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The LZ Experiment: Krypton Removal via Gas Charcoal Chromatography S1 S2 E field Drift time indicates depth S1 S2 time Fiducial region is well shielded from external backgrounds Introduction: WIMPs in a Liquid Xenon TPC LUX Tritium ER Calibration LUX DD NR Calibration log 10 (S2/S1)x,y,z corrected S1 x,y,z corrected (phe) LZ is a dual-phase xenon Time Projection Chamber (TPC) that collects two scintillation signals for each scattering event to look for single-scatter nuclear recoil (NR) events from dark matter in the form of Weakly Interacting Massive Particles (WIMPs). Since the expected rate of dark matter events is very low — already less than one event per 100 kg of detector mass per 100 days— it is essential to reduce as much as possible all sources of radioactivity in and around the detector. Kr-85, a radioactive isotope of krypton, is one of the background sources of concern, and must be removed down to a very low level so that it does not give too many background events in the experiment. To put this background source into context, we describe briefly how the LZ detector works and how the required level of Kr-85 is determined. LZ uses liquid xenon to generate two scintillation signals, S1 and S2, which provide detailed information about the events: • S1 is from the de-excitation of short-lived xenon molecules, or dimers, and is emitted promptly after the scattering (~10 nsec) and is a “start” signal for the event clock. • S2 is from electrons liberated at the event site that are extracted into the gas phase where they undergo electroluminescence. The location of the S2 event in the upper PMT array gives the lateral (xy) event position. • The S1-S2 time difference give the depth of the event (z) • The combined xyz position allows events near the edge, which are dominated by external sources of radioactivity, to be rejected as backgrounds. TPC vessel Purification Tower below • Electron recoils (ERs) throughout the detector volume can be rejected about 99.5% of the time because their S2/S1 ratio is different than for NRs • The rate of ERs from solar neutrinos sets a lower bound so we work to make reducible backgrounds come in at a lower level. • Kr-85 is a beta emitter, and in LZ, a concentration of Kr at 200 parts per quadrillion (ppq) dissolved uniformly in the xenon is equivalent to the neutrino background and can’t be fiducailized • Our Kr-85 goal is to reduce the level to 15 ppq in the ten tons of xenon in LZ, starting from a concentration of 1-100 parts per billion (ppb) in the raw stock. Kr Xe He Transit time of a krypton-xenon mixture through a charcoal column measured by gas analyzer Xe He Xe Xe/Kr chromatography Xe recovery Xe storage charcoal 500 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 capacity 24 hour cycle (per condenser) 50 SLPM Xe 80 bar 2 bar ~ 120 min 10 tons / 60 days @ 200 kg/day; 85% uptime 250 SLPM He (dry backing pump) To Condensers Bulk LN Supply 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 Recovery 180 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 Gas Panel Circulation Pumps Charcoal Column Condenser Thermosyphon Lines SRV Kr Trap Dewar Source & Accumulator Bottles Chromatography Like pigments of different size separating on filter paper under the action of a carrier fluid, Xe and and Kr atoms migrate at different speeds through a charcoal column under the influence of a helium carrier gas. Development system at SLAC At SLAC, we’ve resurrected and reconfigured the Case Western Kr removal system that was used to reduce Kr in the LUX xenon from 130 ppb to 4 ppt. The system is being used as a development platform to test ideas for improving the separation and to demonstrate chromatography separation down to the LZ spec. Xenon assay: cold-trap assisted RGA Our collaborators led by Carter Hall at the University of Maryland have developed an innovative system to measure trace Kr in Xe down to 10 ppq using a cold-trap assisted RGA, or Residual Gas Analyzer. Commercial RGA units have a baseline of about 1 ppm, far too insensitive for our needs. By passing xenon samples through a cold-trap in liquid nitrogen, the xenon is frozen out to a partial pressure of 10 -3 mbar while allowing the trace contaminants, including Kr to pass through nearly unimpeded. 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 The removal of the helium refrigerator used for the BaBar magnet will clear space on the Bldg 624 cryopad for the installation of the LZ production system. The use of outdoor space mitigates the oxygen deficiency hazard and provides easy loading and unloading of xenon storage packs. A total of 200 cylinderswill be processed twice during the production run. The three main loops for the xenon processing are shown. With two charcoal columns and two xenon condensers, a 16 kg slug of xenon can be introduced every two hours on a continuous basis.
