Post on 21-Sep-2018
TheSudburyNeutrinoObservatory
TheSNOCollaboration
J.Boger,R.L.Hahn,J.K.Rowley
ChemistryDepartment,BrookhavenNationalLaboratory,Upton,NY11973-5000
USA1
A.L.Carter,B.Hollebone,D.Kessler
CarletonUniversity,Ottawa,OntarioK1S5B6CANADA2
I.Blevis,F.Dalnoki-Veress,A.DeKok,J.Farine,
3
D.R.Grant,
C.K.Hargrove,G.Laberge,I.Levine,K.McFarlane,H.Mes,
A.T.Noble,V.M.Novikov,M.O'Neill,M.Shatkay,
C.Shewchuk,D.Sinclair
CentreforResearchinParticlePhysics,HerzbergLaboratory,CarletonUniversity,
Ottawa,OntarioK1S5B6CANADA2
E.T.H.Cli�ord,R.Deal,E.D.Earle,E.Gaudette,G.Milton,
B.Sur
ChalkRiverLaboratories,AECLResearch,
ChalkRiver,OntarioK0J1J0CANADA2
J.Bigu,J.H.M.Cowan,D.L.Clu�,E.D.Hallman,R.U.Haq,
J.Hewett,J.G.Hykawy,G.Jonkmans,
4
R.Michaud,
A.Roberge,J.Roberts,E.Saettler,M.H.Schwendener,
H.Seifert,D.Sweezey,R.Ta�rout,C.J.Virtue
DepartmentofPhysicsandAstronomy,LaurentianUniversity,Sudbury,Ontario
P3E2C6CANADA2
D.N.Beck,Y.D.Chan,X.Chen,M.R.Dragowsky,
21
F.W.Dycus,J.Gonzalez,M.C.P.Isaac,
5
Y.Kajiyama,
G.W.Koehler,K.T.Lesko,M.C.Moebus,E.B.Norman,
C.E.Okada,A.W.P.Poon,P.Purgalis,A.Schuelke,
A.R.Smith,R.G.Stokstad,S.Turner,6I.Zlimen
7
LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720USA1
J.M.Anaya,T.J.Bowles,S.J.Brice,Ernst-IngoEsch,
M.M.Fowler,AzrielGoldschmidt,
5
A.Hime,A.F.McGirt,
G.G.Miller,W.A.Teasdale,J.B.Wilhelmy,J.M.Wouters
LosAlamosNationalLaboratory,LosAlamos,NM
87545USA1
J.D.Anglin,M.Bercovitch,W.F.Davidson,R.S.Storey
18
NationalResearchCouncilofCanada,Ottawa,OntarioK1A0R6CANADA2
S.Biller,R.A.Black,R.J.Boardman,M.G.Bowler,
J.Cameron,B.Cleveland,A.P.Ferraris,G.Doucas,H.Heron,
C.Howard,N.A.Jelley,A.B.Knox,M.Lay,W.Locke,J.Lyon,
S.Majerus,M.Moorhead,M.Omori,N.W.Tanner,
R.K.Taplin,M.Thorman,D.L.Wark,N.West,J.C.Barton,
P.T.Trent
NuclearandAstrophysicsLaboratory,OxfordUniversity,KebleRoad,Oxford,
OX13RH,UK8
R.Kouzes,
10
M.M.Lowry
9
DepartmentofPhysics,PrincetonUniversity,Princeton,NJ08544USA
A.L.Bell,E.Bonvin,1
1
M.Boulay,M.Dayon,F.Duncan,
L.S.Erhardt,H.C.Evans,G.T.Ewan,R.Ford,1
2
A.Hallin,
A.Hamer,P.M.Hart,P.J.Harvey,D.Haslip,C.A.W.Hearns,
R.Heaton,J.D.Hepburn,C.J.Jillings,E.P.Korpach,
H.W.Lee,J.R.Leslie,M.-Q.Liu,H.B.Mak,A.B.McDonald,
J.D.MacArthur,W.McLatchie,B.A.Mo�at,S.Noel,
T.J.Radcli�e,
13
B.C.Robertson,P.Skensved,R.L.Stevenson,
X.Zhu
DepartmentofPhysics,Queen'sUniversity,Kingston,OntarioK7L3N6
CANADA2
S.Gil,1
4
J.Heise,R.L.Helmer,1
5
R.J.Komar,C.W.Nally,
H.S.Ng,C.E.Waltham
DepartmentofPhysicsandAstronomy,UniversityofBritishColumbia,
Vancouver,BCV6T2A6CANADA2
2
R.C.Allen,1
6
G.B�uhler,
17
H.H.Chen
18
DepartmentofPhysics,UniversityofCalifornia,Irvine,CA92717USA
G.Aardsma,T.Andersen,1
9
K.Cameron,M.C.Chon,
R.H.Hanson,P.Jagam,J.Karn,J.Law,R.W.Ollerhead,
J.J.Simpson,N.Tagg,J.-X.Wang
PhysicsDepartment,UniversityofGuelph,Guelph,OntarioN1G2W1CANADA2
C.Alexander,E.W.Beier,J.C.Cook,D.F.Cowen,E.D.Frank,
W.Frati,P.T.Keener,J.R.Klein,G.Mayers,D.S.McDonald,
M.S.Neubauer,F.M.Newcomer,R.J.Pearce,
R.G.VandeWater,R.VanBerg,P.Wittich
DepartmentofPhysicsandAstronomy,UniversityofPennsylvania,Philadelphia,
PA19104-6396,USA1
Q.R.Ahmad,J.M.Beck,2
0
M.C.Browne,
21
T.H.Burritt,
P.J.Doe,C.A.Duba,S.R.Elliott,J.E.Franklin,
J.V.Germani,
22
P.Green,1
5
A.A.Hamian,K.M.Heeger,
M.Howe,R.MeijerDrees,A.Myers,R.G.H.Robertson,
M.W.E.Smith,T.D.Steiger,T.VanWechel,J.F.Wilkerson
NuclearPhysicsLaboratoryandDepartmentofPhysics,Universityof
Washington,P.O.Box351560,Seattle,WA98195USA
1
TheSudbury
Neutrino
Observatory
isa
second
generation
water
� Cerenkovdetectordesignedtodeterminewhetherthecurrentlyobserved
solarneutrinode�citisaresultofneutrinooscillations.Thedetectoris
uniqueinitsuseofD2Oasadetectionmedium,permittingittomake
asolarmodel{independenttestoftheneutrinooscillationhypothesisby
comparisonofthecharged-andneutral-currentinteractionrates.Inthis
paperthephysicalproperties,construction,andpreliminaryoperation
oftheSudburyNeutrinoObservatoryaredescribed.Dataandpredicted
operatingparametersareprovidedwheneverpossible.
Keywords:Solarneutrinos,Cherenkovdetector,heavywater,He3-proportional
counters,waterpuri�cation
PACSNumbers:29.40.Ka,29.40.Cs,26.65,14.60.Pq
3
1
Introduction
TheSudburyNeutrinoObservatory(SNO)hasbeenconstructedtostudythe
fundamentalpropertiesofneutrinos,inparticularthemassandmixingpa-
rameters.SNO
canaccomplishthisbymeasuringthe uxofelectron-type
neutrinos,�e,whichareproducedinthesun,andcomparingittothe uxof
allactive avorsofsolarneutrinosdetectedonearthinanappropriateenergy
interval.Observationofneutrino avortransformationthroughthiscompari-
sonwouldbecompellingevidenceofneutrinomass.Non-zeroneutrinomass
isevidenceforphysicsbeyondtheStandardModeloffundamentalparticle
interactions[1].
Thelongdistancetothesunmakesthesearchforneutrinomasssensitive
tomuchsmallermasssplittingsthancanbestudiedwithterrestrialsources.
Furthermore,thematterdensityinthesunissu�cientlylargetoenhancethe
e�ectsofsmallmixingbetweenelectronneutrinosandmuortauneutrinos.
1
SupportedbytheUSDepartmentofEnergy.
2
SupportedbyNaturalSciencesandEngineeringResearchCouncilofCanada.
3
SupportedbytheSwissNationalScienceFoundationin1997.
4
Presentaddress:InstitutdePhysique,Universit�edeNeuch^atel,Neuch^atelCH-
2000,Switzerland.
5
Presentaddress:PhysicsDepartment,UniversityofCalifornia,Berkeley,CA
94720,USA.
6
PresentAddress:Penngrove,CA,USA.
7
Presentaddress:Berkeley,CA,USA.
8
SupportedbyParticlePhysicsandAstronomyResearchCounciloftheUK.
9
Presentaddress:PhysicsDepartment,BrookhavenNationalLaboratory,Upton,
NY11973USA.
10Presentaddress:PhysicsDepartment,WestVirginiaUniversity,MorgantownWV
26505USA.
11Presentaddress:Membratec,Technop^ole,CH-3960Sierre,Switzerland.
12Presentaddress:DepartmentofPhysics,PrincetonUniversity,Princeton,NJ
08544USA.
13Presentaddress:ESG
Canada,1HyperionCt.,Kingston,Ontario,K7K
7G3
Canada.
14Presentaddress:CNEA,Av.delLibertador8250,1429BuenosAires,Argentina.
15Permanentaddress:TRIUMF,Vancouver,BritishColumbia,V6T2A3Canada.
16Presentaddress:HewlettPackardLabs,MailStop4A-D,1501PageMillRoad,
PaloAlto,CA94304,USA.
17Presentaddress:AT&T,Inc.,Middletown,NJ07748,USA.
18Deceased.
19Presentaddress:SiennaSoftwareIncorporated,Toronto,Ontario,CA
20Presentaddress:VoltServicesGroup,Seattle,WA98101,USA.
21Presentaddress:LosAlamosNationalLaboratory,LosAlamosNM
87545,USA.
22Presentaddress:WRQInc.,Seattle,WA98109,USA.
4
Thismatter,orMSW
[2],e�ectalsoapplieswhensolarneutrinostraverse
theearth,andmaycausewell{de�nedtimeandspectralmodulationsofthe
signalof�e
intheSNOdetector.Measurementofthesee�ects,madepossible
bythehighcountingrateandseparable�e{speci�cand avor{independent
responsesoftheSNOdetector,willpermitthedeterminationofuniquemass
andmixingparameterswhencombinedwiththeexistingresultsofthelight-
water� Cerenkovdetectors,
37Cl,and
71Gasolarneutrinoexperiments[3{5].
TheSNOexperimentisuniqueinthatitutilizesheavywater,D2O,inaspher-
icalvolumeofonekilotonneasatarget.Theheavywaterpermitsdetection
ofneutrinosthroughthereactions
�x
+e�!
�x
+e�
(1)
�e
+d!
e�+p+p
(2)
�x
+d!
�x
+n+p
(3)
where�x
referstoanyactive avorofneutrino.Eachoftheseinteractionsis
detectedwhenoneormoreelectronsproduce� Cerenkovlightthatimpinges
onaphototubearray.Theelasticscattering(ES)ofelectronsbyneutrinos
(Eq.1)ishighlydirectional,andestablishesthesunasthesourceofthede-
tectedneutrinos.Thecharged-current(CC)absorptionof�e
ondeuterons
(Eq.2)producesanelectronwithanenergyhighlycorrelatedwiththatofthe
neutrino.Thisreactionissensitivetotheenergyspectrum
of�e
andhence
todeviationsfromtheparentspectrum.Theneutral-current(NC)disintegra-
tionofthedeuteronbyneutrinos(Eq.3)isindependentofneutrino avorand
hasathresholdof2.2MeV.Tobedetected,theresultingneutronmustbe
absorbed,givinga6:25MeVphotonforabsorptionondeuteriumorphotons
totalling8.6MeVforabsorptionon
35ClwithMgCl 2addedtotheD2O.The
photonsubsequentlyComptonscatters,impartingenoughenergytoelectrons
tocreate� Cerenkovlight.(Oncethespecial{purposeneutral-currentdetectors
havebeeninstalled,theywillprovidetheprimaryneutrondetectionmecha-
nism.SeeSection8fordetails.)MeasurementoftherateoftheNCreaction
determinesthetotal uxof
8Bneutrinos,eveniftheir avorhasbeentrans-
formedtoanotheractive avor(butnotiftoasterileneutrino).Theability
tomeasuretheCCandNCreactionsseparatelyisuniquetoSNOandmakes
theinterpretationoftheresultsoftheexperimentindependentoftheoretical
astrophysicscalculations.
Allexperimentsperformedtodatehavedetectedfewersolarneutrinosthan
areexpectedfromstandardsolarmodels[6].Therearestronghintsthatthis
istheresultofneutrino avortransformationbetweentheproductionpoint
inthesunandtheterrestrialdetectionpoint.However,theseconclusionsare
basedonreferencetoacalculatedprediction.Throughdirectmeasurements
ofthe�e{speci�cCCreactionandthe avor{independentNCreaction,the
5
SNOdetectorwillbethe�rstexperimenttomakeasolar{model{independent
measurementofthesolarneutrino ux.
SNO
canmakecontributions,someofwhichareunique,inotherareasof
physics.Anexampleofthelatterisasearchfortherelicsupernovaneutrinos
integratedoverallpastsupernovae.Fortherelicsupernovaneutrinos,the
interactionof�e
onprotonsinthedeuteronsproducesa� Cerenkovsignalplus
twoneutrons,whichmakesacleansignature.OthertopicsthatSNOwillstudy
includethe avorcompositionoftheatmosphericneutrino ux,searchesfor
certaintypesofdarkmatter,andnucleondecay.Theobservatoryisdesigned
tohandlethehighdataratesinducedbyanintenseburstofneutrinosfroma
supernovaexplosionascloseasonekiloparsecaway.
AnoverviewoftheSNOlaboratoryanddetectorisgiveninFig.1.Thedetector
consistsofatransparentacrylicsphere12m
indiameter,supportedfrom
a
deckstructurebytenropeloopsmadeofsynthetic�ber,asshowninFig.2.
Thesphereholds1000tonnesofheavywater.Surroundingtheacrylicvessel
isageodesicstructure17.8mindiametermadeofstainless-steelstrutsand
carrying9438inward-lookingphotomultipliertubes.Thebarrel-shapedcavity,
22m
indiameterand34m
inheight,is�lledwithpuri�edlightwaterto
providesupportandshielding.
sca
leap
prox
imat
e
chan
ge a
rea
2m
from
min
e
labo
rato
ryen
tran
ce
cont
aine
rw
ash
area
proc
essi
ng
proc
essi
ngH
O
2
2
D O
stor
age
tank
s
dete
ctor
cav
ityne
ckro
omco
ntro
l
deck
clea
nro
omra
cks
elec
tron
ics
phot
otub
esph
ere
vess
elac
rylic
clea
n ch
ange
are
a
show
ers
pers
onne
lro
om
Fig.1.GeneralLayoutoftheSNOLaboratory.
TheSNOdetectorislocatedat46�
2803000
N,81�
1200400
W
intheINCO,Ltd.,
6
D O 2
Fig.2.ThePMTsupportstructure(PSUP)showninsidetheSNO
cavity,sur-
roundingtheacrylicvessel,withlightwaterandheavywatervolumeslocatedas
indicated.
CreightonminenearSudbury,Ontario,Canada.Thecenterofthedetector
vesselis2039m
belowthesurfaceinacavityexcavatedfrom
the\6800-
foot"levelofthemine.Thesurfaceis309m
abovesealevel.Thegranitic
rockoverburden(mainlynorite)correspondstoapproximately6000mwater
equivalent.Atthisdepth,onlyabout70muonspassthroughthedetector
(insidethephotomultiplierarray)perday.
Thesectionsbelowdetailthedesigncriteriaofmanyoftheimportantaspects
ofthedetector.TheextensiveD2OandH2Opuri�cationsystemsaredescribed
inSection2andpurityspeci�cationsaregiventhere.SinceSNOwilladdhigh
7
purityMgCl 2totheD2OtoenhancetheNCdetection,adescriptionofthe
systemrequiredtohandletheheavywaterbrineisalsogiven.Thetechnically
challengingconstructionofthelargeacrylicvessel(\AV")whichholdstheD2O
isdescribedinSection3.ThelargearrayofPMTsandtheirbasicoperating
parametersaredescribedinSection4.Thematerialsandconstructionofthe
PMTsupportstructurearedescribedinSection5.
AdescriptionoftheelectronicreadoutchainisgiveninSection6,followed
byadescriptionofthedownstreamdataacquisitionhardwareandsoftwarein
Section7.Separate
3Heneutral-currentdetectors,whichwillbeinstalledata
laterdateintheD2Ovolume,aredescribedinSection8.Thedetectorcon-
trolandmonitoringsystemisdescribedinSection9.Thecalibrationsystem,
consistingofavarietyofsources,amanipulator,controllingsoftware,andthe
analysisofcalibrationdata,isdescribedinSection10.Theextensivee�ort
tomaintainandmonitorsitecleanlinessduringandafterconstructionisde-
scribedinSection11.Thelargesoftwarepackageusedtogeneratesimulated
dataandanalyzeacquireddataisdescribedinSection12.Briefdescriptions
ofthepresentdetectorstatusandfutureplansareinSection13.
2
WaterSystems
TheSNOwatersystemiscomprisedoftwoseparatesystems:onefortheul-
trapurelightwater(H2O)andonefortheheavywater(D2O).Thesesystems
arelocatedundergroundnearthedetector.ThesourceofwaterfortheH2O
systemisasurfacepuri�cationplantthatproducespotablewaterforthemine.
