Background Characterization and Rejection in the LZ Detector. David Malling Brown University IDM 2012 July 25, 2012

Transcription

1 Background Characterization and Rejection in the LZ Detector David Malling Brown University IDM 2012 July 25, 2012

2 LZ Construction 2

3 Background Sources Ti cryostats 1500 kg <0.2 mbq/kg 238 U / 232 Th (arxiv: ) 222 Rn daughters decay on PTFE creating (α,n) 85 Kr, 222 Rn, cosmogenic isotopes decay in active region PMT arrays 500x 3 R11410 <1 mbq 238 U / 232 Th (arxiv: ) 3

4 Background Sources Muon spallation in rock produces high-e neutrons Gamma flux from rock radioactivity Muon capture in water produces high-e neutrons 4

5 Neutrino Backgrounds 10-5 DRUee (before ER rejection) Limiting ER background ~3 / 1000 days (after ER rejection) GeV WIMP cts/kev ee /tonne/1000 days GeV WIMP (1e 48 cm 2 ) (1e 47 cm 2 ) 7 Be solar 2 (10 22 yrs) pp solar DRU = cts/kev/kg/day 50% NR acceptance 99.5% ER discrimination Electron Recoil Energy (kev ) ee 5

6 Background Rejection - Fiducialization ER Activity (PMTs) NR Activity vs. Fiducial Mass LUX (350 kg) LZ (7000 kg) DRU from PMT background after applying Xe Skin and LS kg 50 cm 150 cm Depth [cm] T 10T 20T Radius [cm] 8 6

7 Background Rejection - Scintillator Veto 34T Gd-loaded LAB volume surrounds active region in 4π >75 cm thickness Warm -- minimizes requirements for outer vessel construction Acrylic OV allows use of water tank PMTs for readout 100 kev threshold assumed for all BG studies 7

8 Background Rejection - Xe Skin Veto Xe gap between internals and IC intentionally enlarged and instrumented 1.4T (4 cm thickness) 100 kev ee threshold used in BG studies Expectation of kev ee threshold from LC studies 8

9 Background Rejection - Water Shield Using 8m x 6m LUX water shield in Sanford Underground Lab 250T shielding (1.3 m vertical, 2.1 m lateral) 5-10% tank area covered by PMTs instrumenting both scintillator and water 9

10 LZ Modeling Geant4-based LUXSim package (arxiv: ) Incorporates NEST for modeling scintillation yield (arxiv: ) Useful for particle sims, light propagation sims, full detector response 10

11 Veto Efficiency Efficiency shown for rejecting a gamma which has already singlescattered in the fiducial region and escaped the active region MeV gamma scattering on edge of ee3.5t fiducial region XS and LS are highly complimentary LS picks up what XS misses Very small degree of overlap Not a fair reflection of the raw detection efficiency of each veto volume Fraction of all SS events NOT vetoed Xe skin Scintillator Combined Xe skin veto threshold [kev ] ee 11

12 Veto Efficiency Individual veto efficiencies better characterized by looking at fraction of gammas rejected which actually enter the veto volume kev Gamma Veto Efficiency for Xe Skin and Scintillator PMT Holder Instrumentation Gammas Incident at Veto Boundary 1.2 MeV gamma scattering on edge of 3.5T fiducial region Fraction of events NOT vetoed Xe skin Scintillator Veto threshold [kev ] ee 12

13 Xe Skin Veto Threshold 13 kevee (3 phe) 33 kevee (6 phe) All BG studies use conservative 100 kevee threshold 13

14 Internal Gamma Rejection mbq U/Th contamination NEW GEOMETRY mbq U/Th contamination dru [cts/kevee/kg/day] Basic cuts Xe skin only LS only Xe skin or LS p p solar activity x1/10 p p solar activity x1/100 p p solar activity Basic cuts: single-scatter in active region; high-e cut at 100 kevee (flat spectrum); fiducial cut Require >100 kev ee deposition in given veto region Fiducial mass [kg] 14

15 Internal Neutron Rejection Basic cuts Xe skin only LS only Xe skin or LS mbq U/Th contamination mbq U/Th contamination Basic cuts: single-scatter in active region; energy cut 5-25 kevnr; EM veto; fiducial cut dru nr (cts/kg/kev nr /day) bkgd event/fiducial mass/1000 days 1/10 below neutron bkgd x25 factor from increased U/Th radioactivity (x5) and from U/Th concentrated in worst PMT material (x5) (Al) Fiducial Mass (kg) Require >100 kev ee deposition in given veto region Do NOT count NR depositions in XS or LS; rely entirely on detection of gammas from neutron capture products 15

16 External BG Rejection External gamma rate (cavern rock radioactivity): x1/400 below PMT gamma rate μ-induced n from cavern rock: x1/25 below PMT n rate μ-induced n from water shield Before veto: ~x2 higher than μ-induced n from rock After veto: negligible 16

17 Intrinsic Contamination Most concerned with naked or semi-naked beta emitters >200 isotopes identified from Xe activation search Naked β β ~107 /kev/kg/day before ER rej, dominated by 137 Xe 137 Xe 137 Cs 85 Kr expected to be controlled by charcoal purification (in use for LUX) - goal <0.1 ppt γ β Semi-naked β 222 Rn must be controlled <0.02 mbq in fiducial 137 Xe 137 Cs 17

18 222 Rn Daughters Plate-out of daughters on PTFE leads to (α,n) from long-lived 210 Pb High raw alpha rate at edges & finite position resolution events reconstructed in fiducial region Careful controls for 222 Rn exposure required Deposition rate and position reconstruction error will be measured in LUX 18

19 Raw Scintillator Rate Component Scintillator Rate [Hz] PMTs 1.5 IV 10 OV 0.4 Readout PMTs Cavern rock gammas 2 Scintillator Composition 99.9% Coincidence Time Window [μs] Dead Time 100% LAB % LAB + 0.3% Gd % LAB + 5% B % Limited by impurities in LAB 19

20 Light Collection Light collection influenced primarily by absorption length PTFE >95% reflective from LUX surface data; no significant change between 95%-99% Lower bound of 10 m from LUX surface data 3 phe threshold in range 4-7 kevnr (dependent on absorption length), with conservative QE / photon yield assumptions 3 phe 20

21 Summary LZ optimized for discovery, with multiple complimentary background rejection technologies Very large fiducial mass enabled with virtually zero background expectation from detector materials or external sources Several intrinsic background sources identified, allowing early mitigation strategies to be developed 21

22 Backup 22

23 23

24 Coherent Neutrino Scattering ~0.5 / 1000 days cts/kev r /tonne/1000 days B solar 100 GeV WIMP E res. conv. (1e 47 cm 2 ) B solar atmospheric 50% NR acceptance 100 GeV WIMP (1e 48 cm 2 ) DSNB Nuclear Recoil Energy (kev ) r L. Strigari, arxiv: