COMMISSIONINGANDCHARACTERIZATIONOFTHENSCL'SLARGEVOLUMELINEARGASCELLByKortneyLynnCooperADISSERTATIONSubmittedtoMichiganStateUniversityinpartialentoftherequirementsforthedegreeofChemistry-DoctorofPhilosophy2016ABSTRACTCOMMISSIONINGANDCHARACTERIZATIONOFTHENSCL'SLARGEVOLUMELINEARGASCELLByKortneyLynnCooperThethermalizationoffastmovingionsplaysapivotalroleintheabilityofprojectilefragmentationfacilitiessuchastheNationalSuperconductingCyclotronLaboratory(NSCL)toprovideexperimenterswithlow-energyionbeamsforprecisionexperiments.TheNSCLproducesradioactiveionbeams(RIBs)withkineticenergiesoftheorder˘100MeV/uandvelocitiesofuptohalfthespeedoflight.Morethantenyearsago,theNSCLproposedanddemonstratedabeamthermalizationtechniqueforprojectilefragments[1].Inorderto
providebeamswithenergiesontheorderof˘10keV,thefastRIBspassthroughadispersiveion-opticalsystem,soliddegraders,andthenamonoenergeticwedgetoremovethebulkoftheirkineticenergy.Theremainingkineticenergyof˘1MeV/uisthendissipatedthroughcollisionswiththegasatomsinalargevolumelineargascell.TheoriginalNSCLgascellwascompact(5cmwidex50cmlong),ranatahighpressure(760torr),and
usedaDCdragandgaswtoguideandextracttheions[1].Thenew,largevolumelineargascellconstructedbyArgonneNationalLab(ANL),islarger(25cmx120cm),operatesatmediumpressures(55to100torr),andemploysanelaborateelectrodestructurewithbothstaticanddynamicelectricaswellasgaswtoguideandtoextractthethermalizedions.Thepresentworkdescribesaseriesofcommissioningexperimentsforthe
NSCL'slargevolumelineargascellthatwereconductedusing76Gabeamsproducedatapproximately90MeV/uintheA1900[2].Sincethecommissioningexperiments,numerousadditionalionbeamshavebeenthermalizedandextractedfromthisgascell,andsomeoftheresultsarealsopresentedhere.Thefastbeamsweredeliveredtothelargevolumelineargascellinanewmomentumcompressionbeamline,andtherangedistributions,extraction,andtheoverallofthesystemweremeasuredasafunctionoftheincidentintensity.Thedatafromthecommissioningrunswerecomparedtopredictionsfromthestoppingandrangeofionsinmatter(SRIM)code[3,4]andtheLISE++code[5,6].Particle-in-cell(PIC)calculations[7]werecarriedouttoevaluatethespacechargeproducedbythestoppingoftheenergeticfragments,and,SIMIONcalculations[8]oftheionmigrationinthegascellwereperformedtoevaluatetheenessofthechangesandupgradesfromthepreviouscell.Resultsfromthesestudiesrevealedthatthelargersize,upgradedelectrodestructure,andadditionaldynamicpotentialonthewallsandconeoftheNSCL'slargevolumelineargascelldid,infact,improvetheextractionofthesystemwhencomparedtopreviousgenerationdevices.TABLEOFCONTENTSLISTOFTABLES....................................viLISTOFFIGURES...................................viiChapter1EvolutionandImpementationoftheGasCell..........11.1IsotopeSeparationOn-Line(ISOL).......................21.2IGISOL......................................41.2.1ComponentsandPrinciplesofISOL-GasCatchers...........5
1.2.2ReactionLossesandMechanismswithintheGas............81.2.3ofVariousReactionProducts................111.2.4AdvancedIGISOLSystems.......................14Chapter2TheNSCLProjectileFragmentationFacilityandLow-EnergyExperiments...............................182.1IsotopeProduction................................18
2.2GasCatchersforEnergeticFragments......................222.2.1ComponentsofPF-GasCatchers....................23
2.2.2In-GasIon-TransportMethods......................252.3Low-EnergyandReAFacilities.........................312.3.1LEBIT...................................322.3.2BECOLA.................................342.3.3ReA....................................35Chapter3TheNSCLBeamThermalizationArea...............383.1AreaOverview..................................38
3.2NSCL'sLargeVolumeLinearGasCell.....................393.2.1CommissioningExperiments.......................463.2.1.1ExperimentDescription....................47
3.2.1.2Measurements..........................503.2.2ImprovementsafterCommissiong....................55
3.2.3SummaryofMeasurements..................57Chapter4SimulatingtheNSCL'sLargeVolumeLinearGasCell.....634.1SRIM.......................................63
4.2LISE++......................................66
4.33DCylPIC.....................................70
4.4SIMION......................................74Chapter5OutlookandSummary.........................785.1NewOperatingPractices.............................78v5.1.1CryogenicOperation...........................795.1.2Ion................................805.2ACGS.......................................81
5.3Cycstopper....................................835.3.1Concept..................................83
5.3.2DesignandCapabilities.........................845.4Summary.....................................86APPENDICES......................................88AppendixA:SRIMExample.............................89AppendixB:3DCycPICExamples..........................91REFERENCES......................................95vLISTOFTABLESTable2.1:Summaryofthelargegascellsdevelopedforenergeticradioactiveion(RI)beams...................................25Table3.1:76Garadioactivitymoleculariondistribution.................54Table3.2:Summaryofvariouslow-energychemistryexperimentsperformedwiththeNSCLlargevolumelineargascell.76Gaisstarredbecauseitistheonlyexperimentshownthatwasconductedpriortotheimprovementsmadetothevacuumsystem................................62viLISTOFFIGURESFigure1.1:ThemostimportantISOLconceptsaredepictedtoshowthevariouswaysthereactionproductsarestopped.Theleftsidedepictstherecoilmethodwhiletherightsideshowsthemethod...............2Figure1.2:SchematicdiagramofalphaemittertargetwithHe-jetassemblyanddetec-torset-up....................................3Figure1.3:Theprincipleoftheionguideinvolvesthermalizingprimaryreactionproductsinagaschamberandthentransportingtheseproductsbygaswandelectriceldsdirectlyintotheacceleratingsectionofthemassseparator5Figure1.4:Schematicdiagram(nottoscale)ofaHelium-jetionguideconnectedtotheacceleratorstageofamassseparatorwhere(1)istheRecoilchamber,(2)isthevacuumchamber,(3)isthecapillary,(4)istheskimmer,and(5)istheextractionelectrode............................8Figure1.5:SchematicofaionguideoperatedbytheUniversityofa..13Figure1.6:Typicalgeometryandelectricpotentialsofasqueezerwiththreegridsop-eratedbetweentheexitholeandskimmer..................15Figure1.7:SchematicofSPIGset-upusedforthestudyofionsproducedbyadischargeionsourceattheUniversityofTokyo'sInstitureforNuclearStudy.....16Figure2.1:SchematicviewsoftheISOLandprojectilefragmentationmethodsusedtoproduceradioactiveisotopebeams.......................19Figure2.2:SchematicoverviewoftheNSCL{aprojectilefragmentationfacilitythatincorporatescoupledcyclotronstoacceleratetheheavybeam........22Figure2.3:PredictedLISE++productionratesplottedonanuclearchartforbothanISOLfacility(left)andfortheNSCLprojectiilefragmentationfacility(right)23Figure2.4:Simplecartoondepictingtheworkingprincipleofamonoenergeticwedge..24
Figure2.5:Schematicofexperimentalset-upfortestingthegenerationgascellatNSCL......................................28viiFigure2.6:AseriesofringelectrodesrotationallysymmetrictothelowerlineprovideanRF-barrier(Eeff)aswellasasuperimposedDC(EDC).TheionsarepulledbyEDCwhileEeffdrivesthemawayfromtheelectrodes..29Figure2.7:RFfunnelexperimentalset-upfortransportingTa+ions......30Figure2.8:TheRF-carpetelectrodeemployedbyRIKENintheRFbasedgascellwasmadeoftwolayersofPCBs.Detailsoftheselayerscanbefoundinthetext(b)Typicaliontrajectoriesdeterminedbyamicroscopicparticlesimulation
for8Liionsin90torrheliumgaswheretheRFvoltagebetweenneighboringelectrodeswas190Vat26MHz.ThesuperimposedDCwas8V/cm..31Figure2.9:Summaryofthedecreaseinextractionncyforavarietyofgascellsystemsasafunctionoftheionization-ratedensity.Thedatawereobtainedfor8Li,38Ca,107Ag,and58NifromtheprojectilefragmentationfacilitiesatRIKEN(squares),MSU/NSCL(circles),andGSI(diamonds),aswellastheISOLfacilityatLeuven(triangles),respectively.............32Figure2.10:(Left)AschematicoverviewoftheLEBITfacility.(Right)TheLEBIThigh-precisionPenningtrapwithitsend-capremoved.Abottlecapisshownwith
thetrapforscale.................................33Figure2.11:AschematicoverviewoftheBECOLAfacilitywithathree-dimensionalmodelshownintheinlet.QSandQDindicatequadrupolesingletordoublet
electrostaticfocusingelements,respectively..................35Figure2.12:AschematicoverviewoftheReAfacilitylocatedattheNSCL........36Figure3.1:AschematicoverviewofthebeamthermalizationarealocatedattheNSCLincludingoptionaldegraderanddetectorpositions..............40Figure3.2:Schematicdepictingthevariouselectromagneticimplementedtoin-creaseandimprovetheextractiontimesandoverallextractioncienciesfortheNSCL'slargevolumelineargascell.Arrowsshowthedirectionthat
theionsaredraggedbytheDCelectricwhilethefocusingpotentialcreatedbytheRFisshownindetailatthetopofthe.....41Figure3.3:Aself-consistentcalculationtoshowthepotentialbuildupfor60millionionsenteringintoandionizingthegaswithinasmallgascatcherisshown.Noticethattheintensityisthestrongestalongthecenterofthebeamaxis.
Theoutlineofthissmallgascatcherisrepresentedbythedashedredline.43Figure3.4:PhotographoftheRFconestructureusedintheNSCL'slargevolumelineargascell......................................44Figure3.5:PhotographoftheRFelectrodestructureusedalongthewallsintheNSCL'slargevolumelineargascell...........................45Figure3.6:Theschematicsontheleftshowtheareasofthegascellsfromwhichra-dioactiveionscanbeextractedgiventheelectricpresentandwhenoperatingathighintensities.TheNSCL'slargevolumelineargascelldesignisrepresentedbythelowerleftschematic.ThegraphontherightcomparesresultsforonlineexperimentswiththeNSCL'slargevolumelineargascell
toresultsobtainedfromgascatcherswithlittletonoRFrefocusing....46Figure3.7:Schematiclayoutoftheexperimentalequipmentandset-upusedtocom-missiontheNSCL'sbeamthermalizationsystem.Distancesbetweenthe
componentsaregivenaboveincm(nottoscale)...............48Figure3.8:PhotographofthelargevolumelineargascellwheninstalledattheNSCL.Notethelargeceramicinsulatoratthefrontofthechamber.....49Figure3.9:Measuredpositive(squares)andnegative(diamonds)ioncurrentdistribu-tionsfor76Gaasafunctionoftheedegraderthickness.Thetotalactivitydistribution(circles,arbitraryscale)isshownforreference..51Figure3.10:Exampleofagrowthanddecaycurveforthetotalextractedradioactivity.Thiscurvewasobtainedwhenthedegraderwassetforthemaximum
extractedradioactivity(34).Thedataareinagreementwiththeknown33shalf-lifefor76Ga...............................52Figure3.11:Partofthemass-to-chargespectrumobtainedforthestableionsextractedfromthegascell,seetextfordetails......................53Figure3.12:NSCL'slargevolumelineargascellwithupgradedinsulator,ports,anddrybox........................................56Figure3.13:Thegraphontheleftshowsamassscanforradioactiveionsfromanearly37Kexperiment.Thegraphontherightshowsasimilarmassscanforthesame37Kradioactiveionfromanexperimentperformedafteraturbomolec-ularpumpwasaddedtothelargevolumelineargascell.Whilethe37Kionwasfragmentedamongnumerousmolecularionscontainingwaterduring
theexperiment,nomolecularionformationwasobservedinthesecond
experiment.Seetextformoredetails......................57Figure3.14:Thepictureontheleftshowsthedeformed10mthickaluminumwindowwhilethenew49mthickwindowisshownontheright...........58Figure3.15:GraphofextractionasafunctionofincomingrateforinitialcommissioningandchemistryexperimentsperformedwiththeNSCLlargevolumegascell..................................59Figure3.16:Measuredpositive(squares)andnegative(diamonds)ioncurrentdistribu-tionsfor33ClasafunctionoftheedegraderthicknessforanincomingrateofA)3.4x104ppsandB)3.1x106pps.A30PcontaminantwaspresentinthefragmentbeamdeliveredbytheA1900.Thiscontaminant
accountsforthesecondobservedpeakinthegraphs.Seetextfordetails..60Figure3.17:GraphofextractionasafunctionofQforpreviousgenerationdevices(orangecircles,redtriangles,andblackdiamonds)aswellasthe
initialcommissioningandchemistryexperimentsperformedwiththeNSCLlargevolumegascell(allsquares)........................61Figure4.1:Summaryofionizationandrangeresultsobtainedforthecenterofthemo-mentumdistributionofthe76GaionswithTRIMprogram..........65Figure4.2:ThemeasuredBraggcurve(squares)andactivitydistribution(circleswithdashedline)for76GaarecomparedtothepredictedcurvesfromtheTRIMcode(solidlineanddashdotline,respectively).Thehatchedarearepresentstheequivalentthicknessoftheheliumgasandisshownforreference....66Figure4.3:Rangeoptimizationingasresultsfor76Gaionbeamthermalizedduringthecommissioningexperiments.TheseresultsweregeneratedwiththeLISE++
programandshowanoptimaldegraderangleof24.5............67Figure4.4:Rangedistributionfor76GafragmentbeamcalculatedwiththeLISE++program.Attheoptimaldegradersetting,theionsofinterestwillstop
inthemiddleofthelargevolumecell(600mm).Noticethatthe78Gecontaminantionissupressedtozeropps/mmattheoptimalsettingwhile
the76Gaionofinterestwillbeextractedatitspeakof19(1)pps/mm...68Figure4.5:EnergydepositioninXvsZ(Top)andYvsZ(Bottom)forthe76GafragmentbeamsimulatedwiththeLISE++program.............69Figure4.6:3DCylPICresultsforthecollectionoftheion(black)andelectron(red)ionizationpairscreatedforincomingbeamratesof102,103,104,and105pps.Thetimerequiredtoreachanequilibriumstatewas0.13,0.13,0.12,and0.10s,respectively.Seetextfordetails..................73Figure4.7:3DCylPICinitialpotential,equilibiumspacechargepotential,andevolutionofthespacechargepotentialforanincomingbeamrateof104pps.ThespacechargepotentialwasdeterminedbysubtractingtheinitialpotentialatTime=0.00sfromthetotalpotentialateachtimestep.........74Figure4.8:Comparisonoftheequilibriumspacechargepotentialscalculatedforvariousincomingbeamrates.Seetextforfurtherdetails...............75Figure4.9:Above:TheSIMIONmodelsofthebodyandconeelectrodesarecomparedtophotographsoftheactualelectrodestructures.Below:TheSIMIONgascellmodelasawhole...............................76Figure4.10:SIMIONresultsobtainedforYvsXwhereXisthelengthofthegascell.Theionbeam(blue)isshowntofollowtheequipotentiallines(red)through
thegascell.Here,theionbeamconsistedof100076GaionsthatweredistributedinaccordancewiththeLISE++results,andtheequipotential
lineswereshapedbyboththeDCandRFappliedpotentialsaswellasthe
calculatedspacechargepotential.Thebeamrate(inpps)associatedwiththespacechargepotentialappliedisshownintheupperrightcornerofeach"NONE"indicatesthatnospacechargepotentialwasincludedin
