Characterization studies of Silicon Photomultipliers and crystals matrices for a novel time of flight PET detector

7.1 MB·English

by  Etiennette Auffray

Characterization studies of Silicon
Photomultipliers and crystals matrices for a
novel time of flight PET detector
EtiennetteAuffray3,FaraahBenMimounBelHadj3,DanieleCortinovis∗1,2,KatayounDoroud3,ErikaGarutti2,PaulLecoq3,
ZhengLiu4,RosanaMartinez4,MarcoPaganoni4,MarcoPizzichemi4,AlessandroSilenzi1,ChenXu1,2 andMilanZvolský1,2
1DeutchesElektronen-Synchrotron(DESY),Notkestrasse85,22607Hamburg,Germany
2UniversityofHamburg,LuruperChaussee149,22671Hamburg,Germany
3EuropeanOrganizationforNuclearResearch(CERN),1211,Geneva23,Switzerland
5 4UniversityofMilano-Bicocca,PiazzadellaScienza3,20126Milano,Italy
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Abstract
J

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1 This paper describes the characterization of crystal matrices and silicon photomultiplier arrays for a novel Positron
EmissionTomography(PET)detector,namelytheexternalplateoftheEndoTOFPET-USsystem. TheEndoTOFPET-US

] collaborationaimstointegrateTime-Of-FlightPETwithultrasoundendoscopyinanovelmultimodaldevice,capableto
t
e supportthedevelopmentofnewbiomarkersforprostateandpancreatictumors. Thedetectorconsistsintwoparts: aPET
d headmountedonanultrasoundprobeandanexternalPETplate. Thechallenginggoalof1mmspatialresolutionforthe
- PETimagerequiresadetectorwithsmallcrystalsize,andthereforehighchanneldensity: 4096LYSOcrystalsindividually
s
n readoutbySiliconPhotomultipliers(SiPM)makeuptheexternalplate. Thequalityandpropertiesofthesecomponentsmust
i beassessedbeforetheassembly. Thedarkcountrate,gain,breakdownvoltageandcorrelatednoiseoftheSiPMsaremeasured,
.
s whiletheLYSOcrystalsareevaluatedintermsoflightyieldandenergyresolution. Inordertoeffectivelyreducethenoisein
c thePETimage,hightimeresolutionforthegammadetectionismandatory. TheCoincidenceTimeResolution(CTR)ofallthe
i
s SiPMsassembledwithcrystalsismeasured,andresultsshowavalueclosetothedemandinggoalof200psFWHM.The
y lightoutputisevaluatedforeverychannelforapreliminarydetectorcalibration,showinganaverageofabout1800pixels
h firedontheSiPMfora511keVinteraction. Finally,theaverageenergyresolutionat511keVisabout13%,enoughfor
p
effectiveComptonrejection.
[

1
v
3 1 Introduction inedwithultrasoundprobesthroughnaturalcavities,
3 thereforethediagnosticcapabilitycanbeenhancedby
2
fusingthemorphologicalimagefromultrasoundwith
4
Positron Emission Tomography (PET) is a non inva- aPETmetabolicimage. Theendoscopicapproachand
0
. sive, diagnostic imaging technique for measuring the theuseoftheTOFinformationwithanunprecedented
1 metabolic activity of cells in the human body. Nowa- CTRof200ps(3cmalongthelineofresponse)allow
0
daysitiswidelyusedforcancerdiagnosis[1]. todefineaspecificRegionOfInterest(ROI)andsignifi-
5
1 Amongthetumors,pancreaticcarcinomaisoneofthe cantlysuppressthebackgroundfromtheneighboring
: most aggressive and resistant to current therapies [2], organs.
v
i and most often is detected only on an advanced state TheEndoTOFPET-USdetectorconsistsofaPETheadex-
X ofdevelopment. Ontheotherhand,prostatecanceris tensionmountedonacommercialUSendoscopeplaced
r themostcommoncanceramongmales[3]. close to the ROI and an external PET plate facing the
a
Bothpancreasandprostatearesurroundedbyorgans patient’sabdomen,incoincidewiththePEThead(Fig-
with high uptake: the liver and the heart are close to ure 1). The external plate is a square of 23 x 23 cm2,
thepancreaswhilethebladderisneartheprostate. In anditismadeof256detectorunitmodules. Eachmod-
thesecasesthecurrentfull-bodymultimodalPETscan- uleconsistsina4x4LYSO:Cecrystalmatrixgluedtoa
nershavestronglimitations,duetothenoisefromthe discretearrayof4x4analogSiPMsfromHamamatsu.
neighboringorgansandthelackofspecificbiomarkers LYSO:Cecrystalshavehighlight yieldandfast decay
[4][5]. EndoTOFPET-US[6]aimstoovercometheselim- time, therefore appropriate for TOF PET. SiPMs are
its,allowingtoinvestigatetheperformancesofnewly novel photon detectors also suitable for TOF PET ap-
developedspecificbiomarkersoftumoralprocessesfor plications [7], thanks to their compactness, high gain,
pancreasandprostate. Bothorgansareusuallyexam-
∗Correspondingauthor,[email protected]
1
insensitivitytomagneticfields,lowoperatingvoltage heatgeneratedbytheactivecomponentsontheSiPM
andexcellenttimingproperties. motherboardisdissipatedbyafan. Thetemperatureis
Thefollowingsectionsdescribetheexperimentalsetups monitoredbyasensor(DallasDS18B20)butnotactively
and the results of the characterization of the SiPMs, controlled.
crystalsandthecombinedsystem(SiPM+crystal). All
the setups have been designed for a fast and reliable
measurementofalargequantityofcomponents,rather
thanadetailedstudyofsingledevices.
Figure2: SketchoftheexperimentalsetupfortheSiPMcharacteri-
zation.
Figure1: TheEndoTOFPET-USdetectorwithamagnificationof
the pancreatic endoscope (top circle) and the external
plate(bottomcircle).
2.2 Gain and Breakdown voltage
2 Photodetector Characterization
The integrated charge spectrum given by the light
pulsesinshowninFigure3. Thisisrepeatedfor30volt-
TheSiPMschosenfortheexternalplatearematricesof
agesteps,acquiring200keventssampleperstepwith
4x4discreteMPPCs(MultiPixelPhotonCounter)from
anintegrationgateof100nssynchronizedwiththelaser
Hamamatsuphotonics(S12643-050CN).EachSiPMin
pulse. Thefirstpeakcorrespondstothepedestal(zero
the matrix is a square of 3.0 x 3.0 mm2 with 3464 ac-
pixelfired)whilethefollowingpeakscorrespondtoan
tive pixels (50 x 50 µm2 each). The distance between
increasing number of pixels fired. After the pedestal
two adjacent SIPMs in the matrix (center to center) is
subtraction,thespectrumisprojectedintothefrequency
3.6mm. TheseSiPMsexploittheThroughSiliconVia
spaceusingaFastFourierTransform. Theperiodofthe
(TSV)technology,leadingtolessdeadspaceandare-
first harmonic is taken as the value of the SiPM gain
ducedconnectionlengthascomparedtoconventional
(G)inQDCunits,whichisthenconvertedintonumber
wire-bondedSiPM.Thefollowingsectionsdescribethe
ofelectronsbythefollowingformula:
experimentalsetup,theanalysismethodandtheresults
ofthecharacterizationof256SiPMmatrices.
G·R
qdc
G = , (1)
2.1 Experimental setup A
The layout of the experimental laboratory setup is
showninFigure2. Inalighttightbox,theSiPMmatrix where R is the QDC resolution of 25 fC per QDC
qdc
ismountedonamotherboardincorporatinglinearam- bin and A is the amplification factor of the readout
plifiers(InfineonBGA614)andhighvoltagefilters. The board (19 dB, ∼ 8.9). The error on the gain is about
signalforeachofthe16SiPMsofthematrixisobtained 1%, obtained as one bin in the Fourier space plus an
using low intensity blue light (Advanced laser diode additionaluncertaintyof5 ,evaluatedbyrepeating
system,451nm). TheamplifiedSiPMoutputisreadout themeasurementmultipleti(cid:104)mesunderthesamecondi-
by a VME-based charge-to-digital converter (CAEN tions.
QDC965A)witharesolutionof25fCperQDCbin. The
2
×103
Counts 890000 pede1 sptiaxl Gain 11680000 G U=b d(0 =.5 (2634 .3±7 0 .±0 003.0) 2×) 1V06
700
600 2 pix 1400
500 1200
3 pix
400 1000
300
800
200
600
100
0 400
600 800 1000 1200 1400 1600 1800 65.5 66 66.5 67 67.5 68
QDC channel Bias voltage [V]
Figure3: Singlephoto-electronspectrumforasingleSiPM,each Figure4: Gainasafunctionoftheappliedbiasvoltageforasingle
peakcorrespondstoacertainnumberofpixelfired. SiPM.Theredlineisalinearfitonthedatapointswhose
errorsarewithinthedots. Thebreakdownvoltage(U )
bd
andgain(G)at1Vexcessbiasobtainedfromthefitare
givenintheinlet.
