StudiesontheReductionofRadonPlate-outonCopper

UsingElectricFields

MoezJanmohammad

Fall2016

AcknowledgementsTheauthorwouldliketothankProfessorJodiCooleyandProfessorStephenSekulafortheirguidance

andsupportthroughthecourseofthisresearch.Theirinsightwasgreatlyvaluedandvitaltothe

knowledgegained.HewouldalsoliketothankDr.RobCalkins,TimothyMulone,DanJardin,Dr.Bob

Kehoe,ProfessorSusanKress,andLaceyPorterfortheirassistanceintheirproject.

ThismaterialisbaseduponworksupportedbytheSMUHamiltonScholarprogramandSMUEngaged

Learning.

AbstractAsradioactiveelementsdecay,thedaughterparticlescansticktosurfaces,aprocesscalledplate-out.

Reducingradonplate-outoncopperisextremelyimportanttothenextgenerationofultra-sensitive

particledetectors.Backgroundinteractionsresultingfromdecayingradondaughterparticlescanmimic

signalsseenindirectdarkmattersearches,neutrinolessdoublebetadecaysearches,andotherexotic

particlesearches.222Rnanditsradioactivedaughterparticlescanbeacontributortobackgroundwhen

thedetectorhousingsandshieldingarestoredandexposedto222Rnforlongperiodsoftime.Previous

studiesindicatethatelectricfieldscanbeusedtodeflectdaughterparticlesandreduceradonplate-out

incopper.Asanextensiontothesestudies,Iinvestigatedtheuseofelectricfieldsatapotential

differenceof35kVtofurthermitigateradonplate-outoncopper.Todothis,Iplacedcoppersamples

intoa220Rnrichenvironmentandcreatedanelectricfieldatapotentialdifferenceof35kVtoattempt

todeflectradon’sdaughterparticlestopreventplate-out.Afterexposure,theemissivityofthecopper

samplesweremeasuredusingtheXIAUltraLo1800alphaparticlecounter.

IntroductionRadonisanaturallyoccurringradioactivegasthatexistseverywhereintheenvironment.Itsdaughter

particlescanplate-outontothesurfacesofmaterialsandfurtherdecay.Overtime,allmaterialscan

becomecontaminatedwithradonanditsdaughterparticles.Ifthesematerialsareusedinthe

constructionofanultrasensitiveexperiment,similartotheonessearchingfordarkmatterorother

exoticparticles,thedecayscanembeddaughterparticlesintothedetectorsandthesubsequentdecays

ofthosedaughterparticlescancreateafalsesignal.Thisresearchfocusesoncreatingnewprocedures

toreduceradonplate-outontocopper.Reducingtheplate-outofradioactiveelementsontocopperis

crucialfornextgenerationultra-sensitiveexperimentstoobtaintheclearestsignalpossible.

222Rnisalargecontributortobackgroundinteractions

thatcanmimicdesiredsignalsindarkmatter,

neutrinolessdoublebetadecayandotherexoticparticle

searches.Amajorityofthesedaughterparticlesare

positivelycharged,meaningtheycanbedeflectedusing

astrongenoughelectricfield.Asaresultofthis

deflection,thereshouldbeareductioninradonplate

outoncopper.Totestthis,coppersamplesareplacedin

anexposurechamberandexposedto220Rnsources.The

sourcesarecampingmantlesmanufacturedbeforethe

mid1990swhichcontainthorium.InFigure1,the232Th

decaychainshowsthat232Thdecaysdownto220Rn,and

eventuallytoastablelead-210atom.Radonisagas,so Fig.1-The232Thdecaychain

eventuallytheatmospherewithintheexposurechamberwillbehaveahighconcentrationofradon

particles.Onewouldexpectalltheradononearth,withitsshorthalf-lifeoffourdays,tohavedecayed

awaybynow,butthatisn’tthecase.Allradioactiveelementshavedecaychainstoastableparticle.The

thoriumdecaychaininFig.1hasaseriesofdecaysabovethoriumthatoriginatefromUranium-238

whichhasahalf-lifeover4billionyears.Sincetheseelementsareembeddedinthesoilanddirtandare

continuouslydecaying,therewillalwaysbeaconcentrationofradoninfusedintheatmosphere.

Apreviousstudy[1]showedthatcoppersamplesstoredwithinanelectricfieldhavealowerrateof

plate-out,butthatexperimentwaslimitedto6,000Vwhenproducingtheelectricfieldsduetothe

dielectricbreakdownofairthatoccurredwithintheexposurechamber.Theminimumdistancerequired

topreventthisarcingwas6inches,buttheexposurechamberwasmuchsmaller.220Rnwasusedasa

substitutefor222Rnbecause222Rnrequiresspeciallicensingandtrainingtohandlesafely.220Rnsources

areeasilyobtainedandrequirenolicensingtouse.Inaddition,220Rndecaystoitsstabledaughter

particlewithacoupleofdayscomparedto222Rnwhichtakesmanyyears.

Anelectricpotentialdifferencewillcreateanelectricfield,andachargedparticlewillfollowtheelectric

fieldlines.Mostradondaughterparticlesarepositivelycharged.Thus,apotentialdifferencecanbe

appliedtocreateanelectricfieldcanpushtheseradondaughtersawayfromthecoppersamples.

Motivation

TheSuperCryogenicDarkMatterSearch(SuperCDMS)usescopperintheconstructionofitsdetector

housings.Inordertomaximizethesensitivityoftheexperiment,wemustfindawaytoensurethatthe

copperusedintheconstructioniscleanandremainscleanthroughouttheprocessofstorageanddata

collection.Ifcontaminatedcopperisusedintheconstructionofthehousings,thesubsequentdecaysof

radonanditsdaughterparticlescancauserecoilthatpushesthenucleiintothedetectorcausingafalse

signaltobecreated.SNOLAB,thefuturehomeofSuperCDMSinSudbury,Canada,hasanaverageradon

environmentaround130Bq/m3.Sinceconstructionandassemblyoftheexperimenttakestime,the

materialsusedintheconstructionmustbestorednearby.Inthetimebetweenfabricationand

installation,thecopperhousingsofthedetectorswillbestoredtraditionally,usingnitrogenfilledpurge

boxes.Thisresearchwillhopefullyprovideinsightintonewproceduresforstoragetoreduceplateout.

Oneofradon’sdecayproductsis210Pb,whichhasalonghalf-lifeof22.3years.Itisnotpracticaltowait

forthe210Pbcontaminationtodecayawaysincetheexpectedlifetimeoftheexperimentislessthanthe

half-lifeoftheleadatoms.

Thisresearchwillbenefitexperimenterswhoworkindirect

darkmatterorneutrinolessdoublebetadecaystudies.

SuperCDMSisanexampleofanexperimentthatwill

benefitfromthisresearch,sincethematerialsstudiedare

thesameasthethoseusedinthedesignofthedetector

housingsusedwithinthecollaboration.Theresultsof

understandinghow220Rn,andbyextension222Rn,plate-out

isaffectedbyelectricfieldscanbeusedfordetectorsthat

usesimilarmaterialsandconstructionmethods.

Fig.2–SuperCDMSiZIPDetector

Methodology

Inpreviousstudies,theexposurechamberwasmadefromamodifiedpressurecooker.Thepressure

cooker’sgeometrypreventedthepowersupplyfrombeingusedatitsfullcapabilityof35,000volts.

Sincethechamberwassosmall,andthehousingwasmadeofmetal,arcingoccurredatanypotential

above6,000V.Theminimumseparationfortheanodeandcathodeneededtopreventarcingwasfound

tobesixinches.Withthisinformation,anewexposurechamberwasdesignedandcreated.Itwas

constructedofacrylicandwasmuchlargertopreventanypossibilityofarcing,withtwometalplateson

oppositesidesofthechambertogroundtheelectricfieldwithinthechamber.Insideofthechamber,

eightcampingmantleswereplacedarounda3D

printedsampleholder,whichwasthencoveredwith

Photosofexperimentalsetup

aconductivenickel-copperfabric.Tomeasureplateout,four-inchbyfour-inchcoppersampleswere

used.ThesampleswerecleanedusingRadiacwash,isopropylalcohol,anddeionizedwater.TheRadiac

washandisopropylalcoholhelpedtoremoveanysurfacecontamination.Theiremissivitywasthen

countedintheXIAUltraLo1800alphaparticlecounterpriortoexposure.Thethreecoppersamples

werethenplacedinthesampleholderandsealedintotheexposurechamberfor72hourswithno

electricfieldactiveasacontrol.

Oncethesamplesareremoved,theyarecountedagainintheXIAUltraLo1800,andthecontamination

levelsareinferredfromthecounts.Theentireprocedureisthenrepeatedusinganelectricfield

generatedbyconnectingthepowersupply’shighvoltageleadtothenickel-copperfabricusingananode

clip.

Results

Atotaloftworunswerecompletedusingthenewexposurechamber.Inthefirstrun,theanodeclip

connectingthemetalfabrictothehighvoltagepowersourcedisconnectedfromthefabricandwaslying

onthefloorofthechamberfor60ofthe72hoursoftheexposure.Thiscreatedapointchargewitha35

kVpotentialintheexposurechamber,andcreatedafieldconfigurationthatwasdrasticallydifferent

fromwhatwasintended.Thisexperimentprovidedsomeinterestingresults.Sincethenickel-copper

fabrichadaninducedcharge,itdeflectedparticles,causingareductionintheemissivityofthecopper

samplesofabout35%.Inthisrunoftheexperiment,thecontrolcoppersampleswereplacedintothe

exposurechamberatthesametimeasthethoriumsources,andresultedinaloweremissivity.