thesimulation..................................77Figure5.1:CartoonofionconceptwheretheionsaretransportedbyatravelingwavethatissuperimposedoveranRFpotential................80Figure5.2:ConceptualdesignoftheACGS.........................82Figure5.3:ConceptualdesignoftheCycstopper......................85Figure5.4:Left:Renderingofthemechanicalmodelofthecyclotrongasstopperwithanindicationofthespiralpathofastoppingion.Right:Photographofthecyclotron-stoppermagnetintheopenstateshowingthepolepieces.....86FigureA.1:TRIMsetupwindowwithexampleparametersfromthe76Gacommissioningexperiments....................................90FigureB.1:PICexamplefromthe76Gacommissioningsimulations......92FigureB.2:Evolutionofthespacechargepotential(inVolts)asafuntionoftimeforanincomingbeamrateontheorderof104pps.ThisspacechargepotentialwasdeterminedbysubtractingtheinitialelectrodepotentialatTime=0.00sfromthetotalpotentialateachtimestep..................94xiChapter1EvolutionandImpementationoftheGasCellRadioactivebeamfacilitiescanbedividedintotwocategories{thosebasedonProjectileFragmentation(PF)andthosebasedonIsotopeSeparationOn-Line(ISOL).Projectilefrag-mentationfacilitiesproducesecondaryRIBsbyimpingingahighenergyprimarybeamonarelativelythintarget.Thefragmentsareseparatedandcollectedt"fortransittotheexperimentalareas.Incontrast,ISOLfacilitiesbombardathicktargetwithaveryintense,light-ionbeam.Thesecondaryproductsthenoutofthetarget,andthebeamiscreatedandseparatedfortransporttotheexperimentalareas[9].Duetothenatureofthesecondarybeamproduction,thesesecondarybeamshavelowandpreciseenergiesaswellaslowemittances.Low-energyprecisiontechniquesandexperimentswerethereforeprimarilydevelopedatandexclusivetoISOLfacilities.AlthoughISOLmethodsproduceveryhighqualitybeams,theaccessableionsarelimitedtothosewithlongerhalflifesandthechemicalpropertiesrequiredtosurvivetheprocess.Inorderforprojectilefragmentationfacilites,whoseprocessesareunhinderedbytodeliversimilartypesofbeamsforlow-energyexperiments,thefastfragmentsproducedmustbethermalized.Akeyelementofbeamthermalizationisthegascell.Overtheyears,gascellshaveevolvedfromsimpledesignsthatsolelyreliedongaswand/or1Figure1.1:ThemostimportantISOLconceptsaredepictedtoshowthevariouswaysthereactionproductsarestopped.Theleftsidedepictstherecoilmethodwhiletherightsideshowsthemethod[10].DCdrifttomorecomplexdesignsthatincludeRFcarpetsandevenRFwalls.Bothlowenergyfacilitiesandgascellsarediscussedwithinthischapter.1.1IsotopeSeparationOn-Line(ISOL)ThetraditionalISOLmethod,asmentionedabove,involvesbombardingathicktargetwithanintenselightionbeam.Atomsofinterestthenoutofthetargetandaretransportedbybulkgasormolecularwtotheionsource.andseparationtimesofthismethodlimittheobservableiosotopestothosethatarenotonlymobile,butlongerlived.Giventheserestrictions,moretechniquesweredevelopedtocollectnonvolatileandshorter-livedspecies.Forsuchspecies,twomethodswereemployedthatexploitedthekineticenergyandmotionoftherecoilingreactionproducts.Inbothmethods,athintarget,whosethicknessis2Figure1.2:SchematicdiagramofalphaemittertargetwithHe-jetassemblyanddetectorset-up[11].limitedbytherecoilrange,isbombardedbyalight-ionbeam.Thereactionproductsrecoiloutofthetargetandarethermalizedineitherasolidstopperoragasstopper.AsummaryofthesetechniquesisdepictedinFig.1.1[10].He-jetswereintroducedasameanstoimproveandtransporttimesoftherecoilingnucleifromthegasstoppingtargetstodistantdetectorsoranionsource[12].Scientistsstudyingshortlivedalphaemitters(t1=2˘0.06s)[11]employedthistypeofset-upforexperiments,wherethebeamisproducedbyasource.AschematicofthisHe-jetset-upcanbefoundinFig.1.2[11].Overthenextfewyears,He-jetsystemscontinuedtoevolveandimproveasstudiesofimpurityclustersize[13]aswellasvariousgasandvaporcarriers[14]andaerosolgenerators3[15]wereexplored.ThebestaerosolsfoundtotlytransporttheionswereNaClandcarbonclusters.Afterthesedings,aerosol-loadedHe-jetswereincorporatedintoon-lineexperimentsconductedatOakRidgeNationalLaboratory(ORNL)[16]andtheUniversityofaska[17].Thesesystemswereshowntoachievetotalbetween0.01%and1.00%,andhavesincebeenincorporatedatotherfacilitiesincludingChalkRiverLaboratories(CRL)[18]andJapanAtomicEnergyResearchInstitute(JAERI)[19].AlthoughHe-jetsimprovedontheextractiontimesobservedforthetraditionalISOLmethod,short-livednuclei(t1=210ms)werestillinaccessibleduetothetransportandionizationtimesassociatedwithanionsource.SincereactionproductsareneutralizeduponadheringtothesurfaceoftheaerosolswithintheHe-jet,andionsarerequiredforaccelerationandmassseparation,therealizationofionsfromtheclusterwasaseverelylimitingstep.Assuch,theionsourcewaspivotaltotheoverallperformanceofanyISOLfacility,andawidevarietyofsourceswereavailable,tested,andused.Fordetaileddiscriptionsofthedesignandfunctionofthesesources,see[20]and[21]andreferencestherein.Inordertostudymoreexotic,shorterlivedisotopes,thedependenceofthesefacilitiesontheionsourcehadtobealleviated.1.2IGISOLIonextractionadvancesatISOLfacilitieswerebuiltoftheconceptsestablishedwiththeHe-jetsystem,andanewtechniquewasdevelopedthatwasnolongerdependentonanionsource.TheIon-GuideIsotopeSeparationOn-Line(IGISOL)methodallowedthebulkofthereactionproductstoretainapositivechargewhichtherein,rendersanionsourceirrelevant.4Figure1.3:Theprincipleoftheionguideinvolvesthermalizingprimaryreactionproductsinagaschamberandthentransportingtheseproductsbygaswandelectricdirectlyintotheacceleratingsectionofthemassseparator[22].1.2.1ComponentsandPrinciplesofISOL-GasCatchersThecomponentsofanIGISOLsysteminclude:atargetchamberwherethereactionproductsarethermalized,anexitnozzlethroughwhichthereactionproductsaswellasthehelium,orthermalizinggas,atomsareacceleratedandcolumnated,askimmerdesignedtosimultaneouslyavertheliumatomswhilefocusingtheionsofinterest,andatialpumpingsystemcompletewithelectrictoremovetheheliumatomsandcontinuetoguidetheionsdirectlyintothemassseparator.ThesecomponentsaredepictedinFig.1.3[22].AbriefsummaryofthedevelopmentoftheIGISOLconceptwillbediscussedbelow.Forfurtherdetails,pleaseseereferencesherein.Thestoppingpowerofthegaschamberandtherangeoftheionswerekeytodeterminingchambersizesandoperatingpressures.Duetothelowerenergyofthereactionproducts,theseionslosetheirkineticenergyandarethermalizedthroughcollisionswiththe5gasatomsofthegaschamber.Thelinearstoppingpower,S,foragivenmediumisgivenbytheformula[23]:S=dEdx;(1.1)wheredEdxistherateatwhichenergyislostastheparticlemovesthroughthematerial,orthespenergyloss.Sincreasesastheparticlevelocitydecreases,andforparticleswithagivenchargestate,theBetheformulaforthespenergylossis[23]:dEdx=4ˇe4z2mov2ˆNB;(1.2)whereeisthefundamentalchargeofanelectron,zisthechargeoftheprimaryparticle,moistherestmassofanelectron,visthevelocityoftheprimaryparticle,ˆNisthenumberdensityofthematerial,andBisaparameterdependentuponthemediumandionvelocity[23]:BZln2mov2Iln1v2c2v2c2:(1.3)Here,Zistheatomicnumberofthestoppingmaterial,Iistheaverageenergyforionizationofthematerial,andcisthespeedoflight.TheBetheformula(1.2)showsthetdependenceofspenergylossonaparticle'svelocity.Assuch,formostofitstrackthroughamaterial,aparticle'sspenergylossincreasesroughlyas1/E.Astheparticlecomestorest,however,itsinteractioncrosssectionincreases,andneartheendofitstrack,thechargeisreducedthroughelectronpickupandthedEdxgoestozero.Thischaracteristicbehaviorforstoppingpowerasafunctionofpathlength,orrangeinthematerial,isknownasaBraggcurve[23].Therangeofaparticlethroughagivenmaterial,R,isthereforedependentuponthestoppingpowerand6canbeas[4]:R=Z0EodEdE=dx;(1.4)whereEoistheinitialkineticenergyoftheionand{dE/dxistheenergylossfunctionoftheion.Giventherangeandinitialenergiesofthereactionproducts,theseionswereexpectedtorange-outandtobecompletelythermalizedbythegas.TheIGISOLmethodwastestedbyusingan227Acsource,andtheexperi-mentalset-upcanbefoundinFig.1.4[24].Notonlydidthismethodreducethetransporttimesfromthe˘10mstimesachievedwithHe-jetsdownto˘1ms,butthetotalciency,whichincludesmassseparation,wasreportedtobeaminimumofetimeshigherat˘5%withroughly98%ofthedecayproductsleavingtheionguideina+1chargestate.Sincethiswaspurelyaproofofconceptexperiment,itisimportanttonotethatthisef-wasobtainedwithoutttimeorspentonsystemoptimization[24].Theseresultspromptedresearcherstoconductfurtherexperimentstoexplainthe+1chargerententionpriortotheon-lineexperiments.Threemainprocesseswereproposedtoproducechargedionswithinthegas:internalionization,Augercascades,andstrippinginhelium[10,22].Theseprocesseswerebelievedtocreatehighandverydistributedchargestatesamongthereactionproducts.Thechargestate,whichisgenerallyproportionaltotheion'svelocity,isthereforeconstantlychangingandresetbychargeexchangereactionswhileslowingdowninthegas[10,22].Collisionsbetweenthechargedreactionproductsandtheneutralheliumatomswerethoughttoreducethechargestatesto+2withthereductionto+1occurringfromcollisionswithimpuritymoleculesfoundwithinthehelium[25].Ionsthenremaininthe+1chargestateduetoalackofenergeticallyfavoredreactionswithinthegas[10].Afull7Figure1.4:Schematicdiagram(nottoscale)ofaHelium-jetionguideconnectedtotheacceleratorstageofamassseparatorwhere(1)istheRecoilchamber,(2)isthevacuumchamber,(3)isthecapillary,(4)istheskimmer,and(5)istheextractionelectrode[24].understandingoftheion-gasreactionsandionlosseswithinthegaswasthereforeessentialtofurtherimprovethesesystems.1.2.2ReactionLossesandMechanismswithintheGasAlthoughtheofionsleavingtheionguideina+1chargestateisnearlyperfect,theoveralleofthesystemistlylower.Numerouslossesoccurwithinthegasthateitherpreventionsfromleavingtheguideorfrombeingcollectediftheydoinfactleave.Theseionlossesoccurthroughthreemainprocesses:tothechamberwalls,formationofmolecularionswithimpuritiesinthegas,andneutralization[10,22,26].eithercausesionstoneutralizeortosticktothewallsofthechamber.These8ionsareusuallytrappedaftertheyreachthermalenergiesandarelostformassseparation.Althoughthermalmotionisunavoidable,tlossesfromcanbeavoidedwithcarefulconsiderationofthechambergeometryandwtrajectories,increasingthegaspressureormassofthestoppinggas,anddecreasingtheevacuationtimeofthechamberbyincreasingthewrate[10].Anotherlossmechanismthatoccurswithinthegasismolecularionformation.Impuritiesintheheliumgasallowthereactionproductstoattainthepreviouslymentioned+1chargestate.Largereactioncrosssectionsalsoexistbetweentheimpuritiesandtheionsofinterestthatleadtotheformationofmolecularions[22].Thesemolecularionscanformwithinthegas,butcanalsoformaftertheionsexitthegascell.Whilethesemoleculesarestillchargedandcanreachthemassseparater,theemassoftheionofinterestisunvariablyincreased.Theseincreasedreactionpathwaysleadtoafractionalizedbeamwheretheionofinterestisspreadoverawidemassrangeinsteadofconcentratedatonemassasabareion.Sincestatisticsareparamountandmolecularcompoundsreacttlythanbareions,afractionalizedbeamisnotidealforexperiments.Thelossesfromthemassshiftandfractionalizationvarydependingonthemassrangeorresolutionofthesystem,butreducingthenumberofimpuriteswithinthegaswouldreducetheselosses[26].Despitethetlossesfromandmolecularionformation,theandmostimportantlossmechanismwithinthegasisneutralization,whichoccursatveryhighbeamratesduetotheplasma[26].Astheprimarybeamand/orthereactionprod-uctspropogatethroughthegas,alargedensityofion-electronpairsarecreated(˘105pairs/incomingion).ThelighterelectronsaremuchmoremobilethantheheaviercationsandcanbecollectedalmostimmediatelybyappliedelectricThisionizationandre-sultantbuildupofpositivecharge,orspacechargepotential,createsnumerousproblems,9including:shieldingofanyelectricpushingthermalionsoutofthecentralgaswandtowardthewalls,andfacilitatingthree-bodyneutralization.Inordertounderstandhowspacechargebuildsup,itisimportanttoknowtheantreactionmechanismswithinthegas.Althoughmonomergasions,He+,arecreatedbycollisionswiththeprimarybeamorthereactionproducts,thesemonomersquicklydimerizetoHe+2inthethree-bodyreaction[26]:He++2He!He+2+He:(1.5)Duetotheirlowerionizationpotential,impuritiesintheheliumgascanthencharge-exchangewiththisdimertoionizeandtodominatethechargeoftheplasma.Nitrogenisusedasanexamplebelow[26]:He+2+N2!2He+N+2:(1.6)Thelargertheresidualpositivecharge,themorelikelyanyappliedelectricwillbeshieldedandthemorelikelytheionsofinterestwillbepushedtowardsthewalls.Asdiscussedabove,thisbehavioreitherleadstotheionsofinterestbeingtrappedintheplasmaorlostonthewallsthroughrespectively.Topreventtheimpuritiesfrombecomingthedominantchargecarriers,andtherebydetermingthechargeintheplasma,itisimportanttokeeptheirpresencetoaminimum.Eveniftheheliumdimeristhedominantchargecarrier,apositivespacechargepotentialcanstilldevelopbecauseofthevariousionmobilities{electronsmove˘1000timesfasterthantheircationpartners.Ifthisspacechargepotentialshieldstheappliedelectricaweakplasmaiscreatedwheretheionsofinterest,electrons,andnetralheliumatomsnolongermigratewhich10facilitatesthree-bodyneutralizationasshownbelow[26]:X++e+He!X+He;(1.7)whereX+isanionofinterestorgasion,eisanelectron,andBisargasatom[26].Thespacechargeandplasmaareapparentinthenon-linearreductioninextractionasafunctionoftheincomingbeamintensity.Thisbeamintensitycanbescaledtoshowthesametrendforallgascatcherfacilitiesindependentofgaschosen,volumeofgasused,energyofprojectiles,orelectricpresent[10,22,26].Theparameterusedforthesecomparisonsistheionization-ratedensity,Q[26]:Q=6:251012IAdE=dxW:(1.8)thatgivesthedensityofion-electronpairscreatedforanincomingbeamintensity,I(pA),theareaofthebeamspot,A(cm2),thelinearenergylossoftheion,dE/dx(eV/cm),andtheenergyrequiredtoproduceanion-electronpair,W(41eVforHe)[26].Unnavoidablelossesoccurwithinthegasathighratesnomatterthedesignorset-upofthesystem,althoughvariationsiniencyoccerdependingonthereactionproductsstudiedasdiscussedbelow.1.2.3ofVariousReactionProductsThreemaintypesofionguidesexistthataremeanttoaccomodatethereactionkinematicsofagivenexperiment:light-ionionguides,heavy-ionionguides,andionguides[26].Theionguidesweredesignedtothermalizeandtotransportreactionproducts11fromlight-ioninducedfusionevaporationreactions.Theseionguideshadasimpledesignconsistingofacylinderwithatargetmounteddirectlyintothestoppingvolumeofthecellandanexit-holeattheoppositeend.Productsfromthesereactionsrecoilintheforwarddirectionwithanyacceleratorbeamandthenboththeproductsandbeamenterthestoppingchamber.estoppingvolumesoftheseionguidesareontheorderofafewcm3withatypicalexitnozzlediameterof˘1mmwhichfacilitatesextractiontimesasshortas1millisecondandethatvarybetween1%and10%[10,22,26].Heavy-Ion-GuideIsotopeSeparationOn-Line(HIGISOL)guidesweredesignedforheavy-ioninducedfusion-evaporationreactions.Thesereactionproductsarepeakedintheforwarddirection,andagainboththereactionproductsandtheacceleratorbeampenetrateintothechamber.Here,targetswereinstalledalongthebeamaxisonawater-cooledframeupstreamofthestoppingvolume.estoppingvolumesweresomewhatlarger,ontheorderofhundredsofcm3,withatypicalexitnozzlediameterofstillonly˘1mm[26].Initialresultsfromtheseionguidesshowedreducedcomparedtotheirlight-ioncounterparts.Thisdecreaseinwasattributedtolesststoppingofthemoreenergeticrecoilsandtotheionizationproducedbytheprimaryionbeaminthestoppinggaswhichleadstospacechargeandplasma[22].Threemainmethodswereemployedtoreducethelossesassociatedwiththeprimaryacceleratorbeam[10].Inthemethod,reactionproductswereseparatedfromtheprimarybeambyadipolemagnet,wherebytheionguidereached10%to40%[22].Thesecondmethod,dubbedthe\shadowmethod,"exploitedtheangulardistributionofthereactionproducts,anda\plug"wasusedtosupresstheprimarybeam[10,22,26].Overallforthisset-upreachednearly1%.Thethirdmethodwasmoreadvancedandwillbediscussedlater.Thismethodinvolveslaserionizationwithapulsedprimarybeam12Figure1.5:SchematicofaionionguideoperatedbytheUniversityofJyva[22].