XMASS experiment Dark matter search with liquid Xe detector

Transcription

1 experiment Dark matter search with liquid Xe detector Akihiro MINAMINO (ICRR) for collaboration Contents Direct search of Dark matter experiment Dark matter search with 800kg detector R&D status of prototype detector Summary KEKPH KEK (04/Mar/2004)

2 Direct search of Dark matter Candidate of Cold Dark Matter Neutralino Target of Lightest Supersymmetric Particle (LSP) Stable from R-parity conservation Weakly Interactive Massive Particle (WIMP) Cross section SUSY model dependent

3 WIMPs neutralino search Indirect detection Observe antiproton, proton, γ ray, produced by WIMPs annihilation in the Galactic halo BESS, GLAST, Observe neutrino produced by WIMPs annihilation in the heavy bodies (Earth, Sun, ) SK, AMANDA Direct detection Observe the recoil signal of detector nuclei by WIMPs CDMS, DAMA,,

4 WIMPs signal and Annual modulation Dark halo 232km/s December Maxwell dist., v 0 =220km/s 2D plot of June Sun Revolution 30km/s cpd/kg/kev Variation of makes annual modulation of signal Annual modulation of WIMPs signal 1 Target = Xe, 10-2 SI, mχ=100gev, 10-6 pb for proton June December difference ~ 1% visible energy (kev) Signals concentrate on low E ~ O(10keV

5 Trend of WIMPs direct search experiments Direct detection of WIMPs Nuclear Recoil Recoiled nuclei are mainly observed by 3 ways Scintillation Photon PMT Ionization Charge Semiconductor, Phonon Heat Bolometer, Reading two signal by Ge detector is the recent trend CDMS, EDELWEISS, are developing these detectors Reduce γ ray BG by comparing two signals choose the different way Make large mass detector (with liquid Xe) Reduce γ ray BG by fiducial volume cut (Self shielding) Observation style is the same as SK, KamLAND,

6 CDMSII experiment at Soudan mine Soudan mine, Minnesota, USA, Depth =2000 m.w.e Ionization and Phonon are obserbed by ZIP detector γ ray BG can be reduced by comparing 2 signal Ge(4kg) & Si(1kg) Sensitivity for SI = GeV (World record) Calibration WIMPs search Ionization signal Electron recoil Nuclear recoil ZIP detector Phonon signal Phonon signal Ionization signal Electron recoil Nuclear recoil

7 Goals experiment Direct detection of Dark Matter Discovery of Dark Matter Real time observation of low energy solar ν (pp, 7 Be) Precise determination of ν oscillation parameters Observation of 0νββ decay Majorana property and absolute mass of ν = Multipurpose Ultra low-background detector with liquid Xe

8 Key idea Self shielding for γ ray background by liquid Xe (Z=54) Volume for shielding Liquid Xe Fiducial volume PMTs Reconstruct the vertex of events from PMTs information γ ray backgrounds are absorbed in outer volume Dark matter can go into fiducial volume

9 Advantages of liquid Xe scintillator Large Atomic number (Z=54) Self shielding for ray background Large photon yield (~42000photons/MeV NaI(Tl)) Energy threshold can be lowered High density (3 g/cm 3 ) Large mass detector with compact volume High boiling point (165 K) Liquid Uniform detector possible (like Super- Kamiokande) Possible to purify during operation

10 Strategy of the scale-up Prototype detector 800kg detector 10ton detector Now, here 30cm (FV 3kg) 80cm 2.5m ood result for next step! R&D Dark matter search Multipurpose detecto

11 Kamioka underground laboratory Depth = 1000 m (2700 m.w.e.) µ flux * reduction of the ground flux Kamioka mine neutron flux ~10-3 reduction of the ground flux Good place for dark matter search (and ν observation) Other groups are also planning to do the dark matter search experiment in this underground laboratory -DP (Waseda Univ.), NEWAGE (Kyoto Univ.), CaF 2 (Eu) exp. (Univ. Tokyo), CANDLES (Osaka Univ.), *1 SuperKamiokade Collaboratio

12 800kg detector 800kg liquid Xe detector Target = Dark matter search 80cm diameter sphere (~800 PMTs) 40cm diameter sphere for fiducial volume Effective self shielding Photo coverage ~ 70% Immerse PMTs into liquid Xe p.e. yield ~ 5 p.e./kev Energy threshold ~ 5keV events [events/(1000events)/p.e.] kev 10 kev 5keV 10keV [p.e.] p.e.

