The LUX Dark Matter Experiment Dan McKinsey Yale University Physics Department July 1, 2009 TAUP 2009, Rome
The LUX detector The LUX Detector ~ 6m diameter Water Cerenkov Shield. Mani Tripathi, June 2009 Dual phase detector - aspect ratio ~ 1.2 350 kg Dual Phase Liquid Xenon Time Projection Chamber, fully funded by NSF and DOE 2 kV/cm drift field in liquid, 5 kV/cm for extraction, and 10 kV/cm in gas phase. 122 PMTs (Hamamatsu R8778) in two arrays 3D imaging via TPC eliminates surface events, defines 100 kg fiducial mass
The LUX-350 Collaboration Brown University: Richard Gaitskell, Simon Fiorucci, Carlos Hernandez Faham, Jeremy Chapman, David Malling, Luiz de Viveiros Case Western Reserve University: Dan Akerib, Adam Bradley, Ken Clark, Mike Dragowsky, Patrick Phelps, Thomas Shutt Harvard University: Masahiro Morii Lawrence Berkeley National Laboratory: Kevin Lesko, Yuen-Dat Chan, Brian Fujikawa Lawrence Livermore National Laboratory: Adam Bernstein, Steven Dazeley, Peter Sorensen, Kareem Kazkaz Moscow Engineering Physics Institute : Alexander Bolozdynya Texas A&M: Rachel Mannino, Tyana Stiegler, Robert Webb, James White UC Davis: Tim Classen, Britt Holbrook, Richard Lander, Jeremy Mock, Robert Svoboda, Melinda Sweany, John Thomson, Mani Tripathi, Nick Walsh, Michael Woods University of Maryland: Carter Hall, Douglas Leonard University of Rochester: Eryk Druszkiewicz, Udo Schroeder, Wojtek Skulski, Jan Toke, Frank Wolfs University of South Dakota: Dongming Mei Yale University: Susie Bedikian, Sidney Cahn,Alessandro Curioni, Louis Kastens, Alexey Lyashenko, Daniel McKinsey, James Nikkel
Gamma Ray Backgrounds LXe is a good self-shielding material, with a scattering length of 6 cm at 1 MeV. X-rays in the energy window of interest (5-25 keVr, or 1.3-8 keVee), are absorbed in less than a mm. Background is then log 10 (DRU) dominated by higher energy gamma rays -60 -50 -40 -30 -20 -10 0 that penetrate the fiducial volume, scatter, and escape. By defining a fiducial volume, gamma ray backgrounds drop enormously, scaling as exp[-L/Ls], where L is the size of the [cm] active volume, and Ls is the gamma ray scattering length. In LUX, the dominant gamma ray background Goal comes from the PMTs. Simulations assume high end of measurements: U/Th/K/Co = 18/17/30/8 (mBq/PMT) -20 -10 0 10 20 [cm] This gives 8.3E-4 events/keVee/kg/day in the 100 kg fiducial mass. above: Monte Carlo of (dominant) PMT activity in LUX After discrimination cut, assuming a conservative efficiency of 99.4%, this gives 4.6E-6 events/keVee/kg/day, or 1 background event in 30,000 kg days.
Other (subdominant) gamma ray backgrounds Cryostat: � We are building a low-background titanium cryostat, with material � � � we have measured to have superior background characteristics. � � � Background rates have been measured to be < a few mBq/kg in U, Th, K. PTFE: � � Bulk PTFE can be purchased extremely radiopure; EXO measures � � � U/Th/K < 0.004/<0.001/0.053 mBq/kg [1] in PTFE pellets. � � � Heusser measures < 0.16/< 0.16/0.7 mBq/kg [2] in PTFE samples, � � � which would give only 6E-8 events/keVee/kg/day after discrimination, � � � or only 1.2% of the maximum expected background level from the PMTs. [1] F. Leport et al., (EXO Collaboration) arXiv:physics/0611183 [2] G. Heusser, M. Laubensteinb, H. Nedera, Low-level germanium gamma-ray spectrometry at the m Bq/kg level and future.developments towards higher sensitivity
Titanium Sample activated in air transport Grade CP1 generally good. Not a problem for construction CP2 had high counts in 2 samples. Materials. 86 days half-life # of Total Counted Sample Type Grade Dim. piece weight At U Th K-40 Sc-46 mBq mBq mBq/ mBq/ ppb /kg ppb /kg ppm kg kg Ti1 3/8" plate CP1 2.5" x 6" 4 1.87 kg Oroville <0.2 < 2.5 <0.4 < 1.6 <0.2 < 6.2 4.8 Ti2 3/16" plate CP2 4" x 6" 20 7.55 kg SOLO 10.4 130 17.5 70 -- Ti3 0.358" plate CP2 ~ 1.3" x 6" 8 1.55 kg SOLO 85 35 Ti6 3/16" plate CP1 4" x 6" 20 7.98 kg Oroville <0.03 <0.4 < 0.2 <0.8 <0.05 <1.6 23 Ti7 1" plate CP1 2" x 6" 8 7.201 kg Oroville <0.02 <0.05 <0.04 2.5 Ti8 0.063" sheet CP1 4" x 6" 40 4.399 Oroville <0.1 <0.4 <0.3 6 Mani Tripathi, June 2009
Fast neutron backgrounds from bulk materials in LUX PMTs are the dominant source of fast neutron background: � fission neutrons negligible (1.5% of goal) � ( a ,n) reactions on light elements dominate log 10 (DRU) -60 -50 -40 -30 -20 -10 0 Assuming U/Th/K/Co = 18/17/30/8 mBq/PMT, => 1.5 neutrons/yr/PMT If the U/Th activity is confined entirely Goal to the PMT glass stem and other glasses [cm] and insulators, this comes to 5 n/PMT/year. After a multiple scattering cut, 5 n/PMT/yr results in a nuclear recoil background well below the goal of 5E-6 events/keVr/kg/day. ( a ,n) reactions in PTFE are subdominant -20 -10 0 10 20 (8/year) assuming Heusser U/Th measurements. [cm] (Even lower assuming EXO numbers). multiple scatter veto for neutrons!
