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Recent Status of XMASS experiment

Recent Status of XMASS experiment. S. Moriyama Institute for Cosmic Ray Research, University of Tokyo For the XMASS collaboration June 3 rd , 2011 @ SPCS2011 Shanghai, China. The XMASS collaborations. Kamioka Observatory, ICRR, Univ. of Tokyo ：

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Recent Status of XMASS experiment

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  1. Recent Status of XMASS experiment S. Moriyama Institute for Cosmic Ray Research, University of Tokyo For the XMASS collaboration June 3rd, 2011 @ SPCS2011 Shanghai, China

  2. The XMASS collaborations • Kamioka Observatory, ICRR, Univ. of Tokyo： • Y. Suzuki, M. Nakahata, S. Moriyama, M. Yamashita, Y. Kishimoto, • Y. Koshio, A. Takeda, K. Abe, H. Sekiya, H. Ogawa, K. Kobayashi, • K. Hiraide, A. Shinozaki, S. Hirano, D. Umemoto, O. Takachio, K. Hieda • IPMU, University of Tokyo： K. Martens, J.Liu • Kobe University:Y. Takeuchi, K. Otsuka, K. Hosokawa, A. Murata • Tokai University:K. Nishijima, D. Motoki, F. Kusaba • Gifu University： S. Tasaka • Yokohama National University： S. Nakamura, I. Murayama, K. Fujii • Miyagi University of Education： Y. Fukuda • STEL, Nagoya University： Y. Itow, K. Masuda, H. Uchida, Y. Nishitani, H. Takiya • Sejong University： Y.D. Kim • KRISS: Y.H. Kim, M.K. Lee, K. B. Lee, J.S. Lee 41 collaborators, 10 institutes

  3. Kamioka Observatory • 1000m under a mountain = 2700m water equiv. • 360m above the sea • Horizontal access • Super-K for n physics and other experiments in deep underground • KamLAND (Tohoku U.) By courtesy of Dr. Miyoki

  4. XMASS experiment ●XMASS ◎ Xenon MASSive detector for Solar neutrino (pp/7Be) ◎ Xenon neutrino MASS detector (double beta decay) ◎ Xenon detector for Weakly Interacting MASSive Particles (DM search) 100kg FV (800kg) 0.8m, DM First phase • It was proposed that Liquid xenon was a good candidate to satisfy scalability and low background. • As the first phase, an 800kg detector for a dark matter search was constructed. 10ton FV (24ton) 2.5m Solar n, 0nbb, DM in future Y. Suzuki, hep-ph/0008296

  5. Structure of the 800kg detector • Single phase liquid Xenon (-100oC, ~0.065MPa) scintillator • 835kg of liquid xenon, 100kg in the fiducial volume • 630 hex +12 round PMTs with 28-39% Q.E. are in LXe. • photocathode > 62% inner surface • Pentakis dodecahedron • Interaction position reconstruction • 5keVelectron equiv. (~25keVnuclear recoil) thre. 1.2m diameter Developed with Hamamatsu

  6. Self shielding effect Compton effect • Photo electric effect starts to dominate @500keV: strong self shielding effect is expected for low energy radiations. • Large photon yield (>~NaI(Tl)) provides a good event reconstruction 10cm water LXe 1cm Photo Electric Effect Attenuation length (cm) ~O(500keV) ~O(50keV) E (keV)

  7. Background reduction 1: g/n from det. parts • /n from detector parts can be reduced by: • Reduction of radioactive contaminations • PMTs: ~1/10 of prev. PMT achieved • OFHC: brought into the mine <1month after electrorefining. • Material selection with a HPGe det. • Self shielding  < 10-4 /keV/day/kg • c.f. XENON100 FV before PID: ~10-2/keV/day/kg g, n g into LXe sphere MC simulation Counts/keV/day/kg n<1.2x10-5/keV/d/kg @5-10keV V.Tomasello, et al., NIMA595(2008)431 keV

  8. Background reduction 2: g and n from rock • and n from rock will be reduced by a • pure water tank  g << g from PMT, n<<10-4/d/kg • - 11m high, 10m diameter, and 72 PMTs (20’’). • - First example for dark matter experiments. • - Active veto for CR m, passive for g and n. • - Applicable for future extensions. g, n 107 n’s Att. vs. thickness y [cm] 2m g n Reduction of gamma rays water 2m Liq. Xe PMT BG level  2m needed >4m 0 1 2 3 (m) X [cm]

