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EURECA Conceptual Design Report
G. Angloher, E. Armengaud, C. Augier, Angélique Benoit, T. Bergmann, J.
Blümeri, A. Broniatowski, V. Brudanin, P. Camus, A. Cazes, et al.
To cite this version:
ContentslistsavailableatScienceDirect
Physics
of
the
Dark
Universe
journal homepage:www.elsevier.com/locate/dark
EURECA
Conceptual
Design
Report
The
EURECA
Collaboration
G.
Angloher
n,
E.
Armengaud
b,
C.
Augier
g,
A.
Benoit
f,
T.
Bergmann
k,
J.
Bl
¨umer
i,j,
A.
Broniatowski
e,
V.
Brudanin
h,
P.
Camus
f,
A.
Cazes
g,
M.
Chapellier
e,
N.
Coron
c,
G.A.
Cox
i,
C.
Cuesta
s,
F.A.
Danevich
l,
M.
De
J
´esus
g,
L.
Dumoulin
e,
K.
Eitel
j,
A.
Erb
o,
A.
Ertl
o,
F.
von
Feilitzsch
o,
D.
Filosofov
h,
N.
Fourches
b,
E.
Garc
´ıa
s,
J.
Gascon
g,
G.
Gerbier
b,
C.
Ginestra
s,
J.
Gironnet
g,
A.
Giuliani
e,
M.
Gros
b,
A.
G
¨utlein
o,
D.
Hauff
n,
S.
Henry
p,
G.
Heuermann
i,
J.
Jochum
r,
S.
Jokisch
j,
A.
Juillard
g,
C.
Kister
n,
M.
Kleifges
k,
H.
Kluck
i,
E.V.
Korolkova
q,
V.Y.
Kozlov
j,
H.
Kraus
p,
V.A.
Kudryavtsev
q,
J.-C.
Lanfranchi
o,
P.
Loaiza
m,
J.
Loebell
r,
I.
Machulin
u,
S.
Marnieros
e,
M.
Mart
´ınez
s,
A.
Menshikov
k,
A.
M
¨unster
o,
X.-F.
Navick
b,
C.
Nones
b,
Y.
Ortigoza
s,
P.
Pari
a,
F.
Petricca
n,
W.
Potzel
o,
P.P.
Povinec
t,
F.
Pr
¨obst
n,
J.
Puimed
´on
s,
F.
Reindl
n,
M.
Robinson
q,
T.
Rol
´on
s,
S.
Roth
o,
K.
Rottler
r,
S.
Rozov
h,
C.
Sailer
r,
A.
Salinas
s,
V.
Sanglard
g,
M.L.
Sarsa
s,
K.
Sch
¨affner
n,
B.
Schmidt
i,
S.
Scholl
o,
S.
Sch
¨onert
o,
W.
Seidel
n,
B.
Siebenborn
i,
M.
v.
Sivers
o,
C.
Strandhagen
r,
R.
Strauß
o,
A.
Tanzke
n,
V.I.
Tretyak
l,
M.
Turad
r,
A.
Ulrich
o,
I.
Usherov
r,
P.
Veber
d,
M.
Velazquez
d,
J.A.
Villar
s,
O.
Viraphong
d,
R.J.
Walker
b,i,
S.
Wawoczny
o,
M.
Weber
k,
M.
Willers
o,
M.
W
¨ustrich
o,
E.
Yakushev
h,
X.
Zhang
p,
A.
Z
¨oller
oa CEA, Centre d ’ ´Etudes Nucl ´eaires de Saclay, IRAMIS, 91191 Gif-sur-Yvette Cedex, France b CEA, Centre d ’ ´Etudes Nucl ´eaires de Saclay, IRFU, 91191 Gif-sur-Yvette Cedex, France c CNRS, Institut d ’ Astrophysique Spatiale, Universit ´e Paris 11, Orsay 91405, France
d CNRS, Universit ´e de Bordeaux, ICMCB, 87 avenue du Dr. A. Schweitzer, Pessac cedex, 33608, France
e Centre de Spectroscopie Nucl ´eaire et de Spectroscopie de Masse, UMR8609 IN2P3-CNRS, Univ. Paris Sud, Orsay Campus, 91405, France f Institut N ´eel, CNRS, 38042 Grenoble cedex 9, France
g IPNL, Universit ´e de Lyon, Universit ´e Lyon 1, CNRS / IN2P3, 4 rue E. Fermi, Villeurbanne 69622, France h Laboratory of Nuclear Problems, JINR, Dubna 141980, Russian Federation
i Karlsruhe Institute of Technology, Institut f ¨ur Experimentelle Kernphysik, Karlsruhe 76128, Germany j Karlsruhe Institute of Technology, Institut f ¨ur Kernphysik, Karlsruhe 76021, Germany
k Karlsruhe Institute of Technology, Institut f ¨ur Prozessdatenverarbeitung und Elektronik, Karlsruhe 76021, Germany l Institute for Nuclear Research, MSP 03680 Kyiv, Ukraine
m Laboratoire Souterrain de Modane, CEA-CNRS, Modane 73500, France n Max-Planck-Institut f ¨ur Physik, 80805 M ¨unchen, Germany
o Physik-Department E15, Technische Universit ¨at M ¨unchen, Garching 85747, Germany p University of Oxford, Department of Physics, Keble Road, Oxford OX1 3RH, UK q University of Sheffield, Department of Physics and Astronomy, Sheffield S3 7RH, UK r Eberhard-Karls-Universit ¨at T ¨ubingen, T ¨ubingen 72076, Germany
s Laboratorio de F ´ısica Nuclear y Astropart ´ıculas, Pedro Cerbuna 12, Universidad de Zaragoza, Zaragoza 50009, Spain t Comenius University, Department of Nuclear Physics and Biophysics, Bratislava 84248, Slovakia
u NRC “Kurchatov Institute”, Kurchatov sq. 1, Moscow, Russia
a
r
t
i
c
l
e
i
n
f
o
Keywords: Dark matter WIMPs Direct detection Cryogenic bolometers EURECA experimenta
b
s
t
r
a
c
t
The EURECA (European Underground Rare Event Calorimeter Array) project is aimed at searching for dark matter particles using cryogenic bolometers. The proponents of the present project have decided to pool their strengths and expertise to build a facility to house up to 1000 kg of detectors, EURECA, consisting in the first instance of germanium and CaWO 4crystals. The shielding will be provided through a large water
tank in which the cryostat with detectors will be immersed. The EURECA infrastructure will be an essential tool for the community interested in using cryogenic detectors for dark matter searches. Beyond European detectors, it will be designed to host other types of similar cryogenic detectors, requiring millikelvin operating temperatures. In particular, this includes the germanium detectors currently in use by the SuperCDMS team, following the current collaborative work performed by the EURECA and SuperCDMS collaborations. EURECA
2212-6864/ $ - see front matter c 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http: // creativecommons.org / licenses / by /
3.0 /).
will have two stages. The first phase aims at a sensitivity of 3 · 10 −10pb and will involve building the
infrastructure, cryostat and shielding, and operating 150 kg of detectors. The second phase will be completed with 850 kg of additional detectors, the relative weight between the different detectors being decided by the collaboration according to the physics reach. A sensitivity of 2 · 10 −11pb is aimed for at the second stage.
EURECA will ideally benefit from the planned extension of the deepest underground laboratory in Europe – LSM. With a site-independent design, it can also be hosted in other locations at similar or deeper sites such as SNOLAB.
c
2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/3.0/).
1. Executivesummary
The EURECA (European Underground Rare Event Calorimeter
Array)projectispartofthequestforunderstandingthecomposition ofouruniverseandinparticularthelargefraction,26.8%[1],ofits un-knownmass-energydensitycalleddarkmatter.Thisquestion,raised 80yearsago,hasstillnoanswerandthereisafierceracetowardsthe directdetectionofdarkmatterparticles.Manyastrophysical obser-vationsconvergeontheassumptionthatitwouldconsistofparticles stillunknown,asyetundetectedandfugitive,presentsincetheearly momentsoftheuniverse.
Manyexperimentsaroundtheworld,alllocatedinunderground environments,aimto trackdown thepresence oftheseparticles. Amongthese,EDELWEISS,locatedattheLSMlaboratory,andCRESST, installedattheGranSassolaboratory,arecryogenicexperiments in-volvingsolidstatedetectorscooledtoverylowtemperatures.
TheexperimentsattheLHChavenotshownanyhintssofar,asof March2013,ofSUSY,thefavouredtheoryprovidinganatural candi-datefordarkmatter.However,thequestionoftheexistenceofdark matterintheuniverseandthequestionofwhichtheorydescribes bestbeyondstandardmodelphysicsaredifferent.Assuch,direct de-tectiondarkmatterexperimentsremainmuchinasituationwhere theattainmentofcontinualimprovementonupperlimitsfor WIMP-nucleoncrosssectionsthroughenhancingthedetectors’sensitivities remainsaveryimportantobjective.