Figure4showsthegaindependenceonthevoltage
biasapplied. Asexpected,thegainislinearlypropor-
tionaltothebiasvoltageapplied,accordingto
G =C ·∆U =C (U −U ), (2)
pixel pixel bias bd
M 400 EEnnttrriieess 44009966
P
where Cpixel is the pixel capacitance, ∆U is the excess Si 350 MMeeaann 00..44882266
bisiatshevoblrtaeagke,dUowbians ivsotlhtaegaep. pHlieendcbeiaUs voisltaogbetaainneddUbbyd # of 300 RRMMSS 00..0011449922
bd 250
applyingalinearfitandextrapolatingtozerogain,with
200
anuncertaintyoftheorderofafewtensonmV.Figure
5showsthedistributionofthegainslope(G at1Vex- 150
cessbias)foralltheSiPMs. Themeanvalueis0.48·106 100
V−1 withaspreadof3%amongtheSiPMstested. 50
It is known that the Ubd depends on the temperature 00.35 0.4 0.45 0.5 0.55 0.6
[8], and some fluctuations have occurred during the Gain slope [106 V-1]
wholesetofmeasurements. Moreover,localdifferences
in temperature have been observed within the SiPM Figure5: Distributionofthegainslope(gainpervolt)forallthe
matrix, due to the inhomogeneous dissipation of the SiPMs.
heatgeneratedbytheamplifiers. Thereforeanoffline
temperature correction (see section 2.5) has been ap-
plied in order to compare the results. Figure 6 shows
thedistributionofthebreakdownvoltagesat25 ◦Cfor
alltheSiPMs.
AlthoughtheU spreadforalltheSiPMisabout2V, M 140 EEnnttrriieess 44009966
bd P
thismustnotexceed0.5VineachsingleSiPMmatrix, Si 120 MMeeaann 6644..2299
becausethisisthemaximumvoltagebiastuningrange of RRMMSS 00..44333344
# 100
of the ASIC developed for the SiPM readout [9] [10].
AsshowninFigure7,alltheSiPMmatricesrespectthis 80
requirement. 60
Finally, the results obtained have been compared to
40
theoperatingvoltages(U )providedbytheSiPMpro-
op
ducer. TheU isdefinedas: G(U ) =1.25·106. The 20
op OP
differencesbetweentheU calculatedusingequation 0
op 63 63.5 64 64.5 65 65.5 66
2withdatafromthemeasurementsandUop provided Breakdown voltage [V]
byHamamatsuareshowninFigure8. Theagreement
isgood,despiteashiftof6%andaspreadof10%. This Figure6: Distributionoftheextractedbreakdownvoltageforall
isduetothedifferentmeasurementmethodimpliedby theSiPMs.
theSiPMproducerandtheuncertaintyofU and G.
bd
3
events n is given by n = DCR ·∆t. The number
0.5pix
ay EEnnttrriieess 225566 of events in the pedestal distribution is given by the
arr 25 MMeeaann 00..11440011 Poissonprobabilitydensityfunctionforzero DCR0.5pix
M RRMMSS 00..0055776655 eventsinthetime ∆t:
P 20
Si
# of 15 P (∆t) = e−DCR0.5pix·∆t. (3)
0
10
5 The DCR isthereforecalculatedusing:
0.5pix
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Ubdmax - Ubdmin [V] DCR = ln(N0/Ntotal), (4)
0.5pix ∆t
Figure7: Distributionofthemaximum∆U ,i.e. thedifference
bd
between maximum and minimum U in one matrix.
bd
Thedeviationisbelowthe0.5Vrequirementspecifiedto where N isthenumberofeventsinthepedestalpeak
0
theSiPMproducer.
and N isthetotalnumberofeventsrecorded. The
total
erroronthecalculatedvaluedependsonthestatisticof
thesampleandonthequalityofthespectrum.
ForeachSiPMthe DCR isevaluatedfor30voltage
0.5pix
levels, in a range of 3 V starting about 1 V above the
M EEnnttrriieess 44009966 breakdown voltage. 500k event samples are collected
P
Si 300 MMeeaann 00..0066445566 foreachvoltagepoint.
of RRMMSS 00..11009944 Also the DCR has a dependence on the temperature,
# 250 thereforeacorrection(seesection2.5)hasbeenapplied
200 inordertocompensatefortemperaturevariations.
150 The distribution of DCR0.5pix, corrected to 25 ◦C and
at 2.5 V excess bias, is shown in Figure 10 for all the
100
SiPMs.
