XMASS

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

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

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

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

6. Double deta decay 136Xe(natural abundance 8.87%), double beta nuclei n e- 2n2b 0n2b e- e- n n 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 ?

7. 2n2b background against 0n2b 2n2b t1/2 theory= 8 x 1021 y

8. 2n2b background against 0n2b Resolution by photon statistics 42000potons/MeV x 2.48MeV x 30%Q.E.  31000 p.e.  0.6 % rms 10ton natural, 5 year t1/2 theory= 8 x 1021 y Signal region 2200 2400 2600 keV

9. Sensitivity of 0n2b 10ton natural, 5 year Both ＋－　sigma Only ＋sigma 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Resolution at 2.5 MeV Resolution at 2.5 MeV 0n2blifetime sensitivity: several ×1027 years mMajoranasensitivity: 0.01 – 0.02 eV

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

11. Precise measurement of oscillation parameters • Expected region using pp neutrinos (90 % C.L.) : • 10 ton Liq. Xe • ne scattering • 5 years data • Statistical error and SSM prediction error(1%) • Accuracy of mixing angle: sin22q = 0.77  0.03(stat.+SSM) KamLAND and pp solar neutrinos will determine precise oscillation parameters

12. Background rejection • (U/Th/K/ Co/Cs/…) m Fiducial volume Self-shield No long life isotope • Self-shield of external gamma rays • rays from 238U chain Distillation H2O, paraffin 6 orders of magnitude reduction for gamma rays below 500keV a,b,g rays from 85Kr, 42,39Ar, U/Th neutron

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

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

15. 100kg detector (1) Low background effort with new PMTs (2) Vertex/energy reconstruction (3) Demonstration of self shielding (4) e/g separation (5) R&D of purification system (6) Possible detection of 2nbb decay purpose 54 2-inch PMTs MgF2 window Liq. Xe (30cm)3=30L=100Kg

16. 100kg detector MgF2 window

17. 100kg detector: cooling Supur-insulation 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 Temperature monitor PMT mouting Supply liq. Xe PMT PMT HV ON detector One PMT per plane 8 days

19. 100kg detector: Gas line Getter Emergency tank(9m3) rupture disk Liq. N2 Original Xe Vacuum chamber Recovery Xe Emergency valve Emergency recovery line

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

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

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

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

24. Vertex/energy reconstruction (MC simulation) 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). • Make PMT hitmaps • Maximize the likelihood: L: likelihood m: F(x, y, z) x (total p.e./total acceptance) n: observed number of p.e. Red: true vertex Green: reconstructed Typical 1MeV alpha ray

25. Vertex/energy resolution (MC) Uniformly distributed source in the fv (20cm cube) 1MeV 300keV s=62keV (gauss fit) s=33keV (gauss fit) 0.88cm (68%) 1.40cm (68%)

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

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

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

29. External background in 800kg detector external g ray (60cmf, 346kg) • Dominant contribution is from PMT • Assuming further 1/10 reduction of PMTs BG external g ray (40cmf, 100kg ) /kg/day/keV 2n2b, 8x1021 yr 7Be pp Dark matter (10-8 pb, 50GeV, 100 GeV)

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

31. Requirements in backgroundfor 800kg detector External Environmental g 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 ~2m of water shield Internal 85Kr: < 0.35 ppt of Kr (by distillation) 42Ar, 39Ar: < 0.1 ppm Ar (by distillation) Radon: < 1microBq/m3 U/Th in Liq.Xe: < 10-14 g/g

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.