Inthepresentsituation,wherewedonotyethaveexperimental proofofSUSY,norconvincingevidenceoftheexistenceofWIMPs bydirectdetectionexperiments,thenextgenerationofdirect detec-tionexperiments,providinganincreaseofsensitivitybytwoorders ofmagnitude,willbeneededtotackletheissueofthedarkmatter nature.TherecentavatarsoflowmassWIMPhints,notconsidered yetasevidence[2],confirmthatseveralexperimentswith comple-mentarydetectionprincipleswillbeneededtoestablishaconclusive demonstrationofsignalidentification.Thespecificstrengthsof cryo-genicdetectors,namelytheirverygoodenergyresolution,excellent discriminationagainstbackgroundandtheredundancyofferedby theuseofmultipletargetnucleiinthesamecryogenicinfrastructure, equipEURECAwithasignificantadvantagefortheidentificationofa darkmattersignal.
Theproponentsofthepresentproject,mostlyoriginatingfromthe EDELWEISS,CRESSTandROSEBUDcollaborations,thereforehave de-cidedtopooltheirstrengthsandexpertisetobuildafacilitytohouse upto1000kgofdetectors,EURECA,consistinginthefirstinstanceof germaniumandCaWO4detectors.Thedetectorswillbemountedin
subunitsoftowers,housedina6-m3cryostat,withshieldingprovided
byalargewatertank.InnovativecryogenicswillrelyonEuropean expertiseinthefield,andtogetherwithadaptablecablingand elec-tronicsoptions,theEURECAinfrastructurewillbeanessentialtoolfor thecommunityinterestedintheuseofcryogenicdetectorsfordark mattersearches.BeyondEuropeandetectors,itwillbedesignedto hostothertypesofsimilarcryogenicdetectors,requiringmillikelvin
operatingtemperatures.Inparticular,thisincludesthegermanium detectorscurrentlyinusebytheSuperCDMSteam,followingthe cur-rentcollaborativeworkperformedbytheEURECAandSuperCDMS collaborations.
EURECAwillhavetwostages.Thefirstphaseaimsatasensitivityof 3· 10−10pbandwillinvolvebuildingtheinfrastructure,cryostatand shielding,andoperating150kgofdetectors.Thesecondphasewill becompletedwith850kgofadditionaldetectors,therelativeweight betweenthedifferentdetectorsbeingdecidedwithinthe collabora-tionaccordingtothephysicsreach.Afinalsensitivityof2· 10−11pb
isaimedfor.
EURECAwillideallybenefitfromnewspaceprovidedbyanew plannedextensionoftheLSM,acavityprojectcalledDOMUS. Fur-thermore,thesiteofFr´ejusisbestsuitedtothisresearchbeingthe deepestinEurope.Withasite-independentdesign,itcanbehosted inotherlocationsatsimilardepthsorevendeepersiteslikeSNOLAB. Suchauniquefacility,plannedtooperatelong-term,willbe de-signedtoensureanexceptionallyradio-pureenvironmentinorderto provideoptimalsensitivityfordarkmattersearchescurrentlyunder design.
2. Science
Thissectiondescribesthecurrentcontextofdarkmattersearches andtheplacethatEURECA,theEuropeanUndergroundRareEvent CalorimeterArray,willoccupywithinthescientificlandscape.Table 1summarisestheobjectivesofEURECA,splitintotwophases,starting withatargetmassof150kginPhase1andreachingthedesignvalue of1000kginPhase2:
Table 1
Objectives of EURECA
EURECA Goals
Phase 1
Cross section (SI) 3 · 10 −10 pb Mass to be operated 150 kg
Residual background (all sources) 10 −2 evts / kg / y in ROI
Duty cycle 70%
Time of operation 1 year
Phase 2
Cross section (SI) 2 · 10 −11 pb Mass to be operated 1000 kg
Residual background (all sources) < 10 −3 evts / kg / y in ROI
Duty cycle 70%
Time of operation 3 years
Figure 1. The sensitivities obtained by the experiments EDELWEISS-II (green) [ 6 ], CDMS (grey) [ 10 ], EDELWEISS-CDMS combined (cyan) [ 11 ], ZEPLIN-III (magenta) [ 12 ] and XENON100 (blue) [ 13 ]; the potential sensitivity of EDELWEISS-III (dashed green) with a 12 000 kg ·d background-free exposure; and the position that the proposed ex- periments of SuperCDMS (dashed grey) [ 14 ], XENON1T (dashed blue) [ 15 ] and EURECA (Phase 1: upper dashed red, Phase 2: lower dashed red) will play in their improvement. The sensitivity for EURECA Phase 2 is as presented in Table 1 , with one event / t / y as- suming a 50:50 mass split of Ge:CaWO4, 5- (15-)keV threshold for 200 kg (300 kg) of Ge and a 10-keV threshold for CaWO 4 as presented in this CDR. Sensitivity for Phase 1 is 15% scaled by mass from Phase 2, for 70% duty cycle of one year. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.1. Scientificenvironment
TheEURECAprojectispartofthequestforunderstandingthe com-positionofouruniverseandinparticularthelargefraction,26.8%,of itsunknownmass-energydensitycalleddarkmatter[1].This ques-tion,raised80yearsago,stillhasnoanswerandthereisafiercerace towardsthedirectdetectionofdarkmatterparticles.
Manyastrophysicalobservationsconvergeontheassumptionthat it wouldconsist ofparticlesstillunknown, asyet undetectedand fugitive,presentsincethebirth ofthe universe.Whiletheoretical models have providedmany candidates,which span50orders of magnitudeofmassandcrosssection,themostmotivatedcandidates aretheweaklyinteractingmassiveparticle(WIMP),theaxionand possiblyasterileneutrino.Inanycase,darkmatterrequiresphysics beyondthestandardmodelofparticlephysics.
ThefavouredcandidatefordarkmatterisaWIMPrelatedtonew physicsattheTeVscale.AmongthevariousWIMPcandidatesatthis scale,themostappealingisthelightestsuper-symmetric(SUSY) par-ticle(LSP),theneutralino,whichisembeddedinaconsistentmodel, e.g.theminimalsupersymmetricstandardmodel(MSSM).TheMSSM postulatesthattheeffectivescaleofSUSYbreakingisaroundaTeV andthatthereisanadditionalsymmetrycalledR-paritywhichwould keepthelightestSUSYparticlestable.Otherextensionsofthe stan-dardmodelprovidesuitableWIMPdarkmattercandidates,e.g.the lightKaluza-Kleinparticle(LKP)inextra-dimensionaltheories[3]and thelightestT-oddparticle(LTP)inLittleHiggsmodels[4].
TheparameterspaceofWIMPsislargelycoveredbytheproposed cryogenicdirectdarkmattersearch,whichcouldprobedarkmatter particles withamass intherangefromGeVto TeVwitha spin-independentWIMP-nucleoncrosssectiondowntoafew10−11 pb.
Currentdata,obtainedattheLHC,donotshowanyhintofSUSY particlesinproton-protoncollisionsforatotalintegratedluminosity of10.5fb−1 [5]up toan energyof√s =8TeV.Noindicationof
particlespredictedbyextra-dimensional modelsorbyLittleHiggs modelshaveshownupeither.However,theremainingparameter spaceisplannedtobeinvestigatedaftertherestartoftheLHCin 2015,withfinalintegratedluminositygoalsof250fb−1by2020and 3000fb−1by2030.
Nonetheless,thediscoveryofacandidateparticlefordarkmatter attheLHCalonedoesnotprovethatitistheCDMparticlerequired bycosmology.Forthatpurpose,thedetectionofcosmologicalWIMPs isnecessary.Conversely,thedetectionofcosmologicalWIMPswould shownewphysicsbutnotprovethattheyareSUSYparticles.Forthat purpose,identificationandinvestigationatacceleratorsisnecessary. ThesynergybetweentheLHCandthenextgenerationofdarkmatter searchesisobviousandconstitutesanexcitingperspective.
Inthepresentsituation,wherewedonothaveexperimentalproof ofphysicsbeyondtheStandardModelattheLHC,norconvincing evidenceoftheexistenceofdarkmatterparticlesbydirectdetection experiments,theEURECAproject,byoperatingcryogenicdetectors ofuptoonetontargetmassandaddressinganincreaseofsensitivity bytwo(three)ordersofmagnitudeathigh(low)mass,asshownin
Fig.1,isanessentialtooltotacklethemysteryofdarkmatter.
2.2. Scientificcase
Manyexperimentsaroundtheworld,alllocatedunderground,aim attrackingdownthepresenceofdarkmatter.EDELWEISS[6,7], lo-catedattheLaboratoireSouterraindeModane(LSM)undertheFr´ejus mountain,andCRESST[8,9],installedattheGranSassolaboratory,are ‘cryogenic’experimentsinvolvingsolidstatedetectorscooledtovery lowtemperatures.Theystillhavenotshownevidenceofthese parti-clesandwillbelimitedinthecomingyearsintheirsensitivitybythe sizeofthecryostatshousingthedetectors.
