1 1. Introduction kg detector design 3. Summary XMASS experiment 15 th Sep A.Takeda for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo IDM2006, Rhodes, Greece

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  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 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  tracking MC from external to Xenon Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume Background are widely reduced in < 500keV low energy region U-chain gamma rays

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

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

9 Expected sensitivities Large improvements will be expected SI ~ cm 2 = pb SD~ cm 2 = 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 Cross section to nucleon [pb] Plots except for XMASS: Gaitskell & Mandic

10 Basic performances have been already confirmed using 100 kg prototype detector  Status of 800 kg 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 Hex 1PMT 12 pentagons / pentakisdodecahedron  Structure of 800 kg detector 5 triangles make pentagon 34cm 31cm 10 PMTs per 1 triangle agonal quarts window Hamamatsu R8778MOD

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

13  Event reconstruction Generated R = 31cm E = 10keV  = 2.3 cm Reconstructed position [cm] Events 5 keV 10 keV 50 keV 100 keV 500 keV1 MeV Fiducial volume Distance from the center [cm]  (reconstructed) [cm] Position of fiducial volume 10 keV ~ 3.2 cm 5 keV ~ 5.3 cm

14 5keV ~ 1MeV Distance from the center [cm] R_reconstructed(cm) Generated VS reconstructed No wall effect Out of 45cm, some events occurring behind the PMT are miss reconstructed In the 40cm~44cm region, reconstructed events are concentrated around 42cm, but they are not mistaken for those occurred in the center 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

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

16 Achieved (measured by prototype detector) Goal (800kg detector)  ray from PMTs ~ 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 1/100 1/33 1/12 1/3  800kg BG study

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

18  Water shield for ambient  and fast neutron wa water Liq. Xe Generation point of  or neutron Necessary shielding was estimated for the estimation of the size of the new excavation MC geometry 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

19   attenuation PMT BG level  attenuation by water shield Distance from LXe [cm] Detected/generated*surface [cm 2 ] More than 200cm water is needed to reduce the BG to the PMT BG level Initial energy spectrum from the rock Deposit energy spectrum (200cm)

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

21  New mine New excavation for XMASS and other underground experiment will be made soon ~5 m ~15 m ~20 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 10 2 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 ~800-2” PMTs immersed into liq. Xe 70% photo-coverage ~80cm diameter ~4 p.e./keV 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 Photoelectrons (p.e.) 5 keV 10 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) 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

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

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

29 100kg prototype With light guide version 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

30 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

31 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

kg Run summary 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

33    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

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

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

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

37 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. Event rate (/kg/day/keV) /kg/day/keV Aug. 04 run preliminary 3.9days livetime ~1.6Hz, 4 fold, triggered by ~0.4p.e.

38 Kr = 3.3±1.1 ppt (by mass spectrometer) → 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

39 Kr 0.1ppm DM signal (10 -6 pb, 50GeV, 100 GeV) 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

40 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

41 HIT ? MC If true vertex is used for fiducial volume cut Energy (keV) 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

42  800 kg detector