13 Development of PMT for 800kg detector and Hamamatsu photonics developed the low background PMT for prototype detector (R8778). Bleeder : Glass epoxy PCB PTFE PCB Glass tube Metal tube We are developing the hexagonal PMT for 800kg detector to increase photo coverage and trying to reduce the radioactive impurities to 10-1 of R8778. R8778 by Hamamatsu for prototype Hamamatsu Hexagonal PMT by for 800kg detector

14 Estimated background of 800kg detector cpd/kg/kev Low background PMTs (10-1 reduction of RI) Low radioactive impurities in Xe (Kr, U & Th-chain) 10-4 reduction of fast neutron flux kev Dark matter signal (SI, 10-6 pb for p) m DM =50GeV m DM =100GeV γ ray background (40cm FV dia 100kg) Fast neutron background

15 Cross section for nucleon (pb) Sensitivity of 800kg detector for DM FV 100kg, 5 year, E th =5keVee~25p.e., 3σ discovery Spin independent interaction WIMP Mass (GeV) 10 6 >10 2 improvement of sensitivity for existing experiments Spin dependent interaction Edelweiss Al2O3 Tokyo LiF NaF Modane NaI CRESST UKDMC NaI NAIAD (Ann. Mod.) (Sepc.) WIMP Mass (GeV)

16 Cross section for nucleon (pb) Compare with theoretical prediction FV 100kg, 5 year, E th =5keVee~25p.e., 3σ discovery Spin independent interaction CDMS II (Ann. Mod.) WIMP Mass (GeV) E. A. Baltz and P. Gondolo, Physical Review D 67(2003) (Sepc.) 10-6 MSSM, cosmology, g-2, b->sγ, e + e - collider 10-8 Spin dependent interaction 10 1 CRESST DAMA 129Xe (Ann. Mod. (Sepc.) WIMP Mass (GeV) J. Ellis et al, Physical Review D 63(2001) MSSM, cosmology, b->sγ, e + e - collider

17 Presentation by Rick Gaitskell at IDM2004 workshop DM Direct Search Progress Over Time

18 R&D status of prototype detector Prototype detector Liquid Xe in ~30cm cube (30liter) High purity copper chamber 3kg fiducial volume Targets Confirmation of 800kg detector performance estimation Reconstruct vertex and energy of events by fitter Demonstrate the self shielding power for γ ray BG Measure photon yield and its attenuation length Understand the environmental BG inside the shield Measure a content of radioactive impurities in Xe

19 Progress from KEKPH meeting 2004 Prototype detector 1 st run (Dec 2003) Confirm performance of Energy & Vertex reconstruction Confirm performance of self shielding power for γ ray Realize the low background environment Measure the internal background concentration Reported at KEKPH meeting 2004 Prototype detector 2 nd run (Aug 2004) Succeed to reduce Kr from Xe by distillation Photo electron yield is increased Rn concentration inside the shield was measured Report now

20 Internal backgrounds in Xe Radioactive impurity in Xe emit radiation in fiducial volume Need to reduce these internal backgrounds Kr = ppt (by Mass spectrometer) Achieved by distillation U-chain = (33 7)x10-14 g/g (by prototype detector) Delayed coincidence search 214 Bi 214 Po 210 Pb β (Q=3.3MeV) α (7.7MeV) τ 1/2 =164µs Th-chain < 23x10-14 g/g(90%cl) (by prototype detector) Delayed coincidence search 212 Bi 212 Po 208 Po β (Q=2.3MeV) α (8.8MeV) τ 1/2 =299ns (radiation equilibrium assumed) (radiation equilibrium assumed)

21 Kr concentration in Xe β decay of 85 Kr makes BG in low energy region Background for DM search (and n observation) Target = Xe 10 Both Kr and Xe are rare gas Kr can easily mix with Xe event rate (event/kev/day/kg) Kr 0.1ppm DM signal 10-4 (10-6 pb, 50GeV, GeV) energy (kev) energy (kev) Commercial Xe contains a few ppb Kr Only succeed to reduce it! (~3ppb 3.3ppt) cpd/kg/kev