Cryogenicdetectors,asdemonstratedbythelatestpublished re-sults,exhibitexcellentcapabilityfortheirexploitationinfacilitiesfor thedirectdetectionofdarkmatterparticles.Thequalityofthenext generationofcryogenicgermaniumdetectors,withsignificantly im-proveddiscriminationpoweragainstbackgroundevents,issuchthat onecananticipatetorunmassesoftheorderof100-1000kgwithout significantbackground.
TheteamsoftheEDELWEISSandCRESSTcollaborations,together withotherEuropeanteams,inparticularfromtheROSEBUD collab-oration,leadinginnovativeR&Donnewscintillatingdetectors,have decidedtopooltheir strengthsandexpertisetobuildafacilityto houseupto1000kgofdetectors:EURECA.
Suchanextremelysensitivesearchshouldbecarriedoutinthe bestpossibleenvironment,thatisinthedeepestsiteinEurope,to reduceasmuchaspossiblethecosmicmuon-inducedbackgrounds. ThisledtheEURECAcollaborationtofavourtheFr´ejussiteastheideal locationoftheexperiment.
Withthecurrentbaselinedesign,consistingofashieldofalarge watertankinwhichwillbeimmersedthecryostathousingthe de-tectors,EURECAistoolargetofitwithintheexistingcavityofthe Fr´ejuslaboratory.Itwillideallybenefitfromtheextensionofthe laboratory,theDeepObservatoryforMultidisciplinaryUnderground Science(DOMUS),plannedtobeduginconjunctionwiththealready startedsafetytunnel(diggingfromFrancereachedthepositionofthe currentLSMlaboratoryinMarch2013)oftheFr´ejusroadtunnel.In caseofcancellationofthisextensionproject,analternativedesign ofamixedwater
/
denseshieldmayallowEURECAtofitwithinthe availablespaceofthelab.EURECAwillconsistofasinglecryostat,installedinadedicated watertankandwillhave twostages.Thefirst phasewill involve acryostatandshieldingwith150kgofdetectorswhilethesecond phasewillbecompletedwith850kgofadditionaldetectors.The two-phasedapproachprovidesuswiththeopportunitytohaveastaged productionofthelargenumberofindividualdetectors.
targetnucleiinordertobeabletoprovethattheobservedratesand energyspectraofapossiblecandidateshowtheexpectedcorrelation ofaWIMPsignal.Itwillbedesignedtooperatevarioustypesof cryo-genicdetectors:germaniumbolometerscurrentlydevelopedwithin EDELWEISS,measuringheatandionisationsignals;andCaWO4
scin-tillatingbolometersallowingmeasurementoflightandheatsignals, developedbyCRESST.R&Disongoingtocharacteriseother scintillat-ingtargets,specificallythoseincludingnucleiwithenhanced sensitiv-itytospin-dependentinteractions,andlow-massWIMPs.Depending ontheachievedperformance,oneofthesematerialscouldbe incor-poratedasanadditionaltargetinEURECA,mostlikelyinthesecond phase.
ForgermaniumdetectorsdevelopedwithinEDELWEISS,the in-creaseofsensitivitywillbeobtainedinthreeways:decreaseofthe localradioactivebackgroundbyselectionofmaterialsandimproved surfacetreatment,increasedrejectioncapabilityagainstgammarays andbetas,andimprovement ofenergyresolutionsallowinglower thresholds.Thesethreeavenuesofdevelopmentarebeingpursued withinEDELWEISS-III,andtheobtainedresultsallowtoextrapolate totherequiredsensitivityforEURECA.
Scintillation-phonondetectorsprovideanexcellentsuppressionof gammabackground.Sofarthispotentialcouldnotyetbefullyshown duetoeffectsrelated toan alphabackground, thisiswhyefforts toincreasethesensitivityarethereforedominatedbyavoidingand taggingthealpharelatedbackgrounds.
Differenttypesofrecoilingnucleicantosomeextentbe distin-guishedwithinonedetectormodule.WiththepresentlyusedCaWO4
detectorsoftheCRESSTexperiment,oxygenrecoilscanclearlybe dis-tinguishedfromtungstenrecoilsbytheirdifferentlightyield(ratio betweenlightandphononsignal).Thus,suchdetectorsarecapableof partiallydiscriminatinganeutronbackgroundfromasignalcaused byaWIMPheavierthan∼20 GeV.
Thedetectorsmentionedaboveusedifferentreadoutschemes, whichwillneedflexibilityinthedesignofwiringandelectronics housingoftheEURECAcryostat.Suchflexibilitywillalsobeakey featureallowingtohouseothertypesofdetectors.
Othertypesofdetectors arebeingdevelopedwithin aparallel projectleadbytheAmericanteamofSuperCDMS,thesuccessorof theCDMScollaboration.Thisprogram,whosegoalistooperate150 kgofgermaniumdetectors,isverysimilartoEURECAPhase1.Indeed, theverysimilarqualityofdataobtainedsofarbytheEDELWEISSand CDMScollaborationsledthetwoteamstocombinebothdatasets toproduceanimprovedlimitrelativetoeachindividualresult[11]. Thesuccessofthiscollaborativeworkledthetwocollaborationsto startmoreconcreteworkonthedesignoftheirnextgenerationof theprojectsdescribedabove,inordertopossiblyexchangedetectors andfurthercombinefuturedatasets.
Other100-kgtotonne-scaleprojectsarebeingpursuedwithliquid noblegases.Self-shieldingeffectsandcapabilityofthelocalisationof interactionsaredistinctivefeaturesofliquidnoblegastwo-phase technology.ThemostadvancedareXENON1T(two-phaseliquidXe, 1000-kgfiducialmass),LUX(two-phase liquidXe,100-kgfiducial mass),andXMASS(single-phaseliquidXe,100-kgfiducialmass).
Withtheirlowthresholdandgoodenergyresolution,the cryo-genicdetectorsexhibitverygoodpotentialforreliablydetecting low-massWIMPs.Theverydifferentdetectiontechniquesmakethetwo approachescomplementary,astronganddecisiveadvantageincase apossiblecandidateisseenbyoneofthedetectorsandneedstobe cross-checkedandconfirmedbytheother.
Insummary,suchauniquesetupinEurope,plannedtooperate long-term,willbedesignedtoprovideanexceptionallygood radio-pureenvironment.Itwillbedesignedtohouse,inthefirstinstance, detectorsdedicatedtothesearchfordarkmatter.Inparticular, elec-tronicsandcablingwillbe designed tooperate differenttypesof cryogenicdetectors,whethersemiconductorssuchasgermanium,or scintillatingtypeasCaWO4,oranyothercryogenicdetectors.
3. Infrastructure
AlthoughEURECAcanbeplacedinanysuitablysizedunderground location,discussionherefollowsthepreferredlocationoftheLSM extension(DOMUS).Aviewoftheoveralllayoutoftheproposednew spaceisshowninFig.2.Herewedescribetheinterfacesbetweenthe hostlaboratoryandtheEURECAfacility.Table2summarisessomeof thekeyfeaturesoftheEURECAinfrastructure.
Figure 2. Position of the new facility, towards the top of the image, with respect to the existing road (lowermost) and laboratory space (shaded).
Table 2
Key features of the EURECA infrastructure
Infrastructure Baseline option Space
EURECA volume 10 × 24-m footprint × 12-m height Cryostat 2-m diameter × 2-m height Water shield tank 8-m diameter × 12-m height Water buffer 6.5-m diameter × 12-m height Infrastructure building 6 × 8-m footprint × 12-m height, 3 or 4
storeys
Cleanroom suite 48-m 2 footprint × 3-m height Cryogenics 6 × 5-m footprint × 3-m height
Power, volume, flows and data rate
Power peak 270 kW
Power running 230 kW Low-Rn air flux 150 m 3/ h Water volume 400 m 3
Cooling need equivalent to 8 m 3/ h of water Shielding water recirculation 2 m 3/ h
Figure 3. Layout of the main water tank (left) containing the cryostat, adjacent infras- tructure and cleanroom tower, and water storage tank (right) within the 50-m-long hall of the DOMUS lab. Proximity cryogenics are shown within the cold box connected to the cryostat. Clearance of at least 1 m has been included between the cavern walls and the experimental infrastructure. The yellow line indicates the floor space available within the current LSM, although with the reduced height of only 10 m. (For interpre- tation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.1. Generalexperimentinfrastructure
AschematicforthehousingoftheexperimentisshowninFig. 3.Thedetectorswillbeplacedinacryostatwithinawatertankto shieldagainstneutronsandgamma-rays.Abuildingislocatednext tothetankthroughwhichaccesstothecryostatwillbeprovided. Anadditionaltankwillbeplacedinthevicinityoftheexperiment forstorageofwaterwhenaccesstothecryostatisrequired.Some elementsofthecryogenicssystemwillbelocatedinimmediate prox-imitytothecryostat,housingthedetectors.Theotherelementsof thecryogenicsystems,especiallylargeplant(compressors,cryogenic machines)pronetocontributenoiseandvibration,willbeplacedat somedistancefromthecryostat.Furtherdetailsareprovidedbelow.