[22].Extractiontimesfortheseionguideswasontheorderofhundredsofmillisecondswith0.04%[26].Fissionionguideswereusedtocollectproductsfromtheproton-inducedofura-nium,orotheractinidetargets[26].TheestoppingvolumesandextractionnozzlesoftheseguidesaresimilartothoseofHIGISOL,ontheorderofahundredcm3and˘1mm,respectively.Unlikethepreviousreactionproducts,though,productsareemit-tedisotropicallyinspace,whichallowedthereactionproductstobephysicallyseparatedfromtheprimarybeam.Severaldesignstoseparatetheproductsfromtheprimarybeamweretestedandused,includingtheuseofaseparationfoil.Anexampleofsuchanionguideset-upisshowninFig.1.5[22].Theplasmawasstillobserved,however,13andwasattributedtotheionizationcreatedbythefragmentsthemselves.Moreover,only˘30%oftheproductsenterthestoppingchamber,andthestoppingncywasreportedtoonlybe˘1%[10,26].Asexpectedwithincreasedspacechargeandplasmaabsoluteofthesesystemswereontheorderofafewtensofthousandths,withtheionsofinterestmoreoftenthannotbeingovershadowedbyunwantedisobars.Laserionizationwasalsoimplementedintheseionguides[26]andwillbediscussedinthenextsectiononadvancedsystems.1.2.4AdvancedIGISOLSystemsNumerousadvancestotheIGISOLmethodweredevelopedandemployedtoovercometheplasmaandimprovethebeamquality.Upgradesincludedthelaserionguide,thesqueezerionguide,andthesextupoleion-beamguide(SPIG)[10,22,26].ForstandardIGISOLset-ups,thevoltageplacedontheskimmeracceleratestheionsthroughthehigherpressureregionlocatedbetweentheexitholeandtheskimmeritself.Ionscancollidewithneutralheliumatomsbeforetheyareremoved,whichcausesanenergyspreadinthebeam[27].Thisenergyspreadtlyreducesthemassresolvingpower(MRP)ofthesystem[10].Here,theMRPisasmmwheremisthemassoftheionseparatedandmisthefullwidthathalfmax(FWHM)oftheion'smasspeak[27].TheeasiestproposedsolutionforimprovingtheMRP,withoutreducingthebeamintensity,wastobringtheionsintohighvacuumbeforetheywereaccelerated,thisconceptwasimplementedbytheUniversityofaintheirso-called\squeezer"ionguidedepictedinFig.1.6[27].Inthisguide,theionswerefocusedthroughtheskimmerwithbothviscousdragoftheheliumwandweakelectric[10].Threerings,allwithvoltageslessthan25V,wereplaced14Figure1.6:Typicalgeometryandelectricpotentialsofasqueezerwiththreegridsoperatedbetweentheexitholeandskimmer[27].betweentheexitholeandtheskimmer.Giventheexperimentallydeterminedgeometriesforthegrids,testsshowedthatthetransportwasabout75%withanenergyspread(FWHM)of2.5eV.Althoughaddingasqueezerdidnotimprovetheonlineyields,theMRPmorethandoubled[10].Withtheintentofproducingwell-focusedbeamswithsmallenergyspreads,anotherimprovementtotheIGISOLmethodwastheadditionofasextupoleion-beamguide(SPIG).TheSPIG,whichwasdevelopedbytheUniversityofTokyo'sInstitureforNuclearStudy,consistsofsixcircularrodsdistributeduniformlyonacircle.Everyotherrodisconnenctedtotheoppositephaseofanalternatingradiofrequencyvoltage[10].AsseeninFig.1.7[28],theSPIGislocatedjustdownstreamofthestoppingchamber,eliminatingtheneedforaskimmervoltage.NoappliedskimmervoltageandtheimplementationoftheSPIGdecreased15Figure1.7:SchematicofSPIGset-upusedforthestudyofionsproducedbyadischargeionsourceattheUniversityofTokyo'sInstitureforNuclearStudy[28].theenergyspread(FWHM)to0.8eVandincreasedthetransportto90%,butitalsoresultedinmostoftheionsleavingtheionguideinmolecularform.Thesmallenergyspreadofthebeamismaintainedbecausetheionsarere-thermalizedinthepartoftheSPIG,whichoperatesatahighheliumpressure.Asforthemolecularion-formation,thevoltageontheskimmerwouldnormallyacceleratetheionstoanenergywherebythemolecularionswouldbebrokenbycollisionswiththergasions.Inordertoobtainsimilarresults,theSPIGwasplaced˘150Vbelowthevoltageappliedtotheionguide[10].Inanattempttoovercomeplasmaaswellasprovideatomicnumberselectivity,aresonantlaserionizationschemewasdeveloped[22].TheLeuvenIsotopeSeparationOn-Line(LISOL)groupinLeuvenmainlypioneeredthistechnique.Inordertoachieveneutralizationoftherecoilingnucleipriortoselectiveionizationofthelaser,theexitholeforthistechnique16couldonlybe0.5mm,notthestandard1.2mm,whichledtolowerextractiontimes.Inadditionlaserionizationonlyproducesaweakly-ionizedplasma,therequiredwratesweremuchlower.Thesereducedwratesnotonlyallowedforhighergaspressures,butalsotheuseofheavierstoppinggases,suchasnitrogenorargon.Bothofthesescenariosleadtohigherstoppingbutalsotoslowerextractiontimes.Incorporatingelectrictofacilitatefastertransporttimeswasproposedandstudiedandwillbediscussedlater[22].Twodistinctlaserionguideshavebeenusedforlaserionizationstudies:oneattheUniversityofLeuvenandtheotherattheUniversityofa.InLISOL'sionguide,thevolumeofthegascellwasdividedintotwoparts{themaincellandasmallchannelleadingtotheexithole.Theexitholeforthissystemwas,aspreviouslymentioned,0.5mmwhichresultedinthepoorerextractiontimesof˘480msforthewholeguideand˘18msfortheextractionchannel[22].Thelaserionguideconstructedbya,however,wasamoHIGISOLsystem.Thismosystemwasoptimizedfortgaswtransportandincludedcapabilitiesforbothwatercoolingandchamberbaking.Theadesignwasalsomademoremodulartofacilitatetheinstallationoftsand/ordcelectrodes.Theevacuationtimeforthenecessary0.5mmexitnozzlewas2.25s,whichismuchlongerthanthe˘500msextractiontimeoftheLISOLsystemandonly390msforthestandard1.2mmexithole[26].tionizationforthesesystemswasachieved;however,similartotheotherIGISOLchambersandsystems,itwasnotedthattheplasmacreatedbythethermalizationprocessstillplayedanimportantroleinthesurvivaloftheions.Itwasalsonotedthatthegasimpuritiesstillimpactedtheproductionandsurvivalofthesingularionsofinterest[22].Aftertheionguidetechniquewasprovenebylow-energyfacilities,waystoimplementsuchtechniquesinhigh-energyfacilitieswereexplored.17Chapter2TheNSCLProjectileFragmentationFacilityandLow-EnergyExperimentsTheNSCLisaprojectilefragmentationfacilitythatemploystwocoupledcyclotronstoproduceavarietyofrareionbeamsoverabroadenergyrangefornuclearphysicsexperiments.Projectilefragmentationcreatesionbeamswithenergiesontheorderof100MeVpernucleon(MeV/u).Whilesuitableforhigh-energyexperiments,thesebeamsmustbedramaticallyreducedinenergybyanintermediateprocess,knownasbeamthermalization,toenergiesof˘50keVforlow-energyprecisionexperimentsorreacceleratedasaverylowemittancebeamforcertainastrophysicalexperiments.Low-energyandreacceleratedexperimentshousedattheNSCLincludetheLow-EnergyBeamandIonTrap(LEBIT)facility[29],theBEamCOolingandLAserspectroscopy(BECOLA)facility[30],andtheReAcceleratorfacility(ReA)[31].TheNSCL'slargevol-umelineargascellplaysacentralroleinthebeamthermalizationprocesswhichultimatelyallowsthisprojectilefragmentationfacilitytocarryouttheselow-energyprojects.2.1IsotopeProductionAspreviouslydiscussedinChapter1,theprojectilefragmentationmethodisachemicallyinsensitivetechniquenotgenerallylimitedbythepropertiesandlifetimesofthenuclides18Figure2.1:SchematicviewsoftheISOLandprojectilefragmentationmethodsusedtoproduceradioactiveisotopebeams[32].produced.ThecomplementaryISOLmethodhaschemicaldependenciestotheofproductsformedbytheinteractionofhigh-energy,light-ionbeamdeepinsideathicktarget.Projectilefragmentationavoidsthisproblemasthehigh-energy,heavy-ionbeaminteractsinathintarget,andproductsarethencollectedt.Peripheralinteractionsbetweentheprojectilenucleiandthetargetnucleiallowsomenucleonsfromtheprojectilenucleitoberandomlyremoved.Theseresiduesthenundergoasmallrecoilandarefocusedforwardandoutofthetargetbythelargeinitialvelocityoftheprojectilebeam[33].AschematicrepresentationofthesetwomethodsisshowninFig.2.1[32].19TheNSCLutilizestwocyclotronsinseriestoaccerlerateheavy-ions.CyclotronsrelyonthefactthatchargedparticlesmoveinacircularorbitwhenplacedinatlylargeanduniformmagneticTheforcecausingtheionstospiralisknownastheLorentzforce,andtheaccelerationineachrevolutionistunedsothatthefrequencyofthecircularmotionoftheparticleisconstant.TheLorentzforceisdescribedbytheequation[32]:FLorentz=Bqv;(2.1)whereBisthe(vector)magneticqisthechargeoftheparticle,andvisthe(vector)velocity.Sincethecrossproductfollowsthe\right-handrule,"theLorentzforceisperpen-diculartotheionmotion.Theradiusoftheionmotion,r,isfoundbysettingthisforceequaltothecentripetalacceleration[32]:FLorentz=Bqv=mv2r;(2.2)andsolvingforr:r=mvBq:(2.3)Thecyclotronresonancefrequency,tcyc,orthetimethatittakesaparticletocompleteoneorbit,isthecircumferenceoftheparticle'spathdividedbythevelocityoftheparticle[32]:tcyc=2ˇrv=2ˇmvBqv=2ˇmBq;(2.4)which,aftersubstitutingforr,isultimatelysolelydependentonthemassandchargeoftheparticleaswellastheappliedmagneticandindependentoftheion'svelocity.This20propertyofthecyclotronfrequencyisalsothebasisforprecisemassmeasurementsthatwillbediscussedinSection2.3.1.Asthevelocityoftheparticleincreases,theorbitalradiiwillincrease,andtheparticleswillappeartospiraloutfromthecenterofthecyclotron.Theparticles,however,canonlyspiraloutsofar.Themaximumorbitalradiusandparticlevelocityis,therefore,limitedbythemaximumphysicalradius,ˆmax,andthemagneticofthecyclotron.Asaresult,mostcyclotronsareidenbytheirKvaluewhichisgiveninMeVandcalculatedas(Bˆmax)2=2.Forgreaterdetailoftheoperatingprinciplesofcyclotrons,pleasesee[32]andreferencestherein.ThetwocyclotronsattheNCSLareaK-500andaK-1200.Bycouplingthebeamfromonemachineintotheother,stableionbeamscanreachvelocitiesofabouthalfthespeedoflightwithbeamenergiesontheorderof˘150MeV/u.Thesestableionbeamsthenimpingeonaberylliumfoil,andtheprojectilefragmentsformedbythenuclearreactionsareseparatedt"andfocusedbytheA1900fragmentseparator[34]fordeliverytotheexperimentalareasofthelaboratory.ThebeamthermalizationareashowninFig.2.2servesasthebridgefromthehigh-energyexperimentalareasofthelabtothelow-energyexperimentalareasofthelab.Withoutthisbridge,low-energyprecisionexperimentsandcertainastrophyisicalstudieswouldbelimitedtoISOLproductionmethodsandfacilities.Agascatcherdesignedforenergeticparticles,whoseprinciplesarediscussedbelow,ispivotaltothesuccessfuloperationofthethermalizationarea.21Figure2.2:SchematicoverviewoftheNSCL{aprojectilefragmentationfacilitythatincor-poratescoupledcyclotronstoacceleratetheheavybeam.2.2GasCatchersforEnergeticFragmentsGiventhesuccessoftheIGISOLmethodforthermalizingandtransportinglower-energyre-actionproductsatISOLfacilities,severallaboratoriesbeganinvestigatingthefeasibilityofimplementingionguidetechniquestocollectthemoreenergeticreactionproductsproducedatprojectilefragmentationfacilities.TheadvantageofprojectilefragmentationfacilitiestoISOLfacilitiesisthattproductionisnotsensitivetothechemicalpropertiesorthelifetimesofthenuclidesofinterest[35].Amajordisadvantageofprojectilefragmentationfa-cilitiesisthehiger-energybeamsandfragments,whichhavehigheremittancesandarehardertocontrolandfocus.Thereisalsoaseparationbetweenproductionandthermalizationinprojectilefragmentationwhichisbothanadvantageanddisadvantage.TheoftheseconditionsisshowninFig.2.3[6],whichcomparesthepredictedLISE++[6]produc-22Figure2.3:PredictedLISE++productionratesplottedonanuclearchartforbothanISOLfacility(left)andfortheNSCLprojectiilefragmentationfacility(right)[6].tionratesforbothanISOLfacilityandtheNSCLprojectilegragmentationfacility.Thesamegeneralfeaturesexistinhigher-energyionguidesystemsthatexistedinIGISOLset-ups,andtheprinciplesestablishedthroughIGISOLresearchstillholdtrue{therepulsiveofspacecharge,thedistortionofanyelectricbytheplasmaandtheinteractionofanionofinterestwithgasimpuritiesallcontributetoylosseswithinthesystem[22].2.2.1ComponentsofPF-GasCatchersSimilartotheirlow-energycounterparts,mediumandhigh-energygascatcherscontainachamber,exitnozzle,skimmerorSPIG,andatialpumpingsystem.Thesehigherenergyset-ups,however,alsogenerallyincludeadegraderandwedgesystemlocatedupstreamofthegascelltoproduceamonoenergeticbeamandaswellastoremovenearlyallofthekineticenergyofthebeampriortogasstopping.Thegascellsalsocontainlargerstoppingvolumesandhighergaspresures,whichnecessitateelectrodestructurestogenerateDCand/orACforfastionextraction.Initialbeamenergiesrangefrom˘1MeV/uto˘1000MeV/u,withenergyspreadsuptoafewpercent.Thehigher-energybeamsrequireadegrader(s)toreducethekineticenergy23Figure2.4:Simplecartoondepictingtheworkingprincipleofamonoenergeticwedge.oftheincomingproductbeamdownto˘1MeV/utoallowforamanageablegasvolume.Theenergyspreadofthebeamalsoneedstobecompensated.Thiscompensationisaccom-plishedbydispersingthebeamonamonoenergeticwedge[35].AschematicdepictionofamonoeregeticwedgeisshowninFig.2.4.Variousfacilitiesemploytdegrader/wedgearrangements,whicharediscussedinsection2.2.2.Higherbeamenergiesalsowarrantlargergasstoppingvolumes.ConventionalIGISOLgaschamberswereonlyafewcminlength.Asdiscussedearlier,reactionproductswerestoppedinthesesmallvolumesandsimplytransportedoutofthecellbygasw.Higherenergygascells,however,haverangedfrom30cmto200cminlength.Thelargervolumeshaveatimpactonextractiontimes.Relyingsolelyongasw,extractiontimesontheorderofminutesareexpectedwithtlossesoccuringfromnotonlyradioactivedecaybutalsochargeexchangereactionsandasdiscussedinsection1.2.2[35].Numerousion-transportmethodsutilizingelectricandrespectiveelectrodestructures24LaboratoryIsotopeSeparatorGasPressureGuidingForceRI-BeamEnergyCellSizeoutofGasRIKEN/JapanRIPS˘130mbarDC+RFFunnel,˘100MeV/u40cmx200cmDC+RFCarpetGSI/GermanyFRS0.5{1barDC+RFFunnel100{1000MeV/u20cmx125cmNSCL/USA1900<5barDC+GasFlow˘100MeV/u5cmx50cmANL/USGARIS˘150mbarDC+RFFunnel˘5MeV/u10cmx20cm+GasFlowGSI/GermanySHIP˘0.1barDC+RFFunnel˘5MeV/u17cmx18cm+GasFlowCYRIC/Japan+IGISOL˘100mbarDC+GasFlow˘1MeV/u24cmx30cmTable2.1:Summaryofthelargegascellsdevelopedforenergeticradioactiveion(RI)beams[35].havebeenexploredinordertoimprovethetransporttimesinsidelargevolumecells.Thesemethodsarediscussedindetailbelow.2.2.2In-GasIon-TransportMethodsSeveralmediumandhigh-energyacceleratorlaboratoriesconstructedandtestedtheirowngascatchers,andasummaryoftheseinitialgascellscanbefoundinTable2.1[35].Themediumenergycatcherswillnotbediscussedhere,andthefocuswillsolelybeplacedonhigh-energyfacilities(beamenergy100MeV/u)andtheirrespectivecatcherswiththeunderstandingthatthesameprinciplesandtechniqueswereemployedforbothenergyranges.Giventhesheervolumeofthesehigher-energycells,theextractiontimesoftheionswouldbedominatedbythedrifttimetothenozzlewithoutthehelpofDCelectricThevelocityofanioningasisdirectlyrelatedtotheappliedDCldandcanbecalculatedusingtheequation[36]:vd=KEPT0P0T(2.5)25wherevdisthedriftvelocity(cm/s),Kistheionmobility(cm2/Vs),Eistheelectric(V/cm),Pistheoperatingpressure(mbar),T0isthestandardtemperature(273K),P0isthestandardpressure(1000mbar),andTistheoperatingtemperature(K).Oncetheionmotionisdampedbythegas,theions'trajectoriesfollowtheelectriclinesproducedbytheappliedDCvoltage.Therefore,theelectricappliedtothecellnotonlydictatestheionmotionbutmoreimportantlytheionvelocityinthegas.Sincetheelectricterminatesattheendwallofthecell,itisnecessarytoemploygaswtoensureionsexitthroughthenozzle.Insidethegascell,adynamic(RF)canbeappliedtorepeltheionsfromthewallandgenerallyguidethemtowardthesmallexithole.Wada[35]expressedtherepellingforceas:Favg=moˆoˆ2V2RFr3o;(2.6)wheremisthemassoftheion,oisthereducedionmobility,ˆoisthenormalgasdensity,ˆisthegasdensity,VRFisthezero-to-peakvoltagefortheRFamplitude,androishalfofthespacingbetweentheelectrodes.Thisaveragerepellingforceexertedonchargedparticlesdecreasesasthedistancefromtheelectrodestructureincreasesandalsodecreasesinthepresenceofgasandcanalsobedescribedbythefollowingequation[35,36]:Fvac=e22mw2EdEdx;(2.7)whereFvacistheaverageforceexertedonchargedparticlesbytheinhomogeneousRFeisthefundamentalchargeofanelectron,misthemassoftheion,wistheangularfrequencyoftheRFandEistheamplitudeoftheRFInthepresenceofgas,itisreduced26totheform[36]:Fdamp=Fvac11+(eKmw)2:(2.8)SinceKistheionmobilityingas,theforcescaleswiththeRFamplitudesquaredoverthepressuresquared.Therepellingforceexperiencedbyanionisalsoapproximatelypropor-tionaltothesquareofthemass.Giventheionsofinterestaremuchmoremassivethanthegasions,oneexpectstheheavierionstomovetowardthenozzlewhilethelighterionstotheelectrodesurfaceandareneutralized.AlsoconsideringtherepellingforceprovidedbyanRFisinverselyproportionaltothesquareofthegaspressure,themax-imumpressureofthegascellislimitedbythedesiredstrenthoftheRFd[35].Twomainiontransportmethodswillbediscussed{staticandgaswonly[37]andDC,RF,andgasw[35].ThegenerationgascellattheNSCLwas50cminlengthandgenerallyoperatedunderaheliumpressureof1bar(760torr).Aschematicoftheexperimentaltestset-upisshowninFig.2.5[37].Thekineticenergyaswellasthemomentumspreadofthebeamwasreducedupstreamofthecellbya1.49-mmthickrotatableborosilicateglassdegrader,a0.74mmaluminummonoenergeticwedge,anda1-mmthickberylliumwindowsuchthattheionswouldrange-outinthegas[37].Asetof21ringelectrodesand4concentricsphericalelectrodesprovidedaDCgradientalongthegascellcylindricalaxistoguideandtofocusthestoppedionstowardtheexithole.Fromthere,theionswerejettedthroughasupersonicnozzleandintoanexpansionchamber.Theionsweretheneithercollectedandmeasuredorcaughtanddeliveredtolow-energyexperimentsbyanRFion-guidesystem[37].Theextractionncyofthegenerationcellwasafewpercent.27Figure2.5:Schematicofexperimentalset-upfortestingthegenerationgascellatNSCL[37].Thisdramaticallydecreasedwithanincreaseintheimplantationrate;similartothereportedinIGISOLsystems.Itwasalsonotedthatalthoughtheextractioncurveasafunctionofdegraderthicknessapproximatelyfollowedthestoppingcurve,themaximumyieldsweremuchlowerthanthosecalculatedforthestopping.Thesamespacechargeandplasmareportedinthelow-energyionguideswereexpectedtobepresentinthenozzleregionofthishigh-energygascellandwasattributedtothesetlosses[37].FurtherdiscussionoftheseresultswillbegivenafterexaminingtheRFbasedgassystemsandproperties.TheideabehindimplementingbothastaticDCandadynamicRFwasthattheDCpotentialwoulddragtheionsthroughthegas,whiletheaverageforceduetotheRFgradientwoulddrivetheionsawayfromtheelectrodesalongthewallandfocusthe28Figure2.6:AseriesofringelectrodesrotationallysymmetrictothelowerlineprovideanRF-barrier(Eeff)aswellasasuperimposedDC(EDC).TheionsarepulledbyEDCwhileEeffdrivesthemawayfromtheelectrodes[35].ionstowardtheexit.ThisconceptisdepictedinFig.2.6[35].TheRF-basedgascellwasconstructedatRIKENandwasusedtotesttheworkingprinciplesoftheRFfunnelinsideagascell.ThisgenerationRIKENgascellwas30cminlengthandwaskeptataheliumpressureof20torr.Thefunnel,aspicturedinFig.2.7,contained80ringelectrodeswithbothanRFvoltageanddecreasingDCoappliedtoeachelectrode.Atransmissionof˘70%wasobtainedforthefunnel[35].ThenextiterationoftheRFgascatcherwastestedon-lineanddubbedthePOP,orproof-of-principle,system.Thisgascellwas70cminlengthandtheheliumpressurewastestedatboth30and60torr.Here,adoubleRFfunnelstructurewasemployed.Thetwofunnelswerebuiltofprintedcircuitboardsandstackedtotransportionstheentirelengthofthecell.TheforthePOPsystemwasreportedas0.43%stoppingwitha2.4%ion-guideency.Heliumgaspressures,aswellasthegeometryofthegascell,limitedthestopping,whiletheRFvoltagelimitedtheion-guide[35].29Figure2.7:RFfunnelexperimentalset-upfortransportingTa+ions[35].BasedontheexperiencewithbothoftheseRFtestcells,athirdRFgascatcherwasconstructedatRIKEN.Itwas2minlengthandwasoperatedataheliumpressureof100torr.NewRFelectrodeswereconstructedtoalleviatelargecapacitiesandhighheatdissipationthatwasexperiencedinthePOPsystem.Theseelectrodesconsistedoftwolayersofplanarprintedcircuitboards(PCB).ThetopPCBhadadiameterof29cmwith280ringelectrodesandacentralholeof10mm.ThebottomPCBhadadiameterof3cmwith48ringelectrodesandanexitnozzleof0.6mm.ThesestructureswerenamedRF-carpetsandareshowninFig.2.8[35].ThetoplayerwasoperatedwithonlyastaticpushwhilethebottomlayerwasoperatedwithbothRFandDCADCdriftwasappliedperpendiculartothecarpetsalongthelengthofthecellwhiletheRF-carpetswerelocatedattheexit,andamaximumoverallof0.2%wasreportedforthelowestbeamintensitytested.Asseenwithallofthepreviousgascells,theofthiscelldecreasedwithincreas-ingbeamintensity[35].Theseresults,aswellastheresultsfromtheNSCLandothergas30Figure2.8:TheRF-carpetelectrodeemployedbyRIKENintheRFbasedgascellwasmadeoftwolayersofPCBs.Detailsoftheselayerscanbefoundinthetext(b)Typicaliontrajectoriesdeterminedbyamicroscopicparticlesimulationfor8Liionsin90torrhe-liumgaswheretheRFvoltagebetweenneighboringelectrodeswas190Vat26MHz.ThesuperimposedDCwas8V/cm[35].catcherfacilitieswerecombinedinanoverallanalysisoftheimpactofbeamintensityonciency.Asummaryofthe\worlddata"isshowninFig.2.9[38].Itisclearthatwerestillneededtoimprovetheextractiontrendobservedforalloperationalgascells.Sincethegreatestextractionlosseswereattributedtospacechargeandplasmabuild-up,mostoftheexperimentalwerefocusedonreducingtheseinnextgenerationdevices.Onesuchdevice,thefocusofthenextchapter,isessentialtothelow-energyandreacceleratedfacilitieslocatedattheNSCL.2.3Low-EnergyandReAFacilitiesLow-energy,so-calledprecisiontechniquesandexperiments,aspreviouslymentioned,wereinitiallydevelopedatISOLfacilities.Thelow-energysecondarybeamsproducedbytheISOL31Figure2.9:Summaryofthedecreaseinextractionforavarietyofgascellsystemsasafunctionoftheionization-ratedensity.Thedatawereobtainedfor8Li,38Ca,107Ag,and58NifromtheprojectilefragmentationfacilitiesatRIKEN(squares),MSU/NSCL(circles),andGSI(diamonds),aswellastheISOLfacilityatLeuven(triangles),respectively[38].methodweresimplymatchedtotheneedsofthetechniquesascomparedtothehigh-energysecondarybeamsproducedbyprojectilefragmentation.Thedevelopmentofgascatchersandbeamthermalizationmadetheselow-energytechniquesavailabletoprojectilefragmentationfacilitiesandprovideawiderrangeofexoticisotopes.ThebeamthermalizationareaattheNSCLprovidesbeamsfortheLEBIT[29],BECOLA[30],andReA[31]facilities.2.3.1LEBITTheLow-EnergyBeamandIonTrapfacility(LEBIT)attheNSCLperformsPenning-trapmassspectrometryonrareisotopes[29].Iontrapsareusedinavarietyofresearchasameanstoortrap,ionsinsmallvolumesinwell-controlledThesedevicescan32Figure2.10:(Left)AschematicoverviewoftheLEBITfacility.(Right)TheLEBIThigh-precisionPenningtrapwithitsend-capremoved.Abottlecapisshownwiththetrapforscale[41].beverysensitive,andinexperimentswherethefrequenciesofrevolutionaremeasured,highprecisioncanbeachievedbytrappingtheionsforextendedamountsoftime.PenningiontrapsionswithastrongstaticmagneticcombinedwithaweakstaticelectricIfthechargestateoftheionandthestrengthofthemagneticareknown,themassoftheioncanbeinferredfromtheion'sresonantcyclotronfrequency(2.4).ThisfrequencycanbedeterminedbymonitoringaninducedcurrentonthetrapelectrodesorthroughtimeoftmeasurementsforionsleavingthePenningtrap.ThemassresolvingpowerofPenningtrapsisdirectlyproportionaltotheobservationtimeoftheionmotion.Thelongerthemotioncanbeobserved,thehighertheresolvingpowerofthetrap[39].Precisemassmeasurementsareimportantfundamentalfeaturesofnuclearstructureandprovideinputfornuclearastrophysicsandfundamentalinteractions[40].Low-energybeamsemergefromtheNSCLlargevolumelineargascellatacontinuousrateandarecooledandbuncheduponenteringtheLEBITfacilityforbatchmodemass33measurements.ThebunchedbeamistheninjectedintoandtrappedbytheLEBITPenningtrap.Ionsareexcitedbyknownfrequencies,andthetimeoftoftheexcitedionsaremeasuredforeachfrequency.Thesetimeoftmeasurementsshowaresonancewhentheionfrequencymatchestheexcitationfrequency.Theexcitationfrequencycanbeusedtocalculatethemassoftheion.AschematicviewofthisfacilityaswellastheLEBIThigh-precisionPenningtrapisshowninFig.2.10[42].TheLEBITfacilitywasthetoperformprecisionPenningtrapmassmeasurementsonrareisotopesproducedbytheprojectilefragmentation[29].Sinceitscommissioning,LEBIThasdeterminedthemassofmanyexoticnucleioverabroadrangeofatomicnumbersandhalf-livesallwitharelativeprecisionsontheorderof108atomicmassunits(amu),or1keVinmc2[40,42,41,43,44,andreferencestherein].2.3.2BECOLATheBEamCOolingandLAserspectroscopyfacility(BECOLA)locatedattheNSCLcanperformbothlaser-hypuremeasurementsandatomic/nuclearspinmanipulationsonexoticions[30].Thesestudiesusecollinearlaserspectroscopy(CLS),opticalpumping,andnuclear-magneticresonance(NMR)techniques.Laser-spectroscopytechniquescande-terminefundamentalpropertiesofnucleisuchasthemagneticdipolemoment,theelectricquadrupolemoment,thenuclearspin,andthemean-squarechargeradius.SimilartotheLEBITfacility,thecontinuouslow-energybeamsfromtheNSCLlargevolumelineargascellarecooledandbunchedfortheBECOLAmeasurements.Alaserbeamisthendirectedcollinearlywiththebunchedionbeam.Sincetheionbeamproducedfromthelargevolumegascellistypicallysinglycharged,acharge-exchangecell(CEC)isutilizedforexperimentsthatrequireneutralbeams.Forhypmeasurements,thelaser34Figure2.11:AschematicoverviewoftheBECOLAfacilitywithathree-dimensionalmodelshownintheinlet.QSandQDindicatequadrupolesingletordoubletelectrostaticfocusingelements,respectively[30].frequencyistunedintoresonancewiththehyptransitionofinterest,andtheresonanttlightiscollectedandanalyzed.Forbeta-NMRmeasurements,thebunchedionbeamispolarizedusinganopticalpumpingtechniqueinconjuctionwithcircularlypolarizedlaserlightbeforeenteringthebeta-NMRdevicewherethebeta-decayoftheexoticionsareanalyzed[30].AschematicviewofthisfacilityisshowninFig.2.11[30].TheBECOLAfacilitywasrecentlycommissionedon-lineusing36Kand37KradioactiveionbeamsfromtheNSCL'slargevolumelineargascell[45].2.3.3ReATheReAcceleratorfacility(ReA)locatedattheNSCLisapost-acceleratordesignedtoreac-celeraterare,thermalizedisotopesuptoenergiesofafewMeV/uwithexcellentemittances.35Figure2.12:AschematicoverviewoftheReAfacilitylocatedattheNSCL[46].Fornuclearreactionstudies,Coulombexcitation,nucleoncapture,andfusionreactionex-perimentscanbeconductedinthisenergyrange.Veryrecently,abeamcoolerandbuncherwasaddedjustupstreamoftheReAfacilitytobettermatchthebeamfromthelargevolumelineargascellintothechargebreeder.AfterenteringReA,theions,typicallyinthe1+chargestate,mustbechargebredintohighlyionizedstatesbythefacility'sElectron-BeamIonTrap(EBIT)fortacceler-ation.Inorderforaspchargestatetobeused,pulsesofthehighlychargedionsareejectedfromtheEBITandsentthroughacharge-over-mass(Q/A)separator.Theselectedionsaretheninjectedintoaradio-frequencyquadrupoleaccelerator(RFQ)andintoasuperconductingradio-frequencylinearaccelerator(SRF-LINAC)forreacceleration.TheReAfacilitywascommissionedusingastable39Kbeam,andlaterwith46Ar,46K,and37Kions.ReAwasthereaccelerationfacilitytosuccessfullyaccelerateionsproducedbytheprojectilefragmentationmethod.36ThepresentReAfacility(ReA3),schematicallydepictedinFig.2.12,contains15reac-celeratorcavitiesandcanaccelerateionswithQ/Aof0.2{0.5toenergiesof0.3{6.0MeV/u,respectively.Theproposedofthefullfacility(ReA12),though,willcontainthirty-ninelinearreacceleratorcavitiesandwillbecapableofprovidingbeamsfrom0.3to12MeV/uforheavyionsandupto20MeV/uforlightions[31].TheReAfacilityenablesavarietyofnuclearstructureandastrophysicalexperimentalprojectsusingequipmentthatincludestheArrayforNuclearAstrophysicsStudieswithExoticNuclei(ANASEN)[47],theActiveTarget-TimeProjectionChamber(AT-TPC)[48],theSummingNaI(Tl)(SuN)detector[49],andtheJetExperimentsinNuclearStructureandAstrophysics(JENSA)[50].Alloftheselow-energyfacilitiesandexperimentsconductedattheNSCLrelyonsuccessfuloperationofthelaboratory'sbeamthermalizationarea.37Chapter3TheNSCLBeamThermalizationAreaTheversionoftheNSCLbeamthermalizationareasuccessfullyranfrom2004-2009andprovedthatprecisionexperimentswerepossibleatthisprojectilefragmentationfacility[1,44,51,andreferencestherein].Overthenextfouryears,thebeamthermalizationareawascompletelyrebuilt,upgraded,andrecommissionedin2013.Upgradestotheareaincludedaddingtwonewhigh-resolution,momentumcompressionbeamlinesandanewlargevolumelineargascellconstructedbyANL[2].Sinceitsinstallation,numerousradioactiveisotopebeamsatavarietyofincomingbeamrateshavebeenthermalizedandextractedfromtheNSCL'slargevolumelineargascell.Thepresentworkwillconcentrateonthecommissioingresultsobtainedwith76Gaandthecharacterizationresultsobtainedwithnumerousotherincomingratesandions.Resultsfromtheseexperimentswereusedtoestablishthestoppingprocessandtheextractionyofthegascellasafunctionoftheionization-ratedensity,Q.ThebehavioroftheNSCL'slargevolumelineargascellisgenerallyconsistentwithprevioussystems,however,thefromthisgascellareshowntobehigheratlargeQvalues.3.1AreaOverviewThebeamthermalizationareaattheNSCLreceiveshigh-energyprojectilefragmentbeamsfromtheA1900separator[34]andreducesthebeamenergiesfrom˘100MeV/udownto38thermalenergiesof1eV[2].Thisareacontainstwoanalysisbeamlines,butthecurrentex-perimentswerecarriedoutusingtheso-callednorthline.Thehigh-energybeamlinelocatedonthenorthsideofthebeamthermalizationareawasdesignedtodispersethefragmentbeaminmomentumontoamonoenergeticwedge[35]andthentoreshapeandfocusthebeamintothelargevolumelineargascell.Duringthisprocess,thehigh-energybeamsarepassedthroughsoliddegradersandthemonoenergeticwedgetoreducethebulkofthebeam'skineticenergydownto˘1MeV/uandtoreducetheenergyspreadofthebeam,respectively.Ionsthenpassintothelargevolumelineargascellwheretheyarethermalizedthroughcollisionswithultra-pureheliumgas[2].Thethermalizedions,whichretaineithera1+or2+chargestateduetothehighionizationpotentialoftheheliumgas,areguidedandconcentratedtowardsasupersonicnozzlewithastaticDCpotenital,dynamicRFpotential,andgaswwheretheyarethenjettedoutofthecellandintoatiallypumpedRFQion-guide[52].Oncethroughtheion-guide,theionsareacceleratedto˘30keV/q,massanalyzed,andsenttothelow-energyexperimentalareasofthelaboratory.AschematicofthebeamthermalizationareaisshowninFig.3.1[2].3.2NSCL'sLargeVolumeLinearGasCellThebeamthermalizationareacentersaroundthegascell.In2012,theNSCLobtainedalargevolumelineargascellfromANLaspartoftheresearchanddevelopmentfortheFacilityofRareIsotopesBeams(FRIB),anextgenerationRIBfacility.ThisgascellreplacedthegenerationNSCLgascell,discussedinSection2.2,thatwas50cminlengthandoperatedat1bar(760torr)helium.TheNSCL'slargevolumelineargascellis1.2metersinlength39Figure3.1:AschematicoverviewofthebeamthermalizationarealocatedattheNSCLincludingoptionaldegraderanddetectorpositions.witha25cmdiameterandisoperatedat˘100mbar(˘75torr)[53].Thelargerstoppingvolumeandwidercellwidthallowsforthecaptureofionswithsubstantialrangestragglingaswellasprovidesalargervolumetomitigatethespacecharge.Largerbeamdimensionsallowtheionizationtobedistributedwithinthegasand,intheory,reducethelossesfromspacechargeandplasmadiscussedinSection1.2.2.Inadditiontothesheersizeofthegascatcher,theNSCL'slargevolumelineargascellutilizesanenhancedion-transportmethod,showninFig.3.2[36],thatconsistsofaDCdragthroughoutthevolumeofthecell,anRFconeandgaswattheextractionregion,andanadditionalRFpotentialappliedtotheelectrodesalongthewallsofthecell.ThisadditionalRFpotentialwasimplementedtokeepthefragmentionsofinterestfromtothechamberwallsastheyarepushedoutwardbyanyspacechargepotential.ThemomadewhencomparedtothegenerationdesignwastheinclusionofanelaborateRFelectrodestucturealongthewallsthatwasincorporatedtoreducethebuild40Figure3.2:SchematicdepictingthevariouselectromagneticimplementedtoincreaseandimprovetheextractiontimesandoverallextractionfortheNSCL'slargevolumelineargascell.ArrowsshowthedirectionthattheionsaredraggedbytheDCelectricwhilethefocusingpotentialcreatedbytheRFisshownindetailatthetopofthe[36].upofthespacechargepotentialbyprovidinglargesurfacestocollecttheionizedgas.RememberthattheRFrepellingpotentialisweakforlowmassionssuchashelium.Asdiscussedinthepreviouschapter,highenergygascatchersemployelectromagnetictoovercometheslowerevacuationtimesassociatedwiththelargerstoppingvolumes.Althoughthegasevacuationtimeisstillontheorderoftensofminutes,welldesignedsystemscanachieveionextractiontimesontheorderoftensofmilliseconds[36].The41NSCL'slargevolumelineargascellhasanappliedDCpotentialtodragtheionstheentirelengthofthecellandtowardtheextractionregion,millimetersfromthenozzle.IonsinthisregionofthegascellarethenpropelledbygaswthroughasupersonicnozzleandintotheRFQionguide.Thegascellwastypicallyoperatedwithadriftof0.75V/cm,apressureof123mbarofhelium,andatemperatureof269Kforthecommissioningexperiments.ByusingEq.