XMASS experiment IDM2006, Rhodes, Greece 15th Sep. 2006 A. Takeda for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo 1. Introduction 2. 800kg detector design 3. Summary 1 1. Introduction 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) Solar neutrino Dark matter Double beta 2 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) 3 Key idea: self-shielding effect for low energy events U-chain gamma rays Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume External g ray from U/Th-chain BG normalized by mass g tracking MC from external to Xenon All volume 20cm wall cut 30cm wall cut (10ton FV) Large self-shield effect 0 1MeV 2MeV 3MeV Background are widely reduced in < 500keV low energy region 4 Strategy of the scale-up 100kg Prototype 10 ton detector 800kg detector With light guide ～30cm ～80cm ～2.5m R&D Vertex & energy reconstruction Self shielding power BG level We are here ! Dark matter search Multipurpose detector (solar neutrino, bb …) 5 Trend of Dark matter (WIMPs) direct searches Recoiled nuclei are mainly observed by 3 ways Scintillation NaI, Xe, CaF2, etc. Phonon Ge, TeO2, Al2O3, LiF, etc Ge, Si Ionization Ge 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 6 Strategy chosen by XMASS Make large mass and uniform detector (with liq. Xe) Same style as successful experiments of Super-K, SNO, KamLAND, etc. Reduce g ray BG by fiducial volume cut (self shielding) Super-K SNO KamLAND 7 2. 800kg detector design Main purpose: Dark Matter search External g ray BG: 60cm, 346kg 40cm, 100kg Achieved pp & 7Be solar n ~80cm diameter 812-2” PMTs immersed into liq. Xe 70% photo-coverage 0 ~4 p.e./ keV 100 200 300 Energy [keV] Expected dark matter signal (assuming 10-6 pb, Q.F.=0.2 50GeV / 100GeV,) 8 Expected sensitivities Cross section to nucleon [pb] 10-4 XMASS FV 0.5 ton year Eth = 5 keVee~25 p.e., 3s discovery w/o any pulse shape info. 106 104 10-6 10-8 102 1 Edelweiss Al2O3 Tokyo LiF Modane NaI CRESST UKDMC NaI XMASS(Ann. Mod.) NAIAD 10-2 XMASS(Sepc.) 10-10 10-4 Large improvements will be expected SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic 9 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 10 Structure of 800 kg detector 12 pentagons / pentakisdodecahedron Hamamatsu R8778MOD 1PMT Hex agonal quarts window 6 2 1 3 7 5 4 8 34cm 10 PMTs per 1 triangle 9 10 5 triangles make pentagon 11 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 12 Event reconstruction @Boundary of fiducial volume Position resolution 10 keV ~ 3.2 cm 5 keV ~ 5.3 cm s (reconstructed) [cm] 60 Generated R = 31cm 50 E = 10keV Events 40 30 20 s = 2.3 cm 10 0 22 26 30 34 38 Reconstructed position [cm] 12 10 Fiducial volume 8 5 keV 6 10 keV 4 50 keV 2 0 100 keV 500 keV 1 MeV 0 10 20 30 40 Distance from the center [cm] 13 R_reconstructed(cm) 50 45 40 35 30 25 20 5keV ~ 1MeV Generated VS reconstructed • 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 • In the 40cm~44cm region, reconstructed events are concentrated around 42cm, but they are not mistaken for those occurred in the center 15 • No wall effect 10 • Out of 45cm, some events occurring behind the PMT are miss reconstructed 5 0 5 10 15 20 25 30 35 40 45 50 Distance from the center [cm] 14 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 15 800kg BG study Achieved (measured by prototype detector) Goal (800kg detector) g ray from PMTs ~ 10-2 cpd/kg/keV 1/100 10-4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) 238U 232Th = (33±7)×10-14 g/g → Remove by filter < 23×10-14 g/g (90% C.L.) → Remove by filter (Only upper limit) Kr = 3.3±1.1 ppt → Achieve by 2 purification pass 1/33 1×10-14 g/g 1/12 2×10-14 g/g 1/3 1 ppt 16 Estimation of g ray BG from PMTs Counts/keV/day/kg All volume R<39.5cm R<34.5cm R<24.5cm • U-chain • 1/10 lower BG PMT than R8778 Statistics: 2.1 days All volume R<39.5cm R<34.5cm R<24.5cm No event is found below 100keV after fiducial cut (R<24.5cm) < 1×10-4 cpd/kg/keV can be achieved (Now, more statistics is accumulating) Energy [keV] 17 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 wa Liq. Xe Configuration of the estimation 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 water MC geometry 18 Detected/generated*surface [cm2] g attenuation Initial energy spectrum from the rock 104 g attenuation by water shield 103 102 Deposit energy spectrum (200cm) 10 1 10-1 PMT BG level 10-2 0 100 200 300 Distance from LXe [cm] More than 200cm water is needed to reduce the BG to the PMT BG level 19 fast neutron attenuation water: 200cm, n: 10MeV • Fast n flux @Kamioka mine: (1.15±0.12) ×10-5 /cm2/sec • Assuming all the energies are 10 MeV very conservatively water < 2×10-2 counts/day/kg Liq. Xe No event is found from the generated neutron of 105 200cm water is enough to reduce the BG to the PMT BG level BG caused by thermal