1. Status of the XMASS experiment S. Moriyama for the XMASS collaboration Kamioka observatory, Institute for Cosmic Ray Research, Univ. of Tokyo, July 27, 2010, IDM2010

2. XMASS project A large scale, multi- purpose, low background detector for astro-particle physics.  Scalability and LowBG As a first phase, an 800kg detector for dark matter search is under construction. ● XMASS ◎ Xenon MASSive detector for Solar neutrino (pp/ 7 Be) ◎ Xenon neutrino MASS detector (double beta decay) ◎ Xenon detector for Weakly Interacting MASSive Particles (DM search) Y. Suzuki et al., hep-ph/0008296 10ton FV (24ton) 2.5m Solar, 0 , DM 100kg FV (800kg) 0.8m, DM

3. Structure of the 800kg detector Single phase liquid Xenon detector – 857kg of liquid xenon, 100kg in the fiducial volume – 630 hex +12 round PMTs with 28-39% Q.E. – photocathode > 62% inner surface – Pentakis dodecahedron – 3D event reconstruction – 5keVee threshold with 4.4pe/keVee Developed with Hamamatsu

4. Background reduction 1:  /n from det. parts  /n from detector parts can be reduced by: 1.Reduction of RI contamination I.PMTs: ~1/10 of prev. PMT achieved II.OFHC: brought in the mine <1month after electrorefining (Mitsubishi Material Co.) III.Material selection: >250 parts were measured by HPGe, ICPMS, Rn det. 2.Self shielding  < 10 -4 /keV/d/kg keV Counts/day/kg/keV  into LXe sphere MC simulation  n n contribution < 2x10 -5 /keV/d/kg BG/PMT in mBq with base parts U chain0.70 +/- 0.28 Th chain1.5 +/- 0.31 40K< 5.1 60Co2.9 +/- 0.16

5. Liq. Xe water X [cm] 10 7 n’s n Background reduction 2:  and n from rock  and n from rock ( ,n) will be reduced by a pure water tank   <<  from PMT, n<<10 -4 /d/kg - 11m high and 10m diameter, 72 PMTs (20’’). - First example for dark matter experiments. - Active veto for CR , passive for  and n. - Applicable for future extensions. - Reduction of n by  ’s in rock will be studied. 3120(m) Att. vs. thickness PMT BG level  2m needed  n  y [cm] Reduction of gamma rays >4m 2m2m 2m2m

6. Background reduction 3: internal radioactiv. Kr LXe intake Kr (Q  =687keV) and Rn can be reduced by: 1.Distillation: Kr has lower boiling point. I.5 orders of magnitude reduction (0.1ppm  1ppt ) can be done with 4.7kg/hr: 10days for 1ton before filling into the detector. K. Abe et al. for XMASS collab., Astropart. Phys. 31 (2009) 290 2.Filtering: by gas and liquid. Under study.  LXe outlet GKr outlet Kr Rn LXe ~a few liter-LXe/m Charcoal Filter GXe <30 liter-GXe/m

7. Expected sensitivity   p >2x10 -45 cm 2 for 50-100GeV WIMP, 90%C.L. 1yr exposure, 100kg FV, BG: 1x10 -4 /keV/d/kg Scintillation efficiency: 0.2 XENON10 CDMSII XMASS 1yr Black:signal+BG Red:BG Expected energy spectrum 1 year exposure   p =10 -44 cm 2 50GeV WIMP Spin Independent Case

8. Experimental hall, water shield, and gas handling syst. Clean booth at the entrance LXe tank GXe buffer tank 10m3 x 2, <10bar GXe compressor Distillation Tower Electronics hut Calibration system water tank Kamioka Observatory

9. Detector Construction

10. Construction of the PMT holder: Nov. 2009

11. PMT installation: 311 for each half, 40 for boundary

12. Cabling: 642 pairs of a coax and HV cable, each 13m length All the work is under Rn free air and clean environment.

13. Joining two halves

14. Filler attachment. Total 2.8ton: end of Feb. 2010

15. Manufacturing detector vessel A challenge: Manufacturing a large flange with soft OFHC copper. Inside: Electropolished Due to insufficient strength of its neck part, it needed to be reinforced by adding ribs. It took four months. On the 29 th of July, it will be delivered.

16. Electronics Analog Timing Modules (ATM) used for Super- Kamiokande record charge and timing of PMTs. FADCs record waveform of PMTs with 500MHz.

17. Water purification system ~5m3/hour <2mBq-Rn/m3 Rn free air generator ~20m3/hour ~a few mBq-Rn/m3

18. Schedule August: Installation of the vessel and PMT holders at the center of the water tank. Cabling and connection of liquid/gas system will be done. Sep.-Oct.: Distillation of xenon to reduce Kr (0.1ppm  1ppt). Evacuation of the detector, water filling test, and liquid xenon filling. Oct.-: gas and liquid circulation starts. Reducing contamination in LXe. Data taking will start.

19. Summary The XMASS project aims to observe pp solar neutrinos, neutrinoless double beta decay, and dark matter signals. The 800kg detector is the first phase of the project. It is expected to have low background of 1x10 -4 keV -1 d -1 kg -1 in the 100kg FV and sensitivity for SI down to 2x10 -45 cm 2 with one year operation. All the parts of the 800kg detector are ready and their assembly work will be finished by the end of August. Data taking for WIMP search will be started in this year.