Development of a novel scintillation-trigger detector for the MTV experiment using aluminum-metallized film tapes

0.54 MB·

by  S. Tanaka

Development of a novel scintillation-trigger detector for the MTV experiment
using aluminum-metallized film tapes
S.Tanaka,S.Ozaki,Y.Sakamoto,R.Tanuma,T.YoshidaandJ.Murata
DepartmentofPhysics,RikkyoUniversity,3-34-1Nishi-ikebukuro,Tokyo171-8501,Japan
4
Abstract
1
0Anewtypeofatrigger-scintillationcounterarraydesignedfortheMTVexperimentatTRIUMF-ISAChasbeendeveloped,which
2usesaluminum-metallizedfilmtapeforwrappingtoachievetherequiredassemblingprecisionof±0.5mm. TheMTVexperiment

rusesacylindricaldriftchamber(CDC)asthemainelectron-trackingdetector. Thebarrel-typetriggercounterisplacedinsidethe
a
MCDCtogenerateatriggersignalusing1mmthick,300mmlongthinplasticscintillationcounters. Detectionefficiencyandlight
attenuationcomparedwithconventionalwrappingmaterialsarestudied.

3
Keywords: PlasticScintillationCounter,WrappingMaterial,Aluminum-MetallizedFilm
1

]
et1. Introduction MTV-CDC setup
d
s- The MTV experiment[1, 2] aimsto performthe finest pre- Beam Stopper Analyzer Foil
n
cisiontestoftimereversalsymmetryinnuclearbetadecayby
i
.means of searching non-zero T-violating transverse polariza-
s Stopping Counter (SC)
ctionoftheelectronsemittedfrompolarizedLi-8nuclei,which
CDC
iisproducedatTRIUMF-ISAC[3]. Thisquantitymayoriginate
s
yfromtheexistenceofaT-violatingtriplevectorcorrelationde-
hfined as R-correlationin beta decayrate function[4]. The ex-
p
istence of the R-correlation can be explored as the electron’s
[
non-zerotransversepolarization,whichisperpendiculartothe

2parentnuclearpolarizationdirection[5]. The electron’strans- Polaized Li-8 Beam
v
verse polarization is measured as a backward-scattering left-
1
right asymmetry from a thin (100 µm) lead analyzer foil us-
6
1ingtheknownanalyzingpoweroftheMottscattering[6]. The Trigger Counter (TC)
5polarized Li-8 beam with 80% horizontally polarized 28 keV
.beam at107 ppsis irradiatedona surfaceof 10µm thickalu-
1 Figure1: DetectorsetupoftheMTVexperiment: triggercounter(TC),stop-
0minumbeamstopperfoilforwhichaspinrelaxationtimeof2.3 ping counter (SC), and cylindrical drift chamber. The lead analyzer foil is
4s is achieved using a pair of permanentmagnets sandwiching placedaroundtheCDC.TheFRPbeamstoppertubeisnotshown.
1the stopperfoilplacedin thehorizontaldirectionto producea
:
vspin-holdingmagneticfieldofapproximately300G.Thestop-
iperisplacedinsidea1.5mmthickvacuumtubemadeoffiber polarizedSr(Y)-90radiationsource. Longitudinallypolarized
X
reinforcedplastics(FRP),designedtoreducethetotalmaterial electronsareemittedinnuclearbetadecay,becauseofthepar-
r
aamount and mean atomic number Z to suppress the multiple ityviolatingweakinteraction. Thislongitudinalpolarizationis
scatteringofthelow-energyelectronsemittedfromthebetade- transferredto transversepolarizationin Coulombscatteringat
cay. thefirstfoil,andthen,itstransversepolarizationismeasuredin
The electronsemitted from the Li-8 nucleifly out from the thesecondaryMottscatteringanalyzerfoil[7]. Themaximum
FRPstoppertube,andtheirpolarizationismeasuredusingthe beta energies of Li-8 and Y-90 are 13.1 MeV and 2.3 MeV,
MTV detectorarray. Inadditionto theLi-8 measurement,we respectively, therefore, the absorbing, multiple scattering, and
alsousedthisdetectorsetuptoperformacalibrationmeasure- back-scattering effects of these low-energy electrons must be
mentusinganartificiallyproducedtransverselypolarizedelec- considered.
