1. 1 1. Introduction 2. R&D status using prototype detector 3. Summary XMASS experiment 8 th June 2005 A.Takeda for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo WIN05

2. 2 1. Introduction Dark matter Double beta Solar neutrino  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

3. 3  Why liquid xenon 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)

4. 4 PMTs Single phase liquid Xe Volume for shielding Fiducial volume 23ton all volume 20cm wall cut 30cm wall cut (10ton FV) BG normalized by mass 1MeV02MeV3MeV Large self-shield effect External  ray from U/Th-chain  Key idea: self-shielding effect for low energy events

5. 5 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

6. 6 Trend of Dark matter (WIMPs) direct searches Scintillation PhononIonization Ge, TeO 2, Al 2 O 3, LiF, etc NaI, Xe, CaF 2, etc. Ge Recoiled nuclei are mainly observed by 3 ways CDMS, EDELWEISS : phonon + ionization Taking two type of signals simultaneously is recent trend   ray reduction owing to powerful particle ID  However, seems to be difficult to realize a large and uniform detector due to complicated technique Ge, Si

7. 7 Strategy chosen by XMASS Make large mass and uniform detector (with liq. Xe) Reduce  ray BG by fiducial volume cut (self shielding) Same style as successful experiments of Super-K, SNO, KamLAND, etc. Super-KKamLANDSNO

8. 8  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 10 -42 cm 2, Q.F.=0.2 50GeV / 100GeV,) pp & 7 Be solar Achieved

9. 9 Geometry design A tentative design (not final one) Total 840 hex PMTs immersed into liq. Xe 70% photo-coverage Radius to inner face ~43cm 12 pentagons / pentakisdodecahedron This geometry has been coded in a Geant 4 based simulator

10. 10 Hamamatsu R8778MOD(hex) 12cm 5.4cm 5.8cm (edge to edge) 0.3cm (rim) c.f. R8778 U 1.8±0.2x10 -2 Bq Th 6.9±1.3x10 -3 Bq 40 K 1.4±0.2x10 -1 Bq Hexagonal quartz window Effective area:  50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778 Prototype has been manufactured already Now, being tested

11. 11 Expected sensitivities Large improvements will be expected SI ~ 10-45 cm 2 = 10-9 pb SD~ 10-39 cm 2 = 10-3 pb XMASS(Ann. Mod.) XMASS(Sepc.) Edelweiss Al2O3 Tokyo LiF Modane NaI CRESST UKDMC NaI NAIAD XMASS FV 0.5 ton year E th = 5 keVee~25 p.e., 3  discovery w/o any pulse shape info. 10 -6 10 -4 10 -8 10 -10 Cross section to nucleon [pb] 10 -4 10 -2 1 10 2 10 4 10 6 Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic

12. 12 100kg prototype With light guide version 2. R&D status using prototype detector  Main purpose Confirmation of estimated 800 kg detector performance BG study  Vertex and energy reconstruction by fitter  Miss fitting due to dead angle of the cubic detector (“wall effect”, will be explained later) can be removed with light guide  Self shielding power  Understanding of the source of BG  Measuring photon yield and its attenuation length ~30 cm cube 3 kg fiducial

13. 13 In the Kamioka Mine (near the Super-K) Gamma ray shield OFHC cubic chamber Liq. Xe (31cm) 3 MgF 2 window 54 2-inch low BG PMTs 16% photo- coverage Hamamatsu R8778 2,700 m.w.e.  100 kg prototype detector

14. 14 1.0m 1.9m Rn free air (~3mBq/m 3 ) materialthickness Polyethylene 15cm Boron 5cm Lead 15cm EVOH sheets 30μm OF Cupper 5cm 4  shield with door

15. 15  Progress so far 1 st run (Dec. 2003) 2 nd run (Aug. 2004) 3 rd run (Mar. 2005) with light guide  Confirmed performances of vertex & energy reconstruction  Confirmed self shielding power for external  rays  Measured the internal background concentration  Succeeded to reduce Kr from Xe by distillation  Photo electron yield is increased  Measured Rn concentration inside the shield  Confirmed the miss fitting (only for the prototype detector) was removed  Now, BG data is under analysis

16. 16    PMT n n L) ! )exp( Log()  L: likelihood  : F(x,y,z,i) x total p.e.  F(x,y,z,i) n: observed number ofp.e. === Background event sample === QADC, FADC, and hit timing information are available for analysis F(x,y,z,i): acceptance for i-th PMT (MC) VUV photon characteristics: L emit =42ph/keV  abs =100cm  scat =30cm Calculate PMT acceptances from various vertices by Monte Carlo. Vtx.: compare acceptance map F(x,y,z,i) Ene.: calc. from obs. p.e. & total accept. Reconstructed here FADCHit timing QADC Reconstruction is performed by PMT charge pattern (not timing)  Vertex and energy reconstruction

