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Introduction 800kg detector design Summary

XMASS experiment. IDM2006, Rhodes, Greece. 15 th Sep. 2006. Takeda for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo. Introduction 800kg detector design Summary. 1. Introduction. Solar neutrino. What’s XMASS.

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Introduction 800kg detector design Summary

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  1. XMASS experiment IDM2006, Rhodes, Greece 15th Sep. 2006 • Takeda for the XMASS collaboration • Kamioka Observatory, ICRR, • University of Tokyo • Introduction • 800kg detector design • 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 External g ray from U/Th-chain g tracking MC from external to Xenon All volume 20cm wall cut 30cm wall cut (10ton FV) U-chain gamma rays BG normalized by mass Large self-shield effect 0 1MeV 2MeV 3MeV Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume Background are widely reduced in < 500keV low energy region

  5. Strategy of the scale-up 10 ton detector 100kg Prototype 800kg detector With light guide ～30cm ～80cm ～2.5m R&D • Vertex & energy reconstruction • Self shielding power • BG level Dark matter search Multipurpose detector (solar neutrino, bb …) We are here !

  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. 2. 800kg detector design Main purpose: Dark Matter search External g ray BG: 60cm, 346kg 40cm, 100kg Achieved pp & 7Be solar n ~80cm diameter 0 100 200 300 Energy [keV] • 812-2” PMTs immersed into liq. Xe • 70% photo-coverage Expected dark matter signal (assuming 10-6 pb, Q.F.=0.2 50GeV / 100GeV,) ~4 p.e./ keV

  9. 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

  10. Status of 800 kg detector • Basic performances have been already confirmed using 100 kg prototype detector • Vertex and energy reconstruction by fitter • Self shielding power • BG level • Detector design is going using MC • Structure and PMT arrangement (812 PMTs) • Event reconstruction • BG estimation • New excavation will be done soon • Necessary size of shielding around the chamber

  11. 1PMT 10 PMTs per 1 triangle 31cm 6 2 7 34cm 1 3 5 8 4 9 10 31cm • Structure of 800 kg detector 12 pentagons / pentakisdodecahedron Hamamatsu R8778MOD Hex agonal quarts window 5 triangles make pentagon

  12. Total 812 hex PMTs (10PMTs/triangle×60 + 212 @gap) immersed into liq. Xe • ~70% photo-coverage • Radius to inner face ~44cm Each rim of a PMT overlaps to maximize coverage

  13. 60 Generated R = 31cm E = 10keV 12 50 10 Fiducial volume 40 8 5 keV 30 6 Events s (reconstructed) [cm] 10 keV s = 2.3 cm 4 20 50 keV 2 100 keV 10 1 MeV 500 keV 0 0 10 20 30 40 0 22 26 30 34 38 Distance from the center [cm] Reconstructed position [cm] • Event reconstruction @Boundary of fiducial volume • Position resolution 10 keV ~ 3.2 cm 5 keV ~ 5.3 cm

  14. R_reconstructed(cm) • Generated VS reconstructed 50 45 5keV ~ 1MeV 40 • Up to <~40cm, events are well reconstructed with position resolution of ~2~5cm • Out of 42cm, grid whose most similar distribution is selected because of no grid data 35 30 • In the 40cm~44cm region, reconstructed events are concentrated around 42cm, but they are not mistaken for those occurred in the center 25 20 15 • No wall effect 10 • Out of 45cm, some events occurring behind the PMT are miss reconstructed 5 50 0 5 10 15 20 25 30 35 40 45 Distance from the center [cm]

  15. Light leak events Some scintillation lights generated behind the PMT enter the inner region It is not problem if light shield is installed PMT PMT PMT hit map

  16. 800kg BG study Achieved(measured by prototype detector)Goal(800kg detector) • g ray from PMTs ~ 10-2 cpd/kg/keV10-4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) • 238U = (33±7)×10-14 g/g1×10-14 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 ppt1 ppt → Achieve by 2 purification pass 1/100 1/33 1/12 1/3

  17. Estimation of g ray BG from PMTs • U-chain • 1/10 lower BG PMT • than R8778 All volume R<34.5cm R<39.5cm R<24.5cm Statistics: 2.1 days Counts/keV/day/kg No event is found below 100keV after fiducial cut (R<24.5cm) All volume R<34.5cm R<39.5cm R<24.5cm < 1×10-4 cpd/kg/keV can be achieved (Now, more statistics is accumulating) Energy [keV]