  9. Background reduction 3: internal radioactiv. • Kr (Qb=687keV) and Rn can be reduced by: • Distillation: Kr has lower boiling point. • 5 orders of magnitude reduction was expected to have been done (0.1ppm1ppt) with 4.7kg/hr: 10days before filling into the detector. • K. Abe et al. for XMASS collab., Astropart. Phys. 31 (2009) 290 • Filtering: by gas and liquid. Under study. a, b, g Charcoal Filter GKr outlet LXe intake GXe <30 liter-GXe/m Kr LXe outlet Rn Kr LXe ~a few liter-LXe/m

  10. Expected sensitivity Spin Independent scp>2x10-45 cm2 for 50-100GeV WIMP, 90%C.L. 1yr exposure, 100kg FV, BG: 1x10-4 /keV/d/kg Scintillation efficiency: 0.2 CDMSII XENON100 Expected energy spectrum XMASS 1yr 1 year exposure scp=10-44 cm2 50GeV WIMP Black:signal+BG Red:BG

  11. Detector Construction

  12. Assembly of PMT holder and installation of PMTs

  13. Joining two halves

  14. クリックしてタイトルを入力 • クリックしてテキストを入力 As of Sep. 2010 P-01

  15. Evaluation of detector performance and internal background

  16. Demonstration of the detector performance Stepping Motor • Calibration system • Introduction of radioactive sources into the detector. • <1mm accuracy along the Z axis. • Thin wire source for some low energy g rays to avoid shadowing effect. • 57Co, 241Am, 109Cd, 55Fe, 137Cs.. Linear Motion Feed- through ~5m Gate valve Source rod with a dummy source 0.15mmf for 57Co source Top photo tube 4mmf

  17. Photoelectron dist. at the center of the detector Total photo electron distribution • 57Co source at the center of the detector gives a typical response of the detector. • High p.e. yield 15.1+/-1.2p.e./keV was obtained. • The photo electron yield distribution was reproduced by a simulation well. real data simulation Reconstructed energy distribution 122keV ~4% rms real data simulation 136keV 59.3keV of W

  18. Performance of the vertex reconstruction Real Data Simulation • Reconstructed vertices for various positioning of the source overlayid. • Position resolution was as expected by a simulation: 1.4cm RMS (0cm) 1cm RMS (+-20cm) for 122keV g rays

  19. Evaluation of internal background sources • External background (gammas and neutrons) can be effectively reduced by the water tank and outer part of LXe. Radioactive contaminations (internal background) need to be reduced by other means. • Radon 222, Radon 220 and Krypton 85 are our concern because they give low energy BG (~flat at signal region). U chain Rn222 Th chain Rn220

  20. Radon 222 in LXe • 214Po decays with 164ms half life. • Identified by a short time coincidence. • Gain of 321 PMTs (1/2 of all PMTs) are reduced to have larger dynamic range. • 8.2+/-0.5mBq in the inner volume 1st event (214Bi b) 2nd event (214Po a) Fitting with an expected decay curve Tail due to saturation 100 500 1000 Time difference (ms)

  21. Radon 220 in LXe • 216Po decays with 0.14sec half life • Identified by two a’s with short coinc. • Result was consistent with chance coincidence of daughters of 222Rn. • Upper limit <0.28mBq (90%C.L.) Consistent with chance coincidence and null hypothesis of Radon 220. 0 500 1000 Time difference (ms)

  22. Kr in LXe • Gas chrom. and APIMS (atmospheric pressure ionization mass spectroscopy) started to work. More sensitive measurement (<1ppt) using a cold trap is under prep. 101 Calibration sample Kr=2.7+/-0.6ppb Sensitivity ~10ppt achieved 100 10-1 Ion current (nA) Background (He) 10-2 Cal x 1/270 (10ppt equiv) 10-3 -1 0 1 Time (min) -2 -3 • Data analysis to look for delayed coincidence events is under study.

  23. Summary on the internal BG • Our target BG level in FV at signal region is in total 2x10-4/kg/day/keV. Half of the budget is for the PMT gamma rays and another half is for the internal BG. • Target for each internal BG corresponds to 1/10 of total: • 222Rn: target 1.0mBq, observed 8.2+-0.5mBq • 220Rn: target 0.43mBq, observed <0.28mBq (90%C.L.) • Kr: target 2ppt, will be evaluated soon • Low 222Rn, 220Rn, and 85Kr levels are realized in our detector and close to our target numbers. • 222Rn would be reduced by a cooled charcoal column in gas phase. 85Kr can be reduced by a second distillation process if necessary.

  24. Conclusion • The XMASS 800kg detector aims to detect dark matter with the sensitivity 2x10-45cm2 (spin independent case). • It utilizes a single phase of LXe target. • Construction of the 800kg detector finished last winter. • Commissioning runs are on going to confirm the detector performance and low background properties. • Energy resolution and vertex resolution were as expected. ~1cm position resolution and ~4% energy resolution for 122keV g. • Radon background are close to the target values and Kr contamination will be evaluated soon.

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