17. 17 → Vertex reconstruction works well 1. Performance of the vertex reconstruction Collimated  ray source run from 3 holes ( 137 Cs, 662keV) DATA MC hole A hole B hole C A B C + + +

18. 18  65keV@peak (  /E ~ 10%) Similar peak position in each fiducial.  No position bias 2. Performance of the energy reconstruction Collimated  ray source run from center hole 137 Cs, 662keV → Energy reconstruction works well All volume 20cm FV 10cm FV

19. 19 z position distribution of the collimated  ray source run → Data and MC agree well γ  Demonstration of self shielding effect

20. 20 Shelf shielding for real data and MC Good agreement (< factor 2) Self shielding effect can be seen clearly. Very low background (10 -2 /kg/day/keV@100-300 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. Event rate (/kg/day/keV) 10 -2 /kg/day/keV Aug. 04 run preliminary 3.9days livetime ~1.6Hz, 4 fold, triggered by ~0.4p.e.

21. 21 Kr = 3.3±1.1 ppt (by mass spectrometer) → Achieved by distillation U-chain = (33±7)x10 -14 g/g (by prototype detector) Th-chain < 23x10 -14 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

22. 22 Kr 0.1ppm DM signal (10 -6 pb, 50GeV, 100 GeV) 0200400 600800 1 10 2 10 -2 10 -4 10 -6 cpd/kg/keV energy (keV) Target = Xe Kr concentration in Xe 85 Kr makes BG in low enegy region Kr can easily mix with Xe because both Kr and Xe are rare gas Commercial Xe contains a few ppb Kr

23. 23 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) Operation@2atm Processing speed : 0.6 kg / hour Design factor : 1/1000 Kr / 1 pass Purified Xe : Off gas = 99:1 Xe purification system Boiling point (@2 atm) Xe178.1K Kr129.4K

24. 24 Now (prototype detector) Goal (800kg detector)  ray BG ~ 10 -2 cpd/kg/keV 10 -4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) 238 U = (33±7)×10 -14 g/g 1×10 -14 g/g → Remove by filter 232 Th < 23×10- 14 g/g (90% C.L.) 2×10 -14 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 Summary of BG measurement

25. 25 HIT ? MC If true vertex is used for fiducial volume cut 10 -2 1 10 -1 Energy (keV) 0 100020003000 No wall effect  Remaining problem: wall effect (only for the prototype detector) Scintillation lights at the dead angle from PMTs give quite uniform 1 p.e. signal for PMTs, and this cause miss reconstruction as if the vertex is around the center of detector Dead angle This effect does not occur with the sphere shape 800 kg detector

26. 26 PTFE light guide (UV reflection) Active veto Fiducial  Prototype detector with light guide 10cm Purpose: remove the wall effect and understand the source of BG in the DM region 6 pieces 10X10X10cm 3 (~3 kg Xe)

27. 27 222 Rn decays ( 210 Pb  64 keV endpoint) implanted in PTFE surfaces might make the dominant BG air PTFE 222 Rn 218 Po 214 Pb 210 Pb α α α Position distribution of 210 Pb (in NaI) Light guide setup Implanted to ~0.1μm N.J.T. Smith et al., Phys. Lett. B 485 (2000) 9 We edged the PTFE inside ~10μm Recoil process implants 30% of the original surface Rn decays Edging of PTFE surfaces 00.050.100.15 Z [  m]

28. 28 ・ MC simulation was done with GEANT3 0 20 40 6080 10 -2 10 -4 1 100 0 20 40 60 80 0 0.4 0.2 0.6 0.8 1.0 Efficiency cpd/keV/kg energy(keV) Efficiency curve BG spectrum Signal window (10-15keV) Efficiency~30% @10keVExpected BG~10 -2 cpd/keV/kg Fast neutron BG (90% C.L. upper limit) 0.48p.e./keV PMT K PMT Th PMT U → Very low BG ~ 10 -2 cpd/keV/kg @ <100keV Expected BG spectrum

29. 29 Energy [keV] Counts 10cm fiducial volume Counts Energy [keV] w/o light guide Reduce the events due to the wall effect Hole-B Result 1: comparing the data taken with and without light guide with light guide Collimated  ray source run from hole-B (137Cs, 662keV)