50 TheDCRhasanegativeimpactonthetimeresolution.
0 The best time resolution is achieved when the timing
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
(Uproducer - Umeasurement) [V] thresholdonthereadoutASICissetaslowaspossible,
op op uptothenoiselevel. Thereforea DCR acceptance
0.5pix
Figure8: DifferencebetweenUop givenbytheSiPMproducerand limitof3MHzforeachSiPMhasbeenagreedwiththe
Uop calculatedfromthemeasurement. SiPM producer. Almost all the SiPMs are within the
limit, however 5 matrices have been sent back to the
producerbecausetheyincludedatleastoneSiPMwith
excessive DCR .
0.5pix
2.3 Dark Count Rate
Eveninabsenceoflight,anavalanchebreakdowncan
betriggeredbyaphotoelectrongeneratedfromthermal
s
excitation or by tunneling effect. This phenomenon, unt 16000 pedestal
usuallycalledDarkCountRate(DCR),isPoissondis- o
C 14000
tributed and it is measured by randomly integrating
thechargewithatimewindow∆tof100ns(sametime 12000
10000
windowasforthegainmeasurement). Atypicalnoise
8000
spectrumisvisibleinFigure9.
0.5 pix threshold
6000
Theprobabilityofhavingtwodarkeventsinthetime
∆t is very low, therefore the events corresponding to 4000 1 pix
more than 1 pixel fired are mainly due to correlated 2000
noise,whichwillbedescribedinthenextsection. 0450 500 550 600 650 700 750 800
The events in the pedestal peak, i.e. the events with QDC channel
chargelessthan0.5pixelthreshold,arenotaffectedby
correlated noise, hence they are used to calculate the Figure9: DCR charge spectrum. The events below the 0.5 pix
uncorrelatedDCR,definedas DCR0.5pix. threshold(pedestalpeak)areusedtocalculateDCR0.5pix.
Inatimerange ∆t, theexpectednumberof DCR
0.5pix
4
M EEnnttrriieess 44009966 M EEnnttrriieess 44009966
SiP MMeeaann 11..444411 SiP 400 MMeeaann 2299..1122
of 102 RRMMSS 00..33667788 of 350 RRMMSS 11..882299
# # 300
250
200
10
150
100
1 50
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 10 15 20 25 30 35 40 45 50
DCR [MHz] Correlated noise probability [%]
Figure10: DistributionoftheDCR0.5pix at2.5Vexcessbiasand Figure11: CorrelatednoiseprobabilityPcn at2.5Vexcessbias.
at25oCforalltheSiPMs. Theredlineindicatesthe
threshold of acceptable DCR. Entries marked in red
correspondtotherejectedSiPMs.
2.5 Temperature dependence
As mentioned in previous sections, SiPM properties
dependontemperature. Therefore,DCR,gain,break-
2.4 Correlated noise
downvoltageandcorrelatednoisehavebeenmeasured
atdifferenttemperaturesforasingleSiPMinanarray.
Usingaclimatechamber(EspecLU-123),severalvolt-
Correlated noise includes after-pulse and inter-pixel agescanshavebeenperformedinatemperaturerange
opticalcrosstalk[11]. between 6 ◦C and 30 ◦C. Figure 12 shows the linear
After-pulsesaregeneratedifanelectronproduceddur- temperaturedependenceofthebreakdownvoltage;the
inganavalancheistrappedandreleasedatalatertime, uncertainty on the temperature is included but it is
with a delay ranging from nanoseconds up to several smallerthanthemarkersize. Thecoefficientobtained
microseconds. Depending on the trapping time con- from the linear fit is 70.1 mV/◦C, and is used for the
stantandthepixelrecoverytime,trappedelectronscan temperaturecorrectionsappliedinsection2.2and4.1.
generateanavalancheofequivalentorsmallercharge The dependence of the DCR as a function of the tem-
withrespecttotheDCRpulses. perature should scale with the density of the thermal
Inter-pixelcrosstalkisduetoopticalphotonsgenerated carriers[12]accordingtoequation:
duringapixelbreakdown. Thesephotonshaveacertain
probabilitytoreachtheneighboringpixels,triggering
newavalanches. n(T) =CpixelT3/2e−kEBaT, (6)
The precise measurement of each of the two effects is
beyondthescopeofthisstudy. However,it’spossible
toestimatethecorrelatednoisetriggeringprobability whereC isthepixelcapacitance, T isthetempera-
pixel
Pcn as: tureoftheSiPMinKelvin, Ea istheactivationenergy
and k istheBoltzmannconstant.