tronsource,andalso,agravityexperimentnamedMTV-G[7], The MTV detector setup consists of the beam stopper, trig-
which probes large spin precession around the nuclei. In the gercounter(TC),leadanalyzerfoil,stoppingcounter(SC),and
MTV-Gandcalibrationmeasurements,theelectron’spolariza- cylindrical drift chamber (CDC), as shown in Figure 1. The
tionismeasuredinadoublescatteringexperiment,usinganun- backward-scatteringangulardistributionisthemeasurementof
PreprintsubmittedtoNuclearInstrumentsandMethodsA March14,2014
transverse polarization, therefore, the CDC [2] is set as the
trackingchambersurroundingthestopper,whichmeasuresthe
incidenttracksoriginatingfromthebeamstopperandthescat- photomultiplier
teredtracksstartingatthepointontheanalyzerfoilplacedout-
5 mm thick light guide
sidetheCDCasV-shaped(”V-track”)events.AtypicalV-track 1 mm thick plastic scintillation counter
eventisshowninFigure2.Thenelectron’stransversepolariza-
tionisobtainedfromtheleftandrightscatteringasymmetry.
Analyzer Foil
Mott Scattering
TC
CDC
Figure 3: Overview of the TC, which consists of 12 identical scintillation
counterbarscombinedinabarrelconfiguration.
positeconfiguration,utilizingparity-violatingemissionangular
SC
distribution.
2. TriggerCounter(TC)Design
The TC needs to satisfy the following requirements: 1) it
Figure2: AtypicaleventdisplayshowingaV-track. ThehitontheTC,scat-
must be as thin as possible to reduce the multiple scattering
teringvertexpositionontheanalyzerfoil,andhitsontheSCandCDCanode
of the low-energy electrons around 1 MeV; 2) it must be fast
wiresareshown.
enoughtogeneratetheLevel-1inputsignalcomparedwiththe
TheTC,whichconsistsof12segmented1mmthickplastic CDC’stimescaleof100ns;3)itmusthaveafinepositioningto
scintillationcounterbars(Figure3),isplacedinsidetheCDC, reducethesystematiceffectsontheR-correlationandMTV-G
surroundingtheFRPstoppervacuumtube.TheSC,whichcon- measurement.
sistsof12segmented70mmthickplasticscintillationcounter To meet these requirements, we built 1 mm thick, 300 mm
bars, is placed outside of the CDC, which is designed to stop long thin plastic scintillation counter bars and assembled 12
andmeasurethetotalkineticenergyofthescatteredelectrons. such bars to form the TC barrel. Each scintillation bar has 5
The lead analyzer foil is placed between the CDC and SC in mmthick,150mmlongacryliclightguidetotransportthelight
a cylindricalconfiguration,anditslocationisshowninFigure outputtothephotomultipliers(PMTs)placedoutsidetheCDC.
