1. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Recent status of the XMASS project Physics goals at XMASS Overview of XMASS Current status of R&D Summary Yasuo Takeuchi (Kamioka Observatory, ICRR, Univ. of Tokyo) for XMASS Collaboration XMASS = a multi purpose detector to search rare phenomena under an ultra low background environment by using ultra pure liquid xenon

2. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Physics goals at XMASS Xenon MASSive Detector for Solar Neutrinos (pp/ 7 Be) Xenon Detector for Weakly Interacting MASSive Particles (Dark Matter Search) Xenon Neutrino MASS Detector (Double Beta Decay) XMASS FV 50 ton year (90%CL) G. Gratta @Neutrino2004 http://www.sns.ias.edu/~jnb/ pp +-1% Measure pp via + e + e 2  life time should be measured Isotope separation would be needed

3. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Expected signal Physics goals at XMASS Xenon MASSive Detector for Solar Neutrinos (pp/ 7 Be) Xenon Detector for Weakly Interacting MASSive Particles (Dark Matter Search) Xenon Neutrino MASS Detector (Double Beta Decay) Direct search via nuclear elastic scattering XMASS FV 0.5ton year E th =5keV, 3  discovery E th = 5keV ~200 events/day/ton E th = 20keV ~3 events/day/ton Spin Independent

4. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Xenon MASSive Detector for Solar Neutrinos (pp/ 7 Be) Xenon Detector for Weakly Interacting MASSive Particles (Dark Matter Search) Xenon Neutrino MASS Detector (Double Beta Decay) Physics goals at XMASS Search for 0  (2  ) decay of 136 Xe (na 8.87%) High purity and enriched Xe can be used. Energy region is different from solar / DM. PMTs should not be placed near the detector. Need another design of the detector! (low priority, at moment…) 136 Xe 136 Ba + e - + e - Q-Value: 2.48 MeV 2   1/2 theory = 8 x 10 21 y

5. Overview of XMASS Strategy Key ideas (self shielding, distillation)

6. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Strategy of the XMASS project Dedicated detector for Double beta decay search ~1 ton 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 NOW Confirmation of feasibilities of the ~1 ton detector Analysis techniques Self shielding performance Low background properties Purification techniques

7. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Self shielding PMTs Liquid Xe Volume for shielding Fiducial volume Quite effective for the events below ~500 keV (pp & DM) Not effective for double beta decay experiment Reconstruct the vertex and energy based on PMTs information (light pattern) 30cm 10 5 reduction for < ~500keV

8. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Distillation to remove Kr Very effective to eliminate internal impurities ( 85 Kr, etc.) We have processed 100kg Xe in March ‘04 Boiling point (@1 atm) Xe165K Kr120K ~3m 13 stage of Operation: 2 atm Processing speed: 0.6 kg / hour Design factor: 1/1000 Kr / 1 pass Lower temp. Higher temp. ~1% 2cm  ~99% Purified Xe: < 5 ppt Kr (measured) Off gas Xe: 330±100 ppb Kr (measured) Raw Xe: ~3 ppb Kr

9. Current status of R&D Prototype detector Results from test runs Self shielding Internal background External background

10. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 XMASS prototype detector 30 litter liquid Xenon (~100kg) Oxygen free copper: (31cm) 3 54 of low-BG 2-inch PMT  Photo coverage ~16% MgF 2 window 0.6 p.e. / keV Polyethylene (15cm) Boric acid (5cm) Lead (15cm) EVOH sheets (30mm) OFC (5cm) Rn free air (~3mBq/m 3 ) 1.0m 1.9m n  Rn

11. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Test runs with the prototype detector December 2003 run  First test run  ~6 days (~2day normal runs for BG estimation)  Test analysis tools  Confirmation of the self shielding performance  Measurements of the internal and external BGs August 2004 run  August 3 ~ 11, 2004, 9days (~6day normal runs)  Used purified xenon (by distillation)  Longer baking time of the system  New electronics (TDC, etc.)  Re-measurements of the internal and external BGs NEW Photon yield x ~1.7

12. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Data MC Data MC  Z= +15Z= -15 Remove events (PMT saturation) 60 Co (1173 & 1333keV) 137 Cs (662keV) Self shielding performance Reconstructed vertex position of collimated source runs Dec.03 run MC reproduces data very well We have demonstrated the self-shield actually works

13. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Internal BG source: 222 Rn 2 separate runs to check 222 Rn decay (  1/2 =3.8day) 4 th Aug. 0.8day 238 U=(72+-11)x10 -14 g/g 10 th Aug. 1.0day 238 U=(33+-7)x10 -14 g/g Consistent with expected 222 Rn decay ((30+-5)x10 -14 ) Aug.04 run Preliminary  T:  1/2 = 141+-51  sec  T < 1ms (1.8days) 3.5MeV 67ev 214 Bi 214 Po 210 Pb  1/2 =164  sec  (E max =3.3MeV)  (7.7MeV) 238 U 222 Rn LL

14. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Internal background sources Current results  238 U: = (33+-7)x10 -14 g/g  232 Th: < 63x10 -14 g/g  Kr: < 5ppt Goal (~1ton) 1x10 -14 g/g Factor <~30 (under further study) Almost achieved by the distillation process 2x10 -14 g/g 1 ppt Factor ~30, but may decay out further Preliminary NEW

15. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 [count/keV/day/kg] (=dru) External background sources Background level was estimated from known sources MC estimation for full volume  rays from outside shield PMTs origin  238 U series  40 K  232 Th series 210 Pb in the lead shield Energy (keV)

16. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Measured background level Self shielding works Good agreement with expectation (< factor 2) Measurements All volume 20cm FV 10cm FV Simulation All volume 20cm FV 10cm FV Aug.04 run Preliminary Geometrical effect only for prototype detector

17. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Alpha vs Gamma separation Aug.04 run Preliminary Alpha-gamma separation by using FADC wave form would be possible (under further investigation) Pulse width (ns) Charge Alpha-like Gamma-like FADC data

18. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Summary XMASS is aiming to search rare phenomena under an ultra low background environment by using ultra pure liquid xenon. 2 nd test run with the prototype detector was just finished.  The data were taken using distilled xenon with low level krypton (Kr/Xe < 5ppt).  Some part of remaining 222 Rn in liquid xenon looks contaminated in outside of the chamber.  The background level is consistent with expectation within factor about 2. The next step (~1ton scale) would be feasible, and a dark matter search around 10 -44 cm 2 level would be possible.

19. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Supplement

20. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Why liquid xenon scintillator High photon yield  Low threshold, good energy resolution, … Can be directory read by PMT Large atomic number  Radiation length ~2.4cm  Self shielding against external backgrounds  Compact (R=1.22m for 23 tons) Easy to liquefy  Liquid N 2 can be used Various purification method  Distillation, circulation during experiment, …  Effective reduction against internal backgrounds No long life radioactive isotopes 136 Xe is a  decay candidate Scintillation light~42photon/keV Scintillation light wave length 175nm Scintillation light width ~40nsec Atomic number54 Atomic weight131.29 amu Density3.0 g/cm 3 Melting (boiling) point 161.4K (165.1K) Chemical seriesNoble gases

21. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 U 1.5±0.3x10 -3 Bq Th 3.2±4.6x10 -4 Bq 40 K 1.7±2.9x10 -3 Bq Development of the low BG PMT Aiming for another order of magnitudes improvement Hexagonal PMT to accomplish 70-80% PMT coverage Q.E. ~ 30% @ 175nm; Collection eff. ~ 90% Quartz window & Metal tube (Low BG) Selection of the parts （ measured by HPGe ）  Low BG PMT base ~1/10 of the usual ones

22. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Energy/vertex reconstruction 137 Cs 662keV Gamma ray ( from a collimator) F(x,y,z,i): hitmap made by MC VUV photon characteristics: L emit =42ph/keV  abs =34cm  scat =30cm Reconstructed here L: likelihood  : F(x,y,z,i)/  F(x,y,z,j) x(total p.e.) n: observed number of p.e.. Real data j Using photoelectron map made by MC (not timing, but charge information) Vertex:MC hitmap Energy:Hitmap scale Dec.03 run

23. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Event reconstruction ABC +++ Collimated gamma rays for three different positions Hole AHole BHole C Real data MC Reconstruction works well 137 Cs Dec.03 run

24. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Stability of the energy scale Aug.04 run Preliminary No degrading of the energy scale Stable within +-0.5% 60 Co calibration data Peak position from simple gaussian fit +-0.5%

25. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Observed light yield Observed number of photons for source runs are increased by factor ~1.7  xenon purification  longer baking time  removal of unnecessary material in the chamber Aug.04 run Preliminary

26. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 85 Kr: 687keV beta analysis The event rate around 200~400 keV in the Normal runs could be explained by 2~3ppb of Kr. Dec.03 run

27. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Measured background level Excess in 200-400keV in Dec. 2003 run may be due to 85 Kr Simulation Aug.04 run Preliminary Dec. 2003 runAug. 2004 run All volume 20cm FV 10cm FV

28. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 232 Th series 212 Bi 212 Po 208 Pb  1/2 =299nsec  (E max =2.3MeV)  (8.8MeV)  -tagged beta events of 212 Bi and 212 Po  High- and Middle-gain normal runs: 1.66day  20cm fiducial volume cut (to reject external events)  1 st peak: < 2000p.e. (efficiency ~100%)   T=160~6000nsec in Flash ADC (efficiency = 69%)  2 nd peak: 500 ~ 4500p.e. (efficiency ~100%) (BR=64%) 1 event remained Dec.03 run

29. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Typical FADC data & peak search 54 PMT analog sum 1 FADC Range: -8 ~ +8  sec 80~240nsec window Threshold: 70 count (4~12p.e.) Most of peaks = after pulses from PMT nsec FADC count Peak position Pedestal (80nsec) Trigger timing Dec.03 run

30. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 232 Th series: Bi-Po analysis (FADC) 1 candidate event 232 Th < 63x10 -14 g( 232 Th)/g(Xe) (90%CL) Trigger timing 1 st peak 2 nd peak (  T~700nsec) nsec FADC count Keep this event conservatively, for now (only stat. error) (OK)(very high energy?) Dec.03 run

31. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 An idea of dedicated detector for  Put room temperature LXe into a thick, acrylic pressure vessel (~50atm). Wavelength shifter inside the vessel. We already have 10kg enriched 136 Xe. Test vessel held 80 atm water (symbolically)

32. Y.Takeuchi @ICHEP04 in Beijing August 18, 2004 Expected sensitivity Assume acrylic material U,Th~10 -12 g/g, no other BG. Cylindrical geom. (4cm dia. LXe, 10cm dia. Vessel) 10kg 136 Xe 42000photon/MeV but 50% scintillation yield, 90% eff. shifter, 80% water transparency, 20% PMT coverage, 25% QE  57keVrms @ Q  =2.48MeV 1yr, 10kg measurement 1.5 x 10 25 yr  =0.2~0.3eV If U/Th ~ 10 -16 g/g + larger mass  ~0.02-0.03eV 2  will not be BG thanks to high resolution U+Th normalized for 10kg, 1yr