Underground,thewaterispretreated,puri�edanddegassedtolevelsaccept-
ablefortheSNOdetector,regassedwithpureN2,and�nallycooledbeforeit
isputintothedetector.Ultrapurewaterleachesoutsolublecomponentswhen
incontactwithsolidsurfaces.Itmayalsosupportbiologicalactivityonsuch
surfacesoronsuspendedparticles.TheH2Oisthereforedeoxygenatedand
continuouslycirculatedtoremoveions,organicsandsuspendedsolids.Both
liquidsarealsoassayedcontinuouslytomonitorradioactivecontaminants.
Incomingpotablewatercontainssandandsiltparticles,bacteria,algae,inor-
ganicsalts,organicmoleculesandgasses(N2,O2,CO2,Rn,etc.).Afterfalling
atotalof6800feet,thewaterissupersaturatedwithair,so�rstitentersa
deaeratortank(see�g.3)whereitspendsafewminutessothatsomeofthe
dissolvedO2
andN2
comesout.Itthenpassesintoamultimedia�ltercon-
sistingofabedofsandandcharcoaltoremovelargeparticlesfollowedbya
10-micron�ltertoremove�neparticles.Thewaterthenentersthelaboratory
waterutilityroom.Acharcoal�lterisusedtoreducethelevelsoforganic
contaminantsandtoconvertfreechlorineintochloridesincechlorinewould
damagethereverseosmosis(RO)unitfurtherdownstream.
8
2
µµµ
254
nmU
V
10 C
Wat
er
To
Cav
ity
MnO
col
umn
Deg
asse
rC
harc
oal
Zeo
lite
Sof
tene
rs10 F
ilter
sM
ultim
edia
Filt
er
Chi
ller
Filt
ers
Dea
erat
or
10 to
nne
leve
l con
trol
tank
Fro
mM
onito
rP
oint
sF
rom
Cav
ity
Rev
erse
Osm
osis
Mon
itor
Filt
ers
3
INC
O
the
min
eT
anks
in
ED
TA
Pro
cess
Deg
asse
r
UV
185
nmIo
n
Col
umns
Exc
hang
eN
R
e-F
ilter
s0.
12
gass
er
Fig.3.Thelightwatersystem.
Afterthecharcoal�lter,thewaterpassesintosoftenersconsistingoftwo
0.14m
3
bottlescontainingstrong-basePuroliteC100-Ecationexchangeresin.
HeredivalentionssuchasCaandMgareexchangedforNaions.Thesofteners
alsoremoveironandstabilizecolloidalparticlessotheydonotcoagulatewhen
concentratedbytheROmembranes.
A9.1%solutionofsodiumethylenediaminetetraacetate(EDTA)andsodium
bisulphateisinjectedat9ml/min.tocomplexvariousionspecies(e.g.,Al)
andtoreduceOandClintoaformthatcanberejectedbytheRO.Thentwo
�lterunits,eachcontainingtwelve25-cmlong,3-�m�lters,removesuspended
particles.Asiltdensityindextestisdoneatthispointontherunningsystem.
Thereverse-osmosisprocessistheworkhorseofthepuri�cationsystem.Twelve
spiral-woundthin�lmcomposite(polyamideonpolysulfone)membraneseach
5.6m
2
inareareduceinorganicsaltlevelsbyafactorofatleast20andreduce
organicsandhencetheEDTAandparticleslargerthanmolecularweight200
withgreaterthan99%e�ciency.TheROperformanceismonitoredonlineby
percentagerejection(typically97.5%)andconductivitymonitors.Afterthe
detectorisfulltheROdoesnothavetobeusedagainunlessSNOrequires
substantialamountsofmake-upwaterinthedetector.
AftertheRO,thewaterentersa185-nmUVunitconsistingofmercurylamps
andquartzsleeveswhereanyremainingorganiccompoundsarebrokenapart
intoionicform.Thewaternextgoestoanion-exchangeunitthatremoves
remainingdissolvedionizedimpuritiesleftbytheRO.Thesearetwosetsof
sixbottlesinparallelcontaining0.1m
3
ofPurolitenucleargradeNWR-37
mixed(cationandanion)bedresins.Theexitingwaterhasaresistivityof
18.2M-cm.
9
Acustom{designedProcessDegasser(PD)isusedtoreducetheO2
andRn
levelsbyfactorsofabout1000and50,respectively,inthewater[7].ThePD
consistsofalargeelectropolishedstainlesssteelvessel(81cmdiameterby6m
high)containingshowerheads,sphericalpolypropylenepackingandheaterel-
ementsandpumpedwithamechanicalboosterpump(EdwardsEH500A)
backedbyafour-stagepositivedisplacementrotarypump(EdwardsQDP80
Drystar).Vacuum
ismaintainedat20torrandwatervapour owrateat
onekg/hr.ThePDremovesallgasesfrom
thewaterandcancause,bydif-
fusion,lowpressuresinsidetheunderwaterPMTconnectors.Lowpressure
compromisesthebreakdownvoltageoftheconnectors,andinasuccessfulef-
forttoreducebreakdownsthatoccurredwithdegassedwater,thewaterwas
regassedwithpurenitrogentoatmosphericpressureatthe2000-mdepthus-
ingagaspermeablemembraneunit.Thisunitisfollowedby0.1-�m�ltersto
removeparticulates.Thena254-nmUVunitisusedtokillbacteria.Finallya
chillercoolsthewaterto10�
Cbeforewaterisputintothedetectoratarate
of150l/min.
ThewatermassbetweentheAVandthePMTsisabout1700tonnesandthe
massbetweenthePMTsandthecavitywallsis5700tonnes.Waterentersthe
detectorbetweentheAVandPMTs.Becausethisregionhastobecleaner
thantheregionoutside,a99.99%leak-tightplasticbarriersealsthebackof
thePMTs.Waterintheouterregionisdirtierduetoitslargecontentof
submersedmaterial(cables,steelsupport,cavityliner,etc.).Waterisdrawn
fromthisregionbacktotheutilityroomandintoarecirculationloop.This
loopconsistsofaten-tonnepolypropylenetank(usedbythecontrolloop
tomaintainconstantwaterlevelinthedetector),the�rstUVunit,theion
exchangecolumns,theprocessdegasser,theN2
regassingunit,the0.1-�m
�lters,thesecondUVunitandthechiller.Therecirculatedlightwateris
assayedregularlyforpH,conductivity,turbidity,anions,cations,suspended
solids,dissolvedgases,andradioactivity.Thisisaccomplishedbymeansofsix
samplepipesintheH2Ovolume.
Theheavywatersystemisdesignedtoperformthefollowingfunctions:
{ReceivetheD2Oandmakeaninitialpuri�cationtoreducetheamountof
contaminationreachingthemainsystem;
{PurifytheD2OwithorwithouttheMgCl 2additive;
{AssaytheD2OorD2Obrinetomakeanaccuratebackgrounddetermina-
tion;
{ManagetheadditionandremovaloftheMgCl 2additiveonatimescale
shortrelativetotheexpectedrunningtimes;
AdiagramofthesystemisshowninFig.4.
ThesourceofD2OwastheOntarioHydroBruceheavywaterplantbesideLake
10
From
Sam
ple
Bot
tle
Ass
ay
Poin
tsSa
mpl
e
Con
trol
tank
Lev
el2
Ton
ne
60 T
tank
Low
Sal
t Out
UFR
D2OFR
RO
F
RO
F
FR
RO
F
RO
F
AC
FR
FR
Hea
tE
xcha
nger
MD
GA
CU
FR
AC
Salt
Tan
k60
T
UFR
RO
F
FR
Nor
mal
Mod
e L
ines
Des
alin
atio
n M
ode
Lin
es a
dded
AC
Abs
orpt
ion
Col
umns
(M
nOx
bead
s)FR
0.1
mic
ron
filte
rsM
onito
r D
egas
ser
Proc
ess
Deg
asse
rM
DG
PDG
SUF
Seed
ed U
ltraf
iltra
tion
unit
UFR
Ultr
afilt
ratio
n un
it
RO
FR
ever
se O
smos
is F
ilter
SUF
UFR
PDG
Ass
ay
Fig.4.Theheavywatersystem.
Huron.TheD2OwastruckedtotheSNOsiteandtransportedunderground.
Itwascleanedandstoredtemporarilybeforeitwasputintotheacrylicvessel
(AV).Heavywaterdeliveredtothelabwas�rstpassedthroughion-exchange
columnstoreduceitsioniccontent,inparticularitsK
content.TheD2O
thenwentintoalargepolypropylene-linedtank.Duringthe�llingoftheH2O
andD2O,theD2O
was�lledatsucharateastomaintainzeropressure
di�erentialacrossthebottomoftheAV,whichassuredthatthevesselsurface
wasgenerallyincompression.
WiththeSNOdetector�lled,theD2Oisrecirculatedtomaintainitspurity.
ThisisaccomplishedbyanRO
system
consistingof�veseparatepressure
housingunitsandthemembranescontainedwithin.Twolarge(22cmdiam-
eterand5.5m
length)unitsareusedinparallelforthepuri�cationofthe
recirculatingheavywater.Theconcentratestreamcontainingtheradioactive
ionspassesthroughadsorptioncolumnstodecreasetheTh,RaandPbcon-
centrations.
AllthewaterreturningtotheAVpassesthroughanultra�ltrationunit.This
isparticularlyimportantfollowingMnOx
andseededultra�ltrationassays(see
below)toinsurethat�nesfromtheseabsorbersarenotcarriedintotheAV.
TheD2Oisassayedregularlyfordensity,pH,conductivity,turbidity,anions,
cations,totalorganiccarbonandsuspendedsolids.Table1givestheisotopic
compositionoftheheavywater.
11
Table1
IsotopiccompositionofSNOheavywater.
Isotope
Abundance
Isotope
Abundance
2H
99.917(5)%
17O
0.17(2)%
3H
0.097(10)�Ci/kg
18O
0.71(7)%
1H
Balance
16O
Balance
TheD2O
andH2O
havetobeisolatedfrom
thelaboratoryairtoprevent
radon(ataconcentrationof3pCi/l)from
gettingin,eveninthepresence
oflargepressuretransientsthatoccurduringmineoperations.Acovergas
systemprovidesnitrogengasthatfunctionsasaphysicalbarrierbetweenthe
waterandtheradon-richlaboratoryair.Thenitrogencomesfrom
theboil-
o�ofliquidnitrogenstoredina1000-literdewar.Theboilo�gashasbeen
measuredtocontainlessthan10�
5
pCiradonperliterofnitrogengas.
Radoninthelabcanalsoenterthewatersystem
inotherways.Therefore,
whena�lterischangedontheD2Osystem,radon-freeD2Oalreadyinthe
D2Osystemwillbeusedto ushthe�ltertogetridoftrappedlaboratoryair
beforebringingthe�lterbackonline.IntheH2Osystem,H2Olossesfromthe
processdegasser,�lterchangesandsmallleaks(ifany)intheUryloncavity
linerismadeupwithradon-freewaterthathasbeen\aged"inastoragetank.
Theradioactivityrequirementsaresetprimarilybythedesigngoalspecifying
thatthecontributiontotheneutron-productionratethroughphotodisinte-
grationofthedeuteron,withaninteractionthresholdof2.2MeV,shouldnot
exceedone-tenththatofthemodelsolar-neutrinorate(5000y�
1).Thisleads
tocombinedlimitsof3�
10�
15g/gofThand4:5�
10�
14g/gUintheD2O.
SNO'sassaysystemsareexpectedtohaveanuncertaintyofnotmorethan
20%intheirmeasurementattheseimpuritylevels,althoughplatingmaygive
risetoasystematicuncertainty.Therearesixsamplepipesinvariouslocations
insidetheAVthatallowD2Osamplestobewithdrawnforradioassay.
ForTh,RaandPbassaytheseededultra�ltration[8]technique(SUF)willbe
used.TheSUFunitcontainsfourlargemicro�ltersthinlycoatedwithDTiO.
Onehundredtonnesofwateratarateof150l/minwillbepassedthrough
them.Thenthe�ltersareremovedandtheTh,Ra,andPbstrippedo�and
counted.AcrylicbeadscoatedwithMnOx
havebeenshowntobeanexcellent
adsorberforRawithanextractione�ciencyofabout90%[9].Waterispassed
at20l/min.througha1-lcolumncontainingtheMnOx
whichisremovedo�-
lineforcountingoftheRadaughtersusinganelectrostaticdeviceinwhich
chargedionsaredepositedonthesurfaceofasilicondetector[10].
Thenumberof
222Rn(3.8-dhalf-life)ismeasuredinsixtonnesofliquidtaken
fromoneofsixregionsintheH2OoroneofsixregionsintheD2O.Thesix
12
tonnesofwaterareputthroughasmallvacuum
degasser[11]at20l/min.
ThegasseswhichareremovedfromthewaterareprimarilyN2,O2,Ar,CO2
andafewhundredatomsof
222Rn,aswellas10ml/minintheformofwater
vapour.RnissubsequentlyfrozenoutinaU-shapedtrapcooledwithliquid
nitrogen(-192C)andtransferredtoaZnScoatedscintillationcell[12]for
counting.
Toenhancetheneutral-currentdetection,MgCl 2willbeaddedtothewater
totakeadvantageofthelargerneutron-capturecrosssectionofClrelativeto
deuterium.Aconcentrationofapproximately0.2%willbeused.AD2Obrine
solutionthathasbeenpuri�edpriortotransportundergroundisputintoa
largepolypropylenelinedtank.WhenitistimetoaddtheMgCl 2totheAV,
thistankwillslowlybeemptied.Atthesametime,anequivalentvolumeof
salt-freeD2OwillbetakenoutoftheAVandputintoasecondpolypropylene
linedtank.
TotaketheMgCl 2outoftheAV,thethirdROunitwillbeusedincombination
withthetwomainonestodesalinatethewatertoabout100partspermillion
(ppm).Inaseconddesalinationpass,afourth,smallerROunitwillbeadded
totheprocesstodesalinatetheD2Otoaboutoneppm.
3
AcrylicVessel
TheD2Ocontainmentvesselmustmeetdiverserequirements,someofwhich
presentopposingdesignconstraints.Theprimarydesigncriteriaforthecon-
tainmentvesselare:
{Isolate1000tonnesofD2OfromsurroundingH2O.
{Maintainstructuralintegrityandperformanceovertenyearswhileim-
mersedinultrapureD2O
andH2O
andsubjectedtotheseismicactivity
expectedinanoperatingmine.
{Minimizethetotalmassofradioactiveimpurities.
{Maximizeopticalperformance.
{Designforconstructioninthemine.
Thedesigncriterialistedaboveresultedinthecontainmentvesselshownin
Fig.5.
A12meterdiameterspherewaschosenastheoptimumshapeforthecontain-
mentvessel.Aspherehasthelargestvolume-to-surfaceratio,andoptimum
stressdistribution.Thisreducesthemassofacrylicneeded,andhencethe
amountofradioactivity.Thesphereissuspendedfromtenloopsofrope,which
areattachedtothevesselbymeansofropegrooveslocatedaroundtheequa-
13
CH
IMN
EY
SU
SP
EN
SIO
NR
OP
ES
AC
RY
LIC
PA
NE
LS
BO
ND
LIN
ES
Fig.5.TheSNOacryliccontainmentvessel.
torofthesphere.Thechoiceofropesuspensionwasdrivenbyradioactivity
andopticalconsiderations:aropeundertensiono�ersexcellentloadbearing
capacityforaminimummass,anditpresentstheminimumobstructionofthe
14
� Cerenkovlightthatneutrinointeractionsproduce.
Toallowtheinsertionofcalibrationdevicesandtheinstallationofthe
3He
neutrondetectorstrings,thesphereisprovidedwitha1.5-mdiameterby6.8-
mtallchimney.Pipingalsoentersandexitsthevesselthroughthischimney.
For�llingandpuri�cationrecirculationrequirements7.6-cm
diameterpipes
introduceD2Oatthebottomofthevesselandremoveitfromthetopofthe
chimney.D2Omaybeextractedvia3.8-cmdiameterpipesfromfourdi�erent
levelsinthevesselandoneinthechimneyformeasurementsofbothchemical
andradioactivewaterquality.
A
totalof122ultraviolettransmitting(UVT)acrylicpanelswereusedin
theconstructionofthesphericalpartofthecontainmentvessel.Thesepanels
werenominally5.6cmthick,withtheexceptionofthetenequatorialpanels
containinggroovesforthesuspensionropes.Theseropegroovepanelswere
nominaly11.4cmthick.Acrylicsheetwaschosenastheconstructionmaterial
foranumberofreasons.Asimplehydrocarbon,UVTacryliccanbemanu-
facturedwithverylowintrinsicradioactivity.ThelighttransmissionofUVT
acrylicmatchesreasonablythespectralresponseofthePMTs.Castacrylic
sheetiscommerciallyavailableinsizesacceptableforminetransportation.It
isreadilythermoformedintosphericalsegmentsandiseasilymachined.Itis
capableofbeingbondedtogetherwithbondstrengthsclosetothatofthe
parentmaterial.
The atcastacrylicpanelswere�rstthermoformedintosphericalsections
byslumpformingintoafemalemoldformedfrom
polishedaluminumplate
witharadiusof6.06m
(theouterradiusofthesphere).Thepanelswere
thenmachinedtothecorrectshapeona�ve-axismillingmachinewhileata
constanttemperatureof21
�
C.Toavoidcontamination,onlycleanwaterwas
usedasalubricantduringmachiningoperations.