2. Therefore,an electron emittedfrom the beamstoppergoes The detailsof the componentsof the TCare listed in Table 1.
outintotheair,penetratesoneoftheTCandCDC,andisthen, ThephotographoftheTCisshowninFigure4. PMTreadout
backwardly scattered at the analyzer foil, from which it goes atoneendissufficientforthepresentexperiment,sincetiming
backtotheCDCagain. Finally,theelectronstopsatoneofthe resolutionbelowcoincidencewindowwidthisnotrequired. It
SCbarsafterpenetratingtheanalyzerfoilatapositiondifferent is because timing selection is required only for reducing the
fromthescatteredposition. Level-1triggerrate. Inaddition, we donotreaddrifttime in-
The event trigger signal consists of two levels of triggering formationfromtheCDC,therefore,theTCdosenotrequested
logics. TheLevel-1triggerisgeneratedwithcoincidentsignals toprovidestartingsignalwithgoodtimingresolutiontoachieve
betweenthesignalsofaTCandSCwiththetimewindowof25 goodpositionresolutionfromthedriftchamber,whichisdom-
ns,whiletheLevel-2triggerrequiresmultiplehitsintheCDC inated bymultiple scattering. However,the TC is designedto
anode wires after the Level-1 trigger. The 12 segmentations bepossibletosetPMTsonbothendsasanupgradeplan.
of the TC and SC enable a rough selection of the backward- BeforefixingtheTC’sdesign,inordertooptimizethethick-
scatteringeventintheLevel-1triggering. ness of the TC, we performed a Monte Carlo simulation to
The prime requirementsfor the TC are supplying the input evaluate the energy loss and multiple scattering effects from
signal for the Level-1 trigger and working as a hodoscopear- the scintillation counters (without wrapping materials) for the
ray to select the rough direction of emission. In addition, the electronsemitted from the Li-8 and Y-90 beta decays. In this
TCworksasabeamintensitymonitorbyprovidingacounting custom-madeMonteCarlocode,energylossandmultiplescat-
rate as well as an on-targetbeam polarimeter. The transverse teringeffectsareincluded. Sincetheenergyofelectronsemit-
beam polarizationcan be determinedby measuringthe count- ted from Sr-90 (maximumbeta energy= 0.55MeV) is small,
ingasymmetrybetweenanpairoftheTCcounterssetinanop- it is ignoredin this estimation. In Figures5 and 6, the results
2
material size(mm)
Scint. counter BC-408 T.1×W.50×L.300
Lightguide acrylic T.5×W.50×L.150
PMT HamamatsuH-7415 Dia.33×L.130
ent] (a) energy spectra for Y-90 ent] (b) energy spectra for Li-8
Table1:DescriptionoftheTCscintillationcounterbar,consistingofscintilla- v v
e e
tioncounter,lightguide,andphotomultiplier(PMT).(T.:thickness,W.:width, nt [ nt [
L.:length,andDia.:diameter) cou 1 mm initial energy cou102
102
Stopping Counter (SC)
10
2 mm
Trigger Counter (TC)
10
1
0 1 2 3 0 2 4 6 8 10 12 14
energy [MeV] energy [MeV]
CDC
Figure5: MonteCarlosimulationoftheenergyofoutgoingelectronsthrough
1mmand2mmthickscintillationcountersforY-90andLi-8betadecays.The
electron’sinitialenergyisalsoshownforcomparison.Samenumbersofevents
aregeneratedforeachcase.
Figure4:PhotographoftheTCinstalledinsidetheCDC.TheFRPbeamstop-
pertubeisnotshown.
oftheMonteCarlosimulationfortheenergyofoutgoingelec-
tronsafterenergylossandtheirscatteredangulardistributions
are shown, respectively. Only those electrons whose energy
waslargerthan0.5MeVwereconsidered.Thenumberofpen-
etratingelectronsthatwerenotstoppedorbackscatteredatthe
scintillation counterswas estimated. Thepenetratingratesare ent] (a) scattering angle for Y-90 ent] (b) scattering angle for Li-8
v v
listed inTable2. Notethatonly35%oftheincidentelectrons nt [e 1 mm nt [e103
can penetrate the 2 mm plastic for Y-90. The mean energy u u
o o
c c
lossandscatteringanglesofforwardscatteringelectronswere
102
calculated and shown in Table 2. In the Y-90 measurements,
2 mm 102 2 mm
wefoundslightdifferenceinthemultiplescatteringeffectsbe-
1 mm
tweenthe1mmand2mmthicksamples,however,alargedif-
ference is expected on the stopping rate. Therefore, the TC
10
shouldbeasthinaspossible(approximately1mm)tomaintain
10
a goodpenetratingrateforthe Y-90measurements. Consider-
ingthedetectionefficiencyandmachineability,atleast1mm
thicknessisrequired.Fromthisestimation,wedecidedtouse1 backscattered
1 events 1
mmscintillationcounters. IncaseofSr(Y)-90measurements,
wedonotusethe1.5mmthickFRPvacuumtubetoreducethe 0 20 40 60 80 100120140160180 0 20 40 60 80 100120140160180
materialamount.TheSr(Y)-90sourceissetinair. angle [deg.] angle [deg.]
