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Recent status of the XMASS project

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.

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Recent status of the XMASS project

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  1. 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 Y.Takeuchi @ICHEP04 in Beijing

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

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

  4. 2nbb t1/2 theory= 8 x 1021 y Physics goals at XMASS • Search for 0nbb (2nbb) decay of 136Xe (na 8.87%) • High purity and enriched Xe can be used. • Energy region is different from solar n / DM. • PMTs should not be placed near the detector. 136Xe 136Ba + e- + e- Q-Value: 2.48 MeV Need another design of the detector! (low priority, at moment…) • Xenon MASSive Detector for Solar Neutrinos (pp/7Be) • Xenon Detector for Weakly Interacting MASSive Particles (Dark Matter Search) • Xenon Neutrino MASS Detector (Double Beta Decay) Y.Takeuchi @ICHEP04 in Beijing

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

  6. Strategy of the XMASS project NOW ~1m ~2.5m ~30cm ~1 ton detector (FV 100kg) Dark matter search ~20 ton detector (FV 10ton) Solar neutrinos Dark matter search Prototype detector (FV 3kg) R&D Confirmation of feasibilities of the ~1 ton detector Analysis techniques Self shielding performance Low background properties Purification techniques Dedicated detector for Double beta decay search Y.Takeuchi @ICHEP04 in Beijing

  7. Self shielding 30cm 105 reduction for < ~500keV Liquid Xe Volume for shielding Fiducial volume PMTs Reconstruct the vertex and energy based on PMTs information (light pattern) • Quite effective for the events below ~500 keV (pp n & DM) • Not effective for double beta decay experiment Y.Takeuchi @ICHEP04 in Beijing

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

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

  10. 1.0m 1.9m XMASS prototype detector • 30 litter liquid Xenon (~100kg) • Oxygen free copper: (31cm)3 • 54 of low-BG 2-inch PMT • Photo coverage~16% • MgF2 window • 0.6 p.e. / keV n Polyethylene (15cm) Boric acid (5cm) g Lead (15cm) EVOH sheets (30mm) OFC (5cm) Rn Rn free air(~3mBq/m3) Y.Takeuchi @ICHEP04 in Beijing

  11. 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 Y.Takeuchi @ICHEP04 in Beijing

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

  13. Aug.04 run Preliminary 238U Internal BG source: 222Rn 222Rn DT < 1ms • 2 separate runs to check 222Rn decay (t1/2=3.8day) • 4th Aug. 0.8day 238U=(72+-11)x10-14 g/g • 10th Aug. 1.0day 238U=(33+-7)x10-14 g/g • Consistent with expected 222Rn decay ((30+-5)x10-14 ) 214Bi214Po210Pb (1.8days) b (Emax=3.3MeV) a (7.7MeV) 3.5MeV t1/2 =164msec 67ev DT: t1/2 = 141+-51msec DL Y.Takeuchi @ICHEP04 in Beijing

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

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

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

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

  18. Summary • XMASS is aiming to search rare phenomena under an ultra low background environment by using ultra pure liquid xenon. • 2nd 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 222Rn 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 cm2level would be possible. Y.Takeuchi @ICHEP04 in Beijing

  19. Supplement Y.Takeuchi @ICHEP04 in Beijing

  20. 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 N2 can be used • Various purification method • Distillation, circulation during experiment, … • Effective reduction against internal backgrounds • No long life radioactive isotopes • 136Xe is a bb decay candidate Y.Takeuchi @ICHEP04 in Beijing

  21. Development of the low BG PMT 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 U 1.5±0.3x10-3 Bq Th 3.2±4.6x10-4 Bq 40K 1.7±2.9x10-3 Bq Aiming for another order of magnitudes improvement Hexagonal PMT to accomplish 70-80% PMT coverage Y.Takeuchi @ICHEP04 in Beijing

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

  23. + + + C B A Collimated gamma rays for three different positions Event reconstruction Real data Hole C Hole B Hole A MC 137Cs Reconstruction works well Dec.03 run Y.Takeuchi @ICHEP04 in Beijing

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

  25. Aug.04 run Preliminary 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 Y.Takeuchi @ICHEP04 in Beijing

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

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

  28. Dec.03 run 232Th series • a-tagged beta events of 212Bi and 212Po • High- and Middle-gain normal runs: 1.66day • 20cm fiducial volume cut (to reject external events) • 1st peak: < 2000p.e.(efficiency ~100%) • DT=160~6000nsec in Flash ADC (efficiency = 69%) • 2nd peak: 500 ~ 4500p.e.(efficiency ~100%) 212Bi212Po208Pb b (Emax=2.3MeV) a (8.8MeV) (BR=64%) t1/2 =299nsec 1 event remained Y.Takeuchi @ICHEP04 in Beijing

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

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

  31. An idea of dedicated detector for0nbb Put room temperature LXe into a thick, acrylic pressure vessel (~50atm). (symbolically) Test vessel held 80 atm water Wavelength shifter inside the vessel. We already have 10kg enriched 136Xe. Y.Takeuchi @ICHEP04 in Beijing

  32. Assume acrylic material U,Th~10-12g/g, no other BG. Cylindrical geom. (4cm dia. LXe, 10cm dia. Vessel) 10kg 136Xe 42000photon/MeV but 50% scintillation yield, 90% eff. shifter, 80% water transparency, 20% PMT coverage, 25% QE  57keVrms @ Qbb=2.48MeV Expected sensitivity U+Th normalized for 10kg, 1yr 1yr, 10kg measurement 1.5 x 1025 yr  <mn>=0.2~0.3eV If U/Th ~ 10-16 g/g + larger mass  <mn>~0.02-0.03eV 2nbb will not be BG thanks to high resolution Y.Takeuchi @ICHEP04 in Beijing

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