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Introduction R&D status using prototype detector Summary

XMASS experiment. WIN05. 8 th June 2005. Takeda for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo. Introduction R&D status using prototype detector Summary. 1. Introduction. Solar neutrino. What’s XMASS. Multi purpose low-background experiment with liq. Xe.

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Introduction R&D status using prototype detector Summary

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

  2. 1. Introduction Solar neutrino • What’s XMASS Multi purpose low-background experiment with liq. Xe • Xenon MASSive detector for solar neutrino (pp/7Be) • Xenon neutrino MASS detector (bb decay) • Xenon detector for Weakly Interacting MASSive Particles (DM search) Dark matter Double beta

  3. Why liquid xenon • Large Z (=54) Self-shielding effect • Large photon yield (~42 photons/keV ~ NaI(Tl)) Low threshold • High density (~3 g/cm3) 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. Key idea: self-shielding effect for low energy events Single phase liquid Xe External g ray from U/Th-chain Volume for shielding 23ton all volume 20cm wall cut 30cm wall cut (10ton FV) Fiducial volume Large self-shield effect BG normalized by mass PMTs 0 1MeV 2MeV 3MeV

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

  6. Trend of Dark matter (WIMPs) direct searches • Recoiled nuclei are mainly observed by 3 ways Scintillation NaI, Xe, CaF2, etc. Phonon Ionization Ge Ge, TeO2, Al2O3, LiF, etc Ge, Si • Taking two type of signals simultaneously is recent trend CDMS, EDELWEISS: phonon + ionization • g ray reduction owing to powerful particle ID • However, seems to be difficult to realize a large and uniform detector due to complicated technique

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

  8. 800 kg detector Main purpose: Dark Matter search External g ray BG: 60cm, 346kg 40cm, 100kg Achieved pp & 7Be solar n ~80cm diameter Expected dark matter signal (assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,) • ~800-2” PMTs immersed into liq. Xe • 70% photo-coverage ~5 keVee threshold

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

  10. Hamamatsu R8778MOD(hex) • Hexagonal quartz window • Effective area: f50mm (min) • QE <~25 % (target) • Aiming for 1/10 lower background than R8778 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 40K 1.4±0.2x10-1 Bq 5.4cm • Prototype has been manufactured already • Now, being tested 12cm

  11. Expected sensitivities XMASS FV 0.5 ton year Eth = 5 keVee~25 p.e., 3s discovery w/o any pulse shape info. 10-4 106 • Large improvements will be expected SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb 104 Edelweiss Al2O3 10-6 Tokyo LiF 102 Modane NaI Cross section to nucleon [pb] CRESST 1 UKDMC NaI 10-8 XMASS(Ann. Mod.) NAIAD 10-2 XMASS(Sepc.) 10-10 10-4 Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic

  12. 2. R&D status using prototype detector • Main purpose 100kg prototype • Confirmation of estimated 800 kg detector performance • 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 ~30 cm cube 3 kg fiducial With light guide version • BG study • Understandingof the source of BG • Measuring photon yield and its attenuation length

  13. 54 2-inch low BG PMTs Hamamatsu R8778 16% photo- coverage Liq. Xe (31cm)3 MgF2 window • 100 kg prototype detector In the Kamioka Mine (near the Super-K) 2,700 m.w.e. OFHC cubic chamber Gamma ray shield

  14. 4p shield with door 1.0m Rn free air (~3mBq/m3) 1.9m

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

  16. - m m n exp( ) å = Log( L ) Log( ) n ! PMT • Vertex and energy reconstruction Reconstruction is performed by PMT charge pattern (not timing) Reconstructed here 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. QADC L: likelihood F(x,y,z,i) m : x total p.e. S F(x,y,z,i) n: observed number of p.e . FADC Hit timing F(x,y,z,i): acceptance for i-th PMT (MC) VUV photon characteristics: Lemit=42ph/keV tabs=100cm tscat=30cm === Background event sample === QADC, FADC, and hit timing information are available for analysis

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

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

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

  20. Shelf shielding for real data and MC All volume All volume 20cm FV 20cm FV 10cm FV (3kg) 10cm FV (3kg) Aug. 04 run preliminary ~1.6Hz, 4 fold, triggered by ~0.4p.e. REAL DATA 3.9days livetime MC simulation Event rate (/kg/day/keV) 10-2/kg/day/keV Miss-reconstruction due to dead-angle region from PMTs. • Good agreement (< factor 2) • Self shielding effect can be seen clearly. • Very low background (10-2 /kg/day/keV@100-300 keV)

  21. 214Bi 214Po 210Pb a (7.7MeV) b (Q=3.3MeV) t1/2=164ms 208Po 212Bi 212Po a (8.8MeV) b (Q=2.3MeV) t1/2=299ns • Internal backgrounds in liq. Xe were measured Main sources in liq. Xe are Kr, U-chain and Th-chain • 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) Delayed coincidence search (radiation equilibrium assumed) Delayed coincidence search (radiation equilibrium assumed)