A key concern on using such thin and long plastic scintil-
lation countersis theirexpectedlowefficiencydue tolightat-
Figure6:MonteCarlosimulationofthescatteringangulardistributionsofthe
tenuation and small initial light yield. On the other hand, the outgoingelectrons(angle<90deg.),andofthebackscatteredelectrons(angle
requirementoffinepositioninghasadrawbackwhenwetryto >90deg.)forY-90andLi-8betadecays.
rely on total reflection by producing an air-layer surrounding
thescintillationbar,whichisrequiredtomakelargerefractive
indexdifferenceatthesurface.
Therefore, we tried to use a new wrapping material made
3
Y-90 Li-8 age is negligible. Direct taping of the aluminum tape without
penetratingratefor1-mm 86% 99% Wrappy is not good because its opaqueglue with poor reflec-
penetratingratefor2-mm 35% 98% tion,whichishardtoremovefromtheplasticsurface.
meanscatteringanglefor1-mm 37deg. 12deg. Lightattenuationwasstudiedforthe1mmthickTC.Inthis
meanscatteringanglefor2-mm 38deg. 17deg. test study, the output of the TC was measured in a test appa-
meanenergylossfor1-mm 0.30MeV 0.19Mev ratus using an SC bar to produce trigger signals, as shown in
meanenergylossfor2-mm 0.38MeV 0.31MeV Figure8. TheSr(Y)-90radiationsource(3.7MBq)isattached
ontheTCwithacollimatoratfivedifferentpositions(positions
Table2: MonteCarloestimationoftheelectron’spenetratingrate,meanscat- AtoE,5cmspacing)tomeasurethetransmittinglengthdepen-
teringangleandmeanenergylossfor1mmand2mmthickplasticscintillation denceontheoutputpulseheightoftheTCsignal. Theoutput
countersforY-90andLi-8betadecays.
signalsofthePMTsfromtheTCandSCweremeasuredusing
aVME-QDC(CAENV792),withtheQDCgategeneratedby
theSCplacedbehindtheTC.Figure9showsthepulseheight
ofaluminum-metallizedtape,whichhasagoodmirror-likere- distributionsfromtheTCandSCpositionedatalongdistance
flecting surface on both sides of the tape. We use Wrappy (positionA)andshortdistance(positionE).Thedifferencebe-
(CEMEDINECo.,Ltd.,[8])asthewrappingtapebecauseofits tweenthemareconsideredastheresultofthelightattenuation
mirror-like glossy back side using excellent transparent adhe- intheTC.
sive,whichisalsoeasilyremovable. Wrappyisanaluminum-
metallizedpolyesterfilmtape,whichconsistsofa25µmthick
SC
polyester film andgoodtransparentacrylic adhesive. The To- PMT
tal thickness of the Wrappy tape is 55± 25µm. The Wrappy
tape was directly glued on the surface of the plastic scintilla-
tioncounters,asshowninFigure7,whereitiscomparedwith
TC
a conventional aluminized Mylar wrapping with black vinyl
Sr(Y)-90 source A B C D E PMT
sheet. The geometrical precision after using the Wrappy tape
position
inoverlapping-wrappingisabout±0.1mm,whichissufficient
forourapplication. 3cm5cm5cm5cm 5cm7cm
Figure8: Setupofthetestmeasurementforstudiesofthestudyofthesource
positiondependence.TheQDCdatafromtheTCandSCareshown.
