D O - Queen's Universityowl.phy.queensu.ca/sno/papers/nim_paper_99_2up.pdf · This matter, or MSW...

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�edeNeuchâtel,NeuchâtelCH-

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,Technopôle,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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