A�nalcheckofthedimensionswasmadeby\dryassembling"thepanels
onaspecialframeworktoform
�rsttheupperhemisphere,thenthelower
hemisphere.Thisensuredthatallthepanelscouldbe�ttedtogetherwithin
thespeci�edtoleranceof�
25mmonthesphericalcurvaturerequiredbythe
bucklingcriteria.Duringconstructionthecurvaturewastypicallymaintained
to�
6mm.
Thepanelswerebondedtogetherusingapartiallypolymerizedadhesivethat
curesatroomtemperature.ThisadhesivewasformulatedbyReynoldsPoly-
merTechnologyInc.,thefabricatorsoftheacrylicvessel.Thebondsareap-
proximately3mm
thickandspecialallowancehastobemadeforthe20%
shrinkageduringpolymerizationoftheadhesive.IttookconsiderableR&D
andconstructiontimetoobtainover500m
ofadequatequalitybonds.All
bondstrengthsexceeded27.5MPa.Priortobondingthemainvesselthebond-
15
Table2
Principalfeaturesofthecontainmentvessel(allnumbersareat20�
C).
Capacityofsphere
997.91tonnesofD2O
Vesselinternaldiameter
12.01m
Nominalwallthickness
5.5cm
Massofsphere
30.0tonnes
Capacityofchimney(normaloperation)
8.61tonnesofD2O
Chimneyinternaldiameter
1.46m
Chimneyheight
6.8m
Massofchimney
2.53tonnes
Bulkabsorptioncoe�.oflightinacrylic
.01cm�
1
@400nm
.04cm�
1
@360nm
.18cm�
1
@320nm
ingtechniqueswereprototypedbybuildingtwosmall(seven-panel)sections
ofthesphere.
Firsttheupperhemisphereofthevesselwasbuiltandthechimneyattached.
Thechimneyofthecontainmentvesselwasmadeof�vecylindersofultraviolet
absorbing(UVA)castacrylictoreducethe\piping"ofunwanted� Cerenkov
lightfromtheseregionsintotheinnervolumeofthedetector.
Thehemispherewasthenraisedintoits�nalpositionandsuspendedonits
tensupportingropes.Vectran�berswerechosenasthematerialforthesus-
pensionropes,notonlyfortheirlowradioactivitybutalsofortheirability
toretainstrengthduringlongterm
exposuretoultrapurewater.Atotalof
300kgofVectran�lamentwassuppliedbyHoechst-CelaneseCompany.The
�lamentsweretwistedintoropebyYaleCordage.Duetothelargesurface
areaofthe�lamentsthisrepresentsasigni�cantcontaminationpotentialdue
todustdeposition.Thereforethetwistingmachineswerecarefullycleaned
andtentedinplasticpriortouse.Eachofthetenloopssupportingthevessel
isapproximately30m
longand24.4mm
indiameter.Innormaloperation,
eachloopiscontinuouslyloadedto�vetonnes,orapproximately10%ofits
ultimatestrengthof500,000N.
Workingfrom
asuspendedconstructionplatform,subsequentringsofthe
lowerhemispherewereconstructedandattachedtothehangingupperhemi-
sphere.Asconstructionprogressedtheplatform
waslowereduntilthe�nal
\southpole"diskwasinstalled.
TheprincipalfeaturesofthecontainmentvesselarelistedinTable2.
16
Table3
Mechanicalmeasurementsoftheacrylicpanels.
Tensilestrength
79MPa
Tensileelongation
5.05%
Tensilemodulus
3.5GPa
Compressivedeformation
0.36%
Residualmonomer
1.22%
Ifacrylicissubjecttoexcessivestressforextendedperiodsoftimeitwill
developcrazingcrackswhicheventuallywillleadtoprematurefailure.The
ten-yeardesignlifeofthevesselrequiresthatthelong-term
tensilestresses
notexceed4MPa.Inordertoreducefurtherthetensilestresses,itwasdecided
toplacethevesselincompressionduringnormaloperationbyadjustingthe
H2OlevelwithrespecttotheD2Olevel.Optimizationofthedesignwascar-
riedoutwiththeANSYS[14]�nite-elementanalysiscode.Thestructurewas
studiedunderavarietyofsimulatedconditions,bothnormalandabnormal
(e.g.,withabrokensuspensionrope).Forallthesestudiesitwasassumedthat
themechanicalpropertiesofthebondswereidenticaltothatoftheacrylic,
anassumptionsupportedbymeasurementsofbondedtestspecimens.
ThePolycastCorp.measuredmechanicalpropertiesoftheacrylicpanelsto
insurethateachsheetmetdesignspeci�cationsforthicknessandmechanical
integrity.Theaveragesof�vemeasurementsarelistedinTable3.Thelow
percentageofresidualmonomerlistedinthetableindicatesthatthepolymer-
izationprocessisessentiallycompleteandthatthemechanicalpropertiesof
theacrylicwillnotchange.
Stresslevelsinbondedpanelswerealsorecordedbypositioningcrossedpo-
larisersoneachsideofthebondandphotographingfringes.Attheend,the
vesselwassuccessfullyproof-testedbyreducingtheinternalpressure7kPa
belowambientpressuretosubjectthespheretobucklingforcesandbypres-
surizingitto14kPaaboveambientpressuretosubjectallthebondstotensile
stresses.
Theradioactiveandopticalrequirementsfortheacrylicwereestablishedby
MonteCarlosimulationofthedetector.Toensurethattheacrylicwould
meettherequirementsofthedetector,theradioactivepropertiesweremea-
suredthroughoutthesupplyofmaterialsbymeansofneutronactivation,
massspectrometry,andalphaspectroscopy.ConcentrationsofThandUin
eachacrylicsampleweremeasuredtobelessthanthespeci�ed1.1pg/geach.
Fortheelevenropesproduced,twelve2.5-kgsamplesweretakenfrom
the
beginningandendofaproductionrunandbetweeneachrope.Directgamma
countingyieldedupperlimitsof200pg/gThandU,inagreementwiththe
averagevalueofsmall-sampleneutron-activationresults.
17
Opticalabsorptioncoe�cientsforeachproductionbatchofacrylicsheetswere
measuredforsamplescutfromthesheets.Over300measurementswereper-
formed.Auseful�gureofmeritistheratioofthelightdetectedwithacrylic
tothelightdetectedwithoutacrylicbetween300nmand440nmatnormal
incidence.The�gureofmeritforthetenthickerequatorialropegroovepanels
wascalculatedfrom
theabsorptioncoe�cientasiftheywere5.6cm
thick.
Forthe170sheetsmanufacturedtheaverage�gureofmeritis0.73.
4
PhotomultiplierTubes
TheprimarydesignconsiderationsforthePMTsystemare
{Highphotondetectione�ciency,
{Minimalamountofradioactivitiesinallcomponents,<
120ng/gU,<
90ng/gTh,<0:2mg/gK,
{Lowfailureratefora10-yearlifespansubmergedinultrapurewaterata
pressureof200kPaandfortheseismicactivityexpectedattheSNOsite,
{Fastanodepulserisetimeandfalltimeandlowphotoelectrontransittime
spread,forasingle-photoelectrontimingresolutionstandarddeviation<
1:70ns,
{Lowdarkcurrentnoiserate,<8kHz,atachargegainof10
7,
{Operatingvoltagelessthan3000V,
{Reasonablechargeresolution,>1:25peaktovalley,
{Lowprepulse,late-pulseandafter-pulsefractions,<1:5%,
{LowsensitivityofPMTparameterstoexternalmagnetic�eld:at100mG,
lessthan10%gainreductionandlessthan20%timingresolutiondegrada-
tion.
RawmaterialsfromthemanufacturersofthePMTcomponents,bases,cables,
andhousingswereassayedforradioactivity[15].Theleachratesofdi�erent
typesofglassandplasticwerealsomeasured.
Thephotomultipliertubes(PMTs)areimmersedinultrapurewatertoamax-
imumdepthof22m,correspondingtoamaximumwaterpressureof200kPa
aboveatmosphericpressure.ThefailurerateofthePMTcomponents,which
consistofthePMT,theresistorchainandtheHV/signalcable,mustbecom-
patiblewiththeexpectedapproximatetenyeardetectorlifetime.Thisputs
severecontraintsonthechoiceofglassforthePMTenvelope,theshapeof
theenvelope,thetypeofcable,andthedesignofthewaterproofenclosure
thatprotectstheresistorchain.Rawmaterialsfrom
allmanufacturerswere
assayedforradioactivity[15].Thedominantcontributortothecontaminant
contentofthePMTistheglasswhichhasamassof850gandThandU
impuritylevelsofroughly40ppb.Theleachratesofdi�erenttypesofglass
18
andplasticwerealsomeasured.
Followingdetailedstressanalysisoftheproposedglassenvelopes,amush-
room
shapedenvelopewasjudgedtobemostsuitableforthisunderwater
applicationandleastlikelytodeteriorateunderlongtermhydraulicpressure
inUPW.SchottGlaswerkeproducedanewborosilicateglass(Schott8246)
forSNO.TheThandUimpuritiesinthisglassarelessthan40ppb.Itswa-
terresistanceisratedasclass1(sameasPyrex).Inaddition,thisglasshas
outstandingopticaltransmissionpropertiesatshortwavelengths,withabulk
lightabsorptioncoe�cientofbetterthan0:5cm
�
1
at320nmwavelength,and
lowHepermeability.Schottre�ttedafurnacewithaspeciallow-radioactivity
linertoproduce16000mouthblownbulbsforSNOandtheLSNDProject[16].
ThemostimportantPMTparametersarethenoiserate,thee�ciency,the
transit-timespreadandtheamountofK,UandThineachPMT.Theenergy
resolutionandtheeventvertexspatialresolutionarelargelydeterminedbythe
�rstthreeparametersandthedetectorenergythresholdisstronglya�ected
bytheradioactivitiesinthePMTcomponents.
TheHamamatsuR1408PMTwasselectedforuseinSNO.Aschematicdraw-
ingoftheR1408isshowninFig.6.From
themeasuredUandThconcen-
trationsintheinternalpartsandglassenvelope,thetotalweightofThand
UineachPMTisestimatedtobeabout100�g,aboutafactorof14below
speci�cations.The9438inwardfacingPMTscollectthe� Cerenkovphotons,
providingaphotocathodecoverageof31%.Toimprovethelightcollection
e�ciency,a27cmdiameterlightconcentratorismountedoneachPMT,in-
creasingthee�ectivephotocathodecoveragetoabout59%.There ectivity
oftheconcentratorreducesthis�gureto54%.Oftheinwardfacingtubes,
49haveadynodetapandasecondsignalcable.These\lowgain"channels
extendthedynamicrangeofSNOathighlightintensitiesbyaboutafactorof
100.Another91PMTswithoutconcentratorsaremountedfacingoutwardto
detectlightfrommuonsandothersourcesintheregionexteriortothePMT
supportstructure.
ThePMTanodeisatpositivehighvoltage,typicallyintherange1700Vto
2100V,andthephotocathodeisatgroundvoltage.AsingleRG59/Utype
cableisusedtocarrythehighvoltageandthefastanodepulsewhichiscapac-
itivelycoupledtothefront-endelectronics.Theprimaryconcerninselecting
materialsfortheresistorchaincircuitboardisradioactivity.Eachtwo-layer
Kaptoncircuitboard,withsurfacemountedcomponentsandathrough-hole
0.0047-�F�lmcapacitorsfor�lteringandsourcetermination,containsatotal
ofabout2�gofthoriumand0.6�gofuranium,aboutafactorof�velower
thanthedesigngoal.
19
5.4 E
ndca
p
20.4
2.8
7.5
Hea
t Shr
ink
Glu
e
Cab
le
O-R
ings
Plug
Fig.6.AschematicoftheHamamatsuR1408PMTanditswaterproofenclosure.
The9dynodesareshownassolidhorizontallines.Thefocusinggridisshownas
thedashedlinesabovethedynodes.Thevolumeinsidetheenclosureis�lledwith
softsiliconegel.Theendcapis exibletoallowforthermalexpansion.Theplugis
sealedwithheatshrinkandthermaladhesive.Dimensionsareincm.
ThemainfunctionofthewaterproofenclosureistokeepthePMTbasedry
whenthePMTisdeployedunderwater.Therearetwowaterbarrierspro-
tectingthecircuitboardtoensurealowfailurerate.Theouterenclosure
consistsofaplastichousing,waterproofmodi�ed\TNC"connectorandheat
shrinktubingwiththermaladhesive.(Aconnectorwasusedtosimplifycon-
structionandcabling.)Theshrinktubingandadhesiveareusedtoholdthe
plastichousingontothePMT.Thecavityinsidetheenclosureis�lledwith
asiliconedielectricgel(GERTV6196)whichactsasthesecondwaterseal.
Thesiliconegelcontainslessthan23ng/gofThandlessthan12ng/gofU.
Thepolypropyleneplastichousingcontainslessthan10ppbofThandless
than6ppbofU.Theheatshrinktubingismadeofclearpolyole�nwhichhas
lowThandUimpurities.TheTNCconnectorwasprovidedbyM/A-COM
withmodi�cationsspeci�edbySNO.Theseconnectorsexhibitedbreakdown
atnominalhighvoltagesettings,aproblemwhichwasovercomebytheaddi-
tionofnitrogengastothewater,asdescribedintheprevioussection.Heat
shrinkandthermaladhesivesealthecabletothemaleconnector.
TheRG59/UtypecablewasdesignedbyBelden,Inc.forSNO.Itconsistsof
acoppercladsteelcentralconductorsurroundedinsuccessionby1.85mm
thicksolidpolyethylenedielectric,95%tinnedcopperbraid,Duofoilbonded
metallicshieldand1.3mmthickhigh-densitypolyethyleneouterjacket.The
colouringcompoundmixintheouterjacketisreducedto0.1%carbonblack,
theminimum
amountneededtomakethejacketopaque.EachPMTcable
is32m
longandcontainsbetween10{17�gofThandafew�gofU.The
20
attenuationat400MHzis7.3dB=30.5m.Thedelayis4.9ns=m.
TheHamamatsuR1408PMThasunusuallystablegaincharacteristicswith
respecttothein uenceofweakmagnetic�elds[17].IntheSNOdetectorthe
earth'smagnetic�eldisabout55�Tandpointsapproximately15degreeso�
theverticalaxis.Insucha�eld,thephoton-detectione�ciencyaverageover
allthePMTswouldbeabout82%ofthedetectione�ciencyatzeromagnetic
�eld.BecausethePMTe�ciencyisreducedbylessthan3%atmagnetic�eld
intensitiesof20�T,onlytheverticalcomponentofthemagnetic�eldinthe
SNOdetectoriscancelledwith14horizontal�eld-compensationcoilsembed-
dedinthecavitywalls.Withthepropercurrents,themaximumresidual�eld
inthePMTregionis19�T,andthereductioninphoton-detectione�ciency
isabout2.5%fromthezero-�eldvalue.
Thesinglephotoelectrontestsystem
andtestresultsaredescribedindetail
inrefs.[18,19].Atotalof9829PMTspassedtheacceptancetest.Theaverage
RMStimingresolutionisfoundtobe1.7ns,inlinewiththespeci�cation.
Themeanrelativee�ciency,de�nedinRef.[18],is10%betterthanspeci�ca-
tion.Themeannoiserateis2.3kHzat20�
C.Thecoolertemperatureinthe
detector(approximately10�
C)reducesthemeannoiseratetoapproximately
500Hz,includingsignalsduetoresidualradioactivityinthedetector.The
meanoperatingvoltageis1875V.
5
PhotomultiplierTubeSupportStructure
Thephotomultipliertubes,theirlightconcentratorsandassociatedhardware
arecollectivelyreferredtoasthePMTSupportStructureorthePSUP.The
geometryofthisplatformisfundamentallyestablishedbymaximizingphoton
collectionwhileminimizingbackgroundsignals,fabricationcosts,complexity,
andtransportationandinstallationintricacy.TheD2Otargetgeometry,the
PMTspeci�cations,lightconcentratorperformance,andcavitygeometryall
a�ectthe�nalPSUPcon�guration.
ThePSUPservestheadditionalfunctionofprovidingabarrierbetweenthe
coreoftheexperiment(theD2Otargetandlightcollectionsurfaces)andthe
outerregionsoftheexperiment.ThisbarriershieldsthePMTsfromlightgen-
eratedintheouterregionsoftheexperiment.Theseregionsincludethecavity
wallsandthesupportpipingandcablingfortheexperiment.Nearthecavity
wallstheradioactivityinthesurroundingrockcreatesasigni�cantphoton
background.Thecomplexityofthecablingandpipingmakestheouterregion
di�culttocleanandkeepcleanduringconstructionandtheradioactivepurity
ofthematerialsneededforsomeofthesefunctionscouldnotbereasonably
controlledtothesamelevelsasotherdetectorcomponents.ThePSUPalso
21
functionsasahighlyimpermeablebarriertowaterbornecontamination,en-
suringthatthehighlypuri�edwaterinthesensitiveregionbetweenthePMTs
andtheAVise�ectivelyisolatedfromthedirtierwaterintheouterregions.
ThePSUPalsosupportstheoutwardlookingPMTs,LEDcalibrationsources,
andwater-recirculationandmonitoringpiping.