B
Figure13showstheDCRasafunctionoftemperature
DCR at2.5Vexcessbias. Anactivationenergy E =0.6eVis
1.5pix a
P = , (5)
cn DCR obtainedbyfittingthedatawithequation6. Thisvalue
0.5pix
iscompatibletopreviousstudies[13],andithasbeen
usedforthetemperaturecorrectionsappliedinsection
2.3.
where DCR iscalculatedbysettingthethreshold Thechangeinthegainisfoundtobelessthan1%/10◦C,
1.5pix
to1.5pixelfired. Figure11showsthecorrelatednoise which is negligible for the temperature variations oc-
probability at 2.5 V excess bias for all the SiPMs. The curredduringthegaincharacterizationofalltheSiPMs.
averageresultsobtained,about30%,iscompatiblewith Alsothecorrelatednoiseshowsamilddependenceon
measurements on similar devices and does not repre- thetemperature,butitisnotrelevantforthedetector
sentanissueforthedetectoroperation. performances.
5
theenergyresolution. Figure14showsthedistribution
V] oflightyieldobtainedwithMiniACCOSsetupforall
[Ubd 6644..68 ∆U/∆T = 70.1 mV/ oC the256matrices. Thedistributionspreadis6.4%,which
subtractingtheuncertaintyofthesetupgivesa4.3%as
64.4
lightoutputdispersion. Figure15showsthedistribu-
64.2
tionoftheenergyresolutionforallthemodules,with
64
a mean value of 13.4%. The spread is comparable to
63.8
thebenchuncertainty,thereforethevariationsinenergy
63.6
resolutonarejustduetothesetup.
63.4
In the final configuration, each matrix will be glued
63.2
to a SiPM array. The absolute values of light output
63
5 10 15 20 25 30 obtainedwiththeMiniACCOSsetuphavethereforeto
Temperature [ oC] be rescaled to this configuration. In order to obtain
this,lightoutputforasubsetof13matriceswasmea-
Figure12: Breakdownvoltagemeasuredatdifferenttemperatures.
suredonastandardXP2020QPMT,usingRhodosil47V
Theredcurveisthelinearfittothedatapoints. The
greaseasopticalcouplingandteflonasbackwrapping.
coefficientobtainedformthefitisdisplayedintheinlet.
Thescalingfactorbetweenthismeasurementcondition
and MiniACCOS was found to be 13.15, with a corre-
lationcoefficientof0.89betweenthetwodatasets,as
showninFigure16.
Theaveragelightyieldforallthecrystalmatricescan
Hz] 1.4 thereforebederivedas32500 ± 2700Ph/MeV,includ-
M
[pix 1.2 Ea = 0.6 eV itnemgaintictshaeriesvinaglufartoiomnthofetchaeliburnatcieorntapinrotyceaslss.oBathseedsyosn-
5 1
R0. thestudiesreportedin[15],itcanbeestimatedthatat
C 0.8 least20000 − 25000Ph/MeVarenecessarytoachievea
D
0.6 CTRof200psFWHM,whichmakeoursetofmatrices
perfectly suitable for this purpose. The homogeneity
0.4
on the crystals performance is also ensured within a
0.2 4.3% level for all 256 matrices tested with an energy
resolutionof13.4 ± 1.3%asmeanvalue.
5 10 15 20 25 30
Temperature [ oC]
Figure13: DCR calculatedatdifferenttemperatures. Thered
0.5pix
lineisthefittodatapointsusingequation6. Thevalue
ofEa obtainedfromthefitisdisplayedintheinlet.
3 Crystals characterization
The crystals for the external plate are grouped in 256
matrices of 4x4 LYSO:Ce scintillators, produced by
Crystal Photonics Inc. The volume of each crystal is es 18 EEnnttrriieess 225566
3.5x3.5x15 mm3 and within the matrix they are sepa- ric 16 MMeeaann 22550044
at RRMMSS 115599..99
rated by a reflector foil (ESR Vikuiti by 3M). In order m 14
toensurethesystemuniformity,measurementsoflight of 12
#
outputandenergyresolutionhavebeenperformedon 10
bothfacesforeachmatrixusingtheMiniACCOSsetup 8
[14], which allows a fast, automatic and highly repro- 6
ducible data acquisition process. In this setup, each 4
matrix is placed on a custom made teflon plate, for 2
a total of 25 matrices per plate. An air gap of 5 mm 10000 1500 2000 2500 3000 3500 4000
is left between the matrix face and the PMT window. LY [Ph/MeV]
Systematicmeasurementswereperformedtoevaluate
uncertainties arising from the bench, yielding a total Figure14: Lightyielddistributionsforallthe256crystalmatrices.
relativeerrorof4.7%forthelightoutputand12.8%for
6
es 18 EEnnttrriieess 225566
ric 16 MMeeaann 1133..4444
at RRMMSS 11..332244
m 14
of 12
#
10
8
6
4
2
0
10 11 12 13 14 15 16 17 18 19 20
Energy Resolution [%]
Figure15: Energyresolutiondistributionsforallthe256crystal
matrices.
