1. XMASS ・ Physics motivation of XMASS ・ R&D using １００ｋｇ detector ・ Next step: 800 kg detector Kamioka observatory, ICRR, Univ. of Tokyo M. Nakahata for XMASS collaboration LowNu 2003, May 20

2. XMASS collaboration ICRR, Kamioka observatory Y. Suzuki, M. Nakahata, Y. Itow, M. Shiozawa, Y. Takeuchi, S. Moriyama, T. Namba, M. Miura, Y. Koshio, Y. Fukuda, S. Fukuda, Y. Ashie, A. Minamino, R. Nambu ICRR, RCNN T. Kajita, K. Kaneyuki, A. Okada, M. Ishitsuka Saga Univ. T. Tsukamoto*, H. Ohsumi, Y. Iimori, Niigata Univ. K. Miyano, K. Ito Tokai Univ. K. Nishijima, T. Hashimoto, Y. Nakajima Gifu Univ. S. Tasaka, Waseda Univ. M. Yamashita, S. Suzuki, K. Kawasaki, J. Kikuchi, T. Doke, TIT Y. Watanabe, K. Ishino, Yokohama National Univ. S. Nakamura, T.Fukuda, J.Hosaka, Seoul National Univ. Soo-Bong Kim, In-Seok Kang, INR-Kiev Y. Zdesenko, O. Ponkratenko, UCI H. Sobel, M. Smy, M. Vagins, P.Cravens

3. XMASS Scintillation detection by PMTs Dark M: Xenon detector for weakly interacting MASSive Particles  : Xenon neutrino MASS detector Solar : Xenon MASsive detector for Solar neutrinos 10 ton class Liq. Xe

4. Dark matter search E th = 20keV; 30 events/day E th = 5keV; 2000 events/day Expected signal of dark matter

5. Sensitivity of dark matter search 10 ton Liq. Xe Spin independent Spectrum only Seasonal variation 10 -6 pb

6. Double deta decay e-e- e-e- e-e- e-e- Majorana mass of neutrinos It is quite important to investigate whether the origin of the neutrino mass is Majorana type See-saw mechanism is correct ? 136 Xe(natural abundance 8.87%), double beta nuclei 2 2  0 2 

7.  background against    1/2 theory = 8 x 10 21 y

8. Resolution by photon statistics 42000potons/MeV x 2.48MeV x 30%Q.E.  31000 p.e.  0.6 % rms 10ton natural, 5 year 2600 keV24002200 Signal region  1/2 theory = 8 x 10 21 y  background against 

9. Sensitivity of  10ton natural, 5 year Both ＋－ sigmaOnly ＋ sigma m Majorana sensitivity: 0.01 – 0.02 eV 0 1 2 3 4 5 6 Resolution at 2.5 MeV 0 1 2 3 4 5 6 Resolution at 2.5 MeV 0 2  lifetime sensitivity: several ×10 27 years

10. 10t Liq. Xe 5yr (Cf. SK-I, ~13 events/day) Solar neutrinos pp: 10ev/day 7 Be: 5ev/day (> 50keV, SSM) Expected spectrum of solar neutrinos +e + e ~1% stat. error measurement of pp flux Quite High statistics

11. KamLAND and pp solar neutrinos will determine precise oscillation parameters Precise measurement of oscillation parameters Expected region using pp neutrinos (90 % C.L.) :  10 ton Liq. Xe  e scattering  5 years data  Statistical error and SSM prediction error(1%) Accuracy of mixing angle: sin 2 2  = 0.77  0.03(stat.+SSM)

12. Fiducial volume  rays from 238 U chain Background rejection 6 orders of magnitude reduction for gamma rays below 500keV  (U/Th/K/ Co/Cs/…) neutron   rays from 85 Kr, 42,39 Ar, U/Th Self- shield No long life isotope H 2 O, paraffin Distillation Self-shield of external gamma rays

13. Development of low background PMTs New type 2-inch PMT(developed with Hamamatsu Co.) Q.E. ~ 30% @ 175nm Collection efficiency ~ 90% Quartz window Metal tube (low background) Parts were selected using HPGe Single p.e. distribution

14. Bq/PMTequiv. 10 -3 10 -2 10 -4 New 2-inch PMT Metal for PMT bulb capacitor (film) Chip resistor Quartz (PMT) Cables PCB(PTFE) Usual PMTs Resistor capacitor Glass Epoxy PCB Selection of parts for the new type PMT Low BG PMT 238 U 1.8x10 -2 Bq 232 Th 6.9x10 -3 Bq 40 K 1.4x10 -1 Bq 60 Co 5.5x10 -3 Bq Bulb for PMT

15. 100kg detector purpose MgF 2 window 54 2-inch PMTs Liq. Xe (30cm) 3 =30L=100Kg (1) Low background effort with new PMTs (2) Vertex/energy reconstruction (3) Demonstration of self shielding (4) e/  separation (5) R&D of purification system (6) Possible detection of 2  decay