3.1.1. Housingoftheexperiment
Thedetectorcryostatwillbesurroundedbyatleast3mof ultra-purewater,ineachdirection,withinastainlesssteelwatertank.Next tothiswillbetheinfrastructuretowerwhichwillincludespacefor storageofmaterials,slowcontrolandDAQsystems,water purifica-tionplant,andacleanroom.Thecleanroomisplacedatthetopofthe towerandservesasaccesstothewatertankwithcleanroomintegrity extendingintothewatertank.Thetankissealedatthepointof en-trybymeansofanairlocktopreservethelow-Rnatmosphere.All connectors(electrical,cryogenic,vacuum)throughthewatercontain bends,orincludesufficientsolidshielding,topreventadirectpath forneutrons.Anadditionalwatertankislocatedbythesideofthe towerforuseasawaterstoragelocationwhenaccesstothecryostat isneeded.Theextratankwillbematchedinheighttothewatertank tominimisetheamountoffloorspaceused,butwillbeofasmaller volumesincenointernalaccessisrequired.
Foracryostatwithaheightanddiametereachof2m,thewater tankwouldneedtohaveadiameterof8mandaheightof12m.This includesaclearanceofabout4mabovethewaterlevelforaccess andcranes.Analternativesizeforthewatershieldispresentedin
AppendixA.Thethicknessofthetankwallswillbeupto20mm, withtheprecisethicknesstobedeterminedthroughstandarddesign procedures(forexampleFEAanalysis)intheTDR.Theinternalsofthe watertankwillrequiresurfacepassivationafterthoroughcleaning, withelectro-polishingbeingalikelymethodforthelatter.
Withawatertankof8-mdiameter,theadjacenttowercouldhave similardimensions.Afootprintof8× 6mprovidesadequate clean-roomspace(seeSection3.3).Averticalspaceof12mcould accom-modatefourfloors.Abuildingthistallwithinacurvedcavernshown inFig.4,wouldallowspaceforthecleanroomoverpressurefanstobe placedontopofthebuilding.Sufficientspacewouldthenalsoexistto allowaflowofairtocooltheoverpressurefans.Thelowermostfloor
Figure 4. Position of the water tank within the cross section of the new DOMUS laboratory, approximately to scale. The widest part of the hall is 18.2 m. The cylindrical envelope for the water tank is to scale within the cross section, and has a height of 12 m and a diameter of 8 m.
couldhousemostofthewaterpurificationplant.Anexceptiontothis wouldbetheRn-strippingcolumn(ifused),whichwouldlikelybetoo talltobehousedwithinasinglelevel.Themiddlefloororfloorscould bededicatedtoslowcontrolandDAQ,whilst theuppermostfloor orfloorscouldcontainthecleanrooms.Alllevelscouldalsoprovide someformofstorage.
Anon-detailedanalysisusingthelaboratorycross-sectionshown inFig.4andassumingaregularcylindricalshapeforthewatertank suggestssitingthetankwitha1-mclearancefromthecranebeam wouldallowahorizontaldistanceof2.0mtothewallononeside,and 5.0montheotherside.Notethatthe1-mclearanceshowninFig.4is mostlimitedintheverticaldirection,leavingahorizontalclearance of1.1mbetweenthetopofthetankandthecranebeam.Structural integrityusingFEAwillbeinvestigated,withcloseattentiontothe topofthewaterstoragetankwherethewaterlevelmayapproach closertothetopofthetank.Iffloorspaceisconsideredmore pre-cious,adifferentshapeforthetopofthewatershieldtankshouldbe considered,allowingtheexperimentalhousingtobesituatednearer tothesideofthecavern.Sloping,orstepping,thetopsectionofthe watertanksabovethewaterlevelwouldallowthetanktobeplaced nearerthecavernwall,butdependsinpartontherequirementson thereachofthecraneinternaltothattank.Thisdependsonwhether thecraneneedsthecapabilityofhorizontalmovementacrossthefull radiusofthetank,andwhetherthisisneededaroundthefullarcof 2
π
.3.1.2. Cryogenicsystems
Acryogenicunit,positionedimmediatelyadjacenttothewater tankforcloseproximitytothecryostat,housesthe1-Kpotandstill, theheatexchangersandthe3Hecondenser.Thevalvesforthefast
precoolingofthedetectorarraywillalsobesituatedinsidethis cryo-genicunit.Theunitwillhavedimensionsofabout1.5× 2.5m,and willbemountedonthesamebaseplateasthedetectorarray, con-nectedbyarigidcryo-line.
Theremainderofthecryogenicsystemconsistingofpumps, com-pressorandgasstoragewillbeplacedinaseparatecavernupto100 mfromthedetectorstoreducemicrophonics.Possiblelocationsare thesidesofaccesstunnelsoraseparatecavitycutintotherock.5 m3isrequiredforgasstorage,withthecompressorapproximately
2.5× 2× 2m,andafurther1.5× 1× 2mrequiredforoilfiltration. Afootprintofatleast6× 5mwouldberequiredfortheseitems,if allofthemareonthesamelevel.
Thelinestothe3Heand4Hepumps,aswellastheliquid
he-lium(LHe)transferline,willpassbetweenthemaincavernandthe acousticallydecoupledcryogenicfacility.Thecryogenicunitwillbe decoupledfromvibrationbyflexiblesectionsanddampingstructures thatareoptimisedforthespectrumofthevibrationofthepumpsand theheliumliquefier.Theseflexiblesectionswillalsoallowthe nec-essarymovementoftheplatformsbyseveralcmwhenemptyingor fillingthewatershieldingtanks.
Fulldetailsofthecryogenicsystemareprovidedinthededicated
Section4.
3.1.3. Undergroundstorageofdetectorsandothermaterials
Cosmicrays(neutronsandprotons)interactingwithdifferent ma-terialsat thesurfaceproduceradioactive isotopes.Some ofthese isotopeshavearelativelylonghalflifeand,aftermaterialsand com-ponentsaremovedundergroundandinstalledinthedetectors,may contributeto thebackground eventratecompromisingthe detec-torsensitivity.TheprimeconcernfortheEURECAexperimentisthe Gecrystalsandcopper.Boththesematerialswillbepresentinlarge quantities.Irradiationofcopper bycosmicrayswillproduce 60Co
amongstotherradionuclideswithshorterhalflives,whereas expos-ingGetocosmicrayswillgiveseveralisotopes,themostproblematic being68Geand65Zn.
Theestimatesofneutronflux intensityatdifferentdepthsand copperactivationshowthatcoppercanbesafelystoredatashallow depthofabout10m.w.e.Thiswillgiveasaturationactivityofless than0.3
μ
Bq/
kgwhichiswellbelowthetoleratedupperlimitof about10μ
Bq/
kg.Coppercanbestoredinanundergroundstorage placeinoneoftheinstitutionsandquicklymovedtotheprocessing siteandthenundergroundreducingthetimeofirradiationbycosmic rays.InducedactivityinGecrystalsismoreproblematicandagreater depthforstorageisrequired.Theexposuretocosmicraysatthe sur-faceshouldbeminimisedandthecrystalswillbestoredunderground beforeandafterdetectorpreparation.Thedecaysof68Geand65Zn
givepeaksatabout10keV.Incaseofahigheventrateatthese en-ergies,wewillremovetheregionaroundtheseactivationlinesfrom thedarkmatterdataanalysis.
MoredetailsaregiveninAppendixB.
3.1.4. Heavyliftingequipment
Cranagewillbeneededbothinsidethewatertanksforremovalof thecryostatthermalshielding,andaccesstothebolometertowers; andexternallyforuseasthetanksarebuiltupandcomponentsof theinfrastructureareadded.Thecraneinsidethetoweristherefore likelytoberatedtoasafeworkingloadofnogreaterthan1t.This maybeincreasediftheinternalshieldingcannotbesplitintosmaller components(seebelowregardinginternalgammashielding).For ex-ample,a15-cm-thickCushieldofdiameter1.6mwouldweighnearly 3t.FurtherdetailsofthecleanroomcraneareinSection3.3.Thecrane
outsidethewatertank,inthemainexperimentalhall,willneedtobe ofahigherrating,upto8t.
Placementofaliftpassingalongtheoutsideofthebuildingto allowtheeasytransportofgoodsuptotheupperfloorswouldbe beneficial.Aworkingloadof1tmaybesufficientforthis.
Accesswillbeneededforheavyequipmenttobeplacedin loca-tionssuchasthecleanroom(laminarflowcabinets,otherfurniture), orsidecaverns(cryogenicplant).Somemovementofequipmentwill be withportable floorcranesandpallet-trucks,whichare readily availablewithworkingloadsofuptoatleast3t.
3.2. Interfaceswiththehostlaboratory 3.2.1. Electricity
Powerconsumptionandheatloadsfromanumberofitems de-tailedinthissectionaretobeconsidered.Thepowerconsumptionof thecurrentLSMisapproximately200kW,withelectricitysupplied at400kVA.Anestimationforthepowerdrawnbymanyitemsis includedinAppendixC.Thissuggeststhepowerrequirementsfor EURECAwillbearound450kVA(270kW).Itemsincludedinthatlist aresummarisedhere.