V] Figure17: Thebenchforthemoduleassembly. Thebinocularwith
h/Me45000 χχχχ2222 //// nnnnddddffff 3333....666699999999 //// 11112222 thecameraisplacedoverthe5supportsforthecrystal
T [P pppp0000 11113333....11115555 ±±±± 0000....2222555588884444 matrices, the SiPM matrices (not visible) are located
M
_P40000 below.
Y
L
35000
4.1 Detector light output
30000
The detector light output for the 511 keV photon (de-
25000
fined here as LO ) is of primary importance for the
511
detectortimeresolution. Asdemonstratedinotherstud-
20000
ies [15], the number of detected photoelectrons must
2000 2200 2400 2600 2800 3000 bemaximizedinordertoachievethebesttimeresolu-
LY_MiniACCOS [Ph/MeV]
tion. Moreover,allthechannelsshouldhavethesame
response for the 511 keV photon, allowing an easier
Figure16: CorrelatedMiniACCOS-PMTdataandthelinealfit-
tingfunctionwithits95%confidenceintervalband. detectorcalibration.
A22Na sourcewithanactivityofabout1MBqisused
asaβ+emitter. Thesetupissimilartotheonedescribed
insection2.1buttheSiPMsignalisnotamplifiedand
4 Detector Module Characteriza- theresolutionoftheQDCissetto200fC.Thecharge
integrationgateissetto450nsanditistriggeredbythe
tion
SiPMsignalitself,usingadiscriminator(CAENmodel
96)andagategenerator(LeCroymodel222). Achan-
Adedicatedbenchhasbeendevelopedtogluethecrys-
nel switcher (Keithley 7002 switch system) provides
talmatricesontheSiPMarrays,allowingtoassemble5
automaticscanthroughthe16channelsineachmatrix.
modulesperday. Figure17showsthesetup: thecrystal
Thetemperatureisrecordedbeforeeachchannelmea-
matricesareplacedonamovablesupportandacamera
surement. EachSiPMisoperatedatU asdefinedin
op
allowsthevisualalignmentwiththeSiPMmatriceslo-
section 2.2, eventually corrected for temperature vari-
catedunderthecrystals. Theamountofglue(RTV3145) ationsaccordingtothecoefficientof70.1mV/◦C(see
iscalculatedinordertohaveamaximumthicknessof
section2.5).
0.1 mm, and it is spread over the SiPM matrix with a Fig 18 shows a typical 22Na spectrum for a single
dispenser. TheSiPMmatrixisthenpushedagaintsthe
channel. Thepeakcorrespondingtothe511keVphoton
crystalsandavisualinspectionisperformedtocheck
inthechargespectrumisfittedbyaGaussianfunction.
ifanyairbubbleoccurred. Ifnone,themodulesareleft
Hencethelightoutput LO ,expressedinnumberof
511
onthesupportforthecuringtime(24h).
pixels fired on the SiPM, is obtained by the following
Everymoduleisthenmeasuredregardinglightoutput,
relation:
energyresolutionandCTR,whicharedescribedinthe (Q−P)·r
qdc
nextsections. LO511 = , (7)
G
7
where Q is the mean value of the Gaussian fit, r is 450nsintheQDC.Thechannelswitcherprovidesthe
qdc
200 fC per bin, P is the pedestal position, and G is automaticscantroughthe15channels,eachoperated
1.25x106. The uncertainty on LO , of the order of atitsownU .
511 op
about 30 pixels, is given by statistical error and tem- Ineverychannel,onlytheeventscorrespondingtothe
peratureuncertaintyof0.5 oC.NoerrorfortheU is photopeakinthetriggerareselected. Therefore,divid-
op
available. ingthemeanchargeoftheseeventsbythephotopeak
The light output distribution for all the channels is position in the trigger, the cross-talk for the 511 keV
shown in Figure 19, the average value of about 1800 photon is obtained for all the remaining 15 channels
pixelsshouldallowtoreachtheCTRof200psFWHM. inthemodule. Themeasurementisrepeatedbyselect-
A preliminary detector calibration will be based on ingdifferenttriggerchannelsinthemodule. Figure20
thesemeasurements. showstheaveragecrosstalkobtainedfrom4different
measurements: everytimeoneofthecentralchannels
inthemoduleisusedasatrigger. Thecross-talkdoes
nts not exceed 20%, and can be suppressed by setting a
ou 500 thresholdat100keV.