ThedesigncriteriaforthePSUPinclude:
{MaximizingthecollectionofopticalsignalsfromtheD2Otargetwhilemin-
imizingavarietyofbackgroundsources;
{Maintainingperformanceandintegrityofthearray,withnorequiredmain-
tenance,foratleastatenyearspansubmergedinultrapurewaterandfor
avarietyofseismicconditionsanticipatedattheSNOsite;
{Minimizingthemassofthecomponentstoreduceintrinsicbackgrounds;
{Producingandfabricatingthearrayfrom
documentedlowradioactivity
materials;
{Maintainingahighimpermeabilitywaterbarrierbetweentheinnerand
outerlightwaterregions;
{Maintainingane�ectivebarriertolightgeneratedintheouterregions;
{Minimizingthecomplicationsoftransportingandinstallingthearrayatthe
undergroundsite;
{Producingandfabricatingthearrayfrom
materialsinimicaltobiological
growthwithlowleachingcharacteristics,lowmagneticsusceptibilityand
lowelectrolyticcharacteristics;
{Utilizingmaterialsandinstallationprocessescompatiblewiththeunder-
ground(mining)environment;
{Installingtheupperhalfofthegeodesicsphereandsupportingtheloadsof
roughlyhalfthedetectorarrayduringwhichtimetheAVisconstructed.
ThePSUPislogicallyviewedastwosystems{ageodesicspherethatfunctions
asthemainsupportsystem
andthepanelarraysthathousethePMTsand
concentrators.Thegeodesicsphere,an889-cmradiusthree-frequencyicosahe-
dron,isshowninFig.2.Normallythisgeodesicspherewouldutilize92nodal
connectionsbetweenthe270struts.Thischoiceofgeodesicstructureuses
threedi�erentstrutlengthswithsimilarlineardimensions.Thetopmostnode
isreplacedwithahollowtoroidalringandtheconnectingstrutsshortenedto
accommodatetheacrylicvesselchimney.Additionaltoroidalringsguidethe
acrylicvesselsupportropesthroughthePSUP.Ninety-oneoutwardviewing
PMTsaresupportedonthePSUPstruts.Thedesignofthegeodesicsphere
andpanelarraysisfullydocumented[20].
Thegeodesicsphereissuspendedon15stainlesssteelwireropecablesfrom
thecavitydeck.Thesecablesterminateinasphericalthrustbearingonthe
deckandaremonitoredwithloadcells.Thepositionofthearrayisadjustable
22
Fig.7.PhotomultipliermountedinahexagonalABScell,whichalsoformsthe
light-concentratorhousing.Dimensionsareincm.
inthez-direction(up-down)by�
10cm.Whensubmerged,thearraybecomes
buoyant,andisthereforesecuredtothecavity ooratasetofanchorpoints,
eachwithapulleyandcablethatattachestothemainPSUPsuspension
brackets.
Wireropecannote�ectivelybemanufacturedwithoutlubrication.Themain
suspensioncablesareplasticcoatedandencasedinanadditionalplastichous-
ingtopreventcontaminationofthewater.Anchorcablesareplasticcoated.
ThePMTsaresecuredinhexagonalABS(Acrylonitrile-Butadiene-Styrene)
blackplastichousingsthatalsosupportlightconcentrators(re ectors)for
eachinward-lookingPMT[21]asshownin�g.7.
Theconcentratorsincreasethee�ectiveareaofthephotocathodetomaximize
thenumberofphotonsdetected,andlimittheangularacceptanceofthepho-
tomultiplierssothatonlythecentralpartofthedetector(wherebackgrounds
arelowest)isinview.Theyaredesignedwithathree-dimensionalcriticalan-
gleof56:4�
,whichmeansthedetectorisviewedbyeachPMTouttoaradius
of7m.Theincreaseinlightcollectione�ciencyisapproximately75%[21].
Aconcentratorismadeof18piecesofthindielectric-coatedaluminumsheet
23
curvedandplacedaroundtheholderofABSplastic.Theopticallyactivecoat-
ingconsistsofalayerofspecularaluminumcoveredbyalowrefractiveindex
layerofmagnesium uoride,whichisinturncoveredbyahighrefractiveindex
layerofmixedtitaniumandpraeseodymiumoxides.Thecoatingthicknesses
areoptimizedtomaximizere ectanceinwateroverthevisibleandnear-UV
rangeofwavelengths,andoverawiderangeofincidentangles.Themean
e�ectivere ectanceinwateris82�
3%.
Thetopdielectriclayeralsoformsaprotectivebarrieragainstcorrosionin
waterandwasappliedtothebackexposedsurfaceofthealuminum
pieces
aswell.Aprotectivelayeroftitanium
oxidewasappliedtotheedges.The
materialwassubjectedtoextensiveagingpriortoconstruction.Itwasfound
tohaveausefullifeofmanyyearsunderdeoxygenateddeionizedwaterwith
apHaround7,thenormalenvironmentinSNO.
ThehexagonalPMTassembliesarecollectedtogetherinto atarraysofbe-
tween7and21PMTstoform
asetofrepeatingarraysusedtotessellate
thesphericalsurfaceofthegeodesicstructure.Eachpanelarrayissecured
tothegeodesicstructurewiththreeadjustablemounts.Thesemountspermit
thedelicatepanelarraystoaccommodatethegeodesicstructurestruts exing
underchangingloadconditions(mostnotably,whengoingfromafullyinstru-
menteddryarraytoafullysubmergedarray).Thisalignmentiscriticalto
ensurethateachPMTviewsthefullD2Ovolume.Atotalof751panelarrays
tessellatethesphereproviding9522hexagonalcells.Oftheselocations,the
�naldesignused60cellstoaccomodatethe20AVropesandsixtoprovide
accessatsixlocationsforcalibrationsourcespreservingtheopticalandwater
owrequirementsforthearray.Duringthewater�ll18PMTswereremoved
fordiagnostictestingandtheirlocationsappropriatelycapped,leaving9438
cellsoccupiedwithPMTs.Thehexagonalcellscover�
85%
ofthesurface
areaofthegeodesicsphere.
WaterpipingandsamplingtubesaswellasseveralpulsedblueLED
cali-
brationdevicesareaccommodatedinthesmallspacesbetweenPMTpanel
arrays.
ThePMTsupportstructureissubjectedtoavarietyofloadingconditions
andstresses.Exposuretoultrapurewaterisknowntoreducethemechanical
performanceofmanymaterialsovertime.Thedesignisoptimizedwiththe
criterialistedabovewithbothhandcalculationsandadetailedANSYS�nite
elementanalysisofthePSUPunderavarietyofconditions.Ahalf-scalemodel,
partialfull-scalemodelanddry�t-upoftheentiregeodesicspherewereused
tocon�rmthedesign.
Theanalyses(bothstressandbuckling)studynumerouse�ects,suchasthe
variationofmechanicalandphysicalpropertieswithtimeandimmersionin
24
water,thee�ectsofwater�llonthede ectionsandstressesinthePSUPin
ordertoestablishacceptable�lling-ratelimits,theconsequencesofoneofthe
PSUPsupportsfailing,thee�ectsofvariousconstructiontolerances,andthe
responseofthenaturalfrequencyofthePSUPunderdynamicconditions[22].
Variousstainlesssteelalloysareusedinthefabricationofthegeodesicsphere
andpanelarrays,primarilySS304LforweldedcomponentsandSS304forparts
thatareonlymachined.Thesealloyshaveanexcellentdocumentedhistoryof
long-termexposuretodeionized(ultrapure)water.Weldingwasaccomplished
withinert-gasprocesses.Avacuum-depositedAgcoatprovidesacleananti-
gallingagentforthreadedfastenerswithoutcompromisingwaterpurity.
Inordertoreducethelikelihoodofstresscorrosionandcracking,plateand
tubematerialispickledandpassivated.Steelfastenersarefabricatedfromraw
materialsmeetingthesamespeci�cationsandareinadditionheat-treatedand
quenched.
ABSplasticisusedintheinjectionmoldedcomponentsofthePMThous-
ings.GeneralElectricresinGPX5600color4500wasselectedforthemolding.
Thisselectionofplasticprovidesexcellentimpact,thermal,chemicalresis-
tance,andinjectionmoldingcharacteristics.SinceABSplastichasalimited
documentedhistoryofexposuretodeionizedwater,weconductedextensive
acceleratedagingteststocon�rmthebehavioroftheABSGPX5600tolong
term
ultrapurewaterexposureandtoinsureanadequatesafetyfactorfor
thedesignlife[23].Plasticcomponentswereinjectionmoldedusingcontrolled
rawmaterialsandprocesses.Themachineswerethoroughlycleanedpriorto
andduringthefabricationcycleofthecomponents.Theassemblyareaswere
tentedandmachineoperatorsusedcleanglovesduringtheproductionofthe
items.
Radioactivityofallcomponentsorsamplesofrawmaterialsusedinthefab-
ricationofthePSUPweresubjectedtodirectcountingtodetermineUand
Thconcentration.Speci�edlimitsof15ng/gforeachofthesespeciesare
metbyallmaterials;measuredupperlimitsaretypicallyafactorof3to5
timeslower.Allcleancomponentswerestoredincleanplasticbagspriorto
assembly.AssemblyofthePSUPcomponentswasaccomplishedincleanroom
assemblyareaswithconditionstypicallybetterthanClass1000.Assembled
componentswerestoredincleanplasticbagspriortoinstallation.
6
Electronics
Theelectronicschainisrequiredtoprovidesub-nanosecondtimeandwide-
dynamic-rangechargemeasurementforthePMTpulses.Whilethesolarneu-
25
trinoeventrateisverylow,theelectronicschainmusthandlebackground
ratesinexcessof1kHzandburstratesfrompotentialsupernovaeinexcess
of1MHzwithoutsigni�cantdeadtime.Thechainisimplementedusingthree
fullcustom,applicationspeci�cintegratedcircuits(ASICs)andcommercial
ADCs,memory,andlogic.TheDataAcquisition(DAQ)interfaceisVME
compatible.TheASICscarryoutvirtuallyalltheanalogsignalprocessing
andpartofthedigitalsignalprocessing.TheASICchipsetconsistsofa
wide-dynamic-rangeintegrator,afastandsensitivediscriminator/gatingcir-
cuit,andananalog/digitalpipelinedmemorywithatimingcircuit.Theuse
oftheseASICsprovidesincreasedreliabilityanddecreasedper-channelcost.
Furthercostreductionandimprovedfunctionalityisobtainedbyusinghigh-
volume,large-scale-integrationcommercialcomponentsfortheremainderof
thecircuitry,includingnumerous�eldprogrammablegatearrays(FPGAs).
Thesignalprocessingelectronicsoccupies19crates,eachprocessingsignals
from512PMTs.ThesignalandhighvoltageforeachPMTarecarriedona
singlecableandconnectedingroupsof8attherearofeachcratetooneof
sixteenPMTinterfacecards(PMTICs).Thesignalthenentersoneofsixteen
front-endcards(FECs),whereitisprocessedbythecustomASICs.TheASICs
resideonfourdaughterboards(DBs),eachofwhichhandles8channels.One
FECprocesses32channels,digitizesthesignals,andstoresthedigitalresults
in4MBofon{boardmemory,largeenoughtobu�ereventbursts(roughly
speaking,thememoryinalltheFECscouldholdaone-million-eventsuper-
novaburst).TheFECsareattachedtoacustombackplane(\SNOBus")that
implementstheSNOsignal-andpower-distributionprotocol.Alsoattached
tothebackplaneareatranslatorcardandatrigger-formationcard.TheXL1
translatorcardoperatesinconjunctionwithacompanionXL2translatorcard
intheDAQVMEcratetocommunicatewiththesingle-boardembeddedpro-
cessor(eCPU)oftheDAQVMEcratedownstream(seeSection7).Thetrigger
cardandtheassociatedtriggersystem
aredescribedbelow.Programmable
test,calibration,anddiagnosticfacilitiesareimplementedthroughoutthe
crate.AnoverviewofthefullsystemisprovidedbyFig.8.
Theanalogpartofthechain,fromPMTtoASICchipset,operatesasroughly
9500independent,asynchronous,data-drivenpipelines.Thesubsequentread-
outpartofthechainoperatesasaseparate,clock-drivensystemthatmoves
thedatafromtheFECstotheDAQVMEsystem.Eachelementofthechain
isdescribedbelow.
Theanalogpulsefrom
eachPMTtravelsthrough32m
of75RG59-like
waterproofcoaxialcabletothePMTIC.Onecommercialhigh-voltagepower
supplyprovidesabout70mADCforthe16PMTICs(512PMTs)inone
crate.CurrenttothePMTspassesthroughindividualisolating,�ltering,and
trimmingnetworks.ThePMTICplugsintotherearoftheSNOBuscrateand
connectsdirectlytoitscompanionFECinthefrontofthecrate.ThePMTIC
26
DA
Q
SYST
EM
db dbdb db
dbdb db db
BA
CK
PL
AN
E HV
PO
WE
R
LO
W V
PO
WE
R
TRIG.
VM
EB
AC
KP
LA
NE
TR
IGG
ER
CE
NT
RA
L
FEC #16
SNO
BU
S
FEC #1
eCPU
(GP
S) T
IMIN
G
MA
STE
R
TRANS.
TRANS.
XL2XL1
CRATE
PM
T(1
)P
MT
(512
)
1 C
RA
TE
x 1
9
PM
TIC
#16
PM
TIC
#1
Fig.8.OverviewoftheSNOcustomdataacquisitionelectronics.
alsoprovidesdisconnectsforindividualcables,HVblockingcapacitorsforthe
PMTsignal,overvoltageandbreakdownprotectionfortheintegratedcircuits,
limitedreadbackofthePMTcurrent,andaprogrammablecalibrationpulse
sourceforeachchannel.ThePMTsignalistransmittedthroughaconnector
totheFECwhereitisproperlyterminated,splitandattenuated.
OntheFEC,thesignalcurrentisdeliveredtooneoffourdaughterboards.
TheDBsphysicallyseparatemostoftheanalogsignalsfrom
mostofthe
digitalsignalsinordertoreducepotentialcrosstalk,andsimplifytheoverall
FEC+DBdesign.Thesignalisthenfedtoafour-channeldiscriminatorchip
(SNOD)whereanyleadingedgeisobservedbyafastdi�erentiator.Thecur-
rentissplitintotwobranches(approximatelyintheratio1:16)andfedinto
twoseparatechannels{onelowgain,theotherhighgain{ofaneight-channel
chargeintegrator(SNOINT).TheSNODandSNOINTchipswerefabricated
ascustom
designsinAT&T'sCBICU-2process[24,25].TheSNOINTchip,
whichuseshigh-qualityexternalcapacitorsfortheactualintegration,hasdual
integratorchannelsinordertoincreasethee�ectivedynamicrangetomore
than14bits.TheSNOINTalsoprovidesshapedlowandhighgainanalog
sumsofthePMTinputsignalsforuseindetectortriggering.Eachchannelof
theSNODchiphasindependentdiscriminatorandgategeneratorstoprovide
thetimingfunctionsnecessaryfortheSNOINTchip.Atimingdiagram
for
theSNODandSNOINTchipsisshowninFig.9.TheSNOINTandSNOD
chipoutputsarethendeliveredtotheCMOSASICanalogmemory.
TheCMOSmemberoftheSNOchipset(QUSN7)providesanalogmemory,
27
Dis
crim
inat
or (
Inte
rnal
Sig
nal)
RE
SE
T
Glo
bal T
rigge
r
T Qw
ith G
T
no G
T
SA
MP
LE
050
100
150
200
250
300
350
400
PM
T O
utpu
t
ns
∆Q
∆T
Fig.9.Singlechanneltimingcycle.WithnoGTpresent,achannelresetsautomat-
icallyattheendofatimingcycle(�400ns).
atime{to{amplitudeconverter(TAC),andchannelandtriggerlogicforthe
SNOdetector.The16{deepmemorysamplesthelow-andhigh-gainchannels
oftheSNOINTatearlyandlatetimes,foratotaloffourpossiblecharge
samples[26].TheQUSN7chipalsoreceivesasignalfromtheSNODwhichis
usedtoinitiateatimemeasurementcycleandthecreationofsystemtrigger
primitives.Thesesingle-channeltriggerprimitivesconstitutethe�rststageof
a9438-inputanalogsum.Thismixedanalog/digitaldeviceusescustomanalog
blockscombinedwithauto-placedandroutedstandardcellsandisfabricated
intheNorthernTelecomCMOS4Sprocess[27].
ThetimingsequenceandTACinQUSN7areinitiatedontheleadingedgeofa
signalfromSNOD.TheTACforagivenPMTisstartedwheneverthatPMT
�res,andiseitherstoppedbyacentrallygeneratedGlobalTrigger(GT)signal
orresetsitselfifnoGTarrivesafteraninternaltimeoutperiod.Onavalid
GT,fouranalogvoltages(correspondingtoonetimeandthreeofthefour
selectablechargemeasurements)arestoredinoneofthe16analogmemory
banks,anassociateddigitalmemoryrecordsthesequencenumberoftheGT
(foreventbuilding),thememorylocation(forsecondordercorrectionsofthe
data)andanyassociatedcondition ags,andadata-available agisset.The
overalltriggerdeadtimeislessthan10ns,andsinceeachchannelisself-
resetting,theonlyotherrelevantdeadtimeistheperPMTdeadtime,which
28
issettoabout400nstoallowforlightre ectionacrosstheSNOdetector
volume.Thesetwocontributionstothedetectordeadtimearenegligible.
TheQUSN7chipalsogeneratestwotriggerprimitivesignals,ashortcoin-
cidence(roughly20ns)andalongcoincidence(roughly100ns),somebasic
utilityfunctions,andextensiveself-testcapability.Separateinternalcounters
areusedtokeeptrackofPMTnoiseratesandcounterrorconditions.All
countersandlatchesareaccessiblefortestingviaanexternalscanpathand
avarietyofprogrammableadjustmentsarebuiltin.
Adataavailable agfromanyQUSN7initiatesanexternal32{channelreadout
sequencer.Thesequencerisaclocked,synchronousstatemachineimplemented
inastandard�eld-programmablegatearray(FPGA).Atthesequencer'scon-
venience,aQUSN7chipwithavailabledataisselectedandpresentsfour
individuallybu�eredanalogvoltagestothefouron-board12-bit2-�sADCs.
AftertheADCshavesampledtheanalogvoltages,thesequencerreadsthree
bytesofdigitalinformation:the16-bitGTsequencenumber,fourbitsofcell
address,andfour agbits.Afourthreadstrobeclearsthememorylocation.
Attheendofthesequencer'scycle,a3-word,12-byte,�xed-formatdatastruc-
tureisloadedintotheon{boardmemory.ThememoryisastandardSIMM
DRAMunderthecontrolofacommercialdualportcontroller.Thesequencer
operatesoneportofthecontrollerasaFIFOwhilethesecondportisaccessed
viathedownstream
DAQinterface.Eachthreeworddescriptoriscomplete
inthesensethatthemeasuredtimeandchargeandthegeographicchan-
nelnumberandtemporalGTsequencenumberarefullyspeci�ed,sowhile
itispossiblethathitdescriptorswillbeloadedintomemoryoutoftempo-
ralsequence,theycanbeproperlysortedoutinthedownstreamDAQevent
builder.
Thepairoftranslatorcards(XL1,XL2)implementsahighspeed(>8MB/s)
RS485linkbetweeneachSNOBuscrateandthecentralDAQ
VMEcrate
(showninFig.8containingaMotorola68040single-boardcomputer,or\em-
beddedCPU"(eCPU)).ThetranslatorcardlocatedintheSNOBuscrateper-
formsaTTL$
GTLconversion.TheuseofGTL(GunningTransceiverLogic,
witha0.8-Vswing)ontheSNOBusbackplaneforalldata,addressandcontrol
linesreducesthepossibilityofcrosstalkwiththesensitive,lowvoltagePMT
signals.Forthesamereason,clock,DTACK(theVMEdatahandshakesig-
nal)andotherimportantsignalsaretransmittedontheSNOBusbackplaneas
low{levelsingle{endedemitter-coupledlogic.Allsignalsareterminated,and
theuseofa2-mmgrid,165-pinconnectoronthiscustombackplanemakesfor
amorecompactand exiblesystemandpermitstheuseofagreatlyincreased
numberofgroundlinesrelativetostandardVME.
EachMeVofenergydepositedintheSNOdetectorisexpectedtoresultin
29
roughlyeightcontemporaneousphotoelectronsdetectedbythePMTarray.
Thusthemostpowerfulhardwaretriggerfordetectionofsolarneutrinosisa
simplecountofthenumberofPMTsthathave�redintherecentpast.Because
thePMTarrayis17mindiameter,di�erentphotonsfromaninteractionin
theD2O
coulddi�erintraveltimetothePMTsbyasmuchas66ns(or
longer,iftheyundergore ection(s)).Forthisreasonthetriggerresolving
timeissetdigitallytoabout100nstoallowallunre ectedphotonsfrom
a
� Cerenkoveventtobecountedtowardapossibletrigger.Theactualcounting
ofhitsisdoneviaachainofanalogsummations.
AsignalfromSNODisusedinQUSN7toinitiateapairofindependentcurrent
pulses.Thelongerpulse(nominally100ns)issenttothe�rststageofa9438-
inputanalogsum.SummationsattheFEC,crate,andfulldetectorlevel
follow.The�nalsummationiscomparedagainstaprogrammablethreshold,
andasumabovethisthresholdwillgenerateaGT.Thesecondcurrentpulse
generatedbyQUSN7isnominallymuchnarrower(about20ns),hasadigitally
setwidthanddelayand,inanidenticalsummingtree,generatesaseparate
triggerusefulforstudiesinwhichonewishestoscrutinizeaselected,smaller,
�ducialvolumeofthedetector.
Inadditiontothesetwodiscreteanalogsums,theSNOtriggersystem
can
triggerontheanalogsumofthe9438shapedPMTsignalsproducedbythe
SNOINTchips,andcopiesofeachofthesumsaresenttoadigitaloscilloscope
forrealtimemonitoringofthestateofall9438channels.Forthe100ns
coincidencetrigger,uptothreeseparatethresholdscanbesetsimultaneously,
andtriggersfromthelowestthresholdcanbeprescaledwithafactoraslarge
as65,534toprovideamonitorofverylowenergybackgrounds.Abuilt-in
pulserprovidesa\pulsed{GT"trigger,whichisusedasa\zerobias"trigger
forthedetector.Alltriggertypescanbeenabledsimultaneously,andbits
correspondingtoeachtypethat�redinagiveneventarelatchedina\trigger
word"thatispartofthedatastream.
Twoseparateoscillatorsareusedtokeeparecordofabsoluteandrelative
time.AcommercialGPSsystemprovidesa10MHzsignalaswellasprecision
timemarkersatrequestedUniversalTimes.Thissystemisnominallyaccurate
toorder100nsandwillallowcorrelationofSNO
datawiththatofother
astronomicaldetectors.SincetheGPSreceivermustbeonthesurfaceand
alltheotherelectronicsisbelowground,communicationdelaysduetothe4
kmlong�beropticsmustbecontinuouslymonitored.Thisisdonebyusinga
separate�berinthesamebundleandmeasuringtotalroundtrippropagation
time.Ahighlystablequartzoscillatoroperatingat50MHzo�ersahigher
precisioninter{eventtimingstandard.
Extensivemeasurementshavebeenmadeoftheelectronicsallthewayfromthe
integratedcircuitstothefulloperating9728channelsystem
(19crateswith
30
512channelseach,anumberofwhichareconnectedtonon-PMTsources)
whichhadbeeninstalledundergroundandwascompletelyoperationalbythe
springof1998.TestsofthefrontendcustomASICsrunningassingledevices,
asacoupledchipset,andonfullfunctionalityproductionboardsrunningwith
theprototypeandthenproductionDAQsoftwarewereverysuccessful.The
performanceofthebipolarchipsisveryclosetothatpredictedinthepre-
fabricationsimulationsandtheCMOSchipisadequatefortheexperiment.
Thechargelinearityfullysatis�esourdesigngoals.Thelinearityandpreci-
sionofthetimemeasurementarealsosigni�cantlybetterthantheminimum
requirementsofSNO.TestswithpulsersandactualSNOPMTshaveveri�ed
stableandreliableoperationofthechipsetandthesystem
evenunderthe
unanticipatedchallengeofhighratefullvoltagebreakdownsindegassedH2O
atsomeoftheunderwaterPMTconnectors.Asingle-photoelectronspectrum
withnodiscriminatorthresholdsetisshowninFig.10.
Fig.10.Singlephotoelectronspectrum
usingthefullelectronicschainwithanin
situSNOPMT.Thetruncatednoisepeakreachesroughly2000counts.
31
7
DataAcquisition
InthissectionthedesignandfunctioningoftheSNODataAcquistionSystem
(DAQ)isoutlined.First,ageneraloverview
ofthesystem
architectureis
presented,followedbyamorein-depthdiscussionofthedetailsofthereadout
andcontrol,event-building,andlow-andhigh-levelmonitoringsoftware.
TheprincipalhardwarecomponentsoftheSNO
DAQ
system
areasingle
VMEcratecontainingaMotorola68040single-boardcomputerwithoutan
operatingsystem(theembeddedCPU,oreCPU),butwith32MByteofon-
boardmemory.APCIbus-VMEinterface(theMacintoshDual-PortMemory
orMDPM)with8MByteofmemory,anSBus-VMEinterface(theSunDual-
PortMemoryorSDPM)with8MByteofmemory,theMasterTriggerCard
(MTC),andthesevenXL1cardsneededtoaccessthe19\SNOcrates"con-
tainingthefront-endelectronicsalsoresideinthiscrate.
EachSNOcrateisreadoutsequentiallybytheeCPU,whichaccessesonecrate
inturnthroughthetransparentXL1/XL2interfacebus.Withineachcrate,
theFECsarereadoutsequentially,byprogrammingtheslot-selectregister
onthecrate'sXL2card.
OncethecontentsoftheFECmemoryhavebeenreadbytheeCPU,they
aretransferredtoacircularbu�erontheSDPM.TheeCPU
additionally
hasaccesstotheMasterTriggerCardmemory;onceithascheckedevery
enabledFEC,itreadsouttheMTCmemoryandtransfersthetriggerdatato
aseparatecircularbu�erontheSDPM.
TheSDPM
issimultaneouslyandasynchronouslyaccessedinread-onlymode
byaSunUltraSparc1/170workstation,whichisresponsibleforsortingthe
FECandMTCdataandbuildingthemintoevents.TheSunwritesfullevents
todisk.A
separatemechanism
isresponsibleforcopyingcompletedevent
�lestoatapedrivelocatedonthisSunaswellascopying,viaEthernet,
these�lestoaSunonthesurface,wheretheyarecopiedagaintotape.The
systemalsosupportsmonitoringprogramsrunningonavarietyofplatforms.
Thesemonitoringprogramsaccessasampleddatastream
inrealtimeover
thenetwork.
Thereadoutoftheelectronicsisbothcontrolledandmonitoredviaauser
interfaceProgram,the\SNO
HardwareAcquisitionandReadoutControl
(SHARC),"runningonaPowerComputing250MHzPPCMacintoshcom-
patible,whichusestheMDPMtocommunicatewiththeeCPU.Asetofsmall,
prede�neddatablockstransfercontrolandlow-levelmonitoringinformation
suchasusercommandsandeCPUstatusbetweenSHARCandtheeCPU.
SHARCinitializesthehardware,takesindividualhardwareelements(chan-
nels,cards,crates)inandoutofthereadout,storesthecurrenthardware
32
con�guration(whichelementsarepresentandwhicharebeingreadout),and
ensuresthatthisinformationissaved.
SHARCcontrolsSNO'scalibrationsourcesthroughamanipulatorcomputer
thatmovesthesourcesaroundinsidethedetector,andalsoreceivesinfor-
mationregardingindividualsourcestatusfrom
thevariouscomputersused
tooperatethedi�erentsourcesthemselves.Themanipulatoror\calibration"
computerinsertsthisslowly-varyingdataintothedatastreambysendingit
toSHARC,whichwritesittoacalibrationdatablockintheMDPM.Source
informationthatchangesmorequickly(additionaltriggerinformation,more
precisetiming)isgeneratedifnecessarybyroutingtherelevantsignalsfrom
thesourcestospareFECsandintegratingtheresultingdataintothedata
streamexactlyasifitwerecomingfromthePMTs.Interpretationofthedata
islefttotheo�ineanalysis.
TheeCPUreadsthecurrentcon�gurationfromacontrolblockintheMDPM
thathasbeenloadedbySHARCduringinitialization.Itloopsoverthedesired
hardware,incrementingerrorregistersintheMDPM
ifreadorwriteerrors
aredetected,andattachinginformationidentifyingthebadcrateorcard.The
eCPUperiodicallychecksthecon�gurationblockforchanges,whichifdetected
willalterthereadoutloop.Inthismanner,individualcardsorevenentire
SNOcratescanberemovedfromthereadoutloop.AMDPM-residentcontrol
structurecontaining\start"and\stop"semaphoresisalsopolledperiodically,
andenablesglobalcontrolofthereadout.
Removinghardwarefromthereadoutlooporpausingthereadoutentirelydoes
notpreventdatafrombeingstoredintheFECmemory.EachFECholdsupto
�
350;000PMThits,severalhoursofdataundernormaldetectoroperation.
Individualchannels(badPMTs)aretakeno�inethroughsoftwarebywriting
toaFECregisterthatpreventsthechanneldatafrom
goingintotheFEC
memory.
TheeCPU
transferstheFECandMTCmemorycontentstotwocircular
bu�ersintheSDPM,7and1MByteinsize,respectively.TheeCPUupdates
abu�erwritepointer,whiletheSunupdatesabu�erreadpointer,enablingthe
eCPUtotracktheSun'sreadoutprogress.TheeCPUwillpausethereadout
ofthehardwareifeitherbu�er�llsupandwillwaituntiltheSuncatchesup.
Wheneveritischanged,acopyofthecon�gurationcontrolblockiswrittentoa
blockintheSDPMbytheeCPU,whereitisreadbytheSunandincorporated
intotheeventdatastream.
TheSun,onceithasreadouttheSDPM,sortsthroughtheMTCandFEC
dataandaggregatesPMT
hitsplusMTC
triggerinformationintoevents.
Eventsarestoredinabu�erforsomeperiodoftimetoensurethatanylate
PMThitscanstillbeassociatedwiththeproperevent.Oncethelatencyperiod
33
expires,theeventsarepassedtoaFORTRANprocess,whichtranslatesthe
dataintoZEBRAformat[28]forusebytheo�ineanalysisandwritesthe
resulting�lestodisk.
InordertomonitorthestateoftheeCPUcode,theMDPM
controlblock
contains,inadditiontothestart-stopsemaphores,apairof\heartbeats,"
countersthatareincrementedinturnwhentheeCPU
isloopingoverthe
readout,orinatightloopwaitingforthestartsemaphoretobeset.Theuser
interfaceforSHARCdisplaystheseheartbeats.
DatafromtheSun,eitherinraw,pre-event-builtformorascompletedevents,
aretransmittedoverthenetworktoaSunresidentprocess,calledtheDis-
patcher,whichrebroadcaststhedatatoanyclientsrequestingit.Asthis
rebroadcastisalsooverthenetwork,clientscanbelocatedanywhere.The
DispatcheralsoreceivesdataregardingrunconditionsfromthecontrolMac.
Thedataarelabelled,anddi�erentclientscanrequestdatawithdi�erent
labels.Clientsreceiveonlyasmuchdataastheycanprocess,andanydata
unsentbytheDispatcherisoverwrittenaftersometime.Forhigh-ratedata,
e.g.,certaintypesofcalibrations,themonitorsystembehavesasasampling
system.Thesystem
isextremelymodular,andclientsonvariousplatforms
canconnectanddisconnectfromtheDispatcheratwill.Becauseofthissepa-
rationbetweentheDispatcherandthemonitors,theDispatcheritselfisvery
stable.
ToolsavailableforbothUNIXandMacplatformsallowonetoexaminethe
rawPMTdata,MTCdata,orfulleventdata,tocheckfordataintegrity,and
toexaminedataeitheronacrate-by-cratebasisorforthefullcomplementof
channels.ThereareseveraltoolsforeventdisplayintheSNOdetectorgeom-
etry.Inaddition,thereareWeb-basedmonitoringtoolsthatupdateHTML
�lesonadesignatedserverfordisplayingmorestaticinformationofinterest
totheo�sitecollaborator.
8
NeutralCurrentDetectorArray
Thedetectionofafreeneutronisthesignalthataneutral-current(NC)event
hasoccurred,whilethedetectionofaneutron(ortwoneutrons)incoinci-
dencewithapositronindicatesa�e
interaction.Heavywaterisanexcellent
moderator,andanumberofpossiblestrategiesfordetectingneutronscanbe
devised.Onesuchstrategy,discussedinSections1and2,involvestheaddi-
tionofachloridesalttotheD2O.Whenaneutroncaptureson75%abundant
35Cl,itemits8.6MeVin rays,whichmainlyComptonscatter.Theresulting
� CerenkovradiationemittedbytheelectroncanbedetectedbythePMTar-
rayinthesamewayCCeventsaredetected,andthe� Cerenkov-lightpatterns
34
producedby raysdistinguishedfromthoseofelectronsonthebasisoftheir
greaterisotropy.
Alternatively,neutronscanbedetectedwithhighe�ciencyin
3He-�lledpro-
portionalcounters.Anarrayofsuchcounters,withatotallengthof770m,
andconstructedofhighlypuri�edmaterials,hasbeendesignedforSNO[29].
Inthisway,theNCandCCeventscanberecordedseparatelyanddistin-
guishedonanevent-by-eventbasis.Event-by-eventsortingmakesoptimum
useofavailablestatistics,andsimpli�esinterpretationofsparsesignals,such
asfromadistantsupernova.
Detectionofthermalneutronsbythereaction
3He+n!
p+
3H+764keV
takesadvantageofanenormouscrosssection,5330b,andisawelldeveloped
artthathasincludedapplicationsinneutrinophysics[30{32].Nonetheless,
thedesignofanarrayof
3HeproportionalcountersforSNOposesunusual
problems.
TheneutronproductionrateinSNO
isexpectedtofallintherange6to
42neutronsperdaybasedonthemeasurementsbytheSuperKamiokande
Collaboration[33]oftherateof�-escattering,butnotthe avorcomposition.