  22. Kr concentration in Xe • 85Kr makes BG in low enegy region Target = Xe 102 cpd/kg/keV Kr 0.1ppm 1 10-2 DM signal (10-6 pb, 50GeV, 100 GeV) 10-4 10-6 • Kr can easily mix with Xe because both Kr and Xe are rare gas 0 200 400 600 800 energy (keV) • Commercial Xe contains a few ppb Kr

  23. Xe purification system • XMASS succeeds to reduce Kr concentration in Xe from ~3[ppb] to 3.3(±1.1)[ppt] with one cycle (~1/1000) • Processing speed : 0.6 kg / hour • Design factor : 1/1000 Kr / 1 pass • Purified Xe : Off gas = 99:1 Lower ~3m Raw Xe: ~3 ppb Kr (178K) Off gas Xe: 330±100 ppb Kr (measured) ~1% Purified Xe: 3.3±1.1 ppt Kr (measured) Higher ~99% Operation@2atm (180K) (preliminary)

  24. Summary of BG measurement 1/100 Now (prototype detector) Goal (800kg detector) • g ray BG ~ 10-2 cpd/kg/keV 10-4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) • 238U = (33±7)×10-14 g/g 1×10-14 g/g → Remove by filter • 232Th < 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 1/33 1/12 1/3 Very near to the target level!

  25. Remaining problem: wall effect (only for the prototype detector) HIT HIT HIT HIT HIT ? MC If true vertex is used for fiducial volume cut 1 Dead angle 10-1 10-2 • 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 1000 2000 3000 0 Energy (keV) No wall effect This effect does not occur with the sphere shape 800 kg detector

  26. Active veto Fiducial PTFE light guide (UV reflection) 10cm 10cm 10cm • Prototype detector with light guide Purpose: remove the wall effect and understand the source of BG in the DM region 10X10X10cm3 (~3 kg Xe) 6 pieces

  27. Light guide setup • Edging of PTFE surfaces 222Rn decays (210Pb b, 64 keV endpoint) implanted in PTFE surfaces might make the dominant BG α 222Rn air We edged the PTFE inside ~10μm PTFE 218Po α 214Pb α N.J.T. Smith et al., Phys. Lett. B 485 (2000) 9 Position distribution of 210Pb (in NaI) 210Pb Recoil process implants 30% of the original surface Rn decays 0 0.05 0.10 0.15 Z [mm] Implanted to ~0.1μm

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

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

  30. Result 2: Obtained energy spectrum outside the light guide Outside the light guide(Data) Outside light guide(MC) 10 10 Live time (3.3days) 1 1 events/keV/kg/day events/keV/kg/day 10-1 10-1 10-2 10-2 10-3 10-3 10-4 10-4 1000 2000 3000 0 1000 2000 3000 0 energy(keV) energy(keV) • Good agreement (< factor 2) • Trigger rate is same as the measurement witout guide (Aug. 2004)

  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 102 improvement of sensitivity above existing experiments is expected • R&D with the 100 kg prototype detector Most of the performancesrequired for 800 kg detector are confirmed

  32. 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

  33. Electronics of light guide run (Mar. 2005) ADC Fan-out delay 400ns PMT Inner PMT×6 Sum Amp PMT ID sum OD sum ID x8 x8 PMT FADC(8ch/500MHz) 1μs Sum Amp Outer PMT×48 ID sum OD sum PMT Gate generator Gain : 8.25×106 FADC(2ch/250MHz) 16μs Discri(VME) Discri(NIM)~1/4p.e.thres. ID ≧ 2hit Trigger module x8 OD ≧ 4hit

  34. 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 : 137Cs / 60Co / 57Co / 133Ba source run ID hit event OD hit event TDC FADC ID sum

  35. Background study Expected spectra in all volume • Outside of the shield 0.71 cm-2 s-1 (>500keV) • RI sources in PMTs 238U : 1.8×10-2 Bq/PMT 232Th : 6.9×10-3 Bq/PMT 40K : 1.4×10-1 Bq/PMT • 210Pb in the lead shield 250Bq/kg 1 cpd/kg/keV Outside of the shield 238U in PMTs 232Th in PMTs 10-1 40K in PMTs 210Pb in lead shield 10-2 2000 3000 1000 0 keV

  36. Scintillation photons Liq. Xe • Low energy calibration source (1) X-ray D • 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 • EC decaying nuclei preferable  X-rays • Candidates : 71Ge(463d), 153Gd(263d), 103Pd(17d) Irradiate neutrons to natural Pd wire of 10 μm diameter 102Pd(n,γ) 103Pd  EC decay of 103Pd produce 20 keV x-ray

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

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

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