  18. Water shield for ambient g and fast neutron Necessary shielding was estimated for the estimation of the size of the new excavation Generation point of g or neutron Configuration of the estimation wa • Put 80cm diameter liquid Xe ball • Assume several size of water shield 50, 100, 150, and 200cm thickness • Assume copper vessel (2cm thickness) for liquid Xe Liq. Xe water MC geometry

  19. 104 g attenuation by water shield 103 102 Detected/generated*surface [cm2] 10 1 PMT BG level 10-1 10-2 0 100 200 300 Distance from LXe [cm] Initial energy spectrum from the rock • g attenuation Deposit energy spectrum (200cm) More than 200cm water is needed to reduce the BG to the PMT BG level

  20. fast neutron attenuation • Fast n flux @Kamioka mine: • (1.15±0.12) ×10-5 /cm2/sec water: 200cm, n: 10MeV • Assuming all the energies are • 10 MeV very conservatively water < 2×10-2 counts/day/kg Liq. Xe 200cm water is enough to reduce the BG to the PMT BG level No event is found from the generated neutron of 105 BG caused by thermal neutron is now under estimation

  21. New excavation @Kamioka mine New excavation for XMASS and other underground experiment will be made soon ~5 m ~20 m ~15 m

  22. 3. Summary • XMASS experiment: Multi purpose low-background experiment with large mass liq. Xe • 800 kg detector: Designed for dark matter search mainly, and 102 improvement of sensitivity above existing experiments is expected • Detector design of 800 kg detector is going • BG estimation • Shielding • New excavation

  23. Backup

  24. 800kg detector: Main purpose: Dark Matter search External g ray BG: 60cm, 346kg 40cm, 100kg ~80cm diameter Achieved 5 keV pp & 7Be solar n 10 keV Photoelectrons(p.e.) Expected dark matter signal (assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,) • ~800-2” PMTs immersed into liq. Xe • 70% photo-coverage ~4 p.e./keV

  25. XMASS collaboration • ICRR, Kamioka Y. Suzuki, M. Nakahata, S. Moriyama, M. Shiozawa, Y. Takeuchi , M. Miura, Y. Koshio, K. Abe, H. Sekiya, A. Takeda, H. Ogawa, A. Minamino, T. Iida, K. Ueshima • ICRR, RCNN T. Kajita, K. Kaneyuki • Saga Univ. H. Ohsumi • Tokai Univ. K. Nishijima, T. Maruyama, Y. Sakurai • Gifu Univ. S. Tasaka • Waseda Univ. S. Suzuki, J. Kikuchi, T. Doke, A. Ota, Y. Ebizuka • Yokohama National Univ. S. Nakamura, Y. Uchida, M, Kikuchi, K. Tomita, Y. Ozaki, T. Nagase, T. Kamei, M. Shibasaki, T. Ogiwara • Miyagi Univ. of Education Y. Fukuda, T. Sato • Nagoya ST Y. Itow • Seoul National Univ. Soo-Bong Kim • INR-Kiev O. Ponkratenko • Sejong univ.Y.D. Kim, J.I. Lee, S.H. Moon

  26. Hamamatsu R8778MOD(hex) • Hexagonal quartz window • Effective area: f50mm (min) • QE <~25 % (target) • Aiming for 1/10lower 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

  27. c.f. R8778 (used for 100kg chamber) U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq 40K 1.4±0.2x10-1 Bq ※measured by HPGe detector in Kamioka Th U 40K Ceramic dielectric parts to support dynodes For R8778mod using quartz Glass parts for feed through & containment For R8778mod Reduce glass material Improvement result will be coming soon!

  28. Current XMASS (new improvement!) XMASS 800kg • BG levels DAMA NaI ZEPLIN before PSD cut Kamioka Ge Heidelberg Moscow CDMS II After PID Events/kg/keV/day KamLAND (>0.8MeV) DM signal for LXe 100GeV 10-6pb Super-K

  29. 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

  30. 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

  31. 4p shield with door 1.0m Rn free air (~3mBq/m3) 1.9m

  32. 100 kg Run summary • 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

  33. - 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

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

  35. All volume 20cm FV 10cm FV 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

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

  37. 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)

  38. 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)

  39. 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

  40. 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)

  41. 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

  42. 800 kg detector

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