Wrappy tape
4000
(a) far position (position A) (b) near position (position E)
3500
103
3000
2500
Al. Mylar + vinyl 102
2000
bare scintillation counter
1500
10
1000
Figure7: PhotographoftheTC,wrappedwiththeWrappytape,theconven-
500
tionalaluminizedMylarwithblackvinylsheet,andthebarescintillationbar.
0 1
0 1000 2000 3000 4000 0 1000 2000 3000 4000
3. Testmeasurement
Figure9: PMT’spulseheightdistributionsfortheTC(vertical)andSC(hori-
The main concern in using a direct taping on the surface zontal)at(a)farand(b)nearpositions.
of the scintillation counters without producing an air volume
aroundthesurface,istheexpectedreductionoflighttransmis- The light attenuation behaviour can be clearly observed in
sion to the PMTs. For our usage, we needed to rely not on one dimensional histograms shown in Figure 10, where the
the total reflection at the index boundary between plastic and pulse height distributions are shown for five different source
air,butonthereflectionatthetape’sevaporatedsurface. Since positions. As expected, a clear attenuation is observed, how-
thelight-shieldingabilityofWrappyisnotsufficientforourre- ever,theattenuationofthepulseheightitselfisnotanissuefor
quirementsevenwithmultipleoverlapping,a70µmthickalu- thepresentapplication,becausethe TCisnotrequiredto pro-
minumtapeisaddedasacoversothattheremaininglightleak- videenergyinformation,butisexpectedtogeneratethetrigger
4
signal. Therefore,itismoreimportanttoinvestigatethedetec- %] 100
tionefficiency.Figure11showsthepositiondependenceofthe y [ 90
c
measured efficiency, defined as (NTC×SC)/NSC after subtract- cien 80 source distance from light guide
ingbackgrounds. Theobtainedresultshowsa relativelysmall effi 70 E (7 cm)
efficiency reduction compared to the light attenuation shown
60 D (12 cm)
in Figure 10, indicatingthat the TC signal remainssignificant C (17 cm)
50 B (22 cm)
above the noise level. Althoughthe obtained efficiencyis not A (27 cm)
40
close to100%, itisa problemneitherfortheMTVnorMTV-
30
Gmeasurements,becausetheseexperimentsdonotmeasurean
20
absolute event rate, but measure the asymmetry during beam
polarization direction flipping. Therefore, in the double ratio 10
analysis,mostofthesystematiceffectsfromtheefficiencydif- 0
0 10 10 15 20 25
ferencearecancelled.However,timevariationoftheefficiency source position [cm]
causessystematic effectsonthe physicsmeasurements,which
should be carefully investigated. Also, anisotropic efficiency
distribution in azimuthaldirection should be minimizedto re- Figure11:DependenceofthesourcepositionondetectionefficiencyoftheTC.
ducethesystematicerror.
Itshouldberemarkedthat,althoughadirecttapingwasused
onthesurfaceofthescintillationcounterswithoutproducingan studiedonfivedifferentsourcepositionsatadistanceof2cm
air-layer, no severe efficiency drop was observed in this mea- from each other. It was found that Wrappy (a) was as good
surement. As a result, we concludedthatthe thin 1 mm thick as the conventional aluminized Mylar wrapping (b). More-
and300mmlongplasticscintillationcountersdirectlywrapped over, black vinyl sheet without aluminized Mylar (c) showed
withWrappyhavesufficientefficiencyforourapplication. almostthesamepropertiesasthatoftheconventionalmaterial
(b). Itsuggeststhatthetotalreflectionbetweenplasticandair
nt] 2500 isthedominantcontributiontothelighttransmission. Among
ve allofthem,thewhitepaintingontheplasticsurfaceshowsthe
e
nt [ 2000 source positions (distances from light guide) strongestattenuation.
