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

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. pp +-1% Measure pp via + e + e 2  life time should be measured Isotope separation would be needed

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

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 y

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

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

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

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 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

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

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

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

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

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)x g/g 10 th Aug. 1.0day 238 U=(33+-7)x g/g Consistent with expected 222 Rn decay ((30+-5)x ) Aug.04 run Preliminary  T:  1/2 =  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

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

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)

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

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

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 cm 2 level would be possible.

in Beijing August 18, 2004 Supplement

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 weight amu Density3.0 g/cm 3 Melting (boiling) point 161.4K (165.1K) Chemical seriesNoble gases

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. ~ 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

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

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

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%

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

in Beijing August 18, 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

in Beijing August 18, 2004 Measured background level Excess in keV in Dec run may be due to 85 Kr Simulation Aug.04 run Preliminary Dec runAug run All volume 20cm FV 10cm FV

in Beijing August 18, 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

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

in Beijing August 18, Th series: Bi-Po analysis (FADC) 1 candidate event 232 Th < 63x 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

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)

in Beijing August 18, 2004 Expected sensitivity Assume acrylic material U,Th~ 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  Q  =2.48MeV 1yr, 10kg measurement 1.5 x yr  =0.2~0.3eV If U/Th ~ g/g + larger mass  ~ eV 2  will not be BG thanks to high resolution U+Th normalized for 10kg, 1yr