The LZ Experiment: Krypton Removal via Gas Charcoal Chromatography

S1

S2 Efield

Drifttimeindicatesdepth

S1

S2time

Fiducialregioniswellshieldedfromexternalbackgrounds

Introduction: WIMPs in a Liquid Xenon TPC

LUXTritiumERCalibration

LUXDDNRCalibration

log 1

0(S2/S1)x,y,z

corrected

S1x,y,zcorrected(phe)

LZisadual-phasexenonTimeProjectionChamber(TPC)thatcollectstwoscintillationsignalsforeachscatteringeventtolookforsingle-scatternuclearrecoil(NR)eventsfromdarkmatterintheformofWeaklyInteractingMassiveParticles(WIMPs).Sincetheexpectedrateofdarkmattereventsisverylow—alreadylessthanoneeventper100kgofdetectormassper100days—itisessentialtoreduceasmuchaspossibleallsourcesofradioactivityinandaroundthedetector.Kr-85,aradioactiveisotopeofkrypton,isoneofthebackgroundsourcesofconcern,andmustberemoveddowntoaverylowlevelsothatitdoesnotgivetoomanybackgroundeventsintheexperiment.Toputthisbackgroundsourceintocontext,wedescribebrieflyhowtheLZdetectorworksandhowtherequiredlevelofKr-85isdetermined.LZusesliquidxenontogeneratetwoscintillationsignals,S1andS2,whichprovidedetailedinformationabouttheevents:• S1isfromthede-excitationofshort-livedxenonmolecules,ordimers,andis

emittedpromptlyafterthescattering(~10nsec)andisa“start”signalfortheeventclock.

• S2isfromelectronsliberatedattheeventsitethatareextractedintothegasphasewheretheyundergoelectroluminescence.ThelocationoftheS2eventintheupperPMTarraygivesthelateral(xy)eventposition.

• TheS1-S2timedifferencegivethedepthoftheevent(z)• Thecombinedxyzpositionallowseventsneartheedge,whicharedominated

byexternalsourcesofradioactivity,toberejectedasbackgrounds.

TPCvessel

Purification

Tower

\ vesselsbelow

• Electronrecoils(ERs)throughoutthedetectorvolumecanberejectedabout99.5%ofthetimebecausetheirS2/S1ratioisdifferentthanforNRs

• TherateofERsfromsolarneutrinossetsalowerboundsoweworktomakereduciblebackgroundscomeinatalowerlevel.

• Kr-85isabetaemitter,andinLZ,aconcentrationofKrat200partsperquadrillion(ppq)dissolveduniformlyinthexenonisequivalenttotheneutrinobackgroundandcan’tbefiducailized

• OurKr-85goalistoreducethelevelto15ppqinthetentonsofxenoninLZ,startingfromaconcentrationof1-100partsperbillion(ppb)intherawstock.

KrXe

He

Transittimeofakrypton-xenonmixturethroughacharcoalcolumnmeasuredbygasanalyzer

Xe

He Xe

Xe/Kr chromatography

Xe recovery

Xe storage

charcoal500 kg

condensers

thermosyphon

16 kg

500 SLPM He(RIX compressor)

filter

U RGA

sampling

feed/purge ~ 120 min

100 mbar

1000 mbar

250 SLPM He(Roots booster)

10 mbar

cooled Kr traps

200 kg capacity24 hour cycle(per condenser)

50 SLPM Xe

80 bar2 bar

~ 120 min10 tons / 60 days @ 200 kg/day; 85% uptime

250 SLPM He(dry backing pump)

To Condensers

Bulk LNSupply

automation of all 3 cycles

Scaling LUX LZ

Column / slug size

2 kg Xe slug in 60 kg charcoal

16 kg Xe slug in 500 kg charcoal Saturation: fix MXe/Ma

Chroma-tography

120 min: 100 LPM 50 SLPM @ 0.5 bar

120 min: 1000 LPM 500 SLPM @ 0.5 bar up to

2000 SLPM @ 2 bar

Transit time ~ Ma/(vol. flow); higher pressure reduces diffusion

Recovery180 min: 1500 LPM

15 SLPM He @ 10 mbar 120 SLPM Xe at peak

120 min: 25000 LPM 250 SLPM He @ 10 mbar

Match chromatography time; conservative scaling since 1.5

faster × 8 Ma = 12x volume flow, or 18000 LPM

Processing rate

2 kg / 5 hours 10 kg/day

50 kg/week, incl. storage

16 kg / 2 hrs 192 kg/day

20T / 120 days (85% uptime)

Continuous processing in LZ - no downtime for

storage; 2 passes of 10 T

GasPanel

CirculationPumps

CharcoalColumn

Condenser

ThermosyphonLines

SRV

KrTrapDewar

Source&AccumulatorBottles

Chromatography Likepigmentsofdifferentsizeseparatingonfilterpaperundertheactionofacarrierfluid,XeandandKratomsmigrateatdifferentspeedsthroughacharcoalcolumnundertheinfluenceofaheliumcarriergas.

Development system at SLAC AtSLAC,we’veresurrectedandreconfiguredtheCaseWesternKrremovalsystemthatwasusedtoreduceKrintheLUXxenonfrom130ppbto4ppt.ThesystemisbeingusedasadevelopmentplatformtotestideasforimprovingtheseparationandtodemonstratechromatographyseparationdowntotheLZspec.

Xenon assay: cold-trap assisted RGAOurcollaboratorsledbyCarterHallattheUniversityofMarylandhavedevelopedaninnovativesystemtomeasuretraceKrinXedownto10ppqusingacold-trapassistedRGA,orResidualGasAnalyzer.CommercialRGAunitshaveabaselineofabout1ppm,fartooinsensitiveforourneeds.Bypassingxenonsamplesthroughacold-trapinliquidnitrogen,thexenonisfrozenouttoapartialpressureof10-3mbarwhileallowingthetracecontaminants,includingKrtopassthroughnearlyunimpeded.

1.5 ppt Kr

open leak valve

Detection of krypton at the part-per-trillion level

arXiv:1103.2714v3

Signal integration window Average here to get pressure baseline

Conceptual layout of production system

Scaling parameter from LUX to LZ

LZ production system

TheremovaloftheheliumrefrigeratorusedfortheBaBarmagnetwillclearspaceontheBldg624cryopadfortheinstallationoftheLZproductionsystem.Theuseofoutdoorspacemitigatestheoxygendeficiencyhazardandprovideseasyloadingandunloadingofxenonstoragepacks.Atotalof200cylinderswillbeprocessedtwiceduringtheproductionrun.

Thethreemainloopsforthexenonprocessingareshown.Withtwocharcoalcolumnsandtwoxenoncondensers,a16kgslugofxenoncanbeintroducedeverytwohoursonacontinuousbasis.