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The XMASS 800Kg Experiment

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The XMASS 800Kg Experiment The XMASS 800kg Experiment XMASS collaboration: Kamioka Observatory, ICRR, Univ. of Tokyo： K. Abe, K. Hieda, K. Hiraide, Y. Kishimoto, K. Kobayashi, Y. Koshio, S. Moriyama, M. Nakahata, H. Ogawa, H. Sekiya, A. Shinozaki, Y. Suzuki, O. Takachio, A. Takeda, D. Umemoto, M. Yamashita, B. Yang IPMU, University of Tokyo： K. Martens, J.Liu Jing LIU Kobe University: K. Hosokawa, K. Miuchi, A. Murata, Y. Ohnishi, Y. Takeuchi Tokai University: F. Kusaba, K. Nishijima Gifu University： S. Tasaka Kavli IPMU, Univ. of Tokyo Yokohama National University： S. Nakamura, I. Murayama, K. Fujii Miyagi University of Education： Y. Fukuda NPB2012, Shenzhen STEL, Nagoya University： Y. Itow, K. Masuda, H. Uchida, H. Takiya Sejong University： Y.D. Kim Seoul National University: S. B. Kim KRISS: Y.H. Kim, M.K. Lee, K. B. Lee, J.S. Lee Xenon MASSive detector for Solar neutrino (pp/7Be) Xenon neutrino MASS detector (double beta decay) The XMASS Experiment Xenon detector for weakly interacting MASSive Particles PMT LXe inside ~100 kg LXe ~800 kg LXe multi ton scale prototype direct dark matter search Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe XMASS 800kg Jing LIU @ NPB 2012 2 Xenon MASSive detector for Solar neutrino (pp/7Be) Xenon neutrino MASS detector (double beta decay) The XMASS Experiment Xenon detector for weakly interacting MASSive Particles PMT γ, n, α,… or χ? LXe LXe inside scintillation PMT ~100 kg LXe ~800 kg LXe ~26 ton LXe prototype direct dark matter search Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe XMASS 800kg Jing LIU @ NPB 2012 3 Xenon MASSive detector for Solar neutrino (pp/7Be) Xenon neutrino MASS detector (double beta decay) The XMASS Experiment Xenon detector for weakly interacting MASSive Particles PMT LXe LXe inside PMT ~100 kg LXe ~800 kg LXe ~26 ton LXe prototype direct dark matter search Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe XMASS 800kg Jing LIU @ NPB 2012 4 Xenon MASSive detector for Solar neutrino (pp/7Be) Xenon neutrino MASS detector (double beta decay) The XMASS Experiment Xenon detector for weakly interacting MASSive Particles PMT LXe inside ~100 kg LXe ~800 kg LXe multi ton scale prototype direct dark matter search Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe XMASS 800kg Jing LIU @ NPB 2012 5 Xenon MASSive detector for Solar neutrino (pp/7Be) Xenon neutrino MASS detector (double beta decay) The XMASS Experiment Xenon detector for weakly interacting MASSive Particles PMT LXe inside ~100 kg LXe ~800 kg LXe ~26 ton LXe prototype direct dark matter search Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe XMASS 800kg Jing LIU @ NPB 2012 6 Xenon MASSive detector for Solar neutrino (pp/7Be) Xenon neutrino MASS detector (double beta decay) The XMASS Experiment Xenon detector for weakly interacting MASSive Particles PMT LXe inside ~100 kg LXe ~800 kg LXe ~26 ton LXe prototype direct dark matter search Multi-purpose LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe XMASS 800kg Jing LIU @ NPB 2012 7 Detector XMASS 800kg Jing LIU @ NPB 2012 8 Structure of XMASS 800kg detector PMT XMASS 800kg Jing LIU @ NPB 2012 9 Structure of XMASS 800kg detector ∅~0.8 meter Pentakis dodecahedron PMT photo-cathodes cover ~62% inner surface PMT ~10 PMTs in one triangle 642 PMTs in total XMASS 800kg Jing LIU @ NPB 2012 10 Reconstruct interaction point from PMT hit pattern Colored photo cathodes indicating number of p.e. (photoelectrons) recorded by PMTs Interaction point (vertex) can be reconstructed from the PMT hit pattern. XMASS 800kg Jing LIU @ NPB 2012 11 LXe self-shielding Simulation: γ into LXe ] [cm γ length of of length water LXe Attenuation Attenuation Eγ [keV] XMASS 800kg Jing LIU @ NPB 2012 12 Where is it? Kamioka underground observatory 1000 m rock overburden (2700 m water equiv.): Muon: 6.0x10-8 /cm-2/s/sr Neutron: 1.2x10-6/cm-2/s 360m above the sea Horizontal access: 15 minutes drive from office, too easy to get in! No excuse to avoid 24-hour shift XMASS 800kg Jing LIU @ NPB 2012 13 PMT mounting finished, Feb. 2010 XMASS 800kg Jing LIU @ NPB 2012 14 Water filling, Sep. 2010 XMASS 800kg Jing LIU @ NPB 2012 15 Scintillation light yield :: calibration system Z position of source is controlled by a motor on top at <1 mm accuracy Source changed here Top PMT 57Co, 241Am, can be 109Cd, 55Fe, pulled 137Cs out z y Φ∼4mm x Φ∼0.15mm for 57Co XMASS 800kg Jing LIU @ NPB 2012 16 Scintillation light yield: 