Thetotalpowerdrawnfromthecryogenicplantisestimatedtobe 130kW.Itemsconsideredforthisarethecompressor,vacuumpumps, refrigerator,electronicsandthepossibilityofwatercooling,ifneeded, atatotalrateof8m3
/
h.Otheritemslikelytodrawlargeamountsofpowerarethewatershieldpurificationplant(20kW),low-Rnair supply(30kW),DAQandslowcontrol(12kW),andcleanroomplant (9 kW).Furtherdetailsabouteach ofthesearecontainedintheir dedicatedSectionsbelow.
3.2.2. Computinginfrastructure
ComputingwillbeneededforcontrolofDAQ(detectorandveto), slowcontrol(detectorandwaterpurification)andcryogenicsaswell asforgeneraluseforcommunications,monitoring,logbookentries, etc.EDELWEISS-IIIwillprovideatestbedfortheEURECADAQ,which willinturndictatepartofthecomputinginfrastructurerequirements. TheDAQwilloperateatroomtemperatureandrequireaircooling. MoredetailsaregiveninSection7.1.
Allocationmustbemadeforlocaldatastorageandslowcontrol machinestoincludedatalogging.Bothcryogenicandwater purifica-tiondata,alongwithinformationaboutambientconditions (temper-ature,pressure,Rnlevels,etc.)willbestored.
3.2.3. Coolingwateranddrainage
Theamountofcoolingwaterrequireddependsontheamountof heatproduced,andcannotbeaccuratelyknownuntiladesignofthe cryogenicsystemhasbeenfinalised.Initialestimateshavequoted thatpartsofthecryogenicplantneedeitherwatercoolingatarateof 8m3ofwaterperhour,or14000m3ofairperhour.Thefire-fighting
circuitshavepreviouslyprovidedwaterforcoolingsystemswhich wasdisposedofafteruse.
3.2.4. Laboratorycooling
Theheatproducedbythecryogenicplant,vacuumpumps, com-puting,DAQ,waterandairpurification
plantswillneedtobeextractedfromthelaboratory.Thefirstorder estimateofpowerconsumptionisshowninSubsection3.2.1.Apower consumptioncorrespondingto30%ofthetotalelectricalpowerhas beenassumedforprovisionofcooling.Itwillbepossibletoprovidea moreprecisefigureoncedetaileddesignsofthecryogenics,DAQand waterpurificationplanthavebeenfinalised.Thesewillbeincluded withinthefutureTDR.
3.2.5. Low-radonairsupply
Figure 5. Schematic of the low Rn air facility.
withinaregimewherebyonlythevolumeabovethewateris sup-pliedwithlow-Rnair,witharesidualRnconcentrationinairofa “few” mBq
/
m3[18].Inthisinstance,asystemofthesamescaleasthatoperatingintheLSMforNEMOandEDELWEISScouldprovide thenecessaryflowoflowRnair.
Thecurrentsystemconsistsofacompressor,airbufferanddryer atonelocation,withafootprintofapproximately6× 1m;and sev-eralfilters,anaircoolerandaseriesofcarbonadsorptioncolumnsat anotherlocation,withafootprintofapproximately5× 2m.Theair cooler,andthetonneofcarbon,arecooledto−50◦C.Powerdrawn bythisisestimatedtobearound30kWwhenrunningforboth EDEL-WEISSandNEMO.Thesystemprovides150m3
/
hofairwithafactor1000reductionforRn,tolessthan15mBq
/
m3.AschematicofthecurrentRnreductionfacilityisshowninFig.5.
Further details about Rn progeny deposition are provided in
Section3.3.
ThecurrentRnmonitoringsysteminuseattheLSMcouldbeused formonitoringthelocalenvironmentofEURECA.Ifagreater sensi-tivityisrequired(dependentonsimulationstoshowhowlowaRn concentrationinairweneed),itwouldbepossibletoincreasethesize oftheelectrostaticcollectionvesselandoperatethePINdetectorata highervoltage,aswasdonewiththeSuperKamiokandeRndetector. Further,anumberofsimilarsystemsatdifferentlocationsaroundthe caverncouldbeinoperationsimultaneously.
3.2.6. Low-radonwatersupply
Simulationshaveshownthatitisnecessarytoincludea3-m thick-nessofwatertoefficientlysuppressthegammabackground.Thisis inexcessofthe70cmofwaternecessarytoreducetheneutron back-groundtobelowthesensitivityofthedetector.RequirementsforRn inwateraretobelessthan100mBq
/
m3.GERDAusedasimilarvol-umeofwaterandobtainedU,Th,K,Pbcontaminationsbelowppt levels,withaveryefficientremovalofparticleswithsizes
>
1μ
m [19].EURECAwilloperatewithasimilarlylowlevelofcontaminants inthewater.Workingwiththebaselineofawatertankofdiameter8mand aheightof12mcontaining8mdepthofwater,EURECArequires around 400m3ofultra-purewaterforonecryostat.Anadditional
tankcapableofstoringtheentirecapacityofwaterfromtheshielding tankwillalsoberequired.
Wewillrequireatwo-stagepurificationprocess.Aschematicof thewaterpurificationplantisshowninFig.6.Feedwaterisinitially passedthroughasystemconsistingoffiltrationandreverseosmosis severaltimesbeforeenteringasecondstage.Upto70%ofthe wa-termaybelostduringtheinitialstage.Thesecondstageconsistsof ultrafiltration,electrodeionisationandmembranedegasification,and formspartofthecontinuouspurificationloopwhichwillbe neces-sarytomaintainthecleanlinessofthewater.UVtreatmentand
/
or coolingmayalsobenecessaryduringthesecondstage,tolimit bac-terialgrowthwhichwouldreduceopticaldepth.Forthefirststage purification,afootprintofaround4× 2misenvisaged,dependingFigure 6. Schematic of the water purification plant. The abbreviations are: UF, ultra- filtration; RO, reverse osmosis; EDI, electrodeionisation; and UV, ultraviolet.
onfinaldesign.Thesecondstagemay(againsubjecttofinaldesign) fitintoafootprintofaround6 × 4m.However,ifabuffertankis usedwithinthe(secondstage)purificationloop,anadditionalspace oftheorder10m3maybeneeded.
OfparticularconcernarethelevelsofRninwater.Thisisnotonly forRndaughtersactingasasourceofbackgroundwithinthewater butalsoforthepossibilityofoutgassingofthewaterincreasingtheRn concentrationwithintheairofthecleanroom,whichwillbeattimes opentothewatertank.Borexinousea6-mtall,approximately40-cm diameterRn-strippingcolumn.Waterflowsinonedirection (down-wards),withacounterflowofN2,toachieveaRnlevelof3mBq
/
m3(reductiontothislevelachievedaftermanycyclesthroughthe purifi-cationloop)[20].Thefootprintfortheirsystemisabout1× 1m.An alternative,asusedbySuperKamiokande,wouldbetouseavacuum degasifierwiththepre-treatmentofRn-reducedairtoincreasethe degasificationefficiency.
FormonitoringtheRnconcentrationinwater,asimilarsystemto thatcurrentlyemployedattheLSMformonitoringRninaircanbe used,whichisbasedonthedesignemployedbySuperKamiokande [21].Withthisdetector,waterispassedthroughamembrane degasi-fierallowinggastoflowintoachamberofRn-reducedairwhichis fedintothemonitoraswiththe‘standard’Rn-in-airdetector.Quoted sensitivityisaround1mBq
/
m3ofRninwaterforaone-daymea-surement.Itwouldbebeneficialtohaveatleasttwosuchdetectors; onemonitoringtheRnconcentrationwithinthewatertank,andthe otherattheoutletoftheRndegasificationunit.
Itmaybenecessarytoinstallanadditionalresin-basedRnfilter withinthefirst-stagepurificationprocess.Suchastepwoulddepend onthequalityofthefeedwater,likelydependentonitsorigin.
Aninputflowrateofupto10m3
/
hisforeseenwithrecirculationdependentoncontaminationarisingfromthesystem.
Forthecryostatoutershell,anumberofpossibilitiesexist.Ideally Cuwouldbeused.However,Cu isknownto corrodeindeionised water[22],necessitatingapolymer(orother)coating.Hence,PMMA (acrylic,Perspex),asdiscussedinSection4ischosenasoutershellfor thecryostat.
Itmaybepossibletousestainlesssteelforthepipework. How-ever,theuseofpolymersispreferredtopreventmetallic contamina-tionwithinthewater.Metallicparticulatescanaidthetransportof bacteria.PVDFandPEarebothfavouredinultrapurewatersystems. Materialscontainingfluorineusedherewouldbesufficientlyfarfrom thedetectorsforittobeofnoconcernwith(
α
,n)reactions.3.3. Cleanrooms
Weplanforatleasttwocleanroomareas,onewithinthewater tankandtheotheratthetopoftheinfrastructuretower.Thelatter willbeusedfordetectorpreparationanditwillbethroughthisthat accesstothewatertankwillbeprovided.Anoptionexistsforhaving eitherthewatertankandthecleanroomastwoseparatecleanrooms, eachwithitsownlow-Rnairsupplyandvent;orhavingtheairsupply tothewatertank,inturnoverpressurisingthecleanroom.Thetwo mustbemechanicallydecoupledtopreventunnecessaryvibration ofthedetector,butalsosealagainsttheoutsidetomaintain cleanli-nesswithin.Additionalcleanroomspacemaybeavailableforgeneral facilityuseelsewherewithintheundergroundlaboratory.