Water Shield 2.5 meters of instrumented water shielding Gamma rays from rock contribute < 2% of total electronic recoil background. Fast neutrons from rock are moderated and captured extremely efficiently => negligible. Muon-induced neutrons in rock: < 0.01 events/year in detector.
Internal Backgrounds Kr-85: �� Beta decay, 687 keV endpoint. � � � Normally at ppm in commercial Xe, though can purchase at 5 ppb � � � LUX requirement is 5 parts per trillion � � � Achieved by charcoal column separation (< 2 ppt demonstrated at Case) 14 C, T, U,Th:�Removed efficiently by getter Radon:� � Pb-210 daughter removed by getter. Surface daughter backgrounds � � � removed by fiducial cut. Pb-214 makes a "naked" beta, which sets � � � the LUX requirement = 16 mBq, compared to XENON10 measured rate � � � of 1.6 mBq. pp n 's: � � Elastic scattering of neutrinos from electrons � � � gives background of 6E-8 events/keVee/kg/day, after discrimination. Xe-136:�� Double beta decay background of 1.5E-8 events/keVee/kg/day, assuming t 1/2 = 0.8 x 10 22 years (current lower limit). � � � Chemically active cosmogenic activation products removed by getter. Xe-131m, Xe-129m decay away with ~ 10 days half-lives.
LUX Internals Assembly Internal Assembly
Surface Facility at Homestake LUX integration planned for October 2009 Mani Tripathi, June 2009
The Davis Cavern Mani Tripathi, June 2009
Dewatering Milestone De-watering Milestone Mani Tripathi, June 2009
Sanford Lab Mani Tripathi, June 2009
Photomultiplier R&D New 3" PMTs -- Hamamatsu R11065 New 3" PMTs -- Hamamatsu R11065 Single photoelectron resolution obtained Single p.e. resolution obtained from first With x2 collection area of R8778. from first articles of Hamamatsu articles of Hamamatsu With 2x collection area of R8778 Background target for U/Th of 1/1 mBq. Background target for U/Th of 1/1 mBq
Experimental setup BC501-A organic polyethylene scintillator shield water shield θ water 2.8 MeV neutron generator shield n liquid xenon detector water shield cryostat 2 m n M Xe E R = E n ( m n + M Xe ) 2 (1 − cos θ ) Energies: 4 - 66 keVr
Liquid xenon cell SS flanges Teflon PMT Grid Shaping ring Grid PMT Teflon
Results • N o significant dependence on field. • The Leff decreases with decreasing energy. • Escape electrons seem to be an important contributor to Leff
L eff model L eff = q ncl × q el × q esc • nuclear quenching (Lindhard factor), energy goes into q ncl heat. • electronic quenching. Bi-excitonic collisions q el Xe ∗ + Xe ∗ → Xe + Xe + + e − 1 q el = 1 + k dE dx • Escape electrons α + 1 − β N ex + N i − N esc q esc = = ex + N 122 N 122 − N 122 α + 1 − β 122 i esc
L eff model
XENON10 limit -41 10 ] 2 Leff = 0.19 [cm Leff WIMP-nucleon Leff model -42 10 s -43 10 -44 10 3 2 10 10 10 Mass [GeV]
LXe scintillation data from Kr-83m dissolved into LXe 4000 9.4 keV line 32.1 keV line 3500 3000 2500 Counts 2000 1500 1000 500 0 0 10 20 30 40 50 Energy (keV) L. Kastens et al, arXiv:0905.1766
The LZ3 and LZ20 Collaboration Merger with ZEPLIN-III collaboration. Plus, some new US groups joining in. New members: A. Murphy, C. Ghag, E. Barnes, A. Hollingsworth, P. Scovell Edinburgh University, United Kingdom T. Sumner, H. Araujo, J. Quenby, M. Horn, K. Lyons, R. Walker, A. Currie, B. Edwards Imperial College London, United Kingdom N. Smith, G. Kalmus, P. Smith, P. Majewski, B. Edwards STFC Rutherford Appleton Lab, United Kingdom I. Lopes, V. Chepel, J. Pinto da Cunha, F. Neves, A. Lindote, V. Solovov, C. Silva LIP - Coimbra, Portugal D. Akimov, V. Belov, A. Burenkov, A. Kobyakin, A. Kolvalenko, V. Stekanhov ITEP - Moscow, Russia J. Siegrist Lawrence Berkeley National Laboratory H. Nelson Mani Tripathi, June 2009 University of California, Santa Barbara
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