u
o
c A (27 cm)
1500 B (C2 2( 1c7m c)m) ps] 8
c
1000 D (12E c (m7) cm) nt rate [k 67
u (a) Wrappy tape
500 co 5
(b) Al. Mylar
4
(c) air
0
0 200 400 600 800 1000 1200 1400 1600 1800 2000 3
TC output [QDC ch]
2
(d) white paint
1
Figure10:PMT’soutputchargedistributionsoftheTCatdifferentsourcepo-
0
sitionsfromAtoE.SincetheQDCgatesignalisgeneratedusingtheSC,this 0 2 4 6 8 10 12 14
signalrepresents∆EintheTC.
source distance from PMT [cm]
4. Discussion Figure 12: Wrapping material effect on the counting rate measured for a 1
mmthick,25mmwide,and140mmlongplasticscintillation counterfor(a)
The TC built for the MTV experiment proved to have suf- Wrappytapewithaluminumtape,(b)aluminizedMylarwithblackvinylsheet,
(c)air(blackvinylsheetwithoutaluminizedMylar),and(d)whitepaint.
ficient absolute detection efficiency. Independently, from the
applicationfortheMTVexperiment,wealsostudiedthedepen-
denceofthewrappingmaterialonthePMT’soutputin aded- TheresultsofcountingratesshowninFigure12dependon
icated measurement. A plastic scintillation counter (1 mm in thresholdsettings,therefore,QDCspectraarebettertobeused
thickness,25mminwidth,and140mminlength)wasusedto forthecomparison. Indeed,theattenuationdifferenceis more
studythiswrappingmaterialdependence.Wetested(a)Wrappy clearly seen in Figure 13. The QDC spectra of the materials
with a 70 µm covering aluminum tape, (b) conventional alu- containing (a) Wrappy and (b) aluminized Mylar at different
minizedMylarwithcoveringblackvinylsheet,(c)blackvinyl sourcepositionsfromthePMT areshown. AluminizedMylar
sheet, and(d)white acrylicpaint. Figure12showsthecount- showedalightattenuationstrongerthanofWrappy.Therefore,
ingratemeasuredinasetupsimilartothatshowninFigure8. we can conclude that the Wrappy taping has excellent perfor-
Inthismeasurement,thesourcepositiondependencehasbeen mancecomparedwiththeconventionalwrappingmaterials.
5
ent] 105 (a) Wrappy tape (b) Al. Mylar
v
e
nt [
cou 104
3
10
4 cm
102 8 cm6 cm
10 cm
12 cm
10
1
0 1000 2000 3000 4000 0 1000 2000 3000 4000
QDC [ch] QDC [ch]
Figure 13: Wrapping material effect onthelightattenuation. QDCdataare
shownforWrappyandaluminizedMylarcases,fordifferentsourcepositions
of4,6,8,10,and12cmfromthePMT.
5. Conclusion
In summary, we attempted the direct taping on the surface
ofaplasticscintillationcountertoimprovethepositioningpre-
cision for the MTV and MTV-G experiments. Although, as
expected, a slight attenuationwas observed, the efficiencydid
not drastically drop, which would have negatively influenced
theresultsoftheexperiments. Wefoundanew,easy,andreli-
able methodto wrap scintillationcounters, which canbe used
for many other experiments and applications. For example,
this new wrapping technique could be used for building fine
segmentedscintillationcounterarrays,orscintillationcounters
placedinsidevacuumchambers.
6. Acknowledgements
This work was supported by TRIUMF’s staffs under many
technical aspects. This work was supported by the JSPS
KAKENHIGrant-in-AidforScientificResearch(B)25287061,
ChallengingExploratoryResearch24654070,andRikkyoUni-
versity Special Fund for Research (for Graduate Students
2013).
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[8] Product name : CEMEDINE Wrappy, CEMEDINE Co., Ltd., Japan.
http://www.cemedine.co.jp/e/index.html Product Code (SDS No) :
GE0830-1.
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