Inadditiontothecleanarea,extraspacemustbeavailablefor changingrooms,andthestorageofbothcleanroomandeveryday clothing.Aclean-offareaandamaterialshatchwillrequirespaceas theywillneedadoubleairlocksystem.
Externaltothecleanroom,spacemustbemadeavailableforthe interfacesbetweeninsideandoutsidethecleanrooms.Provisionmust bemadeforgascylinderplacement,liquidnitrogen(LN2)dewar
stor-age(ifboil-offofLN2istobeusedasapurgegas),andthesupplyand
disposalofwater.
3.3.1. Cleanroomaroundcryostat
Sincethereexistsarequirementforthecryostattobeshielded byultra-purewater,thenthewatertankmustbeconsideredtobea cleanroom.Accesstothecryostatwillbeviathewatertank,initially throughtheadjacentman-tower,theupperfloorofwhichwill func-tionasacleanroom.AlikelycategorywouldbeISO6to7(class1000 to10000,respectively).
Low-Rnaircouldbeintroducedatthetopofthewatertank,further filteredtotherequirementsofthecleanroom.Anoverpressurewill bemaintainedwithinthetank,although theoptionofusing boil-offfromLN2willbeconsideredduringdetectoroperations.Careful
considerationmustbegiventodeadzonesofairwithinthetank.It maybenecessarytopipethecleanairdowntothebottomofthe tankwhilethecryostatisopen.Thewaterwillalreadyhavealow concentrationofRn.
Theentrancefromthecleanroomwillleadontoaplatforminthe tankabovethe levelofthewater. Thisisenvisagedto beasolid structureofstainlesssteel.Twotrapdoorswillbelocatedinthefloor, oneattheedgeprovidingaccesstothebottomofthewatertank;the otherinthecentreallowingaccesstothecryostat.
Acranewillbeincludedwithinthewatertank,likelywithaload limitof1tonne.Thecranewillbeusedformovingshields,andforthe detectortowers.Theminimummovementnecessaryforthecrane willneedtobedefined(fordetectortowerplacement)aswillthe reachofthecrane.Ifaccessisrequiredtothewholevolume, modifi-cationswillbeneededtotheplatformatthetopofthetanktoenable ittobetemporarilymoved.
Spacemustbeavailableforthesixdetectorthermalshields.One optionwouldbetoincludeafloorstageataroundthelevelofthe openingofthecryostat.Adequatedrainagewouldhavetobe consid-ered,e.g.useofagrilledfloor,aswouldaccessofthecrane.
3.3.2. Cleanroomfordetectormountingandpreparation
Thecleanroomlocatedatthetopfloorofthetowerwillbefor detectormountingandpreparation.ThiswillbeaISO7cleanroom containinglaminarflowcabinetsofISO5.
Sincewewillhavemultipletargetmaterials, andthereforethe possibilityofmultiplegroupsworkingwithinonearea,thereshould beatleasttwoofthesededicatedareas.Returnairfromthecleanroom willbepassedtotheinputoftheairhandlingunit,tominimisethe inputoflow-Rnair.
Storageandmountingspacewithinthiscleanroomwillbeneeded forindividualdetectors,thosemountedontobaseplates,andthose
mountedintowers.Consideringthatindividualtowersofdetectors maybeupto1-mlongandupto30-cmdiameter,adequatestorage spaceneedstobeplanned.
Apparatusforcleaningwillbeneeded.Thisincludesanultrasonic bath,aclean(deionisedandfiltered)watersupplyforrinsing,awater disposalroute,fumehoodsandstoragecabinetsforworkwith haz-ardouschemicalsandsolventssuchasdichloromethane,andagas supply.Mainswatercanbeinputtothecleanroom,withsmallscale filteringoccurringtherein.Gascylindersand
/
orLN2dewarscouldbeplacedexternaltothebuildingandgaspipedintothecleanroom. Abuildingofdimensions6 × 8 mhasbeenpresentedwithin
Section3.1.Ifthisprovidesinsufficientfloorspaceforthecleanroom, alternativesofalargerfootprintorpartiallysubmergingthebuilding willbeconsidered.CleanroomoverpressureandHEPA-filteredfans willbeplacedontopofthebuilding.
3.3.3. Radonprevention
222Rn decay leads to deposition of the relatively long-lived
progeny210PbontosurfacesexposedtoRn.210Pbdecaysmainlyby
emission oftwo
β
s andanα
beforereachingthestable206Pb.Inordertoreducethebackgroundtobelowoneevent
/
tonne/
yearin theWIMP-searchregionofinterest(ROI),specialcaremustbetaken toreducesuchdepositionontosurfaceswhichareindirectviewof thedetectorcrystals.Alow-Rnenvironmentmustbesupplied for mountingthedetectorsintotheEURECAcryostat.The222RnlevelinsidethecurrentLSMlaboratoryis10–15Bq
/
m3duetotheventilationsystemsupplyingairfromoutsideofthe
mountainat7000m3
/
h(twolaboratoryvolumesperhour).Low-Rnairisalsosuppliedtoareasofthecurrentlaboratorytoamaximum rateof150m3
/
hwithameasuredconcentrationattheexitofthelow-Rnfacilityoflessthan15mBq
/
m3.FurtherdetailsofthesystemarepresentedinSection3.1.AnaverageRnlevelintheproximityof theEDELWEISS-IIcryostatattheLSMoflessthan100mBq
/
m3hasbeenachieved,withairsuppliedfromthissystematarateof0.5m3
/
h.
WeplantouseasimilarRnreductionsystemtothatalreadyin placeasaninputtothecleanroomoverpressurefans.Sincethe re-quiredairflowthroughthecleanroomisinexcessofthelow-Rnair flowbyafactorofaround10,wewillrecyclethelow-Rnairsuch thatthefanfilterunitsdrawairinfromtheroomitself,astandard cleanroompractice.
CalculationshavebeenperformedtoestimatetheamountofRn progenydepositedonasurfaceforgivenexposuretimesand concen-trationsofRn.FurtherdetailsaregiveninAppendixD.Foraninitially cleansurfaceexposedto100mBq
/
m3ofRnfor90days,wewouldexpecttoobtain2.5
α
sand5β
sintheROIpertonneperyearwith arejectionpowerforbothof1.7× 104[23,24].Thisactivityisafac-torof∼10toohigh.PossiblemethodsofreducingtheamountofRn withinthecleanroomtoanacceptablelevelwouldbebyincreasing theflowrateofairthroughtheRntrap,andinstallinguptotwo ad-ditionalcharcoalfilters.Thefinalspecificationsforthisimprovement willbedetailedatalaterstage.The90daysprovidesanexampletime forassemblyofthedetectors.
Withinthewatertank,sourcesofRnmayarisefromthePMTsand stainlesssteelofthetank.Amembranedegassingunit(see[21]for anexample)willbeincludedwithinthewaterpurificationcircuitto lowerthelevelofRntobelowtherequired100mBq
/
m3.3.4. ElectromagneticShielding
numbereddocuments.Generalregulationsandgoodpracticeprovide sufficientguidanceforthedesignandimplementationofgeneral ex-perimentinfrastructure.
Specialconsiderationisnecessaryforthespacehousingthe de-tectorsandfront-endelectronicsatthecentreofthecryostat.Here, requirementstypicallygobeyondthegeneralgoodpracticedescribed aboveandaredescribedinmoredetailintherelevantsectionon electronics. Broadly,the detectoraccommodation willneedto be designed andimplementedasa Faradaycage.Inaddition,further shieldingagainstmagneticinterferencemaybecomenecessaryifthis noise contributionis identifiedasbeingtoohigh. Typical sources ofmagneticcontributionsarevacuumandrecirculationpumpsand othermotorsandcompressors.Theseshouldbelocatedatsufficient distancefromthedetectorcompartment.
3.5. Muonvetosystem
Thewatertankhousingthecryostatwillbetheshieldingforany neutronandgammaactivityfromtheconcreteofthelaboratory(see
Section3.6)andwillactasanactiveshieldforcosmicmuons.Forthat purpose,thewatertankwillbeequippedwithPMTs.Thedetailed parametersofthePMTsetup(e.g.PMTsize,number,position,trigger threshold)willbedefinedinanoptimisingprocessbasedonMC sim-ulations.Theoptimisationprocesswillincludetheexactgeometryof thesupportstructureofthecryostat,theinfrastructureandcabling pipesfromthetankwalltothecryostatandwillexaminedifferent solutionsforthetanksurfacesuchashighlyreflectivefoils.Alikely configurationwillconsist oftheorderof1008-inchencapsulated PMTsfixedonamountingstructureatthetankwall,bottomandtop. AllPMTswillhavetobeleakcheckedtoensurecompatibilitywith longtermuse.Fromfirstsimulations(Section8.4),thesignal thresh-oldcorrespondingtoanenergydepositof0.2GeVhastobeachieved tosuppressanymuon-inducedneutronbackgroundinthebolometer array.
