclusterdevel-
thoserayshittingthesensorsurfacearethenweighedbythese opments.Theanalysisofthesedataiscurrentlyongoing.
properties.Theweighedrayspectrumiscomparedtothatfrom
simulationsperformedwithMAGICstandardpixelsettingsby
3. Performancestudies
using the integralof those spectra as a proxyfor the expected
pixelperformance.
Since the total throughput on the pixels of the focal-plane
Our design study is somehow constrained by the existing
cameraofanIACTdependsbothonthemirrordishreflectivity
pixel layout, that is by its entrance aperture and the angular
and on the PDE of the light sensor, we investigated the inter-
acceptance, being ∼30 mm and 40◦, respectively, wherefore
play betweenthese two hardwareelements, for differentener-
oursimulationsforeseeaninsufficientlightcompressioninthe
gies and arrivaldirectionof the primarygammaray including
caseofaclassicalWinstonconeandtoohighabsorptioninthe
theCherenkovlightextinctionthroughtheatmosphere(Fig.3).
ultravioletband in the case of solid opticsdue to its lengthof
Asworkingexample,wecomparedpossibledifferentreflec-
severaltensofmillimeters.Therefore,acombinationofaWin-
tive surfacesfor futureIACTs (Fig. 4). We repeatedthe exer-
stonconewithsolidopticsiscurrentlyinvestigated.
cise considering the PDE of PMTs and SiPMs and compared
The first cluster prototype, for which slow control, optical the totalamountofCherenkovphotonsdetected. Forsimplic-
transmitters, design of mechanics and HV supply were pro- ity, we did not consider the performance of the light guide in
vided by the MAGIC MPI-Munich team, was installed in the thesestudiesandneglectedthecrosstalk. Inthesamemanner,
MAGICcamera onLa Palma nextto the PMT clustersin Oc- westudiedtheimportanceoftheNSB(Fig.5).
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Figure 6: Rate oftheNSBpernanosecond peracamera pixel ofthelarge-
sizetelescope(LST)oftheCherenkovTelescopeArrayprojectweighedwith
individualmirrorreflectivitycurvesandthePDEofaPMT(left)andaSiPM
(right).
We concluded that the choice of the current generation of
SiPMs over PMTs would lead to an enhanced throughput ir-
respective of the mirror coating, due to the enhanced PDE
of SiPMs in a wavelength range of the Cherenkov spectrum
(∼290 - 900nm), in particular at wavelengths larger than 700
Figure4: Topleft: Reflectivitycurvesconsideredinourstudy. TheAl+SiO2 nmwherethePDEofandPMTdropstozero.
(black solid line) is taken from [6] and is considered as reference. The sil-
ThisleadsconsequentlytoanenhanceddetectionoftheNSB
vercoating(longdashedgray)istakenfromMAGICtechnological testsand
reported in [7]. The red dot-dashed and green short-dashed lines represent whosedominantemissionoccursinthewavelengthrangefrom
twometalliccoatings(Al+SiO2+HfO2andCr+Al+SiO2+HfO2+SiO2,respec- ∼720-1000nm(Fig.6),exceptforthedielectriccoating,which
tively), whilethedotedblueindicatesadielectric coating. Allthreecoatings
suppresses the NSB contribution at the cost of a decreased
areundertestfortheCherenkov Telescope Array(CTA)project andarede-
Cherenkovlightdetection.
scribedin[8]. Topright: PDEforareferencePMTandSiPMforthispaper.
ThePMTisaHamamatsu19200(from[9])plannedfortheuseinthelarge- Thismayhaveaneffectontheenergythresholdofthetele-
sizetelescope oftheCTAproject. TheSiPM(from[10])isa30µmcoated scopeinpresenceoflargeNSB,duetothesmallsignal-to-noise
FBKsensor,currentlyunderdevelopmentandalsocandidateforuseinCTA.
ratio atthe lowest energies. To quantifyproperlythe effectof
Bottom:Fractionofphotoelectronsthatsurvivethereflectionontotheprimary
mirrorofthetelescopeandthephoto-conversionintothePMT(left)andSiPM theNSBonthedetectorsensitivity,wearecurrentlyperforming
(right)foraprimarygammaof1500GeVand5◦zenithangledirection. MonteCarlosimulations.
-2]c Acknowledgements
e
s
arc250
y We thankfullyacknowledgethe cooperationof MAGICex-
J
µy [ perimentintheinstallationandtestoftheSiPM clusterproto-
nsit200 typein theirtelescope. Inparticularwe acknowledgethehelp
e of the MAGIC MPI-Munich team in the design of the cluster
nt
I module.
Funds for this study have been provided within the Progetto
150
Premiale TECHE.it (TElescopi CHErenkov made in Italy) by
theItalianMinistryofEducation,UniversityandResearch.
100
References
References
50
[1] F.Aharonianetal.,Rep.Prog.Phys.,71(2008),9.
[2] O.Catalanoet al., in: Proceedings SPIE9147, Ground-based and Air-
0 borne Instrumentation for Astronomy V, Montre´al, Quebec, Canada,
300 400 500 600 700 800 900
λ [nm] 2013,91470D.
[3] T.Bretzetal.,in:Proceedingsofthe19thIEEE,Real-TimeConference,
Nara,Japan,2014,15107044
[4] A.Bilandetal.,in:ProceedingsSPIE9906,Ground-basedandAirborne
Figure5:ThespectrumoftheNSBatLaPalmafrom[11].
TelescopesVI,Edinburgh,UnitedKingdom,2016,99061A.
[5] W.T.Welford&R.Winston,Highcollectionnonimagingoptics,1sted.,
AcademicPress,Inc.,SanDiego,1989.
4
[6] D.Bastierietal.,in: Proceedings ofthe29thICRC,Pune,India,2005,
101–106
[7] J.Barrioetal.,MPI-PhE98-05,1998.
[8] A.Fo¨rsteretal.,in:Proceedingsofthe33rdICRC,RiodeJaneiro,Brazil,
2013,0747
[9] A.Okumuraetal.,in: Proceddings ofthe34rdICRC,TheHague, The
Netherlands,2015,952
[10] N.A.Otteetal.,acceptedforpublicationbyNucl.Instrum.MethodsA,
2016,arXiv:1606.05186
[11] C.Benn&S.L.Ellison,BrightnessofthenightskyoverLaPalma,New
Astron.Rev.,42(1998),503-507.
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