neutron is now under estimation 20 New excavation @Kamioka mine New excavation for XMASS and other underground experiment will be made soon ~5 m ~20 m ~15 m 21 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 22 Backup 23 800kg detector: Main purpose: Dark Matter search ~80cm diameter External g ray BG: 60cm, 346kg 40cm, 100kg Achieved 5 keV pp & 7Be solar n 10 keV Photoelectrons (p.e.) ~800-2” PMTs immersed into liq. Xe 70% photo-coverage ~4 p.e./keV Expected dark matter signal (assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,) 24 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 25 5.8cm (edge to edge) Hamamatsu R8778MOD(hex) Hexagonal quartz window Effective area: f50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778 5.4cm 0.3cm (rim) 12cm c.f. R8778 U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq 40K 1.4±0.2x10-1 Bq Prototype has been manufactured already Now, being tested 26 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 U Th 1 2 3 ※measured by HPGe detector in Kamioka 40K 1 2 3 1 2 1Ceramic dielectric parts to support dynodes 1 For R8778mod using quartz 2 2 Glass parts for feed through & containment For R8778mod Reduce glass material Improvement result will be coming soon! 27 BG levels Events/kg/keV/day DAMA NaI ZEPLIN before PSD cut Kamioka Ge Current XMASS (new improvement!) XMASS 800kg CDMS II After PID Heidelberg Moscow KamLAND (>0.8MeV) DM signal for LXe 100GeV 10-6pb Super-K 28 R&D status using prototype detector 100kg prototype Main purpose Confirmation of estimated 800 kg detector performance ~30 cm cube 3 kg fiducial With light guide version 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 BG study Understanding of the source of BG Measuring photon yield and its attenuation length 29 100 kg prototype detector In the Kamioka Mine (near the Super-K) 2,700 m.w.e. OFHC cubic chamber 54 2-inch low BG PMTs Hamamatsu R8778 16% photocoverage Liq. Xe (31cm)3 Gamma ray shield MgF2 window 30 4p shield with door material 1.0m 1.9m thickness Polyethylene 15cm Boron 5cm Lead 15cm EVOH sheets 30μm OF Cupper 5cm Rn free air (~3mBq/m3) 31 100 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 32 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. exp(- m ) m n Log(L) = Log( ) n ! PMT QADC L: likelihood F(x,y,z,i) x total p.e. m: S F(x,y,z,i) n: observed number of p.e. F(x,y,z,i): acceptance for i-th PMT (MC) VUV photon characteristics: Lemit=42ph/keV tabs=100cm tscat=30cm FADC Hit timing === Background event sample === QADC, FADC, and hit timing 33 information are available for analysis Performance of the vertex reconstruction Collimated g ray source run from 3 holes (137Cs, 662keV) hole C hole B hole A DATA MC + + + C BA → Vertex reconstruction works well 34 Performance of the energy reconstruction Collimated g ray source run from center hole (137Cs, 662keV) All volume 20cm FV 10cm FV s=65keV@peak (s/E ~ 10%) Similar peak position in each fiducial. No position bias → Energy reconstruction works well 35 Demonstration of self shielding effect z position distribution of the collimated g ray source run → Data and MC agree well γ 36 Event rate (/kg/day/keV) Shelf shielding for real data and MC ~1.6Hz, 4 fold, triggered by ~0.4p.e. 3.9days livetime REAL DATA Aug. 04 run preliminary MC simulation All volume 20cm FV All volume 20cm FV 10cm FV (3kg) 10cm FV (3kg) 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) 37 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) Delayed coincidence search (radiation equilibrium assumed) 214Bi 214Po 210Pb a (7.7MeV) b (Q=3.3MeV) t1/2=164ms Th-chain < 23x10-14 g/g(90%CL) (by prototype detector) Delayed coincidence search (radiation equilibrium assumed) 208Po 212Bi 212Po a (8.8MeV) b (Q=2.3MeV) t1/2=299ns 38 Kr concentration in Xe cpd/kg/keV 85Kr makes BG in low enegy region 102 Target = Xe Kr 0.1ppm 1 10-2 DM signal 10-4 (10-6 pb, 50GeV, 100 GeV) 10-6 0 200 Kr can easily mix with Xe because both Kr and Xe are rare gas 400 600 800 energy (keV) Commercial Xe contains a few ppb Kr 39 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 Boiling point (@2 atm) • Design factor : 1/1000 Kr / 1 pass • Purified Xe : Off gas = 99:1 Raw Xe: ~3 ppb Kr Lower (178K) ~3m ~1% Xe 178.1K Kr 129.4K Off gas Xe: 330±100 ppb Kr (measured) Purified Xe: Operation@2atm Higher (180K) ~99% 3.3±1.1 ppt Kr (measured) (preliminary) 40 Remaining problem: wall effect (only for the prototype detector) HIT HIT ? Dead angle 1 MC If true vertex is used for fiducial volume cut 10-1 HIT HIT HIT 10-2 Scintillation lights at the dead angle 0 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 Energy (keV) No wall effect This effect does not occur with the sphere shape 800 kg detector 41 800 kg detector 42
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