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Status XMASS experiment

Dark2007, September 28, 2007. Status XMASS experiment. M. Nakahata for XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo. Solar neutrino. What’s XMASS. Multi purpose low-background and low-energy threshold experiment with liq. Xe.

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Status XMASS experiment

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  1. Dark2007, September 28, 2007 Status XMASS experiment M. Nakahata for XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo

  2. Solar neutrino What’s XMASS Multi purpose low-background and low-energy threshold experiment with liq. Xe • Xenon detector for Weakly Interacting MASSive Particles (DM search) • Xenon MASSive detector for solar neutrino (pp/7Be) • Xenon neutrino MASS detector (bb decay) Dark matter Double beta

  3. Double beta decay option Funded from 2007 Strategy of the XMASS project ~1m ~2.5m ~30cm 800kg detector (FV 100kg) Dark matter search ~20 ton detector (FV 10ton) Solar neutrinos Dark matter search Prototype detector (FV 3kg) R&D Confirmation of feasibility of the ~1ton detector So far

  4. Large self-shield effect Concept of background reductionSelf-shielding External g ray from U/Th-chain Single phase liquid Xe 23ton all volume (d=240cm) 20cm wall cut 30cm wall cut (10ton FV) Volume for shielding Fiducial volume BG normalized by mass PMTs 0 1MeV 2MeV 3MeV pp, 7Be solar n Low Background region near the center of the fiducial volume

  5. 54 2-inch low BG PMTs Liq. Xe (31cm)3 MgF2 window 3kg FV prototype detector In the Kamioka Mine (near the Super-K) OFHC cubic chamber Hamamatsu R8778 g ray/neutron shield 16% photo- coverage • Demonstration of reconstruction, • self shielding effect, and low background properties.

  6. - m m n exp( ) å = Log( L ) Log( ) n ! PMT Vertex and energy reconstruction Reconstruction is performed by PMT charge pattern (not timing) Reconstructed here Calculate PMT acceptances from various vertices by Monte Carlo. Vtx.: compare acceptance map F(x,y,z,i) Ene.: calc. from obs. p.e. & total accept. QADC L: likelihood F(x,y,z,i) m : x total p.e. S F(x,y,z,i) n: observed number of p.e . FADC Hit timing F(x,y,z,i): acceptance for i-th PMT (MC) VUV photon characteristics: Lemit=42ph/keV tabs=100cm tscat=30cm QADC, FADC, and hit timing information are available for analysis

  7. + + + B A C Source run(g ray injection from collimators) Collimator B Collimator C Collimator A DATA • Well reproduced. MC

  8. Source run(g ray injection from collimators) 60Co: 1.17&1.33MeV DATA MC 137Cs: 662keV DATA MC 10-1 10-1 No energy cut, only saturation cut. BG subtracted r=2.884g/cc • Self shield works as expected. • Photo electron yield ~ 0.8p.e./keV for all volume Arbitrary Unit 10-2 10-2 ~1/10 10-3 10-3 ~1/200 PMT Saturation region 10-4 10-4 10-5 10-5 -15 Reconstructed Z +15cm -15 Reconstructed Z +15cm Gamma rays Z= -15 Z= +15

  9. Background data 3.9days livetime MC simulation REAL DATA All volume 1 All volume 1 20cm FV • MC uses U/Th/K activity from PMTs, etc (meas. by HPGe). • Self shield effect can be clearly seen. • Very low background (10-2 /kg/day/keV@50-300 keV) 20cm FV 10cm FV (3kg) 10cm FV (3kg) Event rate (/kg/day/keV) 10-1 10-1 10-2 10-2 10-2/kg/day/keV 0 2000 3000 1000 2000 3000 1000 0 keV keV

  10. Distillation to reduce krypton • Process speed: 0.6kg Xe/hour • Collection efficiency: > 99% • Kr concentration after process: < 1/1000 A distillation system was made and tested. System specification: 178±2K in tower Original Xe: ~3 x 10-9 Kr Off gas Xe: 330±100 x 10-9 Kr (measured) Lower temp. ~1% ~3m Operation: 2 atm(abs.) Processed speed: 0.6 kg / hour Design factor: 1/1000 Kr / 1 pass 13 stage of 2cmf Purified Xe: (3.3±1.1) x 10-12 Kr(by a high sensitivity measurement method) Higher temp. ~99%

  11. Radioactive contamination in LXe(internal background) Achieved with the prototype detector Requirement for 800kg detector 238U: < 1x10-14 g/g (~1decay/100kg/d) 232Th: < 2x10-14 g/g (~1decay/100kg/d) 85Kr: < 1ppt 238U:(9±6) x10-14 g/g Further reduction by filter 232Th: < 23 x10-14 g/g Upper limit, use filter 85Kr: 3.3±1.1 ppt by a prototype distillation tower U, Th, Kr near to the goal. Within reach.

