Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
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.