14.7 ± 1.2 p.e./keV (57Co at center) ~ 2 p.e./keV in XENON100 122 keV Data MC Arbitrary unit Arbitrary 136 keV 59.3 keV (W) Number of photoelectrons XMASS 800kg Jing LIU @ NPB 2012 17 Position & energy resolution (122keV γ from 57Co) Reconstructed vertices for various source positions Position resolution (RMS): Data • 1.4 cm @ z = 0 cm • 1.0 cm @ z = ±20 cm Data MC Arbitrary unit Arbitrary z [cm] 122 keV Data MC RMS ~4% Arbitrary unit Arbitrary 136 keV 59.3 keV (W) Reconstructed energy [keV] y [cm] XMASS 800kg Jing LIU @ NPB 2012 18 Background XMASS 800kg Jing LIU @ NPB 2012 19 Ambient γ and n: pure water tank, ∅~10 meter Pure water tank (large enough for multi ton LXe) equipped with 20 inch PMTs on the wall as • active muon veto and γ • passive ambient and n shielding γ << γ from PMT, n<<10-4/d/kg 20 inch PMTs 107 neutrons, simulation y [cm] Xe Water tanks LXe sphere water X [cm] XMASS 800kg Jing LIU @ NPB 2012 20 PMT radiation: Ultra low background PMTs Calibration pipe γ n LXe copper cryostat copper LXe γ n Neutron: <1.2x10-5 dru @5-10 keV XMASS 800kg Jing LIU @ NPB 2012 21 PMT & PMT holder radiation: LXe self-shielding Simulation: γ into LXe ] dru Counts [ Energy [keV] BG/PMT [mBq] U chain 0.70 ± 0.28 fiducial volume: r<20cm, 100 kg LXe Th chain 1.51 ± 0.31 40K < 5.10 60Co 2.92 ± 0.16 XMASS 800kg Jing LIU @ NPB 2012 22 85 K. Abe et al. for XMASS collab., Kr (Qβ=687keV) : distillation Astropart. Phys. 31 (2009) 290 Kr can be boiled out from LXe 0.1 ppm → ~1ppt (~1 ton in 10 days) LXe Gas Kr intake outlet Kr LXe outlet Kr concentration: < 2.7 ppt (90% C.L.) (Goal: 2 ppt) XMASS 800kg Jing LIU @ NPB 2012 23 222Rn EXO200: 4.5 μBq/kg 214Po decays with 164 µs half life. It can be identified by time coincidence between two consecutive events: 8.2±0.5 µBq/kg 1. 214Bi β decays into 214Po 2. 214Po α decays into 210Pb 1st event (214Bi β) 2nd event (214Po α) Events p0 * exp(-t/τ) + p1, Events τ: decay constant Tail due to saturation x103 Time difference [µs] Number of photoelectrons XMASS 800kg Jing LIU @ NPB 2012 24 220Rn 216Po decays with <0.28 µBq/kg (90%C.L.) 140 ms half life 1st event 2nd event Events Events p0 * exp(-t/τ) + p1, τ: decay constant x103 Time difference [ms] Number of photoelectrons XMASS 800kg Jing LIU @ NPB 2012 25 26 Cherenkov light from PMT window Electrons from 40K in a PMT photo cathode create Cherenkov lights in PMT window --- a major background at low energy head-to-total ratio ≡ (# of hits in 1st 20ns window) / (total # of hits) is used to reject Cherenkov events Cherenkov like Scintillation like 20ns 20ns Total # of hits Total # of hits XMASS 800kg Jing LIU @ NPB 2012 26 Cherenkov cut and efficiency Low energy events from Fe55 calibration data show similar distribution Efficiency as DM MC Cherenkov events Black: data Blue: DM MC ratio ratio total total total total - - to to - - Head Head 0 40 80 120 160 200 (PE) 0 40 80 120 160 200 (PE) XMASS 800kg Jing LIU @ NPB 2012 27 Measured Spectrum after cleanup Observed spectrum PMT γ-ray MC Background ~100 times higher than expectation XMASS 800kg Jing LIU @ NPB 2012 28 Unexpected background XMASS 800kg Jing LIU @ NPB 2012 29 External or internal? Taking data when lowering down the liquid xenon level Rate [Hz] Rate Height from the center of detector [cm] XMASS 800kg Jing LIU @ NPB 2012 30 Particle identification Data taken with ~50 Gamma / electron channels of FADC allow candidates us to fit the pulse shape to get the Alpha candidates Data without scintillation decay time saturated PMTs Alpha energy spectra: Alpha from 214Po. Activity is consistent with Bi-Po coincident from 222Rn from measurement α from Pb210 from from 218Po from Event/day/keV α Event/day/keV α Red: MC Red: MC 2000 4000 6000 8000[keV] 0 2000 4000 6000 8000[keV] XMASS 800kg Jing LIU @ NPB 2012 31 Hint from a low background workshop in SNOLAB Low background from down stream (after Ra226) of U238 Chain XMASS 800kg Jing LIU @ NPB 2012 32 Re-measurement of aluminum activity Old germanium result: consistent with 0 New germanium result (pay special attention to U238 upper stream contamination): background Al sample Th234 63 keV Th234 92 keV keV XMASS 800kg Jing LIU @ NPB 2012 33 Al MC simulation Observed data PMT γ-ray MC PMT Al MC + Surface 210Pb Also consistent with alpha measurement Background above 5 keV being explained! XMASS 800kg Jing LIU @ NPB 2012 34 A background candidate below 5 keVee MC: GORE-TEX Gore-tex Modern C: 7.5% – between PMT and holder LXe absorbed inside – preventing light leak 0.3mm photon att.
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