  12. 800kg(100kg FV) detector for DM search • Low energy threshold  immerse PMTs into LXe • Ext. g BG: from PMT’s  Self-shield effect (demonstrated) • Internal BG: Kr, radon  Removed by distillation, filters… “Full” photo-sensitive,“Spherical” geometry detector g ray BG from PMTs: 60cm, 346kg 40cm, 100kg Achieved pp & 7Be solar n 80cm diameter Expected dark matter signal (assuming 10-42 cm2, Q.F.=0.2, Mc=50,100GeV) 812 hexagonal PMTs 70% photo-coverage~5p.e./keVee

  13. More detailed geometrical design • A tentative design (not final one) 12 pentagons / pentakisdodecahedron Hexagonal PMT ~50mm diameter Aiming for 1/10 lower BG than R8778 R8778: U (1.8±0.2)x10-2 Bq Th (6.9±1.3)x10-3 Bq 40K (1.4±0.2)x10-1 Bq This geometry has been coded in a Geant 4 based simulator

  14. 12 10 Fiducial volume 8 5 keV 6 s (reconstructed) [cm] 10 keV 4 50 keV 2 10keV R=35cm 100 keV 1 MeV 500 keV 0 0 10 20 30 40 Distance from the center [cm] 800kg reconstruction and BG study Vertex resolution Extensive study to optimize the detector ongoing 10keV R=5cm Photon tracking: absorption, scattering, and reflection are taken into account. 5keV(~25p.e.) threshold

  15. All volume Estimated PMT ＢＧ 40cm Fiducial Volume 35cm Fiducial Volume 25cm Fiducial Volume • Activity of PMT • 238U chain 1.8x10-3 Bq/PMT • 232Th chain 6.9x10-4 Bq/PMT • 60Co 5.5x10-3 Bq/PMT • 40K 1.4x10-2 Bq/PMT • Below 300 keV number of events in the 25cm fiducial volume decreases rapidly. • Below 300keV, <10-4 /kg/day/keV BG level. • Below 100 keV, <10-5 /kg/day/keV BG level. (/kg/day/keV) (/kg/day/keV)

  16. Sensitivity of the 800kg detector 10-45cm2 (10-9pb) for SI and 10-39cm2 (10-3pb) for SD 0.5ton・year exposure (100kg, 5yr) 3s discovery Spin Independent(SI) Spin Dependent(SD) 10-40 10-30 SI DAMA 10-32 10-42 CDMS-II 10-34 XENON10 Cross section to nucleon (cm2) XMASS800kg 10-36 10-44 10-38 WARP140kg SuperCDMS Phase A J Ellis et al. benchmark 10-46 10-40 10 102 103 103 WIMP mass(GeV) Plots exept for XMASS http://dmtools.berkeley.edu Gaitskell & Mandic

  17. Water shield for environmental background ~10m diameter, ~10m high pure water tank. • Shield environmental gammas and neutrons by pure water (available from SK system). • Active shield by water Cherenkov detector for muon induced background. 20-inch PMTs for veto counter. 800kg detector

  18. Simulation of environmental g and fast neutron • External gamma and neutron are another large background sources. • To reduce these background, use thick water shield. • Thickness of shield is estimated by a simulation. Generation point of g and neutron wa Configuration of the estimation Liq. Xe • Put 80cm diameter liquid Xe ball • Assume copper vessel (2cm thickness) • Assume several size of water shield 50, 100, 150, and 200cm thickness for liquid Xe water MC geometry

  19. Initial energy spectrum from the rock • g background g attenuation by water shield 104 103 Deposit energy spectrum (200cm) 102 Detected/generated*surface [cm2] 10 1 PMT BG level 10-1 More than 200cm water is needed to reduce the BG to the PMT BG level 10-2 0 100 200 300 Thickness of shield [cm]

  20. Fast neutron background Reach points before thermalized • Fast n flux @Kamioka mine: • (1.15+0.12) x10-5 /cm2/sec - water: 200cm, energy: 10MeV • Assuming all neutron’s energies are 10 MeV very conservatively Generat:107 MC events, no event in Liq.Xe volume water Z [cm] Liq. Xe < 2 x 10-4 counts/day/kg 200cm of water is enough to reduce the fast neutron X [cm]

  21. New experimental halls at Kamioka KamLAND 15m 15m 21m SK 170m The halls will be ready by summer 2008. New Halls

  22. Excavation status As of end of September 2007. The halls will be ready by summer 2008. Hall for XMASS taup2007

  23. PMT1 PMT2 PTFE filler Test PMT immersed in Liquid Xe Cold cap 100 mm • Two PMTs immersed in liquid Xe • Co-57 (122 keV, 136 keV) source between the PMTs SUS304 chamber 40 mm PMT 1 Co-57 Liquid Xe 15 mm 200 mm Blue LED PTFE light guide PMT 2

  24. PMT immerse test: pulse height distribution pe1: p.e. of PMT1 pe2: p.e. of PMT2 selected 122keV peak (122keV) const. (p0) : 74.8 mean (p1) : 2088.7 sigma (p2) : 49.9 (136keV) const. (p3) : 6.1 mean (p4) : 2267.2 sigma (p5) : 73.9 Light yield: 17.1 p.e. / keV Resolution: 2.4 % @122 keV 136 keV peak

  25. Summary • R&D by the prototype detector • Demonstrated vertex reconstruction method and self-shield • Developed background reduction methods • 800 kg detector • Sensitivity of 10-45 cm2 for spin independent • Budget funded in this year and construction started. • Plan to construct detector within 2-3 years.

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