Theratepredictedbyrecentsolar-modelcalculations[6]is13perdaywhenthe
cross-sectioncalculatedbyKuboderaandNozawa[34]isused.Theneutron
signalmustnotbeoverwhelmedbybackgroundsfrom
naturallyoccurring
radioactivityinconstructionmaterials,norcansubstancesbeallowedtoleach
signi�cantlyintotheheavywaterandinterferewithwaterpuri�cation.Helium
permeatesreadilythroughmanyotherwiseacceptablematerials.Thecounters
aretosurviveunderwateratabsolutepressuresupto3.1atmfortenyears
ormore,placingstringentdemandsondetectorlongevityandstability.A
counterarraywithhighneutrone�ciencyisneeded,butnotatthecostof
signi�cantlyobscuringthe� CerenkovlightfromCCevents.Theseconditions
severelyconstraindetectordesign,thematerialsemployedinconstructingthe
detectorarray,andthemethodsofhandlinganddeployment.
Backgroundsofseveralkindscanbepresent.Mostseriousisphotodisintegra-
tionofdeuteriumbygammasabove2.22MeV,astheneutronsproducedare
indistinguishablefromthe�-inducedsignal.
Atthebottom
ofthenaturalThandU
decaychainsaretwosu�ciently
energeticgammas.Manycosmogenicactivitiesalsohavehighenergygammas,
butonlyone,78-d
56Co,hasalongenoughhalf-lifetoberelevant.Energetic
photonscanalsobeproducedbycaptureofneutronsandby(�,p )and(�,n )
reactions.
35
Table4
Capturepercentagesofneutronsbyisotope,forpureD2OandforpureD2Owith
NCDsinstalled.
PureD2O
NCDs
3He
36.6
Ni
1.6
D2O
66.9
36.9
HinD2O
16.5
9.1
DinD2O
32.0
17.7
O(n, )inD2O
5.8
3.2
O(n,�)inD2O
12.6
6.9
Acrylic
28.4
20.9
H2OexteriortoAV
4.7
4.0
Alphaparticlesemergingfromthewalloftheproportionalcountercanleave
thesameamountofionizationinthegasasa
3He(n,p)Tevent,ascanelectrons
from�decayandComptonscattering.Evenverylow-energyevents,suchas
thedecayoftritium,cancompromisethesignalviarandomsumming.
Theprincipaldefenseagainstthesebackgroundsistominimizeradioactivity
inconstructionmaterialsandtoassureahighdegreeofcleanlinessduring
assembly.Tosupplementthosemeasures,techniquesforpulse-shapediscrim-
inationandpositionencodingareused,bothofwhichrequiredigitizingthe
pulseion-currentpro�leseventbyevent.Spuriouspulsesfrom
high-voltage
breakdownandelectromagneticinterferencemustbecontrolled.Backgrounds
ofalltypesmustnotonlybeminimized,butinadditiontheremustberobust
techniquestomeasuretheminsitu.
Becauseofthee�ciencyofheavywaterasaneutronmoderatorandthehigh
cross-sectionforneutroncaptureon
3Hearathersparsearrayofneutral-
currentdetectors(NCD)issu�cient.Anarrayof5.08cm
diameterpropor-
tionalcounterswithatotallengthof770marrangedonasquarelatticewith
1mspacinggivesaneutroncapturee�ciencyofapproximately37%(seeTa-
ble4)withtolerableinterferenceto� Cerenkovlightproducedviathecharged
currentinteraction.Approximately15%
ofuniformlygeneratedopticalpho-
tonsareabsorbed.
Atmodestpressuresof1-3atm
of
3He,thecountersareessentially\black"
tothermalneutrons.For99.85%
enrichedheavywater,themeandistance
fromthepointofgenerationtothepointofcaptureforthermalneutronsis
calculatedtobe110cmandthemeantimetocapture16mswiththeNCD
36
array,comparedwith48cmand4ms,respectively,for0.2%MgCl 2.
Thedetectorsare�lledwithagasmixtureof85%
3Heand15%
CF4
ata
totalpressureof2.5atm
toprovideagoodcompromisebetweengasgain
andstoppingpower(tomitigatea\walle�ect"whereineithertheprotonor
thetritonstrikesthewallbeforetheendofitsrange).Alowerpressure,and
thereforeoperatingvoltage,wouldsimplifymicrodischargemanagementand
increasedriftspeeds,butwouldrequirethick-walledcounterstoresistcollapse.
AtthebottomoftheSNOvessel,theabsolutepressureisthatofacolumnof
D2Oofheight16.8mplus1.3atmofairpressure,foratotalof3.1atm.
Thecopperanodewireofdiameter50�mislowinradioactivityandohmic
losses.At1650V
thegasgainisapproximately100.Forhighergasgains
positive-ionspacechargeatthewirecausestheavalanchemultiplicationto
dependincreasinglyontrackorientation.
Traceamountsofelectronegativecontaminantssuchasoxygenandwaterde-
gradeseverelytheperformanceofthegasinasealedcounter.Consequently,
countersurfacesareelectropolished,acid-etched,bakedundervacuum,and
purgedwithboilo�N2
priorto�ll.Thechemicaltreatmentalsoremovessur-
facedebrisstrippedo�themandrelduringthefabricationprocess.
The
3HegasfromtheDepartmentofEnergyfacilityinSavannahRivercon-
tainsasmallamountoftritium,about0.5mCi/l,whichisreducedto5nCi/l
orlessbypassagethroughacharcoal-loadedcoldtrapandbyrecirculation
throughaSAES,Inc.,St101getter.Atthatlevel,random
coincidencesand
pileupoftritiumdecaypulsesarenolongeraconcernintheenergyregime
ofinterest.Somedetectors,about5%
ofthetotalbylength,are�lledwith
4He:CF4
toprovideacheckonbackgrounds.
TheNCDsarefabricatedinthreedi�erentunitlengthsinorderto�llthe
spheree�ciently.(Anupperlimitissetbythedimensionsofthecageinthe
INCO
Number9shaft,whichpermitsanobject3.7m
longtobebrought
down.)Theactivelengthis13cm
lessthanthemechanicallength.Fig.11
showsthemainfeatures.Theimportantmechanicalparametersofthedetec-
torsaresummarizedinTable5.
Thebodiesoftheproportionalcountersaremadeofultrapurenickeltubes
fabricatedatMirotech(Toronto)Inc.,bythermolysisofNi(CO) 4vaporatthe
surfaceofananodizedaluminummandrelheatedto215C.Alimitednumber
ofelements(Pb,Ra,Th,andUnotamongthem)reactwithCOtoformcar-
bonyls,andthemetalsformedbychemicalvapordeposition(CVD)fromthis
precursorcanbeexpectedtobefreeofthemosttroublesomeradioisotopes.
Thenickeldeposithaspropertiesverysimilartoconventionalmetallurgical
products.RadiochemicalneutronactivationanalysishasshownThlevelsin
thebulkmaterialoforder10�
12
(1ppt)byweightorless.
37
Vec
tran
bra
id
Anc
hor
balls
Fuse
d-si
lica
insu
lato
r
Del
ay li
ne
term
inat
ion
Pinc
h-of
ffi
ll tu
be
Nic
kel e
ndca
pbo
dy
Res
istiv
e co
uple
r(c
able
end
onl
y)
Rea
dout
cab
le
Cab
le e
ndca
pw
ith a
cryl
ic s
pace
r Cou
nter
bod
y(
He-
CF
gas
mix
)3
4
Fig.11.SNOneutral-currentdetector.
Nickelisarelativelyinertmetal.TheexposedmacroscopicsurfaceareainSNO
is125m
2.In17.3M-cmwaterthemeasuredleachrateis�
22�gm
�
2d
�
1,
whichisnotexpectedtocompromisetheoperationofthewaterpuri�cation
plant.
EndcapsarealsomadebyCVD,inthiscaseonstainlesssteelmandrels,and
areweldedintothetubeswithaLumonics50-W
Nd-YAGlaserwelder.In-
sulatorsareHeraeus-AmersilSuprasilT-21syntheticfused-silicatubes.The
insulatorsareinternallycoatedwithalayerofpyrolyticgraphiteatanode
potentialtoeliminateelectric�eldsinsidethem.Theyextend2.5cm
into
thegasvolumetoactas�eldtubesandpreventmultiplicationofelectrons
fromregionswheretheelectric�eldisdistorted.Asilica-nickelsealishighly
38
Table5
ImportantphysicalparametersoftheNCDdetectors.
Item
Value
Variation
Units
Wallthickness
0.036
+0:012
�
0:000
cm
Length
200,250,and300
cm
Diameter
5.08
cm
Wirediameter
50
�m
Gaspressure
2.50
�
0.03
atm
Weight
510
gm/m
mismatched,sothedesignplacesthesilicaundercompressivestressatwork-
ingtemperaturestotakeadvantageofthehighcompressivestrengthofthat
material.TechniquesweredevelopedincollaborationwithIJResearch(Santa
Ana),Inc.tometallizethesealsandsolderthemwitha96.5:3.5eutecticSn:Ag
alloy.Countersare�lledthroughcoppertubes,whicharethenpinchedo�.
AllassemblyiscarriedoutinaClass1000cleanroom.
TheindividualunitsareweldedtogetheratthetimeofdeploymentinSNO
in96\strings"oflengthrangingfrom4mto11m.Atotalmechanicallength
of770mistobedeployedintheheavywater.Thestringsareinstalledbya
remotelyoperatedvehicle(ROV)oncethevesselis�lledwithheavywater.
Eachstringisanchoredtoanattachmentpointa�xedtothebottomofthe
acrylicvesseland oatsupward,restrainedbya exiblebraidedanchorcord
madeofVectran�ber.
Spacesof35to50cm
areleftbetweenthestringendsandthevessel.The
bottomofeachstringisterminatedwitha30nsopen-ended415delayline
tofacilitatepositionreadoutbypulse-re ectiontiming.Asingle91coaxial
cable,madeofcopperandpolyethylenebySouthBayCable(Idyllwyld,CA),
Inc.,carriessignalsfrom
thetopofeachstringuptheneckofthevesselto
preampli�ers.Thecablespeci�cgravityis0.955.
InTable6arelistedsomeofthedecaypropertiesofrelevantisotopes.Since
detectionof� Cerenkovlightistheprincipalmeansofquantifyingthephoto-
disintegrationbackgroundinsitu,theproductionoflightbyotheractivities
isalsoimportant.
Bymeansofasuiteofradioassaytechniques(neutronactivationanalysis,
radiochemicalmethods,directgammacounting,alphacounting)appliedto
samplesandtothecompleteinventoryofsmallercomponents,quantitative
predictionscanbemadeconcerningthelevelsofphotodisintegrationandother
backgroundsthatwillaccompanytheNCD
arrayinSNO.Aneutronpro-
ductionrateof100peryearintheSNO
vesselresultsfrom
1.1�gofTh
39
Table6
Photodisintegrationandhigh-energybetaemitters.
Isotope
E�(m
ax),MeV
E ,MeV
branch,%
( ,n)Probability
212Bi(!
208Tl)
1.8(18%)
2.615
36%
1/470(per )
214Bi
3.26(18%)
2.445
1.5%
1/750(per )
56Co
1.8
var.
-
1/1125(perdecay)
228Ac
2.06(11%)
var.
89%
0
234m
Pa
2.28(100%)
-
-
0
40K
1.3(89%)
1.46(EC)
11%
0
inequilibrium,or12.8�gofUinequilibrium.Slightlymorethanhalfofall
neutron-captureeventscanbeunambiguouslyidenti�edviatracklengthvsen-
ergyasbeingdistinctfromalphaparticles(forthisreasonthearraye�ciency
istakentobe25%ratherthanthenominal45%capturee�ciency).Electron
andComptonbackgrounds,andmicrodischargeevents,havetopologiessepa-
ratedstillfurtherfromneutronevents.Asaresult,photodisintegrationisthe
onlybackgroundrequiringaseparatedeterminationandsubtraction,which
canbeaccomplishedbymeasurementofthe� Cerenkovlightemittedbyra-
dioactivity.AssaysstillinprogresspreliminarilyindicatethattheNCDarray
willcontributefewerthan300neutronsperyear.
9
Control{Monitor{Alarm
(CMA)System
TheControl{Monitor{Alarm(CMA)systemisthedata-acquisitionandcon-
trolsystem
fortheundergroundlaboratoryenvironment.Thesystem
moni-
torsapproximately250analogand220digitalreadings.Majorfunctionsof
theCMAsystemare:
{Hazardousgasdetection;
{Environmentmonitoring(temperature,humidity,airpressure);
{Coolingandair-circulationcontrol;
{Loadandpositionsensingoncriticalstructuralelements;
{Electricalsystemmonitoring;
{Controlofthemagneticcompensationcoilsforreductionofthee�ectofthe
earth's�eld;
{Monitoringofsupplyvoltagesandcurrentsfortheelectronics;
{Loggingofvariousmonitoredparameters.
Thesystem
usesParagonTNTindustrialautomationsoftwarefrom
Nema-
soft[35].ThehardwarewasfurnishedasacompletesystembySciemetricIn-
40
struments[36].Analogreadingsareconvertedbyasinglemultiplexed20kHz
12-bitADC.Voltagesupto�
10VDCcanbeconvertedinsevensoftware
selectedranges.Industrial4{20mAsignalsareconvertedtovoltageswith
precisionresistors.Digitalinputsmaybe24VDCor120VAC.Thereare32
solid-staterelaychannelsfordigitaloutput.Theinput/outputscanrateisup
to2Hz.Systemswithindependentmicroprocessors(suchasthecoolingunit,
andtheuninterruptablepowersupply)haveseriallinksforASCIIoutputto
theCMA.
TheCMAsoftwareispresentlyrunningonadual200MHzPentium-Prosys-
temwith256MBytesofRAMrunningWindowsNT4.0.Remotesystemsrun-
ningParagonTNTsoftwarecanconnectviaTCP/IPwiththeunderground
system.AremotemonitoringsystemiskeptrunningintheSNOsurfacebuild-
ing.Criticalalarmsareannunciatedonthesurfaceaswellasunderground.
DataarerecordedcontinuouslyinaMicrosoftAccessdatabaseonthemain
CMAsystem,andareexportedperiodicallytothecustomSNOdatabase.The
dataloggingprogramentersanewrecordonlywhenasigni�cantchangeina
parameterhasoccurred.
10
Calibration
AccuratemeasurementofthephysicsprocessesintheSNOdetectorrequiresa
chainofcalibrationsandcalculationstoconnecttherawmeasuredquantities
ofPMTchargeandtimetoafulldescriptionoftheinteractionintermsof
energy,direction,andparticletype.Thissetofcalibrationsincludesbuilt-in
electroniccalibrations,severaldedicatedopticalcalibrators,varioustypesof
taggedanduntaggedradioactivesourcesanddetailedradioassaysofthedetec-
tormaterials.TakentogetherwithacomprehensiveMonteCarlosimulation
ofthedetector,thissetofcalibrationscreatesanoverdeterminedmodelofthe
detectorthatcanbeusedtointerpolateenergyscalesandparticleidenti�ca-
tionacrossthefullresponserangeofthedetector.Themajorcalibrationtools
arelistedinTable7andbrie ydescribedinthefollowingsections.
TheconversionofthedigitizedvaluesforPMTchargeandtimetophoto-
electrons(pe)andnanoseconds(ns)isatwo-stepprocess,involvingelectronic
pulsesandlight ashesfromknownpositionswithinthedetector.Theelec-
tronicpulsercalibrationyieldsthetransferfunctionsando�sets(pedestals).
Theestablishmentofacommontimereferenceforallchannels(t0)isper-
formedwithopticalsources.
Thechargeo�sets(pedestals)arethevaluesdigitizedwhenthereisnocharge
inputfrom
thePMT.Thetimeo�setisfoundusingafulltimeramp.Dur-
41
Table7
CalibrationdevicesfortheSNOdetectorandtheirprincipaluses.
Device
Calibrations
ElectronicPulsers
Timeslope(nspercount);Timepedestal;
Chargeslope(pCpercount);Chargepedestal
LaserBall;Sonolumi-
nescent
Source;
LED
Sources
Commontimereference(t0);D2O
absorption
andscattering(�);Acrylicabsorptionandscat-
tering(�,r,�);H2O
absorptionandscattering
(�);Re ectionandscatteringfrom
structures;
Singlephotoelectron(pe)response;Relative
singlepee�ciency;Multiplephotoelectronre-
sponse;Walk(timevs.pulseamplitude)
Radioactive
Sources:
16N,252Cf,8Li,Th,U
Gammaenergyresponse;Electronenergyre-
sponse;Neutron-capture
e�ciency;Angular
response
3H(p, )3HeAccelerator
Source
Gammaenergyresponse
TaggedSources:252Cf,
24Na,228Th
Gamma
energy
response;
Neutron-capture
e�ciency
Radioassay
Backgroundsfromconstructionmaterials;Back-
groundsfromwatercontaminants
ingcalibration,eachcell(thereare16analogmemorycellsperchannelfor
eachofthethreechargeandonetimemeasurement)isgiven10eventtrigger
pulseswithoutanyPMTinput.Becausethepedestalcalibrationisdonedur-
ingnormalrunningthereareoccasionalcoincidenceswithnoiseordetected
lightthatmustberemovedduringtheanalysisstage.Intheo�ineanalysis,
twopassesaremadethroughthedata.The�rstpassdeterminesthevalueof
thedigitizationthatoccursmostfrequently,andtheneventsthataremore
than�
10unitsawayfromthispeakdigitizedvaluearediscardedforasecond
passthatdeterminesthemean(pedestal)andRMSforeachcell.Shouldmore
thanacertainnumberofeventsberejectedwiththiscut,thecellis agged
andnotusedagainuntilrecalibrationorrepair.TheRMSistypicallyless
thanoneADCcountandcellswithanRMSgreaterthanthreecountsare
agged.Thepedestalsarerunconstantlyinbackgroundwhilesolar-neutrino
dataarebeingtaken.
