The ZEPLIN III Two-Phase Xenon WIMP Detector Henrique Araújo Underground Commissioning and Plans
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The ZEPLIN III Two-Phase Xenon WIMP Detector Underground Commissioning and Plans Henrique Araújo Imperial College London & Rutherford Appleton Laboratory, UK On behalf of the ZEPLIN-III Collaboration: Edinburgh University (UK), Imperial College London (UK), ITEP-Moscow (Russia), LIP-Coimbra (Portugal) STFC Rutherford Appleton Laboratory (UK) DARK-2007, 24-28 September 2007, University of Sydney, Australia Direct WIMP Searches DARK 2007 Henrique Araújo 2 ZEPLIN-III Operation Principle Readout of scintillation light and ionisation charge with array of 31 photomultipliers Gaseous xenon Liquid xenon photomultipliers Sensitivity to single ionisation electrons! Edwards et al., arXiv:0708.0768 Primary Scintillation DARK 2007 Secondary Scintillation Henrique Araújo 3 ZEPLIN III: High-Field, 2-Phase Xenon Good light collection for scintillation Slab geometry (35 mm drift height, D/h~10) Photomultipliers immersed in the liquid Better discrimination ‘Open plan’ target, no extraction grids High field operation (3-5 kV/cm) Precision 3D position reconstruction Low background construction DARK 2007 Copper construction, low background Xe Henrique Araújo 4 ZEPLIN III: Entrails DARK 2007 Henrique Araújo 5 Boulby Underground Laboratory We’re all in the gutter, but some of us are looking underground… Depth 1100 m (2.8 km w.e.) DARK 2007 Henrique Araújo 6 The Road to Boulby DARK 2007 After a successful test campaign on the surface at IC, the system was transported to the Boulby Underground Lab earlier this year Henrique Araújo 7 Shielding DARK 2007 Neutron shield (hydrocarbon) already in place (30 cm) Gamma shield (boxed lead) being commissioned (30 cm) Henrique Araújo 8 High-Field Operation Fields ~5 kV/cm achieved (~unity electron extraction from liquid surface) Dual-range (2x31)-channel DAQ system can cope with large S2 response S2 trigger for small signals allows low S1 (energy) threshold (~2 keVee) Analysis software: ZE3RA (ZEplin 3 Reduction & Analysis) Cs-137 event (4.5 kV/cm) DARK 2007 Henrique Araújo 9 Gas Handling System DARK 2007 Purification using SAES getters (non-recirculating): ~ppb O2 eq. required 222Rn emanation not a problem (fiducial volume far from lateral walls) 2 getter passes show improvement (no serious attempt to purify yet) Henrique Araújo 10 Gamma Calibration (57Co) First underground data (below) confirms surface tests Zero-field Co-57 calibration shows light yield of ~5 phe/keV S2 spectrum similar to S1 as expected S1 DARK 2007 S2 Henrique Araújo 11 Gamma Calibration (57Co) Low-energy feature appears as predicted: useful calibration point (Compton backscattering of 122/136 keV gammas near surface) 122 keV 136 keV escape peaks ~33 keV DARK 2007 Henrique Araújo 12 Backgrounds – g-rays (no lead castle) gammas from rock – 0-field data (almost) agrees with simulation Shielded PMT Upgrade DARK 2007 Henrique Araújo 13 Neutron calibration (surface) Surface calibration with Am-Be (a,n) source suggests better discrimination than ZEPLIN-II (99.5% for 50% NR acceptance) ZEPLIN II (u/g) ZEPLIN III (surface) 40 keV (el+in) gammas n elastic DARK 2007 Henrique Araújo 14 3D Reconstruction (Fiducialisation) Z (vertical) coordinate – sub-mm precision from drift time (S2-S1) X,Y – S2 template grid with c2 searched using 2D Sobol sequence FWHM~5 mm for 10 electrons extracted from liquid (~250 phe) 8 kg DARK 2007 Lindote et al, 2007 Henrique Araújo 6 kg 8 kg 12 kg 15 Phase II – Integration with Veto An important tool for both neutron rejection as well as diagnostics! Inner Gd-loaded hydrocarbon surrounded by plastic scintillator veto 52-channel readout using low-background photomultipliers Delayed coincidence detection of capture gammas from Gd and H n DARK 2007 g Henrique Araújo 16 Phase II – Integration with Veto DARK 2007 Plastic being manufactured by Kharkiv Institute (Ukraine) Tests at ITEP for light collection and linearity with g-rays and CR muons Alternative configurations studied: direct coupling vs wavelength shifting On track for design goal: >75% neutron efficiency, ~100 keV threshold Henrique Araújo 17 Phase II – PMT Upgrade DARK 2007 Existing PMTs limit sensitivity of first run (10 DRU in gammas) Custom design for ultra low-background tubes, pin-by-pin compatible Material radioassays nearly finished – 40 mBq/PMT (x30 improvement) Manufacture underway at ET Enterprises (UK) Henrique Araújo 18 ITEP prototype (non low-bk, surface lab): great prospects for discrimination at high-field! 7-PMT prototype being tested at ITEP on d-d neutron beam Operation at 4.5 kV/cm electric field shows good discrimination Full analysis to be reported soon 13-27 keVee DARK 2007 Henrique Araújo 19 Predicted sensitivity PHASE I: short run, present configuration ~20 evt/yr (~1 evt in 240 kg*days) ~1x10-7 pb PHASE II: Long run, after upgrade ~1 evt/yr (~1 evt in 3000 kg*days) ~6x10-9 pb 103:1 (99.9%): Achieved in other xenon experiments Araujo et al., 2006 DARK 2007 Henrique Araújo 104:1 (99.99%): Should be feasible at high field! 20 Predicted sensitivity * single-sided limit ZEPLIN-II PHASE I SHORT RUN PHASE II UPGRADED LONG RUN DARK 2007 Henrique Araújo 21 Summary ZEPLIN-III exploitation phase funded for 2.5 years System successfully re-commissioned underground at Boulby Ongoing: optimisation, calibration, Xe purification, shielding First dark run expected from Oct’07 → ~1x10-7 pb sensitivity Phase II: Veto & PMTs integrated by mid-2008 Second run expected from Oct’08 → 6x10-9 pb sensitivity Next generation: tonne-scale design study (EU proposal) ELIXIR – European Liquid Xenon Identification of Recoils Some ZEPLIN-III publications Instrument design and surface commissioning Simulated response and sensitivity H.M.Araújo et al, Astroparticle Phys. 26:140-153, 2006 Position reconstruction DARK 2007 D.Yu.Akimov et al, Astroparticle Phys. 27:46-60, 2007 A.Lindote et al, Nucl. Instr. & Meth. A 573:200-203, 2007 Henrique Araújo 22