Current status of XMASS experiment 11 th International Workshop on Low Temperature Detectors (LTD-11) Takeda Hall, University of Tokyo, JAPAN 8/1, 2005 Hiroshi Ogawa (ICRR)

 What’s XMASS Xenon MASSive detector for solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (  decay) Xenon detector for Weakly Interacting MASSive Particles (DM search) Multi purpose low-background experiment with liq. Xe Large Z (=54) Self-shielding effect Large photon yield (~42 photons/keV ~ NaI(Tl)) Low threshold High density (~3 g/cm 3 ) Compact detector (10 ton: sphere with diameter of ~2m) Purification (distillation) No long life radioactive isotope Scintillation wavelength ~175 nm, detected directly by PMT Relative high temperature (~165 K)  Merit of using Liq. Xe:

100kg Prototype 800kg detector 10 ton detector ～ 30cm ～ 80cm ～ 2.5m R&D Dark matter search Multipurpose detector (solar neutrino,  …) We are now here  Strategy of the scale-up With light guide

Gamma ray shield OFHC cubic chamber Liq. Xe (31cm) 3 MgF 2 window 54 2-inch low BG PMTs 16% photo- coverage Hamamatsu R8778  100 kg prototype detector In the Kamioka Mine (near the Super-K) 2,700 m.w.e  Vertex and energy reconstruction by fitter  Vertex and energy calibration using radioactive sources 137Cs & 60Co  Self shielding power are measured by 137Cs &60Co Confirmation of estimated 800 kg detector performance PMT are developed for VUV region (Q.E.~ PMT performance (Gain, Q.E.) in low temp. is similar to one in room temp.

z position distribution of the collimated  ray source run → Data and MC agree well γ  Demonstration of self shielding effect

Shelf shielding for real data and MC Good agreement (< factor 2) Self shielding effect can be seen clearly. Very low background (10 -2 keV) REAL DATA MC simulation All volume 20cm FV 10cm FV (3kg) All volume 20cm FV 10cm FV (3kg) Miss-reconstruction due to dead-angle region from PMTs. Reduced in Light guide measurement & future 800kg detector. Event rate (/kg/day/keV) /kg/day/keV Aug. 04 run preliminary 3.9days livetime ~1.6Hz, 4 fold, triggered by ~0.4p.e.

Kr = 3.3±1.1 ppt (by mass spectrometer) β687keV (99.563%) as BG → Achieved by distillation U-chain = (33±7)x g/g (by prototype detector) Th-chain < 23x g/g(90%CL) (by prototype detector)  1/2 =164  s  (Q=3.3MeV)  (7.7MeV) 214 Bi 214 Po 210 Pb  1/2 =299ns  (Q=2.3MeV)  (8.8MeV) 212 Bi 212 Po 208 Po Delayed coincidence search (radiation equilibrium assumed)  Internal backgrounds in liq. Xe were measured Main sources in liq. Xe are Kr, U-chain and Th-chain

XMASS succeeds to reduce Kr concentration in Xe from ~3[ppb] to 3.3(±1.1)[ppt] with one cycle (~1/1000) ~3m Lower Higher ~1% ~99% Purified Xe: 3.3±1.1 ppt Kr (measured) Off gas Xe: 330±100 ppb Kr (measured) Raw Xe: ~3 ppb Kr (preliminary) (178K) (180K) Processing speed : 0.6 kg / hour Design factor : 1/1000 Kr / 1 pass Purified Xe : Off gas = 99:1 Xe purification system Boiling point atm) Xe178.1K Kr129.4K

 800 kg detector Main purpose: Dark Matter search ~800-2” PMTs immersed into liq. Xe 70% photo-coverage ~80cm diameter ~5 keVee threshold External  ray BG: 60cm, 346kg 40cm, 100kg Expected dark matter signal (assuming cm 2, Q.F.=0.2 50GeV / 100GeV,) pp & 7 Be solar Achieved Expected sensitivity SI ~ cm 2 = 10-9 pb SD~ cm 2 = 10-3 pb XMASS FV 0.5 ton year E th = 5 keVee~25 p.e., 3  discovery w/o any pulse shape info.

Detector design Geometry : 12 pentagons / pentakisdodecahedron A tentative design (not final one) Hamamatsu R8778MOD(hex) 12cm 5.4cm 5.8cm (edge to edge) 0.3cm (rim) Hexagonal quartz window Effective area:  50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778 Total 840 hex PMTs immersed into liq. Xe 70% photo-coverage Radius to inner face ~43cm

Now (prototype detector) Goal (800kg detector)  ray BG ~ cpd/kg/keV cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) 238 U = (33±7)× g/g 1× g/g → Remove by filter 232 Th < 23× g/g (90% C.L.) 2× g/g → Remove by filter (Only upper limit) Kr = 3.3±1.1 ppt 1 ppt → Achieve by 2 purification pass Very near to the target level! 1/100 1/33 1/12 1/3 Goal of BG for 800kg detector

Summary XMASS experiment: Multi purpose low- background experiment with large mass liq. Xe Performances required for 800 kg detector are confirmed : self shield effect, distillation and son on using 100kg prototype detector. 800 kg detector: Designed for dark matter search mainly, and 10 2 improvement of sensitivity above existing experiments is expected.