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

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

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

4. Concept of background reduction Self-shielding PMTs Single phase liquid Xe Volume for shielding Fiducial volume 23ton all volume (d=240cm) 20cm wall cut 30cm wall cut (10ton FV) BG normalized by mass 1MeV02MeV3MeV Large self-shield effect External  ray from U/Th-chain Low Background region near the center of the fiducial volume pp, 7 Be solar

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

6.    PMT n n L) ! )exp( Log()  L: likelihood  : F(x,y,z,i) x total p.e.  F(x,y,z,i) n: observed number ofp.e. Vertex and energy reconstruction QADC, FADC, and hit timing information are available for analysis F(x,y,z,i): acceptance for i-th PMT (MC) VUV photon characteristics: L emit =42ph/keV  abs =100cm  scat =30cm 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. Reconstructed here FADCHit timing QADC Reconstruction is performed by PMT charge pattern (not timing)

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

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

9. Background data 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) REAL DATA MC simulation All volume 20cm FV 10cm FV (3kg) All volume 20cm FV 10cm FV (3kg) 10 -2 /kg/day/keV 3.9days livetime Event rate (/kg/day/keV) 0 2000 1000 10 -2 1 3000 keV 10 -1 0 2000 1000 3000 keV 10 -2 1 10 -1

10. Distillation to reduce krypton A distillation system was made and tested. System specification: Boiling point (@1 atm) Xe165K Kr120K ~3m 13 stage of Operation: 2 atm(abs.) Processed speed: 0.6 kg / hour Design factor: 1/1000 Kr / 1 pass Lower temp. Higher temp. ~1% 2cm  ~99% Purified Xe: (3.3±1.1) x 10 -12 Kr (by a high sensitivity measurement method) Off gas Xe: 330±100 x 10 -9 Kr (measured) Original Xe: ~3 x 10 -9 Kr 178±2K in tower Process speed: 0.6kg Xe/hour Collection efficiency: > 99% Kr concentration after process: < 1/1000

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

12. 800kg(100kg FV) detector for DM search Low energy threshold  immerse PMTs into LXe Ext.  BG: from PMT’s  Self-shield effect (demonstrated) Internal BG: Kr, radon  Removed by distillation, filters… “Full” photo-sensitive,“Spherical” geometry detector 80cm diameter 812 hexagonal PMTs 70% photo-coverage ~5p.e./keVee  ray BG from PMTs: 60cm, 346kg 40cm, 100kg Achieved pp & 7 Be solar Expected dark matter signal (assuming 10 -42 cm 2, Q.F.=0.2, M  =50,100GeV)

13. More detailed geometrical design A tentative design (not final one) 12 pentagons / pentakisdodecahedron This geometry has been coded in a Geant 4 based simulator 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 40 K (1.4±0.2)x10 -1 Bq

14. 10keV R=5cm 800kg reconstruction and BG study Extensive study to optimize the detector ongoing 5 keV 10 keV 50 keV 100 keV 500 keV1 MeV Fiducial volume 201030040 8 10 6 4 2 0 12 Distance from the center [cm]  (reconstructed) [cm] 10keV R=35cm Photon tracking: absorption, scattering, and reflection are taken into account. Vertex resolution 5keV(~25p.e.) threshold

15.  Estimated PMT ＢＧ Activity of PMT – 238 U chain 1.8x10 -3 Bq/PMT – 232 Th chain 6.9x10 -4 Bq/PMT – 60 Co 5.5x10 -3 Bq/PMT – 40 K 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. All volume 40cm Fiducial Volume 35cm Fiducial Volume 25cm Fiducial Volume (/kg/day/keV)

16. Sensitivity of the 800kg detector 10 -45 cm 2 (10 -9 pb) for SI and 10 -39 cm 2 (10 -3 pb) for SD 0.5ton ・ year exposure (100kg, 5yr) 3  discovery Plots exept for XMASS http://dmtools.berkeley.edu Gaitskell & Mandic 10 3 XENON10 DAMA SuperCDMS Phase A XMASS800kg WARP140kg J Ellis et al. benchmark WIMP mass(GeV) 10 10 2 10 3 10 -42 10 -44 10 -46 10 -40 Cross section to nucleon (cm 2 ) Spin Independent(SI) 10 -38 10 -40 10 -34 10 -36 10 -30 10 -32 Spin Dependent(SD) SI CDMS-II

17. Sep/12/2007 Shield environmental gammas and neutrons by pure water (available from SK system). Active shield by water Cherenkov detector for muon induced background. Water shield for environmental background ~10m diameter, ~10m high pure water tank. 20-inch PMTs for veto counter. 800kg detector

18.  Simulation of environmental  and fast neutron wa water Liq. Xe Generation point of  and neutron MC geometry Configuration of the estimation 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 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.

19.   background PMT BG level  attenuation by water shield 0100200300 Thickness of shield [cm] 10 10 4 Detected/generated*surface [cm 2 ] 10 3 10 2 1 10 -1 10 -2 More than 200cm water is needed to reduce the BG to the PMT BG level Initial energy spectrum from the rock Deposit energy spectrum (200cm)

20. water: 200cm, energy: 10MeV Liq. Xe water < 2 x 10 -4 counts/day/kg  Fast neutron background Fast n flux @Kamioka mine: (1.15+0.12) x10 -5 /cm 2 /sec Assuming all neutron’s energies are 10 MeV very conservatively 200cm of water is enough to reduce the fast neutron Generat:10 7 MC events, no event in Liq.Xe volume X [cm] Z [cm] Reach points before thermalized -

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

22. Sep/12/2007taup2007 Excavation status As of end of September 2007. The halls will be ready by summer 2008. Hall for XMASS

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

24. PMT immerse test: pulse height distribution (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 Resolution: 2.4 % @122 keV pe1: p.e. of PMT1 pe2: p.e. of PMT2 selected Light yield: 17.1 p.e. / keV 122keV peak 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 cm 2 for spin independent –Budget funded in this year and construction started. –Plan to construct detector within 2-3 years.