Individualvoltagepulsesofvaryingwidthandamplitude(and,thus,charge)
arefedtothePMTinputstomeasurethehighgain,longintegration(QHL),
highgain,shortintegration(QHS),andlowgain,selectableintegration(QLx)
signals.Eachcellispulsedtentimesforagiveninjectedcharge(Qinj).The
chargeinjectionisincrementedinstepsuptoamaximumvalue.Alow-order
polynomial�tofdigitizedvaluevs.Qinjisperformedforeachcello�ine.
42
ThetimecalibrationsendsonepulsetothePMTinputandadelayedpulseto
theeventtrigger;thetimedi�erencebetweenthepulsesisvariedbyaknown
amountinstepsassmallas100ps.Theslopecalibrationprocedureissimilar
tothatofthechargecalibration.Therelationbetweenthetimedigitizedvalue
andnanosecondsbetweentwogeneratedpulsesismonotonicovermostofthe
range.
TherisetimeofthePMTpulsesandleading-edgediscriminationleadtoa
timingdependenceonthePMTanodecharge.Calibrationofthise�ectis
madebyrunningthelaserballatvariousintensitiesfrom
afractionofape
perPMTtohundredsofpeperPMT,andthencreatingchargeversustime
(QvT)histogramsforeachPMT.
Becausemanyoftheoptical-andparticle-calibrationtechniquesareposi-
tiondependent,ageneral-purposemanipulatorhasbeendevelopedtoprovide
sourcedeploymentataccuratelyknowno�-axispositionsintwoorthogonal
planesintheD2O.(Inaddition,sixcalibrationaccesstubesrunfrom
the
decktopositionsinthelightwater.)Thesourcemanipulatorisacarriage
locatedinaplanebytwoVectranropes(see�g.12)thatrundownthevessel
neckthroughpulleysonthecarriageto�xedlocationsontheacrylicvessel.
Athird,centerropeconnectsdirectlytothesourcecarriage.Thesourcecan
beintroducedattheneckandvisit70%oftheplanewithapositionaccuracy
ofbetterthan5cm.
Atthedeckinterfacethecarriageispulledupintoagloveboxthatispartof
thecover-gasvolume.Thereapersoncanworkonasourcewhilekeepingthe
detectorisolatedfrom
lightandairborneradioactivity.Thesourcecanthen
bepulledthroughagatevalveintoaninterlockboxwhereitcanberemoved
orserviced.
Someofthecalibrationsourcesrequirepowercable,signalcableand/orgas
capillaryconnectionstoequipmentondeck.Toprovideuniformandreliable
manipulatoroperation,theseauxiliariesarecabledintoastandardsizedum-
bilicalcable.Theumbilicalcablerunsthroughthecenterofthesourcecarriage
toacompression�ttingatthetopofthesource.Thecableconnectionsare
theninternaltothesource.Theumbilicalcablesarestoredandpaidoutfrom
apulleyarraymechanismheldundertension,therebyavoidingtwistingofthe
umbilicalcable.
Opticalsourcesareusedinboththeopticalcalibrationandtheelectronics
calibration.Theyincludeawavelength-selectabledyelaser,asonoluminescent
source,andlight-emittingdiodes(LEDs).Boththelaserandsonoluminescent
sourcehaveemissiontimeswellsuitedtomakingprecisiontimingcalibrations.
TheLEDsourcesareslightlybroaderintime,butmultiplesourcesinwell-
surveyedpositionso�erausefulandeasilyoperatedcheckonthedetector
43
Fig.12.SNOcalibrationsourcemanipulatorsystem.
geometryandstability.
Thenitrogenordyelaserwitha�ber-opticcabletoadi�userball(\laserball")
istheprimarylightsource[37].Thelaser(LaserPhotonicsInc.LN203C)
providespulsedradiationat337:1nm
witha600pspulsewidthandpeak
powerof150kW.Thelaserpulsesupto45Hzandcanbeuseddirectlyor
asapumpforseveraldyelasersthatprovidewavelengthsintherangeof
360{700nm.Neutral-density�ltersmountedintotwoselectorwheelsallow
remoteintensityadjustment.Thelaserlightisdeliveredtoadi�userballvia
a�ber-opticbundlewithinanumbilicalcable.
Thedyelaserfrequencyandattenuation�lterscanbeadjustedremotelyby
computercontrol.Theoutputfromthedyelasersisfocusedintoquartz�ber
withlength1manddiameter1mm(FiberguideInd.SFS-1000N)toenhance
beamstructurestability.
Thelaserpulsesaretransmittedtothedetectorbyabundleof22high-
OH100�msilicastep-index�bers(FiberguideInd.SFS-100/110/240/310N).
These�berso�eredthebestcombinationofUVtransmission,lowdispersion
andcost.The�ber-opticumbilicalcableterminateswitha10cm
diameter
di�userballthatservestodistributelightquasi-isotropicallythroughthede-
tector.Aplugattheneckoftheballcontainsa�ber-opticpositioningguide
44
tubeandalsohousesaminiaturemonitoringPMT(HamamatsuR5600).The
�ber-opticguidetubeallowstheumbilicalcabletobedetachedfromthelaser-
ballandreconnectedwithnoe�ectonthelightoutputpattern.Thelight
distributionpatternfromthelaserballismappedoutwithmonitorPMTsby
rotatingtheballinadarkroom.Thedi�userballlightpulsewidthisabout
2nsec(FWHM).
Asonoluminescentsourceprovidesbright,isotropiclightofextremelyshort
duration.Singlebubblesonoluminescence(SBSL)canoccurwhenanairbub-
bleisintroducedintowateratthenodeofanintense,sphericallysymmetric
sound�eld.Thelight[38]hasaveryshort(<100ps)pulsewidthandafre-
quencydistributioncorrespondingtoablackbodyatatemperatureofover
20,000K[39],verysimilartothe� Cerenkovspectrum.Therelativelyhighrep-
etitionrate(�25kHz)o�ersagoodtestofthesystem'sburstcapability.The
shortpulsewidthandisotropicoutputmaketheSBSLsourceusefulforPMT
timingcalibrationsandsomeopticalanalysis.Inaddition,theSNOdetector
providedanopportunitytoinvestigateavarietyoffundamentalpropertiesof
SBSL[40].
TheSNOSBSLsourceconsistsofa250mlspherical askwithtwocylindrical
piezoelectrictransducers.The askishousedwithinaspheremadeoftwo
15cmradiushemisphericalacrylicsectionstoprovideanairbu�er.Itis�lled
withpuredistilledwatertothenecklevel,anddrivenatthenaturalmonopole
resonance(28.5kHz)withaninductivelymatchedcircuit.Asmallmicrophone
transduceraidsinscanningthefrequencyto�ndthenaturalresonance.
Abubbleisgeneratedremotelywithanichromeheaterwirewithinthewa-
ter.Thebubblestabilityismonitoredbyobservingthespikesonthemicro-
phonevoltagetrace.Assonoluminescenceoutputbegins,thelightoutputis
monitoredwithasmallPMT(HamamatsuR5600)withintheacrylicsource
housing.Thelightoutputcontainsoforderamillionbluephotonswhichare
isotropicallydistributedinspacewhenaveragedovertimescalesofseconds
orlonger.Theheaterwirealsoprovidessomecontroloverthelightintensity,
whichdecreaseswithincreasingwatertemperature.
Lightemittingdiodesprovideasimpleandrobustopticalsourceforveri-
�cationofwatertransparency,acrylicvesselandsourcepositions,andPMT
response.Duringthe�nalconstructionstagesofthePSUP,newLEDproducts
becameavailablefromNichiaCorporation.Theseindium-galliumnitridesin-
glequantumwell(NSPB500)anddoublehetero-structure(NLPB500)devices
o�erhighintensityblueradiation(420{480nm),andthelatterdevicecan
evenemitintheUV(380nm)whenstronglyoverdriveninpulsemode[41].A
pulseofwidthapproximately7nsisobserved.Sixofthesedeviceshavebeen
packagedwithapulsercircuitandmountedatthelocations,incm,(695,248,
399),(-162,681,-463),(-699,49,-462),(-8,-764,-346),(698,54,-463),and
45
(37,1,-838)onthePSUP,wherethecenterofthePSUPis(0,0,0),thezaxisis
up,andthexandyaxesarede�nedwithrespecttoconstructioncoordinates.
Theopticalcalibrationprocessresultsinthedeterminationofattenuation
andscatteringcoe�cientsfortheheavywater,lightwater,andacrylic[45].
Byusingthevariousmeasuredopticalcoe�cients,itispossibletosimulatevia
aMonteCarlocalculationtheopticaltransportof� Cerenkovlightproduced
byaneutrinoorbackgroundparticle.Thesesimulationscanthenbeusedto
interpolate(inspaceandamplitude)betweenthevariousenergycalibrations
discussedinthenextsession.
Mostoftheopticalanalysisisperformedusingdatafromthequasi-isotropic
laserballatnearsinglephotoelectron(spe)lightintensity,andutilizesthe
detectedlightintensitiesasafunctionofthecalibratedtimebetweenthe
fastlasertriggerandthedigitizedtimevalueofthePMT
discriminators.
Lightre ectedfromvariouspartsofthedetectorproducessubstructureinthe
timedistribution.Thissubstructurecanbeusedtocharacterizetheoptical
propertiesofvariouspartsofthedetector,suchasthePMTre ectorsand
theacrylicvessel,andtodeterminetheRayleigh,Mieandlargeparticulate
contributionstothescatteringoflightinthewatervolumes[46,47].
Bymovingthelaserballtovariouso�-centerpositions,absorptioncoe�cients
canbemeasured.CalibrationsourceaccessportsintotheH2O
volumewithin
thePSUPwillaidindeterminingthelightattenuationintheacrylicpanels.
TherelativePMTanodee�cienciesandtheoveralloptical\gain"ofthede-
tectoraretobemonitoredatthespelevelovertime,andcanbeseparated
fromchangesinotheropticalconstantsbytakingdataatseveralsourceposi-
tionsandat400nmwheretheacrylictransmissionishighanduniformacross
acrylicsheets.
Therearethreegastransportsources:a
16Ngammaraysource,a
17Nneutron
sourceanda
8Libetasource[42],ofwhichthe
16Nisfullydeveloped.Thegas
transportsourcesconsistofaD-Tgeneratorneutronsource,aneutrontar-
getchamber,gascapillaries,anddecaychambersthatcanbedeployedwithin
thedetector.TheD-Tgenerator,producing14MeVneutrons,isenclosedin
concrete30mfromthedetector.Varioustargetchamberscanbemovedinto
theneutron uxbystepper-motorcontrol.Gasesfromthetargetchamberare
routedviacapillariestothedeckareaanddownintothesourceumbilicalca-
bles.Thedecaychamberscanattachtothemanipulatorcarriageandconnect
totheumbilicalcablegaslines.
The
16Nisproducedvia(n;p)ontheoxygeninCO2
gasanddecaysbyabeta
delayed6.13-MeV(66%)or7.12-MeV(4.8%)gammaraywith7.13-shalf-life.
Thedecaychamberis41.9cmlongand5.7cm
radius,andismadefrom5-
mmthickstainlesssteelsothattheelectrons(whichhavea10-MeVendpoint
46
energy)areabsorbed.Abetatagisprovidedbya3-mm
thickscintillator
sleeveontheinsideofthechamberwitha5-cmPMT.Thesourcehasbeen
testedintheSNOdetectorandproducedover300
16Ndecayspersecond.
Toreachgammaenergiesabove6MeVaminiatureacceleratorsourcedrives
the
3H(p, )4Hereactiontoproduce19.8MeVgammas[43].Thesourcecon-
sistsofacold-cathodePenningionsource,electronopticsforbeamextraction
andacceleration,anda�xedtarget.Itishousedina25-cmdiameter,60-cm
longstainlesssteelcapsuleandoperateswithabeamvoltageof20{30kVto
produceapproximatelyonephotonpersecond.
Taggedsourceshavebeendevelopedthatpermitspeci�ckindsofeventsin
SNOtobeidenti�edbytimecoincidencewithatrigger,allowingonetoac-
quirecalibrationdatainanessentiallybackground-freemanner.Suchsources
arevaluablefortestingPMTresponse,particularlyatlowenergies,fordeter-
miningbackgrounds,forcheckingneutrontransportsimulationcode,andfor
calibratingeventreconstruction.
Taggedneutronsourcesarefabricatedusing
252Cfatseveraldi�erentactivity
levels.The2.64-y
252Cfdecayswithaspontaneous�ssionbranchat3.09%,
whichprovidesneutronsandatriggerfromfragmentbetasandgammas.
AnothertaggedsourceisconstructedfromaNaIcrystalcoupledtoaPMT.
Thecrystalisactivatedtoproduce15-h
24Na.Thetriggercanbegatedon
theenergydepositedintheNaIinordertotagthe1.37MeVorthe2.75MeV
gammasinSNOtostudytheresponsetolow-energygammas,andtomea-
surephotodisintegrationneutrons.Athirdsource[44]underdevelopmentis
apolycarbonate-housedproportionalcounterwitharadioactivesource(e.g.,
228Th)depositedontheanodewire.Thesourceisdesignedsothatemitted
betaswilldepositlittleenergyinthecounterandwillmimicthespectrum
producedbynaturalradioactivitypresentinSNO.
Aftercorrectingforrelativeresponseduetopositionanddirection,anevent
isassignedanenergyaccordingtoacalibratedNHITscale.The
16Nsource
setstheabsolutescaleandprovidesameasurementoftheenergyresolution
fora6.13-MeVgammaray.Theenergyresponsetoelectronswillbeinferred
fromthismeasurementsincethedi�erenceinresponsebetweengammarays
andelectronsiswellmodeledwiththeEGS4codeandwillnotcontribute
signi�cantlytotheuncertaintyintheelectronenergyscale.Foreachevent
thereisasigni�cantprobabilityforrecordingunrelatednoisehits.Therateof
backgroundhitsisdirectlymeasuredbyregularlysamplingthedetectorusing
thepulsed{GTtrigger.Thehigh-energyendofthescaleiscalibratedwiththe
acceleratorsource.
Thedetectorangularresolutionisprimarilydeterminedfromtheoreticaltreat-
mentofelectronscatteringandproductionof� Cerenkovlight,togetherwith
47
thegeometryofthePMTarray.Thetreatmentcanbeveri�edthroughthe
MonteCarloagainstdatafromthe
16Ngammasource.Ifthe�ttedeventpo-
sitionistakentobethepointofmomentumtransferfromthegammarayto
theelectron,thentheinitialdirectionofelectronmotionforsingleCompton
interactionsisthevectortothispositionfromthe
16Nsourceposition.Acut
onlargeradialdistancesreducesthesourceangleuncertainty,whileacuton
thelargerNHITeventswillselectmostlythoseeventsoflargemomentum
transferstotheelectroninthe�rstscatter.DeconvolutionoftheCompton
scatteringanglecanbeaccomplishedwiththeMonteCarlosimulation.
11
ControlofRadioactiveContaminants
Theminimizationofbackgroundeventsrequiresahighintrinsicpurityof
materialsandthereductionofradioactivityonthesurfacesofthedetector
components.Thepurityofmaterialswasaddressedatthetimetheywere
selectedforfabricationbymeasuringtheactivityinsamples.Lowbackground
�
raycountingandneutronactivationanalysishavebeenusedextensivelyfor
analysisofdi�erentmaterialsintheSNOdetector.Sincethedetectorislocated
inanactiveminewheretherockandoredustcontainstypically60mg/gFe,
about1.1�g/gU,andabout6.4�g/gTh,whiletheinnercomponentsofthe
detectorrequireatorbelow10�
12
g/gUandTh,dustlevelshadtobekept
verylow.Aprogram
tokeepcontaminationaslowasreasonablyachievable
duringtheconstructionandoperationofthedetectorwasplannedfromthe
inceptionoftheexperiment.Thissectiondescribesthemethodsforcontrolling
andreducingsurfacecontamination.
Radon(2
22Rn)isofspecialconcernintheSNOdetector,sinceitcandi�use
fromcomponents,dustorthecavitywallsintothewaterandmoveintocritical
regionsofthedetector.Radonemanationfromcomponentsdependsondi�u-
sionpropertiesaswellastheUcontent,andhenceitvarieswiththedensity,
hardnessandsurfacepropertiesofthematerials.Extensivemeasurementsof
radonemanationwerecarriedoutinbothvacuum
andwatertestchambers
forallcriticalmaterialsfortheSNOdetector.
Becauseofthepresenceofsmallamountsofradioactivecontaminants{1:1�
0:2�g/gUand5:5�
0:5�g/gThinthehostrockand1:2�
0:1�g/gU
amd2:4�
0:2�g/gThintheconcreteliner{intheSNOdetectorcavity,radon
ingressfromthecavitywallsisapotentiallyseriousbackgroundsource.Radon
di�usionstudieswerecarriedoutonseveralpolyurethanecoatingmaterials,
bothinthelaboratoryandinanundergroundtestfacility,todeterminethe
radonattenuationpropertiesofthesesprayed-oncoatings.Tomeetthede-
signmaximum
of12radonatomspersquaremeterperhourdi�usinginto
thecavityfromrock,concreteandlinermaterials,thecoatingmaterial(Ury-
48
lonHH453and201-25polyurethanes)wasrequiredtobe8�
1mm
thick.