The simulationsshowthat lessthan fivesinglenuclearrecoils inducedbycosmicmuonsareexpectedfora3tonne· yearsexposure. AllthebackgroundeventslinkedtosinglenuclearrecoilsinEURECA crystalsshowenergydeposits
>
0.2GeVinthewatertank.Forsuch energydeposits,anefficiencyfarinexcessof90%istypicalforsuch waterCherenkovvetoes,thusanymuon-inducedsinglenuclearrecoil isexpectedtobevetoed.AsstatedinSection8.4,anupperlimiton theunvetoedsinglenuclearrecoilsis0.3events/
yearassumingan 0.2GeVenergythresholdforthevetosystem.EachPMTwillbesuppliedwithanindividualhighvoltagebased onaHVsystem(MPOD2HLXHV-EXmainframehousing10boards with16or32HVchannelseach)toindividuallyadjustthePMT re-sponse.Calibrationandstability
/
puritycontrolwillbeensuredby laserpulses,movablebulbsandradioactivesources.Severaloptions forthedeadtime-freesignal readoutanddataacquisitionare cur-rentlyunderinvestigation.Themuonvetodatastream(asdigitisedtraces)willbeintegrated intothemaindataacquisitionsystem(seeSection6).
3.6. MonteCarlosimulationsfortheshieldingdesign
Alphaandbetaparticleshaveshortrangeswithinmaterials.The 5-mm-thickcopperscreenswillbesufficient toshieldfromthese sourcesofradiation.
Neutronsandgammas fromrockcanbeefficientlyshieldedby variousmaterials.Asuppressionofgamma-rayandneutronfluxes by aboutsixorders ofmagnitudeis required to reduce the non-discriminatedbackgroundbelowthelevelofdetectorsensitivity.In oursimulationswehaveconsideredbackgroundradiationfromrock andconcreteofthelabwalls.Fig.7showstheattenuationofthe gamma-rayfluxfromconcrete beyondthewatershield,asinthe baseline designofEURECA.Ourresultsshowthat3mofwateris
Figure 7. Gamma-ray spectra from the Th decay chain contained in the concrete of the laboratory walls attenuated by different thicknesses of water shielding (given in metres).
requiredtoattenuatethegamma-rayfluxfromthelabwallssothe remaininggamma-rayinducedbackground(beforediscrimination)in theregionofinterestwillbeabout2000peryearinatonneoftarget. Thisleadstoasizeofthewatertankofabout8mindiameterand8 minheightforatonne-scaleexperiment.With3mofwatershield theneutronfluxfromrockandconcreteofthelabwallswillbe sup-pressedtoalevelwellbelowthedetectorsensitivity.Moredetailsof simulationsaregiveninAppendixF.
4. Cryogenicsandcryostat
4.1. Cryogenics
ThecryogenicsolutionforcoolingEURECAispresentedinthis section.Table3summarisestheaimsofthecoolingsystem.
Table 3
Baseline design for the EURECA cryostat and cryogenics. Cryogenics and cryostat Base temperature 7 mK Cooling power at base temperature 20 μW
Cooldown time 20 days to base temperature Cryostat size 2 m diameter, 2 m height Cold volume 1.5 m diameter, 1.5 m height
ThebasetemperaturetobeprovidedbytheEURECAcryogenic systemwillbe7mKtocooladetectormassof1t.Detectorsequipped withNTD-Gesensorscanoperatewithexcellentsensitivityat20mK; but10mKoperatingtemperature(withmargin)isrequiredforthe operationofCaWO4detectors,equippedwithtransitionedgesensors
(TESs).ThecriticaltemperatureoftungstenTESsisaround 15mK withaspreadofafewmK.Forreliablestabilisationoftheoperating temperatureforTESs,abasetemperatureof10mKisrequiredatthe detectorholders,andthus7mKisspecifiedasthebasetemperature forthecryogenicsystem.
Thislowtemperaturewillbecreatedby adilutionrefrigerator (DR).Circulatedlow-temperaturegaswithintheDRcircuitwill fur-thercooltwointermediatelow-temperaturestages:aheatintercept at50mKandathermalshieldat500mK.Additionalthermal shield-ingwillbemaintainedat1.8Kand60KbyaHerefrigerator,which willalsoprovidepre-coolingtoallcryogeniccomponentsduringthe firststageofcool-down.Adiagramofthecryogeniccircuitisshown inFig.8.
Figure 8. Diagram of the cryogenic system for EURECA. Numbered items refer to: 1, pressurised gaseous helium cooling loop; 2, pre-cooling helium loop; 3, liquid helium supply; 4, superfluid helium supply; 5, pumping line (16 mbar, return from Claudet bath); 6, dilution 3 He pumping line (return); 7, dilution 3 He supply.
polarisationof3He[25]andwiththecurrentdesignoftheCUORE
cryostat[26].AfeasibilitystudyonthedesignoftheEURECAdilution refrigeratorhasbeencarriedoutwithintherecentdesignstudyfor EURECA[27].
Basedoncompletedstudies [17,27,28] ourdesignincludesthe followingfeatures:
• Athermalshieldbelowroomtemperatureat60K;thethermal loadisdrivenbytheradiativeloadfromroomtemperature;the coolingpowerisprovidedbyapressurisedheliumloop.
• A1.8Klevel,cooledbyapressurisedsuperfluidheliumlinkin ordertominimisethemircro-vibrations(avoidingboilinghelium); theheatloadisdrivenby theconductionfromthesupporting strutsandcabling,andtheheatingintroducedbytheelectronics front-end.
• Thedilutionrefrigeratorisoptimisedtominimisetheconductive loadonthecoldeststage:
- Aclosed500mKshieldinterceptsthebulkoftheconductive loadfromstructuralelementsandthedetectorwiring. - Anintermediate50mKstagelimitstheconductiveloadson
thelaststage.
- Anenclosureofthedetectorcompartmentat10mK.
• Low-temperaturedilutionrefrigeratorcomponentsarelocatedin acommoncoldboxwiththeHerefrigerator(i.e.thestillandthe
3Hepre-cooling).
Forcomparison,theCUOREexperiment[29,30]hasasimilar re-quirementforthecoolingof1tofdetectorswithacoolingpower of5
μ
Wat12mK.CUOREusesseveral40-Kand4-Kcryo-coolersto establishanumberofintermediatestages.FortheEURECAdesign,we decidedagainsttheuseofcryo-coolerstominimisepotentialissues withvibrations.Inaddition,theEURECAsolutionofusinga combi-nationofaHerefrigeratorandadilutionrefrigeratorisfavouredfor reasonsoffastcool-downandhighefficiency.4.1.1. Requirements
Themaindriversforthecryogenicdesignare:
• Abasetemperatureof7mKforadetectortemperatureof10mKis chosenfordetectorsensitivity.Thelargecoolingpower(20
μ
W) isdrivenbytheestimatedthermalrelaxationprocessinthepure copperusedforpartofthelow-backgroundshielding.• Acoldspacevolumeofupto1.5-mdiameterand1.5-mheight toaccommodate1tofdetectors,andinternalshieldingagainst radioactivity.
• An allocation of cooling power at a temperature
<
4 K for theSQUID-basedcoldelectronics,orotherfront-endelectronics (1.5Watthe1.8-Kstage).• Materialselectionforthecryogenicpartsinsideexternal shield-ing.
• Minimisationofthevibrationsexportedfromthecryogenic equip-ment,i.e.thecompressorsandpumps,thefluids’circulationand phasechanges.
• Lifetimeoftheequipmentlongerthan20years,withtypical mea-surementperiods
>
1year.• Acool-downtime
<
5daystoreach1Kandafurther<
15daysto reachoperationalconditions.4.1.2. Overallcryogenicsystem
Thecryogenicsystemiscomposedoffivemainelements,which arethecryostat,containingthedetectors,acryo-line,throughwhich thecoolingtothecryostatisprovided,acoldbox,housingcryogenics, aDRandaHerefrigerator.
Thecryostatwillcontainthedetectorsmountedbelowthemixing chamber,withfront-endelectronicsmountedabove.Thecryo-line connectsthecryostat,placedwithinthewatertank,tothecoldbox. Ourdesignisbasedonatotalallowablelengthofthecryo-lineof20m, althoughtheaimistokeeptheoveralllengthbelow10mtoreduce heatandpressurelosses,andtherequiredamountof3He.Further
detailsaregivenbelow.