(2.5),thedriftvelocityofthe76Ga+ionsinthegascellunderthesconditionswas˘19m/s.Inordertosustaintheconcentrationoflinesatthenozzle,astrongergradientisrequiredintheextractionregion.Thestrengthinanyoneareaofthecell,though,islimitedbyelectricaldischargesthatcanoccurwithinthegas,asgivenbythePaschencurvefortheheliumgas.Sincethestrengthappliedtothenozzleregionisthemostlimited,thestrengthoftheappliedintherestofthegascellisalsolimited.However,giventhattheelectricstrengthisdirectlyproportionaltotheions'velocitiesandtherebytheirextractiontimes,theDCgradientiskeptashighaspossibletonotonlydragtheionsofinterestquicklyandtlythroughthegasbuttoalsoremovetheionsresponsibleforthebuildupofspacecharge[36].AsdiscussedinChapter1,thebuildupofspacechargeoccurswhenthecationscreatedbythefastionsarenotremovedquicklyfromthegascell.Eachincomingioncreatesabout105He+/epairs.Theelectronsarerapidlyremovedduetotheirlowmassandhighvelocitybytheelectricwhilethemuchheavierheliumgasionsmustdriftalongwiththeionsofinterest.Thespacechargepotentialisthereforestrongestalongthecenterofthebeamaxis.Anexampleofthisstrongerbuild-upalongthecenterofthebeamaxisisshowninFig.3.3[36].Inthisexample,itwasassumedthatthefullrangeoftheBraggpeakfortheparticlesfellwithinthegascell;however,aswillbediscussedwiththeNSCL'slargevolume42Figure3.3:Aself-consistentcalculationtoshowthepotentialbuildupfor60millionionsenteringintoandionizingthegaswithinasmallgascatcherisshown.Noticethattheintensityisthestrongestalongthecenterofthebeamaxis.Theoutlineofthisgascatcherisrepresentedbythedashedredline[36].lineargascell,onlypartofthisrangefallswithinthecatcher.Theaccumulationofspacechargeinthecenterofthedeviceshieldstheexternallyappliedandcancausethermalionstomovetowardsthewallsandnottowardtheextractionregion.Theso-calledRFwallsandanRFconewereincludedinothergascellsdesignedbyANL[54,53]andwerealsoincludedintheNSCL'slargevolumelineargascelltoovercomethesespacechargebyprovidingbetterfocusingtowardthenozzle.Thesestructuresarecomposedofcloselypacked,thinelectrodeswithanoverallrepellingforcecreatedbyapplyingoppositephasesofanRFelectrictoneighboringelectrodesashighlightedintheinsetofFig.3.2.RecallthattheRFrepellingforceexperiencedbyanionisapproximatelyproportionaltothesquareofthemass,andsincetheionsofinterestaremuchmoremassivethanthegasions,theheavierionsmovetowardthenozzlewhilethelighterionstotheelectrodesurfaceandareneutralized[35,36].TheRFcone,Fig.3.4,iscomposedof280independentconsintricringsofdecreasingradius.Theseelectrodesare0.4mmthickand0.5mmapartfromeachother.Theconeis43Figure3.4:PhotographoftheRFconestructureusedintheNSCL'slargevolumelineargascell.designedinsuchawaytofocustheionsdirectlyontothesupersonicnozzle.TheelctrodestructuredesignedforthebodyoftheNSCL'slargevolumelineargascellwasslightlymoreelaborate,asshowninFig.3.5.Thereareroughly750independentelectrodesthatareeach0.8mmthickandseparatedbya0.6mmgap.Eachelectrodeincludes12spokes30degreesapartfromoneanother.Everysecondspokeis3.8cminlengthwiththeothersalternatingbetween7.5cmand9.8cm.SincetheheliumionsarelesssensitivetotheRFrepellingforce,thespokesalongthewallwereincorporatedintotheelectrodedesigntoshortenthedistancetheseionsmusttravelbeforethiercollection.ThegoalwastoreducetheoverallbuildupofHe+asafunctionoftheincomingbeamrate.44Figure3.5:PhotographoftheRFelectrodestructureusedalongthewallsintheNSCL'slargevolumelineargascell.AswastruewiththemaximumDCvoltage,themaximumRFamplitudeislimitedbyelectricaldischargesandbreakdownvoltageswithinthegas.Alsorememberthattherepellingforceexertedonchargedparticlesdecreasesasthedistancefromtheelectrodestructureincreasesandalsodecreasesinthepresenceofagas.Ahighergaspressure,therefore,leadstohigherstopping,butresultsinadecreaseintheextraction.Alowergaspressureensuresahighertransportbutseverelylimitsthestoppingofenergeticand/orlightions.Theoperatinggaspressureandtheappliedelectricstrengthsare,therefore,acompromisebetweenthemaximumstoppingforthefastionsandthemaximumiontransport.TheDCdragisnecessarytoobtainreasonableextractiontimeswhiletheRFfo-cusingisnecessarytoovercomethedefocusingassociatedwithspacecharge.ByextendingtheRFtoincludenotonlytheextractionregion,butalsothebodyof45Figure3.6:Theschematicsontheleftshowtheareasofthegascellsfromwhichradioactiveionscanbeextractedgiventheelectricpresentandwhenoperatingathighintensities.TheNSCL'slargevolumelineargascellisrepresentedbythelowerleftschematic.ThegraphontherightcomparesresultsforonlineexperimentswiththeNSCL'slargevolumelineargascelldesigntoresultsobtainedfromgascatcherswithlittletonoRFrefocusing[36].thecell,radioactiveionscan,theoretically,beextractedfromthewholecellwhichshouldincreasethe.ANLtestedtheiencieyectsofthisextendedextractionregion,seeFig.3.6[36].Asshown,thewasimprovedforhigherincomingbeamrates.AuniquespokedesignwasalsoincorporatedintotheelectrodestuctureofthenewlargevolumelineargascellwhichshouldreducetheoverallspacechargebuildupasafunctionoftheincomingbeamrateandshouldfurtherincreasethereportedextractionTheresultsfromcommissioningandtestingwith76Gaandotherionsaswellasadetaileddiscussionontheoperationofthenewlargevolumelineargascellarecontainedinthenextsection.3.2.1CommissioningExperimentsTherecentupgradesoftheNSCL'sbeamthermalizationareawerecompletedbytheadditionofthelargevolumelineargascellobtainedfromANL.Severalexperimentswereconducted46tocommissionthethermalizationsystemthatincludesthebeamline,thedegradersystem,thegascell,andtheextractionsystem.Theexperimentalset-up,measurements,andresultsobtainedfromthesecommissioningrunswillbediscussedindetailbelow[2].3.2.1.1ExperimentDescriptionForthecommissioningofthelargevolumelineargascell,thecoupledcyclotronsproducedaprimary82Se34+beamat140MeV/u.Thishigh-energybeamwasthenimpingedupona415mg/cm2berylliumtarget,andawiderangeofprojectilefragmentswereproduced.TheprimarybeamcurrentwasfrequentlymonitoredatthetargetpositionbyinsertingaFaradaycupintothebeam'spath,andtypicalcurrentswere1.1(0.1)pAforfullbeam.TheA1900fragmentseparator[34]employeda299mg/cm2aluminumachromaticwedgetoseparatethefragmentsinordertoprovidethebeamthermalizationlinewitha79%pure76Ga31+secondarybeam.Duringseparation,projectilefragmentsbeganwithBˆ=3.83400Tmwhilethe76GafragmentshadaBˆ=3.43310Tmandenteredthebeamthermalizationareaat90MeV/u.ThefullacceptanceoftheseparatorwassettoamomentumspreadofP=P=0.5%wherethelargestcontaminantioninthesecondarybeamwas78Ge32+.Aschematicdiagramoftheexperimentalset-upforthermalizationisshowninFig.3.7.Thedegradersystemreducedthekineticenergyofthehigh-energybeamdownto˘1MeV/uandincludeda1503(5)mthickaluminumplate(so-calleddegrader)whoseethicknesswasvariedbyremotelyadjustingitsanglewithrespecttothebeam'spath.Italsoincludedamonoenergeticsilicondioxide(glass)wedgethatwas1050(50)mthickinthemiddlewitha5(0.9)mradwedginganglethatfurtherreducedtherelativelysmallmomentumspreadofthesecondarybeam(0.5%P=P),anda37(2)mthickaluminumwindowwithatitaniumalloysupportgrid(85(5)%transmissionlocatedatthe47Figure3.7:Schematiclayoutoftheexperimentalequipmentandset-upusedtocommissiontheNSCL'sbeamthermalizationsystem.Distancesbetweenthecomponentsaregivenaboveincm(nottoscale).entranceofthegascell.Thedispersionofthebeamlinewasmeasuredtobe15.4(0.9)mm/%inmomentum[55].Fragmentsproceedthroughthedegradersystemandintothegasofthecell.Ultrahighpurityhelium(99.999%)servedasthegasandwaspassedthroughaMonotorrandagasregulatorsystempriortoenteringthechamber.Theheliumgaswasthenwedthroughthechamberatarateinwhichaconstantpressureof123(12)mbarcouldbemaintained.Duetothepossibilityofreactionsduringthedrift,allcomponentsofthechamberwereassembledtoultra-highvacuumstandards,whichkeptthelevelofcontaminantsontheorderofppborless[56].Althoughlimitingcontaminantswasapriority,somejointsofthechamberweresealedwithindium,whichpreventedthechamberfrombeingbaked.Asaresult,thechamberwasfoundtocontainarelativelyhighlevelofwatervapor[2].Eventhoughthegascellcouldnotbebaked,itwaschilledtotemperaturesaslowas48Figure3.8:PhotographofthelargevolumelineargascellwheninstalledattheNSCL.Notethelargeceramicinsulatoratthefrontofthechamber.-10C.Therefore,inordertopreventshortcircuitsfromcondensationandtoprotecttheexperimentersfromtheappliedvoltages,thegaswashousedinsideastainlesssteelboxwithdrynitrogen,dubbedthe\drybox."ApictureofthegascellwithoutthesurroundingdryboxisshowninFig.3.8,wherethesixorangebondsarecoppercoolingjackets.Oncetheprojectilefragmentswerethermalizedintheheliumgas,theywereguidedtowardstheexitnozzlebytheDCdragandRFelectricAbiasof900Vwasappliedbetweentheentrancewindow(anode)andtheexitnozzle(cathode)ofthegascellwhichproducedanaverageDCdriftof0.75V/cm.Sincetheelectrodespacingnarrowsinthecone,theRFfrequencyappliedtotheconeelectrodeswas3.180MHzat50Wwherethepeak-to-peakvoltage,Vpp,was31(2)Vwhilethepotentialappliedtothebodyelectrodeswas3.514MHzat90WwithaVppof32(2)V.49Undertheseconditions,thetimeneededforatypicalionofinteresttodrifthalfthelengthofthegascatcherwascalculatedtobe39(2)ms.Ionswerejettedthroughasupersonicnozzle,wherethetotalgaswwasmeasuredtobe0.06(0.03)standard-L/s[2]andintoatiallypumpedexpansionchamberandRFQion-guide[52].Athree-stagetialpumpingsystemwasemployedtoevacuatethechambersinthissectionto7.5x102torr,3.8x105torr,and1.2x107torr.Thegascellandextractionsystemwereonahighvoltageplatformat30(0.005)kVtoacceleratetheionsintoanelectrostaticbeamlinethattransportedtheionstovariousexperimentalareas[2].3.2.1.2MeasurementsTheionizationcreatedbythethermaliztionprocessprovidesimportantinformationontheincomingfastions.Inordertostudytheionization,thenegativeions(electrons)werecollectedinsidethegascatcher,andthepositiveionsthatexitedthesystemwerecollecteddownstreamofthecatcher.Sincetheionsdeposittheirkineticenergyastheymovethroughthegas,thegascellcanbeoperatedasalargeionizationchamberandthusmeasuretheamountofionizationcreatedbytheincidentbeam.Also,asdiscussedinChapter1,thestoppingbehaviorobservedforparticlesingasischaracteristicallyknownasaBraggcurve[23].Therangedistributionofthe76Gasecondarybeaminthegaswasobtainedbymeasuring(1)thetotalnegativeioncurrentcollectedonthewindowofthegascelland(2)thetotalextractedpositiveioncurrentonaFaradaycuplocateddownstreamoftheextractionsystemasafunctionofthedegrader'sethickness.TheexamplesshowninFig.3.9weremeasuredforanincomingbeamintensityof7.6x105particlespersecond(pps),exhibitthetypicalBraggcurveshape,andagreewithintheuncertaintiesofthemeasurements.Asystematicshiftof29(2)minthee50Figure3.9:Measuredpositive(squares,nA)andnegative(diamonds,pnA)ioncurrentdistributionsfor76Gaasafunctionoftheedegraderthickness.Thetotalactivitydistribution(circles,arbitraryscale)isshownforreference.thicknessofthedegraderwasobservedbetweenthetwomeasurementsandwaslaterfoundtobeduetoamechanicallashinthesystemusedtorotatethedegrader.Here,theterm\lash"describesthetendancyofthedegradersystemtoreturntoaslightlydtpositionafterfullrotation.Thetotalradioactivityextractedfromthegascellwascollectedonathinfoil,whichwasalsooperatedasaFaradaycup,andmeasuredbyasiliconsurface-barrierdetectorthatwaslocateddirectlybehindthefoil.Thebetadetector'swasdeterminedtobe33(2)%usinga90Srsourcewithaknownactivity.Inordertoverifythattheextractedradioactivitywasindeed76Ga,growthanddecaycurveswerecollectedwiththesilicondetector.AnexampleofameasureddecaycurveisshowninFig.3.10[2].Thesedecaycurveswerethenusedtodeterminethetotalextractedradioactivityasafunctionofthedegrader's51Figure3.10:Exampleofagrowthanddecaycurveforthetotalextractedradioactivity.Thiscurvewasobtainedwhenthealdegraderwassetforthemaximumextractedradioactivity(34).Thedataareinagreementwiththeknown33shalf-lifefor76Ga[2].ethickness,andanexampleoftheseresultsareshownbytheblackcirclesinFig.3.9.Theactivitydistributionwasnormalizedinthey-axisfordispaypurposesandisbroadwithapeaknearthemid-pointofthefallingedgesoftheBraggcurvesasexpected.Thebroadeningisaresultofrangestraggling,wherestochasticinparticleinterac-tionscauseslightlyerentpathlengthsforindividualparticleswiththesameinitialenergycombinedwithanyresidualenergyspreadafterthewedge.Thepeakistypicalbecausethemid-pointofthefallingedgeofaBraggcurveisusedtonethemeanrangeofparticlesinagivenmaterial[23].Sinceitisimportanttoextractthemaximumamountofradioactivity,theoptimalangleforthedegraderwassetat34(1876(5)m)formeasurementsofthemassdistributionoftheions[2].Themassdistributionsforboththestableandradioactiveionsextractedfromthegascellweredeterminedbyusingamass-analyzingmagnetlocateddownstreamoftheextraction52Figure3.11:Partofthemass-to-chargespectrumobtainedforthestableionsextractedfromthegascell,seetextfordetails.system.Theresolutionofthemagnet,mm,wasontheorderof50whichprovidedtseparationofindividualmassnumbersintheregionofinterest.AsecondFaradaycup/siliconsurface-barrierdetectordevicewasplacedattheimagepositionofthemassanalyzertomeasuretheelectricalcurrentofstableionsandthecountingrateofradioactiveions,respectively,asafunctionoftheirmass-to-chargevalue(m/q)[2].Itisinterestingtonotethatpeakswereobservedatessentiallyallintegralm/qvaluesofpositiveionsextractedfromthegascell.PartoftheobservedmassspectrumisshowninFig.3.11[2].Recallthatthechamberwasnotbakedpriortocommissioning,sowaterwasexpectedtobepresent.Theprominentpeaksinthestablem/qspectrumat37and55werethusattributedtothemolecularwaterions[H3O(H2O)]+and[H3O(H2O)2]+,re-spectively,whilepeaksat33and51wereattributedtothecloselyrelated[CH3(H2O)]+and[CH3(H2O)2]+molecularions.Thesemolecularionsweremostlikelycreatedbycharge53Ionm/qDecayRateFraction(cps)(%)76Ga2+38<20<1(0.3)76Ga+76207024.3(0.6)[76Ga(H2O)]+94275332.3(0.5)[76Ga(H2O)2]+112201223.6(0.7)Table3.1:76Garadioactivitymoleculariondistribution.transferreactionsfromprimaryheliumionstoneutralimpuritiesinthegasthatundergosubsequentchemicalreactionswiththecontaminantwatermoleculesinthegaspriortoextraction.Notethatthetotalradioactivitywasmeasuredpriortomassseparation,butthera-dioactiveiondistributionwasdeterminedform/qvaluesintherangeof30to120.Foranincidentbeamrateof7.3x103pps,thebulkoftheactivitywaslocalizedinthreepeakswhosecountingratesandpercentageofthetotalextractedactivityaregiveninTable3.1.Althoughsomewatercontaminationwasexpected,theamountofwateranditsontheionofinterestwastlyunderestimated.Assuch,thedesignatedmassrangecouldonlyaccountforroughly80%ofthetotalextractedradioactivity.Theremaining20%was,therefore,assumedtolieathighermassvalues.Theyieldofincidentprojectilefragmentsthatareextractedfromthegascellissensitivetotwoindependentthestopping,"stop,andtheextraction,"ex.Theformerdescribesthefractionofthermalizedfragmentsstoppedwithinthegaswhilethelatterdescribesthefractionofthermalizedfragmentsthatareextractedfromthegas.Therefore,theactivitycollectedwouldbeR"stop"ex(1-e)wheretisthecollectiontime,andthesaturatedactivityobservedinthebeta-decaycounterdownstreamoftheextractionsystem,Asat,canbewrittenasAsat=R"stop"exwhereRistheincoming76Gabeam54rateaccountingforbeampurity.Thestoppingwascalculatedtobe0.86withtheLISE++program[6].DetailsofthisprogramandcalculationwillbediscussedinChapter4.TheextractionwastherebyobtainedbydividingAsatby"stopandRaswellasaccountingforthedetectorandtransmissionencies.Extractioneforthecommissioningexperimentswith76Gaweredeterminedforimplantationratesrangingfrom9.3x102ppsto7.6x105ppsandvariedfrom37%to18%,respectively.3.2.2ImprovementsafterCommissiongFollowingthecommissiongruns,severalimprovementsweremadetoenhancetheperfor-manceoftheNSCL'slargevolumegascell.Thesechangesincluded:enhancingthevacuumsystemofthecell,replacingtheentrancewindow,andaddingastableiontestsource.Aspreviouslymentioned,duetotheinabilityofthecelltobebaked,somewaterwasexpectedtobepresentintheheliumgas,whichwouldresultinunwantedchemicalreactions.Infact,resultsfromthecommissioningrunsindicatedthatwaterwasinvolvedinasubstantialamountofreactionsandlessthan25%ofthe76Gaionsextractedfromthelargevolumegascellwerefreefromanattachedwater[2].Amajorchangewasmadeinanattempttoremovethewaterfromthesystemwhenitwasnotinuse:thegasvolumewasextendedtoallowanewturbomolecular(turbo)pumpthatcouldpumpdirectlyonthegaschamber.Priortocommissioning,theonlywaytoremovegasesfromthechamberwastopumpthroughthesmallnozzleofthecellandasmallbypassvalve.Giventhevolumeofthegascatcherandthesizeofthenozzle,thepumpingofthechamberwaslimitedto0.06(0.03)standard-L/saspreviouslymentioned.Byimplementinganewhighvoltageinsulator,thewindowsectionofthegascellwasredesignedtoincludearingwithmultiple(6)ports,asseeninFig.3.12.Aturbopumpwasaddedtooneofthesenewportssothatwhenthe55Figure3.12:NSCL'slargevolumelineargascellwithupgradedinsulator,ports,anddrybox.gascellwasnotinuse,thechambercouldbeleftunderhighvacuum.Thisadditioncould,thereby,reducetheresidualgas.Aresidualmassanalyzer(RGA)wasinstalledatthesametimeastheturbo,andwasplacedjustdownstreamofthegascelltoquicklyanalyzeanycontaminants.ResultsfromtheRGAsuggestedthatthewatercontaminantwasessentiallyremovedfromthecell,whichshouldhaveresultedinverylittlemolecularionformationfromtheionsofinterestinlaterruns.Fig.3.13comparesthemolecularionformationbetweentwosimilarpotassiumexper-iments.Oneexperimentwasrunpriortotheadditionoftheturbopumpwhiletheotherwasrunaftertheadditionoftheturbopump.Theseresultsthattheadditionoftheturbopumpsuccessfullyreducedthewatercontaminentwithinthegascelltotlevels.Thediscrepancyincountsbetweenthetwoexperimentsresultedfromlowerbeamratesfromthecyclotroninthesecondexperiment.Thealuminumwindowlocatedattheentranceofthegascellwasalsoreplace.Thiswindowmustbeabletowithstandthepressureerencebetweenthehighvacuuminthedegraderpotjustupstreamofthecell(˘107torr)andthegaschamberitself(˘10256Figure3.13:Thegraphontheleftshowsamassscanforradioactiveionsfromanearly37Kexperiment.Thegraphontherightshowsasimilarmassscanforthesame37Kradioactiveionfromanexperimentperformedafteraturbomolecularpumpwasaddedtothelargevolumelineargascell.Whilethe37Kionwasfragmentedamongnumerousmolecularionscontainingwaterduringtheexperiment,nomolecularionformationwasobservedinthesecondexperiment.Seetextformoredetails.torr).Theoriginalwindow,10mthickwithatitaniumalloysupportgrid,showedextremedeformationafterthecommissioningexperiments,seeFig.3.14.Thisdeformationcancauseincreasedrangestragglingbecausedependingonthebeam'spositiononthewindow,theionscouldtravelthroughvariablethicknessessofmaterial.Inaddition,thecrinklesincreasetheprobabilityofthewindowfailingunderthepressureToensuretheproperbehaviorofthewindow,anew49mwindowwasinstalled.PhotographsofthenewandoldwindowareshowninFig.3.14.3.2.3SummaryofMeasurementsTheextraction("ex)oftheNSCL'slargevolumelineargascellwastakentobe:"ex=OutgoingRate"det"trans"stopIncomingRateBeamPurity;(3.1)57Figure3.14:Thepictureontheleftshowsthedeformed10mthickaluminumwindowwhilethenew49mthickwindowisshownontheright.where"detisthedetector,"transisthetransporttothedetecor,and"stopisthestoppingwithinthegas.Remember,thatthedetectorwasmeasuredwitharadioactivesourceandwasfoundtobe0.33(0.05),andthetransmissionwasmeasuredwiththecurrentofstableionsandwasfoundtobe0.85(0.05).ThebeampurityforeachexperimentwasprovidedbytheA1900.Datafromthecommissioningexperimentsaswellasotherlow-energyuserexperimentswasusedtodeterminetheextractionoftheNSCL'slargevolumelineargascellwasmeasuredasafunctionoftheincomingbeamrate.Theseresultsweresumma-rizedandareshowninFig.3.15.Theextractioncanbeseentodecreasewith˘1/pIncomingRate[38,57]atincomingrateslargerthan˘5x105pps,whichindicatestheinstanceswherespacechargeandplasmabecometwithintheNSCL'slargevolumelineargascell.Forionization-ratedensitiesabovethisthreshold,thespacechargeareapparentintheobservedBraggcurvesforboththeextractedpositiveioncurrentandinternallycollectnegativeioncurrent.58Figure3.15:Graphofextractionciesasafunctionofincomingrateforinitialcom-missioningandchemistryexperimentsperformedwiththeNSCLlargevolumegascell.Inhigherratecases,notonlyisthepositiveioncurrentcantlysuppressed,butduetorecombinationandotherplasmawithinthegas,thenegativeioncurrentisalsosuppressed.Thissupressedbehaviorwasmoreeasilydiscernableinsomeofthelaterexperiments.Resultsfromoneofthecharacterizationexperimentsinvolving33Cl(seeTable3.2)waschosenasthebestvisualexampleofsupressionwithincreasingratesandshowninFig.3.16.Similartotheresultsshownforthe76Gacommissioningexperiments(Fig.3.9),thesegraphsincludeboththepositiveandnegativeioncurrentsaswellastheactivitydistributionasafunctionofdegraderthickness.GraphAshowstheresultsforanincomingrateof3.4x104ppswhilegraphBshowstheresultsforanincomingrateof3.1x106pps.GiventheincomingratesforgraphBwereaboutafactorof100higherthanthoseofA,theextractionratesobservedingraphBshouldhavealsobeenaboutafactor59of100higher.Duetospacechargeandplasmathough,thenegativeioncurrentissupressedby53%whilethepositiveioncurrentissupressedby96%.Figure3.16:Measuredpositive(squares)andnegative(diamonds)ioncurrentdistributionsfor33ClasafunctionoftheedegraderthicknessforanincomingrateofA)3.4x104ppsandB)3.1x106pps.A30PcontaminantwaspresentinthefragmentbeamdeliveredbytheA1900.Thiscontaminantaccountsforthesecondobservedpeakinthegraphs.Seetextfordetails.InordertodirectlycomparetheresultsobtainedfromtheNSCL'slargevolumelineargascelltopreviousgenerationdevices(Fig.2.9[38]),theionization-ratedensity(Q)foreachexperimentneededtobedetermined.Theionization-ratedensity,discussedin1.2.2,wascalculatedusingEq.(1.4)foreachionanditsrespectiverates.Thecomparisonbetweensimilarmedium-massedmeasurementsmadewithpreviousgascellsandmeasurementsmadewiththeNSCLlargevolumelineargascellisshowinFig.3.17.AlthoughsimilarsuchlossesandtrendsobservedinpreviousdeviceswerealsoobservedintheNSCLlargevolumelineargascell,themoretlossesfromthiscelldidnotoccuruntiltlyhigherQ-valueswereattained.Thisobservationindicatesthatthelargervolume,enhancedelectrodestructure,andaddedRFpotentialsoftheNSCL'slargevolumelineargascellweresuccessfulinimprovingtheoveralliencyofthesystem.VariousexperimentalconditionsandionsthermalizedbytheNSCLlargevolumelineargascellare60summarizedinTable3.2.Detailedsimulationsandconclusionsofthecompletesystemarepresentedinthenextchapter.Figure3.17:GraphofextractionsasafunctionofQforpreviousgenerationdevices(orangecircles,redtriangles,andblackdiamonds)aswellastheinitialcommissioningandchemistryexperimentsperformedwiththeNSCLlargevolumegascell(allsquares).61IonIncomingRateQBeamPurity"stopOutgoingRate"ex(cps)(IonPairs/cm3/s)(cps)29Mg2.1x1052.5x10120.900.511.7x1030.03229P1.5x1071.3x10140.120.381.0x1030.0061.7x1061.5x10134.2x1020.0191.2x1051.1x10121.5x1020.1021.3x1041.2x10111.7x1010.1032.3x1032.1x10103.0x1000.10533Cl2.0x1076.7x10130.210.763.5x1030.0043.1x1061.0x10132.6x1030.0192.1x1057.0x10115.3x1020.0563.4x1041.1x10111.5x1020.09736K7.8x1061.8x10140.030.897.4x1020.0131.2x1062.7x10132.3x1020.0258.6x1041.9x101252.8x1010.04437K6.8x1067.5x10120.270.911.4x1030.0324.1x1054.5x10113.5x1020.13740S1.6x1061.6x10130.830.552.4x1040.0591.1x1051.1x10122.4x1030.0871.4x1041.4x10113.8x1020.10246Ar2.6x1061.2x10140.860.482.1x1040.0352.9x1051.3x10135.1x1030.0771.7x1047.8x10116.0x1020.1522.9x1031.3x10117.3x1010.1092.2x1021.0x10108.0x1000.15976Ga6.2x1051.7x10120.790.861.1x1040.0917.6x1042.1x10112.3x1030.1595.9x1031.6x10101.9x1020.1687.6x1022.1x1092.7x1010.187Table3.2:Summaryofvariouslow-energychemistryexperimentsperformedwiththeNSCLlargevolumelineargascell.76Gaisstarredbecauseitistheonlyexperimentshownthatwasconductedpriortotheimprovementsmadetothevacuumsystem.62Chapter4SimulatingtheNSCL'sLargeVolumeLinearGasCellAsdiscussedinChapter3,resultsfromthecommissioningandotherlow-energyexperimentswereusedtoestablishadecreasingtrendfortheextraction,aswellasthedirectlyrelatedQ-value,asafunctionoftheincomingbeamrate.ThistrendindicatesthatspacechargeandplasmaarestillpresentwithintheNSCL'slargevolumelineargascell.Inordertounderstandhowandwherethelossesfromtheseoccurandalsotoreproduceandtoonedaypredicttheobservedbehaviorofthisdevice,numerousmodelingprogramsandsimulationswereincorporatedandperformed.Thesesimulationsintegratednotonlytheintricateelectrodestructurebutalsothemultipleelectricsandpotentialspresentthroughoutthelargevolumecell.Giventheextensivemeasurementsmadeduringthecommissioingexperiments,76Gawasusedastheexamplecaseforthesimulations.4.1SRIMTheStoppingandRangeofIonsinMatter(SRIM)codecalculatestheenergylossofionbeamsinmatter.Thiscalculationtrackseachindividualionanditsrespectivecollisionswithatomsofvarioustargetmaterials.Here,thestoppingpowersofthetargetcompoundsareestimatedwithBragg'sRule,andthecollisionsbetweenthemovingprojectileionsand63stationarytargetatomsaresimulatedbyMonte-Carlomethods.Thesestatisticalalgorithmsaccountforbothshortandlongrangeinteractionsandallowaniontomakejumpsbetweencalculatedcollisions,whilethecollisionresultsareaveragedovertheinterveninggap[3].Forthepresentwork,theTransportofIonsinMatter(TRIM)program,partoftheSRIMpackage,wasusedtostudytheenergydepositedbythe76Gaionsaswellastherangeoftheseionsthroughthedegraders,wedge,window,andheliumgas[4].Theion,ionmass,andkineticenergyoftheprojectilebeamaswellasthecompounds,thicknesses,anddensityvaluesforthetargetmaterialsusedduringthecommissiongex-perimentswererstloadedintotheprogram.Next,thebeamenergy,wedge,window,andgasmaterialsettingswereheldconstantwhilethedegraderthicknesswasvariedbetween1555{1740m.Finally,anumberof1000ionsweregeneratedfromapoint-sourceandintroducedintothesystem.Theresultingontheionization,orenergydepositedperion,andtherangeoftheionsthroughthesystemwasstudiedforthevaryiousdegraderthicknesses.AnexampleoftheTRIMsetupwindowwithapplicablevaluesisgiveninAppendixA.AsdiscussedinChapter3,thebeamwasdeliveredwitha0.5%momentumspread.Thisspreadwasaccountedforbychangingthekineticenergysettingfortheprojectilebeamandre-evaluatingtheresultsforeachdegraderthickness.ResultsfromonekineticenergysettingareshowninFig.4.1.Theionizationandrangecurvesweregeneratedbycalculatingtheaverageenergylossperionandtheaveragerangeperion,respectively,ateachdegraderthickness.Althoughtheionizationcurvewasplotteddirectly,thestderivativeoftheoriginalrangecurvewascalculatedandplottedinordertodeterminetheoptimalthicknessofdegradermaterial.Theseindividualenergyplotswereintegratedandcomparedtotheexperimentalresults64Figure4.1:Summaryofionizationandrangeresultsobtainedforthecenterofthemomen-tumdistributionofthe76GaionswithTRIMprogram.asshowninFig.4.2[2].Itshouldbenotedthatthesimulateddatawasshifted+136(1)mforthiscomparison.Asidefromthisshift,whichwasattributedtoaslightmiscalibrationinthemagneticrigidityoftheA1900[34]fragmentseparator,thesimulateddatagenerallyagreeswiththeexperimentalresultsinbothshapeandmaximumenergyloss.ThesmallwidthobservedbetweentheexperimentaldataandtheTRIMcalculationswasattributedtoboththeTRIMassumptionofthebeamasapoint-sourceandtheincreasedrangestragglingexperiencedbytheionsduetothedeformationintheoriginalwindow(seeChapter3fordetails).65Figure4.2:ThemeasuredBraggcurve(squares)andactivitydistribution(circleswithdashedline)for76GaarecomparedtothepredictedcurvesfromtheTRIMcode(solidlineanddashdotline,respectively).Thehatchedarearepresentstheequivalentthicknessoftheheliumgasandisshownforreference.4.2LISE++LISE++isaprogramdesignedtopredictthepurityandintensityofradioactiveionbeamsproducedwithtseparators.Althoughnumerousproductionmechanismsandreactionmodelsareincludedinthecode,projectilefragmentationisappropriateforthecurrentwork[6].Inaccordancewithitsmostcommonfunctions,LISE++wasusedforbothexperimentalplanningandon-linetuningduringexperiments.Thefocushere,though,willbeplacedonthevariousutilitiesandtoolsavailablewiththisprogramtostudytherangedistributionofthefragmentbeamions,theenergydepositedbytheseionsintheheliumbugas,andthestoppingiencyofthegasitself.66Figure4.3:Rangeoptimizationingasresultsfor76Gaionbeamthermalizedduringthecommissioningexperiments.TheseresultsweregeneratedwiththeLISE++programandshowanoptimaldegraderangleof24.5.UnlikeSRIM,whichcanonlycalculatetheinteractionsofapuresecondarybeam,LISE++calculationsbeginwithaprimaryionbeam.Thisprimarybeamisimpingeduponatarget,therbyproducingasecondaryfragmentbeamthroughprojectilefragmentation.Giventheproductionmechanism,thesecondaryfragmentbeamisinitiallycomposedofnu-merousionsandisotopes.However,afterseparationwiththeA1900fragmentseparator,thisbeamismostlycomposedofthedesiredisotopeofinterest.AlthoughtheA1900isalreadybuiltintotheprogram,thebeamlinedownstreamoftheseparatormustbeconstructedus-ingthe\set-up"options[6].Onceassembled,completecalculationsandsimulationsfromproductionthroughthermalizationcanbeperformed.67Figure4.4:Rangedistributionfor76GafragmentbeamcalculatedwiththeLISE++pro-gram.Attheoptimaldegradersetting,theionsofinterestwillstopinthemiddleofthelargevolumecell(600mm).Noticethatthe78Gecontaminantionissupressedtozeropps/mmattheoptimalsettingwhilethe76Gaionofinterestwillbeextractedatitspeakof19(1)pps/mm.Asalludedtoabove,fragmentbeamsproducedthroughprojectilefragmentationandsubsequentseparationarerarely100%pure.Assuch,itisimportanttorecognizethatthecompositionofthefragmentbeamwillchangeduetoisotopesvaryingrangedistributionsinthegas.Forthecommissioningruns,an81(1)%pure76Gafragmentbeamwasdeliveredtothebeamthermalizationarea.Themaincontaminantofthissecondarybeamwas78Ge.Suppressionofcontaminantionsisoneofthemoreunderstated,butt,functionsofavariabledegrader.Theoptimalvariabledegradersettingformaximumsuppressionofcontaminantswithminimallosstotheionofinterestcanbequicklydetermined,similartoSRIMmethods,througha\Rangeoptimizer(gascellutility)"functionfoundwithintheLISE++program,seeforexampleFig.4.3.ResultsfromthisoptimizationwerecheckedbytherespectiveSRIMresults(Fig.4.1)andfoundtovarybylessthan1%.Fig.4.468Figure4.5:EnergydepositioninXvsZ(Top)andYvsZ(Bottom)forthe76GafragmentbeamsimulatedwiththeLISE++program.thatthe78Gecontaminantwillbetllysupressedwithminimallosstothe76Gaionofinterestattheoptimaldegraderangle.Alongwithdeterminingtheoptimaldegraderangle,therangeoptimizerfunctioncanalsobeusedtoestimatethestoppingofthegasateachangle.Thenumberofionsstoppedinthegasperangularstepcanbededucedbycomparingthenumberofionspresentinthe76Gabeambeforeandafterinteractionsoccurwithinthegas.Then,thestoppingforthelargevolumelineargascellcanbecalculatedbycomparingthenumberofionsstoppedinthegastothenumberofionsenteringthegascellatthepeak69ofthestoppingcurve.FortheexampleshowninFig.4.3,thetheoreticalstoppingwascalculatedtobe0.86attheoptimaldegraderanglewhichagreedwithinerroroftheexperimentallydeterminedvalueof0.84(0.04).Oncetheoptimaldegraderanglewasdetermined,itwaspossibletosimulatetheenergydepositedinthegas.LISE++includesacalculationoftheion-opticsofthebeamlineandcansimulatethesizeandangulardivergenceofthesecondarybeamatanypositionalongitspath.Inthepresentwork,thisfeaturewasusedandMonteCarlosimulationswereperformedtocalculatetheionizationinthegasasafunctionofposition.ProjectionsofthecalculatedenergydepositioninMeV/particleareshownforthe76GacaseintheX-ZandY-ZplanesinFig.4.5.ThedatawasthenuploadedtoMathematicaandboththeenergydepositedinavolumeelementandthenumberofionpairscreatedperparticleinthatelementweredetermined.Thesevalues,alongwiththevaluesobtainedfromtheprojectionofeachgraphontoitsrespectiveaxes,becameinputsfortheparticle-in-cellcode(3DCylPIC)whichrelatestheenergydepositedtothespacechargecreatedinthegas.4.33DCylPIC3DCylPIC[7]isa3Dparticle-in-cell(PIC)codethatwasdevelopedtostudyandcharacterizethespacechargeinvariouscylindricaliontrapsandtransportdevices.ThisprogramsolvesPoisson'sequationforanarbitrarynumberofchargedparticlesonacylindricalgridateverytimestep.Asaresult,itcancalculateaself-consistentchargedensityandtheresultingspacechargepotentialonthatgrid.Inordertoobtainthemostaccurateresults,3DCylPICallowsfortheinclusionofinternalelectrodes,magneticlds,andhardspherecollisionsandscatteringofionswithaneutralgas[7].Forthepresentlargevolume70lineargascell,thiscodewasusedtocalculatetheself-consistentspacechargepotentialforagivenincoming76Gaionbeamrate,gaspressure,andDCdraginthebodyofthecell.Thedimensionsofthecellandtheelectrodedesignwerehardcodedintothesimulations,whileallotherrelavantparameterswerevariablesinputwithaThesevariableparametersincludedthepressure,temperature,andmassofthegas,theappliedDCdragandtheionandelectronmasses,mobilites,andcharges.Thechargeswerescaledinordertoaccountforthevariousincomingbeamrates,andthisfactorwascalculatedwiththeequation[58]:chargescale=RIEtEIPIP;(4.1)whereRIistheincomingbeamrate(pps),Eistheaverageenergydepositedperparticle(eV/ion),tisthesimulationtimestep(s),IPistheenergyrequiredtocreateanionpair(eV/IP),andIPisthenumberofionpairscreatedpertimestepinthesimulation.Forthesesimulations,thetimestepwaslimitedtovalueslessthanthetheoreticaltimeforanelectrontoadvanceonegridunit.Thisinputparameter,alongwiththenumberofionsreleasedpertimestepandtheionadvancementsteps,controlledtheresolutionofthesimulation.ThephysicalsizeandenergydepositionresultsobtainedfromLISE++forthe76GabeamwerealsousedasinputparametersinthisAfterthecowassuccessfullyuploaded,chargedparticleswerecontinuouslyintroducedintothesystem,andthechargedensitywastrackeduntilanequilibriumwasestablished.AnexampleisshownanddiscussedinAppendixBwhilesomeofthesimulationresultsareshownanddiscussedbelow.Theevolutionofthechargedensityasafunctionoftimeforthevariousincomingbeam71ratesstudiedinthecommissioningexperimentsisshowninFig.4.6.Inthesevisualizations,theredlineshowstherapidmigrationandcollectionoftheelectronswhiletheblacklineshowstheslowermigrationandcollectionoftheheliumions.Althoughessentiallyalloftheionsinjectedintothesystemarecollectedafter˘0.1s,aplasmabeginstoformathigherincomingbeamrates(105pps).Thisplasmaisevidentintheerraticelectron,redline,seeninthebottomrightofFig.4.6.Rememberthatthepositivespacechargepotentialthatbuildsupinsidethegascellastheincomingbeamrateincreases.Athigherrates,thispotentialbecomeslargeenoughtoformaplasmathattrapsboththeionsandelectronsinsidethesystem.Thisincreasedpotentialalsoaddsanextrapushtothepositiveionslocatedontheedgeoftheplasmaandexplainswhythetimerequiredforthesystemtoreachequilibriumdecreasesastheincomingrateisincreased.Inordertobetterunderstandthesespacechargetheevolutionofthespacechargepotentialisalsostudied.Fig.4.7showstheappliedpotential,thespacechargepotentialatequilibrium,andagraphoftheevolutionofthespacechargepotentialovertime.AlloftheseexamplesareshownforaspZpointinthegascellandanincomingbeamrateof104pps.Theupperleftillustrationisatemperaturemapthatshowstheappliedpotentialofthesystemincludingtheoftheelectrodespokes.Theupperrightillustrationisasimiliartemperaturemapbutisrepresentativeofthespacechargepotentialpresentatequilibriumthroughoutthewholegascell.Here,thespacechargepotentialwasdeterminedbysubtractingtheappliedpotentialfromthetotalpotentialcalculatedatthegiventimestep.Theresultantshapeofthespacechargepotentialisanofboththedistributionoftheincomingbeamaswellastheoftheelectrodestructure.Finally,thegraphlocatedinthebottomoftheplotsthecalculatedspacechargepotentialatregulartimeintervalsof0.01suntilequilibriumisestablished.Fortheindividualtemperaturemaps72Figure4.6:3DCylPICresultsforthecollectionoftheion(black)andelectron(red)ioniza-tionpairscreatedforincomingbeamratesof102,103,104,and105pps.Thetimerequiredtoreachanequilibriumstatewas0.13,0.13,0.12,and0.10s,respectively.Seetextfordetails.obtainedfromeachtimestep,pleaseseeAppendixB.Sincehigherincomingbeamratesareassociatedwithhigherspacechargepotentialsandhigherspacechargepotentialscanhavenegativeonextractionciencies,itwasalsoimportanttostudythestrengthofthispotentialatincreasingincomingbeamrates.TheresultsfromthisstudyatthesameZpointareshowninFig.4.8.Notethattheintensityofthespacechargepotentialdramaticallyincreasesatrateshigherthan105pps.Therefore,thespacechargeresultsasawholeindicatedthattheperformanceoftheNSCL'slargevolumelineargascellwoulddecreasesigtlyatincidentbeamrateshigherthan105pps.AlthoughthesecalculatedresultsaregenerallyconsistentwiththeexperimentalresultsshowninFig.3.15,theconelocatedintheextractionregionofthecellwasunaccountedfor73Figure4.7:3DCylPICinitialpotential,equilibiumspacechargepotential,andevolutionofthespacechargepotentialforanincomingbeamrateof104pps.ThespacechargepotentialwasdeterminedbysubtractingtheinitialpotentialatTime=0.00sfromthetotalpotentialateachtimestep.bythe3DCycPICcalculation.Foramorecompletesimulation,thespacechargepotentialcalculatedforthebodyofthecellwasincorporatedintotheSIMIONmodeldiscussedbelow.4.4SIMIONSIMIONisacommercialsoftwarepackagethatcalculateselectromagneticandthetrajectoriesofchargedparticlesthatmigrateinthoseThecodeincludestheelectrodeinitialparticledistributions,appliedelectricandmagneticvoltages,andgaspresence.SIMIONcansimulatetrajectoriesineither2Dor3Dspace[8],andforthe74Figure4.8:Comparisonoftheequilibriumspacechargepotentialscalculatedforvariousincomingbeamrates.Seetextforfurtherdetails.presentwork,itwasusedtopredictthetrajectoriesof76Gaionsthrougha3D-modelspace.Thisspacereplicatedtheelectrodegeometry,appliedelectricspacechargepotential(from3DCycPIC),andheliumgasconditionsofthelargevolumelineargascell.Resultsfromthesesimulationswerethencompiledandcomparedtothevariousexperimentalresultswithrespecttotheextractionasafunctionoftheincomingbeamrate.TheSIMION3D-modelbeganwithrecreatingtheelectrodestructurelocatedwithintheNSCL'slargevolumelineargascell.AsdiscussedinChapter2,thebodyofthegascellcontains˘750thin,closelypacked,independentelectrodes.Eachelectrodeincludes12spokesthatare30degreesapartfromoneanotherandvaryinsize.Thesestructureshelpguidetheionsofinteresttotheextractionregionwhilesimultaneouslycollectingtheheliumionscreatedduringtheionizationprocess.TheextractionregionofthecellcontainsanRFconecomposedof˘280thin,closelypacked,independent,concentricringelectrodesthatdecreaseindiameterfrom˘246mmto˘2.74mm.TheSIMIONgascellasawhole,aswellasacomparisonbetweentheactualandsimulatedbodyandconeelectrodestructuresare75Figure4.9:Above:TheSIMIONmodelsofthebodyandconeelectrodesarecomparedtophotographsoftheactualelectrodestructures.Below:TheSIMIONgascellmodelasawhole.showninFig.4.9.Next,theappropriateRFandDCpotentialsalongwiththeheliumgasatomswereaddedtothemodel.Finally,thespacechargepotentialcalculatedwiththe3DCycPICcodewasapplied,andtheLISE++distributionof76GaionswasmigratedthroughtheSIMIONsystem.Aspreviouslydiscussed,thespacechargepotentialgrewwithincreasedincomingbeamrates.ThispotentialisevidentbytheincreasedvariationintheequipotentiallinesjustupstreamoftheconeintheSIMIONresultsshowninFig.4.10.Althoughthelinesandthereforethepotentialexperiencedbytheionsarebythespacecharge,eventuallyalloftheionsarecollectedinthesimulation.Bycombiningthenear100%collectioninboththe3DCycPICandSIMIONresultsforthesimulatedbeamrateswiththe0.84(0.04)stopping,0.33(0.05)detectoriency,0.85(0.05)transmissionthroughthewindow,andthe0.79(0.02)beampurity,theexpectedextractionforthelargevolumelineargascellcanbecalculatedtobe0.19(0.05).76Figure4.10:SIMIONresultsobtainedforYvsXwhereXisthelengthofthegascell.Theionbeam(blue)isshowntofollowtheequipotentiallines(red)throughthegascell.Here,theionbeamconsistedof100076GaionsthatweredistributedinaccordancewiththeLISE++results,andtheequipotentiallineswereshapedbyboththeDCandRFappliedpotentialsaswellasthecalculatedspacechargepotential.Thebeamrate(inpps)associatedwiththespacechargepotentialappliedisshownintheupperrightcornerofeach\NONE"indicatesthatnospacechargepotentialwasincludedinthesimulation.The3DCycPICandtherebytheSIMIONsimulations,however,onlyaccountforthespacechargewithinthebodyofthegascell.Notonlydothesesimulationsexcludespacechargewithintheconeofthecell,theyalsoexcludegasw,andanylossesfromdecay.Giventhenumberofionsthathavetopassthroughthenozzle,increasedspacechargecouldbeexperiencedbytheionsinthecone.Moreover,largerspacechargepotentialscausetheionstotravelalongthewallsofthegascellwhichcouldincreasethenecessaryextractiontime.Consideringtheseexceptions,itissuspectedthatthelossesathigherincomingbeamrates(105pps)resultfromspacechargewithintheconeofthegascelland/ordecaylosses.Moredetailedsimulationsoftheions'behaviorintheconeandthetimerequiredfortheionstomovedownthewallsofthegascellare,therefore,requiredtofullyunderstandtheobservedexperimentallosses.77Chapter5OutlookandSummaryGiventhesuccessoftheNSCL'slargevolumelineargascellandtheincreasingdemandforexperimentswiththermalizedionbeams,additionalenhancementsandoperatingconditionsarecurrentlybeingexploredthatcouldimprovethethermalizedbeampurities,increasethevarietyofionsthatcanbethermalizedbythesystem,andreducethespacechargeandplasmaexperiendbytheionsofinterest.ThesenextgenerationdevicescanbeincorporatedintoboththepresentNSCLbeamthermalizationareaaswellasinthebeamthermalizationareaforthenextgenerationFRIBfacility.5.1NewOperatingPracticesImpuritiesinthegas,althoughtlyreducedbyadditionalvacuumpumpingwhenthegascellisnotbeingused,arestillpresentandobservableinthebeamsdeliv-eredfromthecurrentlargevolumelineargascell.Operatingatcryogenictemperaturescanpotentiallyeliminateimpuritiesandprovidecleaner,ornon-fractionated,ionbeamstoexperimenters.Alongwithloweringimpuritylevels,theextractiontimecanalsobereducedbyimplementinganewiontransporttechnique{ionIonpromisesshorterextractiontimeswhichwillallowmoreexoticnucleitobethermalizedandstudiedbyex-perimenters[59,60].Finally,newgeometriesandset-upswillallowforthemoretremovalofthegasionsproducedduringthethermalizationprocess.Theseionsare78ultimatelyattributedwiththelossescausedthroughspacechargeandplasmaef-fects.eremovaloftheseunwantedgasionswouldthereforeleadtohigherandratecapabilitiesofthethermalizationsystems.5.1.1CryogenicOperationMolecularimpuritiesinthegasarisefromthegassupply,outgassingofthechamber,andbackstreamingfromthevacuumpumps[56].Asdiscussedinthepreviouschapters,theseimpuritiescanreactwiththeionofinterestwhiletheyaredriftingtowardstheexitandevenafterextraction[56]resultinginadistributionoftheradioactivityoveralargerangeofmasses.Impurityionscanalsochargeexchangewithgasionsandleadtostablecontaminantbeams.Theradioactiveiondistributionandcontaminantbeamscanreducetheoverallextractioncyofthesystemandthebeampurity,respectively.Althoughspecialmeasurescanbetakentoensurethecleanlinessoftheset-upsandsystems{suchasusingultrahighpurityhelium,ensuringultrahighvacuumstandardsaremet,andbakingthechamber{currentroomtemperaturedeviceshavetypicalimpuritylevelsontheorderofppb[56].Suchisthecaseforthepresentchamberwhichcanonlybeheatedto˘60Cduetoitsindiummetalseals.Attemperaturesbelow˘40K,though,themajorityofmolecularcontaminantswillfreezeoutofthegasphase,whichimprovesandalsoallowsawidervarietyofmaterialstobeusedforfabrication.Alongwithimprovingtheandthebeamqualities,theNSCLisalsoworkingtoimprovetheextractiontimesoflargegascells.79Figure5.1:CartoonofionconceptwheretheionsaretransportedbyatravelingwavethatissuperimposedoveranRFpotential[60].5.1.2IonOneoftheimprovementsmadetopreviousiontransporttechniqueswastheadditionofstackedringelectrodesalongthebodyofthegascell.AsdiscussedinChapters2and3,thetransporttimewaslimitedbythemaximumelectricthatcouldbeappliedbeforedielectricbreakdownsoccurwithintheheliumgas.