Thecoatingwasappliedtoasmoothconcretebaseonthecavitywallsand
oorin9separatelayers(ofalternatingwhiteandgraycolour).Thecoating
thicknesswasmonitoredwithaportableprobe,developedespeciallyforthis
work,whichmeasuredtheamountofbackscatteredx-raysfromaradioisotope
sourcemountedintheprobe[48].Thisbackscattersignalwasproportionalto
thecoatingthicknessandastandardcalibrationprocedurewasdevelopedto
givecoatingthicknessesdirectly.Thecoatingisexpectedtoattenuateradon
fromthewallsbyablockingfactorof2�
10�
7.
ContaminationcontrolatSNOhasseveralelements.Atthebeginningofas-
semblyofthedetectoracleanenvironment(airandsurfaces)wasestablished
intheundergroundlaboratoryandmaintainedthroughoutconstruction.The
entirelaboratorybecame,andcontinuestobe,acleanroom.Freshairen-
teringthelaboratoryandtheairthatiscooledandcirculatedwithinmuch
ofthelaboratorypassesthroughHEPA
�lters.Detectorcomponentswere
cleanedabovegroundandappropriatelypackagedforshipmentunderground.
Materialandequipmententerthelaboratorythroughaninterlockedareawith
high-pressurewatercleaningequipment.Personneltakewetshowersandput
oncleangarmentsbeforeenteringthelaboratoryproper.Thesetransitionar-
eas,thecarwashandthepersonnelentry,areindicatedonthegenerallayout
ofthelaboratory(seeFig.1).
Thesurfacesofthelaboratory(walls, oors,andconstructionequipment)were
cleanedrepeatedlyasnecessaryduringconstruction.Personnelpassthrough
airshowersatcriticallocations.Componentsofthedetector(suchasthe
acrylicvessel)weregivena�nalcleaningafterassembly.Componentsthat
couldnotbeaccessedforcleaningorthathavetopologicallycomplicatedsur-
faces,forexample,thePMTsandre ectors,werecoveredwhereverpossible
withcleantarpaulinsorplasticsheeting(dustcovers),whichwereremovedata
latertime.Dustcoverswereusedextensivelyintheconstructionoftheacrylic
vesselandthePSUP.Towardtheendofconstruction,theinfrastructure(plat-
forms,ladderways,ductwork,elevator,etc.)wasremovedinamannerthat
leftthedetectorcavityclean.Carewasrequiredasmuchofthisinfrastructure
wasinstalledbeforethecavitywas�rstcleaned.
Attheendofconstructionthecavitydeckwassealedtoinhibitthemove-
mentofradon,andaseparatecleanroomwasestablishedonthedeckabove
thechimneyoftheacrylicvessel.Thislocalcleanroom
enclosestheinser-
tionpointsanddevicesthatareusedinthecalibrationandoperationofthe
detectorandsubsequentlyintheinstallationoftheneutral-currentdetectors.
Tocontrolsurfacecontaminationonemustbeabletomeasureitandtomoni-
torthefactorsa�ectingit.Commercialair-particlecountersareusedtosample
thenumberandsizesof�neparticlesintheair.Thenumberofparticleslarger
49
than0.5�mindiameterin28lofambientairde�nesthe\class"oftheair
andofthecleanroom.Classvaluesvarydramaticallydependingonthework
goingoninaparticulararea.Ingeneral,thequalityoftheairwithinthelab-
oratoryhasbeenintherangeofclass1000to10,000,dependingonlocation
andlevelofactivity.Theamount(mass)ofdustthataccumulatesdepends
onthenumberversussizedistributionandonthedensityoftheparticulate.
Massversussizedistributionsforminedustweremeasuredwithacascade
impactor,andnumber-versus-sizedistributionswithanopticalmicroscope.
Thedustdepositionrateandthetotalaccumulateddustarethemostrelevant
quantities.Severalmethodswereusedtomeasuretheamountofcontamina-
tiononasurface.Aquickmethodisthe\whiteglovetest,"inwhichawhite
cloth,mountedonthesharpedgeofaneraser,iswipedoverasurfacefora
givendistance.Throughcalibrationmeasurementsandadoptionofconsistent
proceduresthistesthasbeenmadesemi-quantitative,anditissensitivedown
toabout0.5�g/cm
2
ofdust.
X-ray uorescencespectrometry(XRF)wasusedformoresensitiveandprecise
measurements.Inthismethod,onedetectselementsintherangeZ=20to
45bythecharacteristicX-raysemittedfollowing uorescenceofthesample
withanx-raytube.Aspectrometerwasbuiltforthispurposeandlocated
undergroundforconvenientuse.
InonetypeofXRFmeasurement,thin,spectroscopically-pureplastic�lms
wereusedas\witnessplates"tocollectdustoveraknownperiodoftime.
Theamountofminedustdepositedwasinferredbymeasuringtheamountof
iron,whichispresentinthelocalminedustatthelevelof7%.Asensitivity
ofabout0.1�g/cm
2
ispossibleinacountingtimeof20minutes.
Inasecondtype,dustisremovedfromasurfacebyplacingasectionofthin
adhesivetape(lowinFe)onthesurface,pullingito�andplacingitinthe
spectrometer.Whenasinglepieceoftapeisusedtoperform
�veliftsfrom
adjacentareasonasurface,thesensitivitycanbeextendeddowntoabout
0.015�g/cm
2.Thismethodwasusedtomonitorthesurfacecleanlinessofthe
acrylicvessel.
AfewdirectmeasurementsofUandThindepositeddustweremadeusing
neutron-activationanalysistoverifythecorrelationofFewithU/Th.
Duringmorethanthreeyearsofconstructionsincecleanconditionswere�rst
established,fairlyregularpatternsanddepositionratesemerged.Thequality
oftheairwascharacterizedbyanaverageclassvalueof2500�
500.Average
depositionratesrangedfrom1�g/cm
2/monthofminedustinbusyareasto
onetenththatamountininactiveareas.(Theairparticlecountsandmass
depositionratesareroughlyconsistentwiththenumber-sizedistributionsfor
airborneparticulatesmeasuredinsidethelaboratory.)Thelevelofcleanliness
50
onthesurfaceoftheacrylicvesselatthetimeofwater�llwastypicallybetter
than0.1�g/cm
2.(ZincwasalsoregularlyobservedintheXRFspectraata
leveltypically20%oftheminedust.Zinccomesmainlyfromtheuseofgalva-
nizedsca�oldinginconstruction.)Sinceamassdepositionof1�g/cm
2/month
integratedoverathirty-monthperiodwouldexceedthetargetcontamination
limitsinanyregionofthedetector,periodiccleaningofaccessiblesurfaces
andtheuseofdustcoversoninaccessibleareaswasrequiredtoachievetarget
levels.
12
O�ineAnalysisandSimulationCode{SNOMAN
Theo�inesoftwareisrequiredtoperformtwomajorfunctions:theanalysisof
SNOdata,andadetailedandcompletesimulationbyMonteCarlotechniques
ofallsigni�cantsignalsandbackgroundsusingasaccurateamodelofthede-
tectoranditsresponseaspossible.Bothofthesefunctionsarecombinedinthe
SNOMonteCarloandAnalysiscode,orSNOMAN.SNOMANconsistsofa
setoflargelyautonomousprocessorswrittenprimarilyinFORTRAN.Inorder
toeasedistributeddevelopmentandmaintenance(andtoovercomethelack
ofdynamicmemoryallocationinFORTRAN)theseprocessorscommunicate
throughacentraldatastructuremanagedbytheCERNLIBpackageZEBRA.
ZEBRAisalsothebasisforthemanagementofbanksofdata,called\titles."
Titles-managementroutinesinSNOMANinsurethatassuccessiveeventsare
analysedtheconstantsdescribingthedetectorresponsearetheappropriate
onesforthattimeanddetectorcon�guration.Theuseofautonomousproces-
sorsworkingonacentraldatastructureallowedstageddevelopmentofthe
processors.Sinceatthetimeofthiswritingthereareover25processorsin
SNOMAN,onlyselectedoneswillbediscussed.
InadditiontoSNOMANthereisanothermajoro�ineanalysistool:theSNO
database,orSNODB,whichisbasedontheCERNLIBpackageHEPDB.
SNODBisadistributedmaster-slavedatabasethatrunscollaborationwide
viaPERLinstallationandmanagementscripts.ThescriptssetupSNODB
onthelargevarietyofplatformsinuseandmanagetheupdatesthatkeep
thevariousslavecopiesofthedatabaseinstepwiththemastercopy.SNODB
containsZEBRAbanksthatencodealltheinformationaboutcalibrationsand
detectorstatus,con�guration,andperformancewhichisneededtointerpret
SNOdata.
ThebasicunitsofthedatastructurefortheMonteCarloarethevertexandthe
track.Avertexrepresentsanyeventofinterestinthemovementofaparticle
(theoriginoftheparticle,whenitencountersaboundary,anyinteractions
itundergoes,etc.),whiletracksspecifyhowparticlesmovefrom
onevertex
tothenext.TheMonteCarlocanthenbedividedintogeneration(deciding
51
whatseedverticestocreate),propagation(takingtheinitialseedverticesand
creatingsuccessivetracksandverticesbasedonthetransportofthattype
ofparticleintheSNOdetector,andcreatinganyadditionalparticlesuchas
� Cerenkovphotonswhichtheinitialparticlemayspawn),detection(eitherof
photonsbythePMTsorofneutronsbytheNCDs),andelectronicssimulation.
Inordertoachieveaccurate,usablecodewiththeleasttimeande�ortwell-
establishedandtestedsoftwarepackageshavebeenusedwhereverpossible.
Forexamplethepropagationofelectrons,positrons,andgammasishandled
byEGS4[49](modi�edtoproducetracksandvertices),thepropagationof
neutronsishandledbyroutinesliftedfromMCNP[50],themuonphysicsis
largelytakenfrom
GEANT[51],andmuchofthecodethatdescribeshow
thespectramightbemodi�edbytheMSW
e�ectistakenfromtheworkof
Hata[52].Alargeamountofworkhasgoneintoverifyingseveralaspectsof
thesemodi�edexternalcodes,whichareparticularlycriticaltoSNOphysics
butwhichmaynothavebeenextensivelytestedinthepast.Inparticulara
considerableamountofe�orthasbeenexpendedonunderstandingtheangular
distributionof� CerenkovphotonsproducedbyanelectronofafewMeV[53],
whichisimportantforsomeoftheanalysistechniquesdiscussedbelow.
TheMonteCarlocodeforwhichreliable,pre-testedsoftwarelikeEGS4was
notavailablewaswrittenbytheSNO
collaboration.Thephotonpropaga-
tionincludesthee�ectsofattenuation,refraction,re ection,andRayleigh
scattering(Miescatteringisexpectedtobenegligibleforthelevelsofclean-
linessexpectedinSNO)asafunctionofwavelengthandpolarisation.The
lightconcentrator/PMTresponsehasbeenthefocusofsubstantialsoftware
developmentandexperimentale�ort.Adetailedstand-alonesimulationofa
concentratorandPMTwaswritten[54]andtestedagainstexperiment[55],
theninstalledasaSNOMANprocessor.
Thegeometryofthedetectorisacompromisebetweenatotallydata-driven
systemthatwouldbeslowbut exibleandacompletelyhard-wiredstructure
thatwouldbefastbutin exible.Thegeometryconsistsofdetectorelements,
eachofwhichismadeupofprimitives(suchasspheres,cylinders,elliptical
toroids,etc.)forwhichtheactualcalculationsaremade.Theabilitytoal-
terthecomplexityofthegeometrysimulationallowstheusertoswitcho�
featuresthatarecomputationallyexpensiveinsituationswheretheydonot
signi�cantlya�ecttheresults.
Theactualdatafrom
theSNOdetectorarewrittentotapeintheform
of
packedZEBRAbanks.SNOMANprocessorsunpackthedataandthencali-
brateitbyapplyingpedestal,slopeando�setstotheADCdatatoproduce
PMThitswithcorrectedtimesandcharges(otherprocessorsexisttouncal-
ibrateandthenpacktheMonteCarlodata,thusallowingthewholedata
streamtobetestedwithMonteCarlodata).Afterthecalibrationprocessor,
thedatahandlingisidenticalforMonteCarloanddetectordata.Anumber
52
ofprocessorsthenexisttoderiveadditionalinformationfrom
thePMThit
timesandpositions.The�rsttypeofthesearecalled\�tters."Thesederive
aneventpositionanddirectionfrom
thePMTdata.Anumberofdi�erent
algorithmsexistforthispurpose.Inadditiontothe�ttersthereareproces-
sorscalledclassi�ersthatderiveinformationfromtheangulardistributionof
hitsintheevent.Thisinformationhasbeenshowntobeapowerfultoolfor
discriminatingbetweenneutral-currentandcharged-currentevents[56]and
formeasuringthemostimportantbackgroundeventsdirectly[57].
Oncealloftheseprocessorshave�nishedtheresultingdatacanbeviewed
usingtwotools{theeventviewerandtheeventanalyzer.Theeventvieweris
agraphicaldisplayoftheSNOdetectorwhichdisplaysthevariousdetector
elementsandPMThitsin3Dandanumberof2D
formats.Theanalyzer
isadata-driventoolforsiftingthroughtheentriesinthedatastructure(or
throughquantitiesderivedfromtheentriesinthedatastructurebyoneofa
largenumberofsuppliedfunctions)to�ndthosethatsatisfyeitherindividual
cutsorlogicalcombinationsofanumberofcutsappliedtodi�erentquantities.
Anyentriesorquantitieswhichpassthecutsarewrittenasanentryina
CERNLIBHBOOK
ntupleforlateradditionalanalysiswithPAW
[58].A
slightlyunusualfeatureofSNO(fromthepointofviewofatypicalhigh-energy
physicsexperiment)isthatsomeeventsarecorrelatedintimeorposition(the
spallationproductsfollowingathroughmuon,forexample).Thisinformation
canbeextractedfrom
thedatausingthetimecorrelationprocessor,which
allowstheanalyzertocombineinformationfromdi�erenteventsthatsatisfy
conditionsplacedontheirrelativetimeorpositionintoasinglentupleentry.
13
Conclusion
TheSudburyNeutrinoObservatoryisalarge,second-generationneutrinode-
tectorwithactive�ducialvolumesconsistingof1000tonnesofhighlypuri�ed
heavywaterplusabout1000tonnesofpuri�edlightwater.Thedetectorwill
providehighstatisticsdatafrom
naturalneutrinosources,withanempha-
sisonthestudyofsolarneutrinosfrom
8Bdecayandwhetherornotthey
undergo avoroscillationsastheypropagatefromthecoreofthesuntothe
earth.TheabilityofSNOtodistinguishcharged-currentandneutral-current
neutrinointeractionsisuniqueamongpresent-daysolarneutrinodetectors
thatareoperatingorunderconstruction.SNOisalsotheonlyneutrinode-
tectorthatcanexclusivelyidentifyantineutrinointeractions.Theunusually
deeplocationofSNOandparticularattentiontominimizationofradioactive
backgroundsmakespossiblehighsignal-to-backgroundmeasurements.
A
seriesofengineeringrunstotestdetectorperformanceandcommission
variousdetectorsystemsbeganwithoutwaterpresentinthedetectorcavity
53
inSeptember,1997.SNOwascompletely�lledwithheavyandlightwateron
April30,1999,atwhichtimefullsystemoperationcommenced.Thedetector
willberunwithpureD2O
untiltheSNO
collaborationdeterminesthata
physicallyinterestingdatasethasbeenaccumulated.Then,thedetectorwill
bemodi�edtoenhanceitsneutral-currentdetectioncapabilities.
TheresultingdatasetshouldenableSNOtomakeade�nitivemeasurementof
thefractionofelectronneutrinosinthesolarneutrino ux,distinctfromall
previousmeasurementsinthatitwillbeindependentoftheoreticalmodelsof
thesun.Thisandothermeasurementsareexpectedtodeepentheunderstand-
ingofthepropertiesofneutrinosandoftheastrophysicalsitesfrom
which
theyoriginate.
Acknowledgements
Thisresearchhasbeen�nanciallysupportedinCanadabytheNaturalSci-
encesandEngineeringResearchCouncil,IndustryCanada,NationalResearch
CouncilofCanada,NorthernOntarioHeritageFundCorporationandthe
ProvinceofOntario,intheUnitedStatesbytheDepartmentofEnergy,andin
theUnitedKingdombytheScienceandEngineeringResearchCouncilandthe
ParticlePhysicsandAstronomyResearchCouncil.Furthersupportwaspro-
videdbyINCO,AtomicEnergyofCanadaLimited(AECL),Agra-Monenco,
Canatom,CanadianMicroelectronicsCorporationandNorthernTelecom.The
heavywaterhasbeenloanedbyAECLwiththecooperationofOntarioHy-
dro.TheprovisionofINCOofanundergroundsiteisgreatlyappreciated.The
collaborationwishestoexpressgratitudeforallthesupportprovidedtomake
theexperimentpossible.
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