16. 100kg detector MgF 2 window

17. Supur-insulation 100kg detector: cooling Vacuum chamber (this time stainless steel) ＧＭ refrigerator （ 100W @170K ） Cooling and liq.Xe supply, recovery test in February using only 6 PMTs

18. 100kg detector cooling PMT mouting One PMT per plane Temperature monitor PMT PMT HV ON 8 days detector Supply liq. Xe

19. 100kg detector: Gas line Getter Original Xe Recovery Xe Emergency tank(9m 3 ) Vacuum chamber Emergency recovery line rupture disk Emergency valve Liq. N 2

20. Level Monitor 4 days Supply, recovery port 100kg detector: Liq.Xe supply and recovery Supply (2 days) recovery (0.85 days) L ML M HM H

21. We found that the PMTs work even at -90 ℃ 60 Co source on the detector ６ PMT test data Limit vertex position using (  of 6PMTs)/ (average of 6PMTs) 100kg detector: Test data Observed spectrum Simulation The observed spectrum is consistent with the simulation All  <1 All  <1

22. Shield of １００ｋｇ detector OFHC vacuum chamber, OFHC shield(5cm thick), Lead shield (15cm), Boron (10cm), Polyethylene(15 cm), Radon shield (EVOH sheet) Installation in May, June

23. Installation of Shield Door part is already installed. (May, 2003) Moved into clean room in Kamioka mine. Lead shield (15cm thick) OFHC shield(5cm) Full shield will be installed by the end of June, 2003.

24. Vertex/energy reconstruction (MC simulation) Make PMT hitmaps Maximize the likelihood: F(x, y, z; i): acceptance of scintillation light for i-th PMT from (x, y, z). GEANT based simulation gives F(xg, yg, zg, i) on a grid with 2.5cm spacing. F(x, y, z, i) is linearly interpolated by using F(xg, yg, zg, i). L: likelihood  : F(x, y, z) x (total p.e./total acceptance) n: observed number of p.e. Typical 1MeV alpha ray Red: true vertex Green: reconstructed

25. 0.88cm (68%) 1MeV300keV 1.40cm (68%)  =62keV (gauss fit)  =33keV (gauss fit) Vertex/energy resolution (MC) Uniformly distributed source in the fv (20cm cube)

26. 2 phase detector (S.Suzuki et al.) 100kg detector: background level （ expectation by simulation ） 0.1ppm 85 Kr 85 Kr~0.01ppm /kg/day/keV Gamma rays from 238 U, 232 Th, 40 K outside the chamber. 238 U: 2.4Bq, 232 Th: 1.4Bq, 40 K: 0.86Bq, determined by a HPGe. Events were reconstructed by the reconstruction tool. 100kg all volume estimated 100kg 20cm cube, ½ PMT cut Efficiency: 60% @ 100keV 2.4% @ 10keV

27. Purification by distillation R/D of purification system Simple distillation system for Kr Theoretically, 1/1000 reduction Process power: 0.6kg/hr Buffer tank Compressor Heat exchanger Purified Xe Xe input Refrigerators Purification tower ~3m 13 stages Heater Getter GXe flow LXe Kr Tower Boiling point: Xe=165K, Kr=120K, Ar=87K Test this purification system in 2003.

28. Next step: 800kg detector 800kg liquid Xe 642 2-in PMTs 77% photo-coverage ~5 p.e./keV 80cm diameter

29. /kg/day/keV pp 7 Be , 8x10 21 yr External background in 800kg detector Dominant contribution is from PMT Assuming further 1/10 reduction of PMTs BG external  ray (60cm , 346kg) external  ray (40cm , 100kg ) Dark matter (10 -8 pb, 50GeV, 100 GeV)

30. Dark matter sensitivity 800kg detector Spin independent Spin dependent spectrum Seasonal variation

31. Requirements in background for 800kg detector External Environmental  rays: 15cm Pb + 5cm OFHC or water shield Fast neutron recoil at 10 keV : < 1/10000 of environment Thermal neutron absorption: < 1/400 of environment Internal 85 Kr: < 0.35 ppt of Kr (by distillation) 42 Ar, 39 Ar: < 0.1 ppm Ar (by distillation) Radon: < 1microBq/m 3 U/Th in Liq.Xe: < 10 -14 g/g ~2m of water shield

32. Conclusion Large volume liquid Xe detector is quite effective for dark matter, double beta decay and low energy solar neutrinos. R&D is going on using 100 kg detector. Using the 100 kg detector, we will test vertex and energy reconstruction, self-shielding, and purification of liquid Xe. The next step is 800 kg detector. The 800 kg detector is able to search for dark matter with 4 orders of magnitude better sensitivity than current experiments. Final goal of the XMASS project is 10 ton class detector for multi-purpose astro-particle physics.