C
400
300
200
100
0
1000 1500 2000
QDC channel
Figure18: 22Na spectrumforonechannel. Thehigherpeakcor-
respondstothefullabsorptionofthe511keVphotons,
while the peak on the right comes from the 1277 keV Figure20: Averagecross-talkforthe511keVphotonobtainedover
gammaemittedduringthede-excitationofNeontothe 4measurements,usingeachtimeacentralchannelas
groundstate. trigger.
4.3 Energy calibration
els 350 EEnnttrriieess 44009966 It is well known that the SiPM is a non-linear device
nn 300 MMeeaann 11882233 due to the limited number of pixels, therefore a non-
a RRMMSS 111199..11
h linear correction is necessary in order to estimate the
c 250
r energyresolutionforthe511keVgamma.
o
ct 200 Using the setup described in section 4.1, the detector
e
et 150 lightoutputismeasuredfordifferentgammaenergies:
d
of 100 356keVfrom133Ba,511and1277keVfrom22Na,and
r. 662keVfrom137Cs.
N
50
Themeasurementisperformedonasampleof4mod-
0 ules (64 channels): for each channel the light output
1000 1500 2000 2500 3000
LO [pixel fired] data are fitted with the function describing the SiPM
511
saturation:
Figure19: Lightoutputdistributionforallthechannels.
Npix = Nmax(cid:16)1−e−N(cid:101)m·Eax(cid:17), (8)
where N is the average number of pixels fired on
4.2 Inter channel cross-talk pix
thephotodetector, (cid:101) isthenumberofpixelfiredonthe
The cross-talk between the module channels is mea- SiPMperunitofenergydepositedinthecrystaland E
sured on a sample module. Similarly to the setup de- is the energy of the detected gamma photon. N is
max
scribedinsection4.1,the22Na sourceisplacedinfront relatedtothemaximumnumberofpixelsintheSiPM.
ofthemoduleandonlyonechannelischosenastrigger. Thisnumberisactuallyhigherthanthenominalvalue
Anyotherchannelinthemoduleisacquiredsimultane- (3464 pixels) because the pixel recovery time is about
ouslytothetrigger,usingacommonintegrationgateof 20ns,whichishalfofthedecayconstantofthecrystal
8
(40ns).
Figure21showsthenon-linearfunctionforonechannel, x
a
m
howeverthisfunctionisnotuniqueforallthechannels. N 5800
It is expected that the parameter (cid:101) varies among the
5600
channelsaccordingto LO ,whichtakesintoaccount
511
thevariationsintheSiPMproperties(photodetection 5400
efficiency,correlatednoise),crystallightyieldandthe
5200
opticalcoupling.
Thedependenceof Nmax and (cid:101) tothe LO511 forthe64 5000 χχ22 // nnddff 2277..5566 // 6622
channelsisshowninFigure22and23respectively. Al- pp00 11..114477 ±± 00..22881199
4800 pp11 33113399 ±± 553388..99
thoughthelineardependenceof (cid:101) wasexpected,also
N showsadependenceon LO ,maybeduetovari- 1700 1800 1900 2000 2100
max 511
LO [pixel fired]
ationsininthepixelrecoverytimeorcrystaldecaytime. 511
In both cases a linear fit is applied and the relations
obtainedareusedtocalculate N and (cid:101) foranyother Figure22: Distribution of the parameter Nmax of equation 8 de-
max
pendingonLO for64channels. Theredlineisthe
channeldependingonits LO . 511
511 linearfit.
Figure 24 shows the energy obtained using equation
8 and the LO for every channel. Similarly, the cor-
511
responding energy of the 1277 keV peak of the 22Na
spectrumisestimatedforeverychannel(Figure25). In
bothcasesthepredictedenergyisingoodagreement
withthemeasurements,theprecisionworsensonlyif ∈ 5.4
χχ22 // nnddff 2222..5599 // 6622
the channel LO511 is significantly lower than the aver- 5.2 pp00 00..000022883377 ±± 00..00000011338844
age. pp11 00..88115511 ±± 00..22662288
5
Finally, after the energy calibration has been applied,
4.8
theenergyresolutionforthe511keVgammaisevalu-
4.6
atedforallthechannels(Figure26). Onlyonechannel
isoutofthedetectorrequirement: atleast20%isnec- 4.4
essarytoeffectivelydiscriminatethe511keVgammas 4.2
fromtheComptonevents. 4
3.8
1700 1800 1900 2000 2100
LO [pixel fired]
511
Figure23: Distributionoftheparameter(cid:101)ofequation8depending
onLO for64channels. Theredlineisthelinearfit.