30. 30 Outside the light guide(Data) 0 100020003000 10 -4 10 -3 10 -2 10 -1 1 10 energy(keV) events/keV/kg/day Live time (3.3days) Outside light guide(MC) 0 100020003000 10 -4 10 -3 10 -2 10 -1 1 10 energy(keV) events/keV/kg/day Result 2: Obtained energy spectrum outside the light guide Good agreement (< factor 2) Trigger rate is same as the measurement witout guide (Aug. 2004)

31. 31 3. Summary XMASS experiment: Multi purpose low-background experiment with large mass liq. Xe 800 kg detector: Designed for dark matter shearch mainly, and 10 2 improvement of sensitivity above existing experiments is expected R&D with the 100 kg prototype detector Most of the performances required for 800 kg detector are confirmed

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33. 33 XMASS collaboration ICRR, Kamioka Y. Suzuki, M. Nakahata, Y. Itow, S. Moriyama, M. Shiozawa, Y. Takeuchi, M. Miura, Y. Koshio, K. Ishihara, K. Abe, A. Takeda, T. Namba, H. Ogawa, S. Fukuda, Y. Ashie, A. Minamino, R. Nambu, J. Hosaka, K. Taki, T. Iida, K. Ueshima ICRR, RCNN T. Kajita, K. Kaneyuki Saga Univ. H. Ohsumi, Y. Iimori, Tokai Univ. K. Nishijima, T. Hashimoto, Y. Nakajima, Y. Sakurai Gifu Univ. S. Tasaka Waseda Univ. S. Suzuki, K. Kawasaki, J. Kikuchi, T. Doke, A. Ota Yokohama National Univ. S. Nakamura, T.Fukuda, S. Oda, N. Kobayashi, A. Hashimoto Miyagi Univ. of Education Y. Fukuda, T. Sato Seoul National Univ. Soo-Bong Kim, In-Seok Kang INR-Kiev Y. Zdesenko, O. Ponkratenko UCI H. Sobel, M. Smy, M. Vagins, P.Cravens Sejong univ. Y. Kim Ewha Womans Univ. K. Lim Indiana Univ. M. Ishitsuka

34. 34 Electronics of light guide run (Mar. 2005) ADC Trigger module Gate generator Sum Amp delay PMT Fan-out x8 Sum Amp x8 Discri(VME)Discri(NIM)~1/4p.e.thres. FADC(2ch/250MHz) 16μs ID sum OD sum ID sum OD sum ID x8 FADC(8ch/500MHz) 1μs Gain : 8.25×10 6 ID ≧ 2hit OD ≧ 4hit 400ns Inner PMT×6 Outer PMT×48

35. 35 measurement ➢ Data taking : 3/9 ~ 3/19, 2005 ➢ Background run : 3.7days runtime, 3.3days livetime ➢ Trigger rate 1.5Hz (inner 0.2Hz, outer 1.4Hz) ➢ Calibration run : 137 Cs / 60 Co / 57 Co / 133 Ba source run ID hit event FADC ID sum TDC OD hit event

36. 36  Background study keV 0 1000 20003000 1 10 -1 10 -2 cpd/kg/keV Outside of the shield 238 U in PMTs 232 Th in PMTs 40 K in PMTs 210 Pb in lead shield Outside of the shield 0.71 cm -2 s -1 (>500keV) RI sources in PMTs 238 U : 1.8×10 -2 Bq/PMT 232 Th : 6.9×10 -3 Bq/PMT 40 K : 1.4×10 -1 Bq/PMT 210 Pb in the lead shield 250Bq/kg Expected spectra in all volume

37. 37 Low energy calibration source (1) Attenuation length of 20 keV x-ray in liq. Xe is short ~ 50 μm  The overall size of the source itself should be small not to block the scintillation photons Irradiate neutrons to natural Pd wire of 10 μm diameter 102 Pd(n,γ) 103 Pd  EC decay of 103 Pd produce 20 keV x-ray X-ray Scintillation photons D EC decaying nuclei preferable  X-rays Candidates : 71 Ge(463d), 153 Gd(263d), 103 Pd(17d) Liq. Xe

38. 38 Low energy calibration source (2) ☆ 125 I(X-ray source) : 27.5 keV (59.9day) ☆ Temperature Range : -200 ~ +100 in Centigrade ☆ Overall source diameter < 20 mm ☆ Weak source ~ a few kBq Length 60mm 125 I (1kBq) Electrodeposition Material A F= 10mm Coating Material B Thick=3mm 5mm Source position Liq. Xe

39. 39 Plan of prototype detector ☆ Introduce RI source( 103 Pd, 125 I,…) inside the chamber → Source driving system is ready → Detailed study of the energy and vertex fitter Low energy  source ( 103 Pd, 125 I,…) Wire Motor Position accuracy is within 1mm

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