Background Modeling and Materials Screening for the LUX and LZ Detectors. David Malling Brown University LUX Collaboration AARM Meeting February 2011

Transcription

1 Background Modeling and Materials Screening for the LUX and LZ Detectors David Malling Brown University LUX Collaboration AARM Meeting February

2 Summary LUX screening program limits background contributions from all major internal components to <1 WIMP-like BG evt / 300 livedays Emphasis on discovery mode for WIMPs. Small number of BG evts competing Monte Carlo modeling performed for background contributions from all detector components LZ background modeling includes a broad range of topics, from muon spectra through shield and veto design 2

3 The LUX Experiment 2 Thermosyphon 4 Time Projection Chamber Anode and Electron Extraction Grids 350 kg Liquid Xenon Cathode Grid 5 Photomultiplier Tubes Feedthroughs 1 Water Shield 3 Xenon Recirculation and Heat Exchanger Titanium Cryostats 6 Internal Structure PMT Cu Holders 3

4 Sanford Surface Lab Full deployment of ALL detector, external hardware, and electronics that will subsequently be deployed underground 3m water shield at surface reduces ambient gamma BG - improves calibration data quality Precise physical replica of underground lab space Final deployment in Davis Lab (4850 ft. level) 4

5 Background Rejection Xe self-shields very effectively attenuation length (1 MeV γ) ~ 6 cm Exp fall in event rate toward center (fiducialization) Narrow energy window Multiple scatters rejected DRUee = cts/kevee/kg/day 5

6 ER vs. NR XENON10 NR/ER Bands Scintillation/ionization ratio differs for ER, NR Discrimination measured by XENON10: >99.5% rejection of ER evts 50% NR acceptance in WIMP search window 6

7 Internal Backgrounds Construction materials chosen for low radioactivity (Ti, Cu, PTFE) Majority of materials heavily shielded by Cu PMT radioactivity gives dominant background Internal backgrounds dominate over external (from cavern rock) 7

8 External Backgrounds Davis Lab 4.3 km.w.e. rock overburden 300 tonne water shield instrumented as Cerenkov veto Primary external background: muon-induced neutrons in water shield, cavern rock After muon veto and analysis cuts: Summing above sources ~0.01 NR evts / 300 livedays / 100 kg Flux May 14 20:52:50 LdV Flux Attenuation in Water (Normalized to Number of Incoming Particles) Rock Gammas Rock Neutrons mu Neutrons Z [mm] 8

9 Background Modeling Monte Carlo studies required to understand backgrounds from all internals Standardized geometry used -- LUXSim (Kareem Kazkaz s talk) MC studies set required counting goals for various materials 9

10 Modeling with LUXSim Studies performed for all major LUX internals to determine Xe activity per mbq for major isotopes ( 238 U, 232 Th, 60 Co, 40 K...) Analysis cuts applied to Monte Carlo data DRUee / 60 Co mbq (activity summed for entire PMT arrays top and bottom) Scaling factor DRU/mBq found DRUee == cts/kevee/kg/day 10

11 Modeling with LUXSim Generic background modeling performed to account for all low-mass components Each component assigned a BG contribution expectation given its location Region boundaries set by detector geometry, physics 11

12 LUX Material Screening Unit Screening Result U238 Th232 Co60 K40 Sc46 PMTs mbq/pmt 9.5± ± ±0.1 66±2 Ti mbq/kg <0.18 < ±0.3* Cu mbq/kg 2.1±0.19* PTFE mbq/kg <3 <1 HDPE mbq/kg <0.5 <0.35 Stainless steel** mbq/kg 19±1 **Type 304 stainless steel used in electric field grids *Cosmogenic equilibrium at 1 mile above SL; decays below ground 12

13 LUX Material Screening Screening of all major components ensures <1 WIMP-like event in 300 livedays Sums includes applied analysis cuts Energy window Single-scatter Fiducial ER/NR rejection WIMP-Like Events (300 Livedays) ER NR PMTs Cryostats <0.02 <0.002 Grid wires PTFE panels <0.01 <0.001 <0.05 <0.009 HDPE <0.01 <0.002 Cu <0.03 < Kr < Total <0.59 <

14 The LZ Detectors Scaling LUX to the multi-tonne level LZS: tonnes LZD: tonnes LZ20 baseline design Increase in sensitivity by over two orders of magnitude LUX 14

15 Internal Backgrounds for Scaled Detectors Self-shielding properties allow fiducial volume >2/3 of total Xe mass used Large instant discovery zone Assuming 5 n/pmt/yr (x6 higher than R8778s) 15

16 LZ Background Modeling Active area of research Muon energy spectrum <1 GeV at Davis cavern level and lower μ-induced n spectrum in rock, water Attenuation by 12 m water shield Scintillator veto effectiveness Internal background scaling Cosmogenic activation of Xe, construction materials Fundamental neutrino backgrounds Large community effort focused on R&D 16

17 Next-Generation PMTs PMT background contributions dominate total background Require PMT with larger surface area, lower radioactivity per unit area Comparable or better sensitivity as R8778 to 178 nm photons cts/kev r /tonne/1000 days B solar atmospheric 3" PMT (,n) 1/1 mbq U/Th 8 B solar E res. conv. 50% NR acceptance 100 GeV WIMP (1e 48 cm 2 ) 100 GeV WIMP (1e 47 cm 2 ) DSNB 2" PMT (,n) 9/3 mbq U/Th Nuclear Recoil Energy (kev r ) 17

18 3 R11410 MOD Twice the cathode area of the R8778 >35% QE for Xe scintillation light 238 U <0.4 mbq/pmt 90%CL 232 Th <0.3 mbq/pmt 90% CL x1/27 reduction in U238; x1/9 reduction in Th232 (below R8778) Publication in review 18

19 Conclusions LUX screening program projects background expectation ~x1/2 below the LUX proposal goal Detailed Monte Carlo estimates are generated for all major internal items; other items modeled generically by location in detector LZ background modeling includes a broad range of topics, from muon spectra through shield and veto design; ultra-low background PMTs will ensure subdominance of construction material BG 19

20 References L. de Viveiros, Optimization of Signal versus Background in Liquid Xe Detectors Used for Dark Matter Direct Detection Experiments. Ph.D. thesis, Brown University, D.-M. Mei and A. Hime, Muon-Induced Background Study for Underground Laboratories. arxiv:astro-ph/ v2. M. Laubenstein and G. Heusser, Cosmogenic radionuclides in metals as indicator for sea level exposure history. Applied Radiation and Isotopes 67 (2009) LUX and ZEPLIN collaborations, LZS: The LZ Liquid Xenon Dark Matter Search at Sanford Lab. LUX and ZEPLIN collaborations, LZ20 Development: the LUX-ZEPLIN 20 Tonne Dark Matter Experiment Technical Development Plan for DUSEL. D. Malling et al, An Ultra-Low Background PMT for Liquid Xenon Detectors. In preparation. 20

21 Additional Slides 21

22 Dark Matter Sensitivity Cross section [cm 2 ] (normalised to nucleon) WIMP Mass [GeV/c 2 ] 22 Gaitskell,Mandic,Filippini CDMS LUX LZS LZD (10 evts) LZD (1 evt)

23 SOLO 0.6 kg HPGe detector Soudan Lab (2.0 km.w.e.) Workhorse for LUX internals counting DRU Energy [kev]