511
el EEnnttrriieess 44009966
n 200
n MMeeaann 551144..99
x 5000 ha 180 RRMMSS 11..550022
Npi 44050000 χχNN∈∈22 mm //aa xxnn ddff 44.. 55 5555441100 ±±88 88 00±±..55.. 00662288330088 33//00 ..335566 ector c 111246000
3500 et 100
3000 of d 80
2500 # 60
2000 40
1500 20
1000 0
500 505 510 515 520 525 530
500 Energy [keV]
0
0 200 400 600 800 1000 1200 1400
energy [keV] Figure24: EnergyDistributioncalculatedfromthepositionofthe
511keVpeak(from22Na)forallthechannelsafterthe
Figure21: Detectorlightoutputfordifferentgammaenergies,the energycorrectionhasbeenapplied. Thetailontheright
dataarefittedwithequation8. correspondtothechannelswithverylowLO .
511
9
Figure30showstheCTRasafunctionofbiasvoltage
el 200 EEnnttrriieess 44009966 fortwochannelsincoincidencewiththereferencemod-
n
n 180 MMeeaann 11228822 ule. DuetotheincreaseoftheSiPMPhotonDetection
a
h 160 RRMMSS 1166..5511 Efficiency (PDE) with the increase of excess bias, it is
c
or 140 expectedthatalsotheCTRimprovesaccordingly. How-
ect 120 ever, also the DCR increase with the excess bias, and
det 100 at a given point it becomes predominant. In order to
of 80 comparetheCTRofallthe256modules,anexcessbias
# 60
of 2.5 V is used for any channel, the temperature is
40
fixed at 19 oC and the NINO ASIC threshold is kept
20
fixedat55mV,whichisequivalenttothethresholdof
0
1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
0.5photoelectron. TheCTRdistributionforallthechan-
Energy [keV]
nels,measuredwithrespecttothereferencemodule,is
presentedinFigure31andshowsanaverageof239 ±
Figure25: EnergyDistributioncalculatedfromthepositionofthe
1277keVpeak(from22Na)forallthechannelsafterthe 10psFWHM.
energycorrectionhasbeenapplied.
nel 103 EEnnttrriieess 44009966
n MMeeaann 1133..2288
a
h RRMMSS 00..55338855
c
or 102
ct
e
et
d
of 10
#
1
Figure27: Thesetupusedforthetimingmeasurements. Thetwo
0 5 10 15 20 25
Energy resolution (@ 511 keV) [%] detector modules are placed facing each other, 10 cm
apart.A22Nasourceisplacedbetweenthetwomodules.
Figure26: Energyresolutionat511keVforallthechannels.
4.4 Coincidence Time Resolution
The coincidence time resolution of all the channels is
measuredusingthesetupshowninFigure27. Thetwo w[8] {w[5]>200000}
modulesarefacingeachotherat10cmdistance,while Counts330500 CCoonnssttTTaannttoo TT11 229988..33
a22Napointlikesourceisplacedbetweenthemonthe 250 MMeeaann 22..223388ee++0055
SSiiggmmaa 44779944
200
line connecting the module centers. One module has
150
beenselectedasthereferenceforthemeasurementof
100
theother256modules. 50
TheoutputsignaloftheSiPMisamplifiedanddiscrim- 0100 120 140 160 180 200 220 240 260×103
ToT (ps)
inatedbyNINOASIC[16]andsenttoahighprecision w[5] {w[8]>200000}
TDC(HPTDC-25psLSB)[17]. Thedataanalysisisper- Counts330500 CCoonnssttTTaannttoo TT22 229977..77
formedwithatotalnumberof4x106triggers. Asshown 250 MMeeaann 22..224433ee++0055
SSiiggmmaa 44662277
inFigure28,theTime-over-Threshold(ToT)spectraare 200
obtained for any two channels in coincidence; then 150
100
a gaussian fit is applied to the peaks corresponding
50
to the 511 keV gamma events. Hence the time differ- 0100 120 140 160 180 200 220 240 260×103
ToT (ps)
ence for the gamma events of the two channels are
histogrammedandshowninFigure29. AGaussianfit Figure28: TypicalTimeoverThresholdspectrafortwochannels
is applied and the FWHM is taken as the CTR of the incoincidence.
channelmeasured(227.3ps ± 3.2psinthiscase).
10