TheemissivitycalculationisdoneintheXIAsoftwarewiththeformula:

𝜀 =𝛼𝐴 ∙ 𝑡

where𝜀isthefinalemissivity,𝛼isthenumberofalphas,Aistheelectrodesize,andtistheelapsed

time.[2]ThisformulaassumesthattheentirecountingareaofthetrayintheXIAiscoveredbythe

sampleswhichisnotthecase.Theanodeofthetrayis707cm2,butthesamplesthemselvescover

about400cm2.Thenumberofbackgroundparticlesresultingfromtheuncoveredportionofthetrayis

negligiblecomparedtotheemissivityofthecoppersamples.

Inthesecondrun,theanodeclipremainedconnectedtothefabric,andasimilar35%reductionwas

seenintheemissivityofthecoppersamples.Inthiscase,thenumberofinitialcountsseenwas

drasticallydifferent.Theemissivitywashighercomparedtotheemissivityofthefirstcontrolrun.The

spectraldifferencescanbeseeninfigures3and4andacomparisonoftheeventcountscanbefoundin

table1.

Time [h]0 10 20 30 40 50 60 70

Cou

nts

0

500

1000

1500

2000

2500

3000

SMU LUMINA LabPreliminary

Fig.4-Run2,noelectricfieldappliedTime [h]

0 10 20 30 40 50 60 70

Cou

nts

0

100

200

300

400

500

600SMU LUMINA LabPreliminary

Fig.3-Run1,noelectricfieldapplied

AfterExposure(alphas/cm2/hour)

Run1 0.5501

Run2 3.121

Thedifferencesintheemissivitypromptedafollowupstudytolearnmoreabouthowradonreaches

equilibriumwithintheexposurechamber.ARad7RadonDetectorwasconnectedtotheportsofthe

exposurechamberinaclosedloopandthethoriumsampleswereplacedinside.Thelevelof

contaminationinthechamberwasmeasuredasafunctionoftime.Theresultswereinconsistent.

However,anotablecorrelationwasfoundbetweenradonconcentrationandthetimeofday.That

correlationdisappearedwhentheRad7wasconnectedtothepressurecookerexposurechamber,which

indicatedthatthesealsofthenewexposurechambermaynotbecompletelysealed,exposingthe

chambertoenvironmentalchangeslikehumidityortemperature.

Table1–Comparisonofexperimentalvalues

Fig.5–Comparisonofrun2data

Conclusion

Theresultsobtainedallowedustolearnagreatdealaboutthenewexposurechamberandthe

experimentalsetup.Aftertworunsoftheexperiment,a30%reductioninplateoutwasobservedusing

thefull35kVpowerofthehighvoltagepowersupply(figure5).Whilethisisnotthesameasthe98.1%

reductionseeninthefirstiterationofthestudydonebyapreviousstudent,agreatdealwaslearned

aboutthenewexposurechamber.Thenumberofchangesinthedesignandconstructionofthe

exposurechamberledtomanyvariablesthatcouldhavebeenthecauseofthediscrepanciesinthe

results.Intheexperiments,adiscrepancywasseenintheemissivityofthecontrolrunsbothbeforeand

afterbakeintimeforthethoriumsourcestosaturatetheexposurechamberwithradon.Furtherstudies

caninvestigatetheconsistencyoftheresultsbyrunningtheexperimentagainwithnochanges.

Fig.6–Renderingofnewexposurechamber,groundingplatesarecoloredgold

Theoldexposurechamber’selectricfieldwasgroundedtoallsides,butinthenewexposurechamber

(figure6),thegroundingplatesareonlyontwosidesofthechamber.Thiscausesthefieldtobe

drasticallydifferentfromtheoldexposurechamber.Thechambercanbemodifiedtoaddtwomore

groundingplatessothefieldisgroundedonfoursidesinsteadofjusttwo.Groundingtothetopand

bottomofthefieldwouldcreatearcingwithintheexperimentsincethesampleholderandnickel-

copperfabriccannotbesuspendedinthechamberwithasufficientseparationfromthetopandbottom

groundingplates.Futurestudiescanfurtherimprovethedesignoftheexposurechambertobetaller

andaccommodategroundingplatesonthetopandbottom.

Thisinformationandconclusionsontheabilityofelectricfieldstopreventorreduceplateoutofradon

ontocopperwillallowfurtherinvestigationintothefield.Theproblemcurrentlyfacedusingthis

exposurechamberisthereproducibilityandquantificationofthereduction.Itisknownthatelectric

fieldswillreduceplateout,butnarrowingtheexactprocedurestomaximizethisreductionwillrequire

additionalwork.

References

[1]“ArXiv.orgPhysicsArXiv:1506.04050”[1506.04050]StudiesontheReductionofRadon

Plate-Out.Web.13May2015.arXiv:1506.04050[physics.ins-det]

[2]“UltraLo-1800AlphaParticleCounterUser’sManual”Version0.2,February2013.