4.1.3. Heliumrefrigerator
The He refrigeratorwillprovide cooling ofthe thermalstages at60Kand1.8K,andwillalsobeusedfortheinitialpre-cooling ofthecolderstages.Coolingofthe60Kshieldwillbeachievedwith Hegasat40K,pressurisedto7-15bar.Asanexample,aflowof20g
/
s (5mol/
s)withatemperatureelevationof20Kgivesacoolingpower of2200W.For the 1.8-K stage, a direct Joule-Thomson expansion of He to16mbarwillcreatea1.8-Kbath.Thiswillbecombinedwitha Claudetbath(seeAppendixE.1)toprovidepressurisedsuperfluidHe (HeII)tothecryostatat1.25bar.TheuseoftheClaudetbathstops microphonicnoiseproducedbyHeboilingwithinthepipe.Acooling powerof10WextractedfromtheHeIIheatpipecreatesan evapora-tionrateof0.05g
/
s/
W(0.013mol/
s/
W).Thisflowislowcompared tothe40-Kcoolingloop,allowingtohaveadirectextractionofthe 1.8-Kvapourswithoutcirculationintheregenerativeheat exchang-ers.AsmallcommercialHerefrigerator,withmodifications,isan op-timalsolutionfortheEURECAcoolingsystem.Thesmallestliquefiers availablefromthemainmanufacturersprovide100Wat4K[31], whichisneededforcoolingdowntoatemperatureof4Kwithin about4days.Further,ithassufficientcoolingcapacityduring nor-malrunning.At1.8Kthecoolingpoweristypicallyaquarterofthat at4K.Twoweeksofcoolingareexpectedforreachingoperational conditions.IncorporatingsuchaHerefrigeratorintothecooling sys-temallowssimplificationofthethermalarchitectureofthesystem throughaccessofcoolingstageswithintherefrigeratorandlinking themdirectlytocorrespondingshieldsandstageswithintheEURECA cryostat.
TheHerefrigeratorconsistsofa5m3gastankfortheclosedcycle 4Hestorage,acompressorandpumps.Theseitemswillbeplaced
atasignificantdistancefromthedetectorstolimitnoisegenerated bythehighlevelofvibration,particularlyfromthecompressor.The onlylimitisthepumpingspeedforthelow-pressureflows(1.8-KHe bathanddilutionstill).Additionalitemsincludeheatexchangers,two gas-bearingturboexpanders,aJoule-Thomsonvalveandthecontrol system,allcontainedwithinthecoldbox.
AsuitableunitisamodifiedHELIALMLseriesfridge, manufac-turedbyAir Liquide[31].Thisdraws75kWatfullpower,during cool-down,reducingto50kWunderpartloadoncethesystemis attheoperationaltemperature.Anadditionalpumpwillberequired forthe1.8KHe IIwithin theClaudet bath.The compressoris of dimensions2225 × 1960 × 1885mm3,andrequiresadedicated
about7m3isneededwithinthecoldboxfortheitemsrelatedtothe
Herefrigerator.
WiththemodifiedHerefrigerator,thefastcoolingphasecanbe managedentirely by thissystemusingthe samecompressor and pumps.ThecoldboxoftheHerefrigeratorcanalsoaccommodate severalelementsoftheDR:the3Hepre-coolingandthestill.
Com-paredtosmallercryocoolers,thethermodynamiccycleofaliquefier (theClaudecycle)is∼10timesmoreefficient.TheClaudetbath, com-binedwithafastcool-downto1.8K,isthemainjustificationtousea smallHerefrigerator,derivedfromaliquefierdesign.
AnexampleofaHerefrigeratorcoupledwithacryostatwitha superfluidthermallinkistheNeuroSpincryogenicsystem[32].More detailsofthissystemareprovidedinAppendixE.1.
4.1.4. Dilutionrefrigerator
TheDRcoolsthreeprogressivelycolderstagesat500 mK,50mK and7mK.It iscomposed ofa gashandlingunit (placedwiththe compressoroftheHerefrigerator),aheatexchangerandstillwithin thecoldbox,andthemixingchamberlocatedwithinthecryostat.
Due tothe20
μ
W coolingpower requirementcombinedwith thelowtemperatureof7mK,theperformanceofthedilution re-frigeratordependscriticallyonthepropertiesofthesintered mate-rialusedforthediscretecounter-flowheatexchangers.Theclassical choiceistousesinteredAgheatexchangers.However,commercialAg powdersareknowntoexhibitradioactivityincompatiblewith low-backgroundrequirements.PowerconsumptionbytheDRislikelyto beupto20kWduringcool-down,estimatedtobe15daysfor1t ofdetectormaterial,startingfromatemperatureof1.8K.Thiswill reduceto10kWduringlow-temperatureoperation.AdesignbasedonsinteredCuhasbeentestedforEURECA[17], showing10cm3ofsinteredCuisnecessarytoachievethe
require-ments.ExamplesareshowninAppendixE.2.Basedontheseresults weconcludethattheEURECAthermalrequirementsarewellwithin thestate-of-the-art.Theprocurementofcommercialpowderand de-tailsofthesinteringprocesswillbedefinedandcharacterizedduring thetechnicaldesignforEURECA.
Severalcommercialpowdershavebeenidentifiedandthe follow-ingpropertieswillbestudiedbeforethefinaldecisiononpurchasing thepowderismade:
• Porositycharacterisation,optimisationofthesinteringprocess. • MeasurementoftheKapitzaresistance.
• Measurementoftheradioactivitylevel.
4.2. Powerbudgetanddetectortowerthermalrequirements
TheestimatedthermalloadonbothstagesisgiveninTable22. Atthisstageofthedesign,comfortablemarginsforcoolingpower (
>
100%)havebeenincluded.Table4summarisesthecoolingpowers andtheirrequiredgasflowsforthedifferenttemperaturestages.The3Heisotope molarflowisdrivenbythecoolingpower
re-quirementonthelowesttemperaturestage(10mK).Theestimation givenhereincludesa100%margin.Thecontributionstoeachstage areshowninmoredetailinTables22and23inAppendixE.3.
EURECAdetectorswillbemountedinarraysoftowerswhichwill beheatsunktoeachofthetemperaturestages.Thedesignfiguresfor thedifferentstagesofthetowersareat75 K,3 K,700 mK,60mK and10mK.The powerbudgetateachstagebothonthe cooling-systemsideandthedetectorsideisshowninTable5withfurther detailsofthecontributiontothe10mKstagelistedinAppendixE.3. Contributionstotheheatloadarefromdissipationofthereadout electronics,dissipationandheatreleaseinthedetectors,and con-ductionthroughthewiring,neglectingtheradiativeloadfromthe detectortowers.Section5.3providesdetailsconcerningthe mount-ingofdetectorswithintowers.
Figure 9. EURECA cryostat sketch showing the thermal shields, Cu, PE and PMMA shielding against radioactivity and other elements. One example of a top-loading de- tector tower is shown. The cryogenic line connects the He refrigerator with the dilution refrigerator through several cooling loops.
4.3. Cryostatdesign
Adetaileddesignforthecryostatwillbecarriedoutforthe Tech-nicalDesignReport.Herewepresentaconceptofthecryostat em-phasisingonlyitsmainfeatures.
4.3.1. Mainfeatures
Thecryostatwillbeinstalledwithinahigh-puritywatertank.The variouscryogenicstagesarethermallylinkedtotheHerefrigerator anddilutionrefrigeratorthroughacryo-line.
Inadditiontotheoutervacuumcontainer(OVC)atambient tem-perature,thermalshieldsareplacedattemperaturesof60 K,1.8 K, 500 mK,50mKand10mK.Fig.9showsaschematicofthe cryo-statcrosssection.TheHerefrigeratorprovidescoolingforthe60K and1.8Kstagesinadditiontotherequiredcoolingpowerforthe readoutelectronics,operatingatatemperatureofaround100K.The detectorvolumeisprotectedbycopper,polyethylene(PE)andacrylic (PMMA)shielding.Thecold-plate,operatingatthebasetemperature of10mKisonepartofthecoppershielding.Thebulkofthecopper shielding,aswellasthePEandPMMAwillbethermallyconnected tothe50mKtemperaturelevel.Thecoolingpowerforthedetector towersisprovidedbyconductionthroughthecold-plateontowhich thecoolingelementswillbemounted.Around10cm3ofsintered
materialwillbeusedtocouplethecold-platetothelowtemperature Hemixture.
The60Kshieldlimitstheheattransfertothe1.8Kstageand pro-videsalargecoolingpowerforthereadoutelectronics.Itisactively cooledbyhighpressuregaseousHesuppliedat40K,atupto15bar. Themainthermalloadistheradiationfromthe300Kexternalwall. Thetemperaturegradientalongtheshieldisanissuetobeconsidered tocontrolthethermalloadatthe1.8Kstage.Twooptionsare consid-ered:eitherasinglewallcooledbyaheatexchangermountedalong thecylindricalpartoftheshield,oradouble-walledstagecooledby thecold-plate.
Table6presentsanoverviewofalltemperaturestagesofthe cryo-stat.FiguresfortheheatbalancearepresentedinTable7.Calculations forTable7assumeanemissivityofthecoppershieldsof10%,which isaconservativevalueforuncoatedpurecopper.Theexternalwall (PMMA)hasanemissivityof100%.
4.3.2. Cryostatmechanicalinfrastructure