\Ionwasrecentlyproposed[59]andproventobe[60]asuccessfultransportmethodforthermalions.Themethodsimtheelectrodesystemdesignwhilealsotlyim-provingtransportspeeds.Similartothecurrentgascell'smethod,thismethodalsoreliesonanRFrepellingforce.Here,however,theionsaremovedbyatravelingwavepotentialparalleltothesurfaceofamulti-electodecarpet,asseeninFig.5.1.OnlyfourRFsignalsandonestatic(DC)pushvoltagearerequiredtoensurequickandttransportofions.Resultsfromtheionmethodshowedreliableoperationwithiontransportspeedsgreaterthan60m/s[61].Thesespeedsareatleasttwicethatpossiblewithcurrentstoppingsystemswhichcanhaventimplicationsonextractiontimes.Forexample,theaverageionextraction80timesfortheNSCL'slargevolumelineargascellareontheorderof40mswhiletheaverageionextractiontimeswiththeiontechniqueforagascellofsimilarlengthwouldbeontheorderof10msorless[2,60,62].Suchshortextractiontimeswouldallowcollectionofthevastmajorityofproducableexoticnuclei.Giventhesepossibilities,theNSCLisdevelopingnewcryogenicgascellsthatwillalsoincorporatetheiontechniqueaspartofitsthermalbeamdevelopmentprogram.5.2ACGSTheAdvancedCryogenicGasStopper(ACGS)[62],isonesuchcellcurrentlyunderdevel-opmentattheNSCL.SimilartothecurrentNSCLgascell,thisdeviceisalsoalargevolumelineargascellwithanextractionareathatextendsthelengthofthecell.TheACGS,how-ever,willbeoperatedatcryogenictemperatures(˘40K)andwillincorporatetheiongtechnique[59]withasimplebutinnovativegeometry.ThegeometryforthecurrentNSCLlargevolumelineargascellconsistsofconcentricelectrodeslocatedalongthewallsofthestoppingchamberwhichextendstheextractionareatheentirelengthofthecell.Positivelychargedheliumgasionsproducedfromthethermalizationprocessarecollectedonthesechamberelectrodeswhiletheelectronsproducedarecollectedonathinwindow.TheionsofinterestareguidedandfocusedthroughthecellbyappliedDCandRFtoasupersonicnozzlewheretheyarejettedintoatiallypumpedionguidesystem.AdetaileddiscussionoftheseprocesseswasgiveninChapter3.ThegeometryfortheACGS,shownschematicallyinFig.5.2[62],isconsiderablyent.AhorizontalRF-based,ioncapetispositionedinthemiddleofthestoppingchamberwhilepushelectrodesarelocatedonthetopandbottomofthechamber.Electrons81Figure5.2:ConceptualdesignoftheACGS[62].arecollectedonthepushelectrodeswhilethepositivelychargedionsarepushedtowardthecentralcarpet.Oncetheionsreachthecarpetsurface,theyarepreventedfromleavinginthetransversedirectionbyadditionallateralelectrodes.RememberthattheforceexertedbyanRFisbothpositionandmassdependent.Therefore,thelight,unwantedheliumgasionsthatleadtospacechargeandplasmacanbecollectedonthecarpetsurfacewhiletheionsofinterestareguidedtotheextractionregion.Theextractionregioniscomposedoftwovertical,concentriccircularioncarpetsthatareusedtotransporttheionsofinteresttotheextractionoDuetothequickandtneutralizationoftheheliumionsonthecentralcarpetaswellastheexpectedextractionspeeds,thisgeometryisexpectedtoincreasethebeamratecapabilityofthedevicebymorethananorderofmagnitudewithrespecttootherlineargascells[62].Althoughsimilarupgradeshavebeenandwillcontinuetobeimplementedtoimprovetheoflineargascells,thesecellsareinexorablylimitedbytheirsizewithrespecttothethermalizationoflightions.825.3CycstopperInanattempttocreatetheconditionsrequiredtosuccessfullythermalizeionsofallmassesathighincomingbeamrates,theNSCLiscurrentlydeveloppingagas-reversecyclotron,dubbedthe\cycstopper."Thisnextgenerationstoppingdevicewilloperateatcryogenictemperatures,employtheiontechnique,andanstoppingdistanceinlowpressuregas[63,64,65].5.3.1ConceptTheultimatelimitationsoflineargascellsarisefromthesizeoftherangestragglingoflightenergeticprojectilefragmentscomparedtothelengthofthegascells.RecallfromChaper1,thatthelinearstoppingpowerofamaterial,alsocalledspenergyloss,isproportionaltomZ2/E.Thus,higherkineticenergiesandloweratomicnumbersleadtolongerranges.SubstitutingtheBetheformulaforspenergyloss(3.2)intotherangeequation,therelationshipbetweentherangeofanionanditsmass,mcanbedescribedas:R/E32opm;(5.1)whichshowthationswithsmalleratomicnumbersandlightermasseshaveamuchlongerrangewithinamaterial.Therefore,inordertothermalizethelightestions,eitherthestoppingpowerofthegaswouldhavetobeincreasedand/orthelengthofthecellwouldneedtobeincreased.Unfortunately,bothoftheseoptionshavetconsequences.Increasingthestoppingpowerofthegaswouldrequireeitherahigherdensityergasand/orhighergaspressures.83AsdiscussedinChapter2,heliumisthebestgasgiventhatithasthehighestionizationpotentialofanyotherelement.Thishighpotentialleadstomostoftheionsofinterestretainingtheirchargeandexitingthechamberineitherthe1+or2+chargestate.Changingthegascould,therefore,leadtoincreasedlossesfromneutralization.Highergaspressuresorlongergascellswouldbothleadtoincreasedextractiontimes.Increasingtheextractiontime,increasesthehalflifelimitofionsthatcouldsurvivethethermalizationprocessandbedeliveredtootherexperimentalset-ups.5.3.2DesignandCapabilitiesVariousexistbetweenlineargasstoppersandthecyclotrongasstopper,shownschematicallyinFig.5.3[64].Forexample,thedegradersystemsemployedbylineargascellsaretypicallylocatedupstreamofthecellwhereasthedegradersystememployedbythecycstopperispartofthedeviceitself.Also,lineardeviceshavecylindricalgaschambersthatarelocatedonthebeamaxiswhereasthecycstopperhasadisk-likegaschamberthatislocatedtangenttotheincomingbeamaxis.Finally,lineargascellsdonotemployamagneticwhereasthecycstopperexploitsthefocusingproperties,orspiralingthatasectoredcyclotronmagnethasonchargedparticles[63,64,65].Fastionsareinjectedintothecycstopperthroughabeamportlocatedontheouterradiusofthedevice.Astheionsenertheroughly2Tmagnetictheyarepassedthroughanadjustablesoliddegraderandbegintospiral.Thismotionisbothradiallyandaxiallybythemagneticandcarriestheionsintothegasstoppingchamber.Thethermalizationprocess,orcollissionswiththegasatoms,causestheionsradiioftrajectorytodecreaseuntiltheyreachthecenterofthechamberwhichalsocorrespondstothecenterofthedevice.Similarsystemshavebeenusedtosuccessfullythermalizeantiprotons,84Figure5.3:Conceptualdesignofthecycstopper[64].pions,andmuons,andhavealsobeenpreviouslyproposedforstoppinglightions[63,64].Fromhere,thenowthermalionsaremovedtotheextractionregionofthedevicebyRF-carpets[63,64,65].Sincetheionsarefreetomakemultipleturnswithinthegas,themassthicknessencoun-teredbytheionswillvarydependingontheion'srange.Giventhatlightionshavealongerrange,theseionswillsimplymakemoreturnsinthegasbeforethermalizationisachieved.ThisstoppingvolumewillallowlowergaspressurestobeusedwhichnotonlyreducesthespacechargebutalsoensurestfunctionalityoftheRFtechnichesemployed[63].Aspreviouslydiscussed,spacechargeandplasmaareattributedwiththemosttlosses.Thesearisefromthetremovalofheliumionscreatedduringthethermalizationprocess.Byapplyinganelectricinthesamedirectionasthemagneticthecycstoppershouldbeabletotlyremoveboththeheliumionsandelectronscreatedduringthethermalizationprocess.tremovalofthesepairswillalsoantlydecreasethespacechargeandplasmainsidethegasandthereby,85Figure5.4:Left:Renderingofthemechanicalmodelofthecyclotrongasstopperwithanindicationofthespiralpathofastoppingion[65].Right:Photographofthecyclotron-stoppermagnetintheopenstateshowingthepolepieces.furtherincreasetheextractionofthesystem[64,65].Simulationresultsforthecystoppertlyenhancedstoppingandextrac-tionforlightandmediummassedionswhencomparedtocurrentlineargascells[65].Constructionofthisdeviceisnearlycomplete,andcharacterizationtestsarecurrentlyunderway.Figure5.4showsamechanicaldrawingofthecyclotrongasstopper[65]alongsideaphotographofthedeviceinitscurrentstate.5.4SummaryThepresentworkhasexploredthecapabilitiesofanewlargevolumelineargascellforthethermalizationofprojectilefragments.Experimentalresultsfromthisdeviceindicatedimprovedperformancecomparedtoearlierdevices,andsimulationswereemployedtoun-86derstandtheobservedbehavior.Itcanbeconcludedthatthisimprovedperformancewasaresultofthelargerstoppingvolume,improvedelectrodestructure,andadditionalRFpotentialwhichincreasedthestopping,decreasedthespacechargepotential,andincreasedtheextraction,respectively.Althoughthesimulatedresultsaccountedforamajorityoftheexperimentallosses,moredetailedsimulationsoftheions'behaviorintheconeofthegascellandthetimerequiredfortheionstomovedownthewallsofthecellarerequiredtoaccountforthelossesobservedathigherincomingbeamrates(105pps).Afewissuesalsoremainwithregardtobeampurity,extractiontimes,andthecollectionofthelightestionsthatnextgenerationdevicesarebeingdevelopedtoaddress.87APPENDICES88AppendixASRIMExampleTheTRIMsetupwindowshowninFig.A.1containstheexperimentalparametersuseddur-ingthe76Gacommissioningexperiments.Theseparametersincludeanadjustablealuminumdegrader,1050m-thicksilicondioxide(glass)wedge,10m-thickaluminumwindow,120cm-longheliumgasvolume,anda1000m-thickaluminumbackwallforcompleteness.Whilemostofthematerialdensitieswerestandardandsetaspictured,thedensityofthegaswascalculatedtobe2.06x105g/cm3.Thisdensitywascalculatedfortheoperatingtemperatureandpressureof-4Cand123mbar,respectively.Theparametersincludetheidentityandmassoftheionofinterestaswellasitskineticenergy.Thekineticenergywascalculatedtobe6864870keVbythe\physicalcalculator"intheLISE++program.Duringcommissioning,thedegraderthicknessnorthekineticenergyofthebeamre-mainedconstant.Assuch,theseparametersarehighlightedwithagoldboxinFig.A.1.Althoughtheethicknessofthedegraderwasintentionallyvaried,thekineticenergyoftheionsinherentlyvariedduetoa0.5%momentumspread.Thesevariationswereac-countedforbymanuallychangingthesettingsandre-runningthesimulation.TheionizationandrangeresultsfromalloftheassociativesimulationswerecombinedandcomparedtotheexperimentallydeterminedvaluesasshowninChapter4.89FigureA.1:TRIMsetupwindowwithexampleparametersfromthe76Gacommissioningexperiments.90AppendixB3DCycPICExamplesExampleFileThe3DCycPICusedforthehighestincomingbeamrateof6.2x105isshowninFig.B.1.AsimpledescriptionofeachvalueisgivenintheThevalueshighlightedinyellowarethescalefactorforthechargevalues.Thisfactor,asdiscussedinChapter3,iscalculatedwiththeequation[58]:chargescale=RIEtEIPIP;(B.1)ThevalueshighlightedinblueareobtainedfromtheLISE++projectionsintheX,Y,andZplanes,andthevalueshighlightedingreenarethemain,variableexperimentalparameters.Finally,thevalueshighlightedinpurplearethemain,adjustableparameterssptothePICsimulation.Thenumberofionsreleasedpertimestep,aswellasthetimestepitselfandionadvancementsteps,controltheresolutionofthesimulation.Astheresolutionisincreased,though,theamountoftimerequiredtorunthesimulationalsoincreases.Therefore,thesevaluesareoptimizedforresolutionandsimulationtime,wheretheelectronscannotadavancefurtherthanonegridunitpertimestepandthetimerequiredtorunthesimulationislessthantwodays.91FigureB.1:PICexamplefromthe76Gacommissioningsimulations.92ExampleTemperatureMapsSincethespacechargepotentialevolvesovertime,itispossibletotrackitsdevelopmentuntilequilibriumisreached.TemperaturemapsshowthepotentialatagivenZslicethroughthegascell.Examplemaps,alongwithagraphthatsummarizedtheevolutionofthespacechargepotentialforanincomingbeamrateof104pps,wasshowninChapter4(Fig.4.7).TheindividualtemperaturemapsusedtocreatethisgraphareshowninFig.B.2.Whiletheappliedpotentialisshowninthetopleftmap,onlythespacechargepotentialisshownintheremainingmaps.Inordertosolelyviewanddeterminethespacechargepotential,theappliedpotentialwassubtractedfromthetotalpotentialcalculatedforeachtimeinterval.Theresultantshapeofthespacechargepotentialisanofboththedistributionoftheincomingbeamaswellastheoftheelectrodestructure.93FigureB.2:Evolutionofthespacechargepotential(inVolts)asafuntionoftimeforanincomingbeamrateontheorderof104pps.ThisspacechargepotentialwasdeterminedbysubtractingtheinitialelectrodepotentialatTime=0.00sfromthetotalpotentialateachtimestep.94REFERENCES95REFERENCES[1]L.Weissman,P.A.Lofy,D.A.Davies,D.J.Morrissey,P.Schury,S.Schwarz,T.Sun,andG.Bollen.Firstextractiontestsofthensclgascell.Nuc.Phys.A,746(2004)655c{648c.[2]K.Cooper,C.S.Sumithrarachchi,D.J.Morrissey,A.Levand,J.A.Rodriguez,G.Savard,S.Schwarz,andB.Zabransky.Extractionofthermalizedprojectilefrag-mentsfromalargevolumegascell.Nuc.Instr.andMeth.A,763(2014)543{546.[3]J.F.Ziegler,M.D.Ziegler,andJ.P.Biersack.SRIM-thestoppingandrangeofionsinmatter(2010).Nuc.Instr.andMeth.B,268(2010)1818{1823.[4]J.F.Ziegler.TheSRIMenergy-losspackage.http://www.srim.org/.[5]O.TarasovandD.Bazin.LISE++:Radioactivebeamproductionwithtsepa-rators.Nuc.Instr.andMeth.B,266(2008)4657{4664.[6]O.B.TarasovandD.Bazin.LISE++:designyourownspectrometer.Nuc.Phys.A,746(2004)411{414.http://lise.nscl.msu.edu/lise.html.[7]R.Ringle.3DCylPIC-a3Dparticle-in-cellcodeincylindricalcoordinatesforspacechargesimulationsofiontrapandiontransportdevices.Intl.J.MassSpectr.,303(2011)42{50.[8]D.A.Dahl.SIMIONforthepersonalcomputerinIntl.J.MassSpectr.,200(2000)3{25.http://simion.com/.[9]D.J.MorrisseyandB.Sherrill.Radioactivenuclearbeamfacilitiesbasedonprojectilefragmentation.Phil.Trans.R.Soc.Lond.A,356(1998)42{50.[10]P.Dendooven.ThedevelopmentandstatusoftheIGISOLtechnique.Nuc.Instr.andMeth.B,126(1997)182{189.[11]R.D.MacfarlaneandR.D.oen.Asystemforstudyingacelerator-producedshort-livedalphaemitters.Nuc.Instr.andMeth.,24(1963)461{464.96[12]JosephCerny(editor).NuclearSpectroscopyandReactions:PartA,volume40-A(Aca-demicPress,NewYorkandLondon1974).[13]W.Wiesehahn,G.BiscandJ.D'auria.Theroleofclustersizeinagasjettransportsystem.Nuc.Instr.andMeth.,129(1975)187{192.[14]J.o,P.Puumalainen,andK.Valli.Thecarrier-loadedhelium-jettransportmethod.Nuc.Instr.andMeth.,115(1974)65{73.[15]H.Wollnik,H.G.Wilhelm,G.Robig,andH.Jungclas.Theimprovementofagas-jetsystembytheuseofanaerosolgenerator.Nuc.Instr.andMeth.,127(1975)539{545.[16]W.D.Schmidt-Ott,R.L.Mlekodaj,E.H.Spejewski,andH.K.Carter.He-jeton-lineionsourceoftheunisormassseparator.Nuc.Instr.andMeth.,124(1975)83{91.[17]J.o,V.Rantala,K.Valli,S.Hillebrand,M.Kortelahti,K.Eskola,andT.Raune-maa.ofanon-lineisotopeseparatorsystememployingcooledandNaCl-loadedHe-jetmethods.Nuc.Instr.andMeth.,139(1976)325{329.[18]V.T.Koslowsky,M.J.Watson,E.Hagberg,J.C.Hardy,W.L.Perry,M.G.Steer,H.Schmeing,P.P.Unger,andK.S.Sharma.TheChalkRiverhigh-temperaturehelium-jetionsource.Nuc.Instr.andMeth.B,70(1992)245{253.[19]S.Ichikawa,M.Asai,K.Tsukada,A.Osa,T.Ikuta,N.Shinohara,H.Iimura,Y.Nagame,Y.Hatsukawa,I.Nishinaka,K.Kawade,H.Yamamoto,M.Shibata,andY.Kojima.Massseparationofneutron-richisotopesusingagas-jetcoupledthermalionsource.Nuc.Instr.andMeth.A,374(1996)330{334.[20]R.Kirchner.Reviewofisoltarget-ion-sourcesystems.Nuc.Instr.andMeth.B,204(2003)179{190.[21]P.VanDuppen,P.Decrock,M.Huyse,andR.Kirchner.Highyionsourcesforon-linemassseparators:Abriefreview.Rev.Sci.Instrum.,63(1992)2381{2386.[22]J.o.DevelopmentandapplicationsoftheIGISOLtechnique.Nuc.Phys.A,693(2001)477{494.[23]GlennF.Knoll.RadiationDetectionandMeasurement(JonWiley&Sons,Inc.,NewJersey2010),4thedition.97[24]J.ArjeandK.Valli.Helium-jetionguideforanon-lineisotopeseparator.Nuc.Instr.andMeth.,179(1981)533{539.[25]J.Arje.Chargecreationandresentmechanismsinanionguideisotopeseparator(IGIS).PhysicaScripta,(1983)37{40.[26]I.D.Moore,T.Kessler,T.Sonoda,Y.Kudryavstev,K.Parvi,A.Popov,K.D.AWendt,andJ.o.Astudyofon-linegascellprocessesatIGISOL.Nuc.Instr.andMeth.B,268(2010)657{670.[27]A.Iivonen,K.Riikonen,R.Saintola,K.Valli,andK.Morita.Focusingionsbyviscousdragandweakelectricinanion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