22 Xe purification system 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 Raw Xe: ~3 ppb Kr Lower (178K) ~3m ~1% Xe Kr Boiling point (@2 atm) 178.1K 129.4K Off gas Xe: 330±100 ppb Kr (measured) Operation@2atm Higher (180K) ~99% Purified Xe: 3.3±1.1 ppt Kr (measured) (preliminary)

23 Summary of BG measurement Now(prototype detector) Goal(800kg detector) 1/100 γ ray BG ~ 10-2 cpd/kg/kev 10-4 cpd/kg/kev Increase volume for self shielding Decrease radioactive impurities in PMTs (~1/10) 1/ U = (33 7) g/g g/g Remove by filter 1/ Th < g/g (90% C.L.) g/g Remove by filter (Only upper limit) 1/3 Kr = ppt 1 ppt Achieve by 2 purification pass Very near to the target level!

24 Self shielding for real data and MC cpd/kg/kev Data 3.9days livetime cpd/kg/kev MC (U/Th/K from PMTs, etc) All volume 20cm FV 10cm FV Wall effect (only prototype detector) Good agreement real data and MC (<factor 2) Self shield effect can be seen clearly Very low background (~10-2 /kg/day/kev@ kev) energy(kev)

25 Wall effect (Only for the prototype detector) Scintillation light at the dead angle from PMTs give quite uniform 1 p.e. signal for PMTs This cause missreconstruction as if the vertex is around the center of the detector Immersing PMTs into liquid Xe and using spherical design solve this problem. 800kg detector HIT HIT? No wall effect! HIT HIT HIT

26 Prototype detector 3 rd run (Mar 2005) BG measurement with PTFE light guide Remove wall effect and understand the low energy BG Confirm no unknown BG source in low energy region Shaving Washing 10cm 10cm 10cm Drying Installation

27 Efficiency Expected BG of PTFE light guide run MC simulation was done with GEANT Efficiency curve 0.48p.e./keV cpd/kev/kg Signal window (10-15keV) BG spectrum PMT K PMT Th PMT U Fast neutron BG (90% C.L. upper limit) energy(kev) Very low BG ~ 10-2 <100keV energy(kev) Expected BG~10-2 cpd/kev/kg

28 Summary experiment Multipurpose low-background experiment with liquid Xe 800kg liquid Xe detector Designed for Dark matter search 10 2 improvement of sensitivity above existing experiments R&D with prototype detector (3kg FV) Good result for next 800kg detector 3 rd data taking are started soon for low energy BG study Start very high sensitivity Dark matter search experiment within few years!

29 collaboration ICRR, Kamioka Y. Suzuki, M. Nakahata, Y. Itow, S. Moriyama, M. Shiozawa, Y. Takeuchi, M. Miura, Y. Koshio, K. Ishihara, A. Takeda, T. Namba, H. Ogawa, S. Fukuda, Y. Ashie, A. Minamino, R. Nambu, J. Hosaka, K. Taki ICRR, RCNN T. Kajita, K. Kaneyuki, M. Ishitsuka Saga Univ. H. Ohsumi, Y. Iimori, Tokai Univ. K. Nishijima, T. Hashimoto, Y. Nakajima 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

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

31 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 Liq. Xe Material A F= 10mm 125 I (1kBq) Electrodeposition 5mm Length 60mm Source position Coating Material B Thick=3mm

32 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 Motor Wire Low energy γ source ( 103 Pd, 125 I, ) Position accuracy is within 1mm

33 Isotope separation of Xe Isotope separation is relatively easy ( 136 Xe = ββ nuclei) has 136 Xe enriched and reduced Xe Abundance measured by massspectrometer [%] cpd/kg/kev Natural abundance kev Solar ν pp 7 Be ββ ( 136 Xe) 2νββ : T 1/2 = year 136 Xe reduced Xe 136 Xe enriched Xe 0νββ : T 1/2 = year 0.06 ev < m ν < 0.09 ev

34 Window & PMT of prototype detector Photo coverage = 16% (through MgF 2 window) 0.6p.e./keV(10% of 800kg detector) 2-inch low background PMT 54 Bleeder : Glass epoxy PCB PTFE PCB Glass tube Metal tube R8778 by Hamamatsu

35 Shield for 100kg detector 1.9m 1.0m Outer Polyethelene(15cm) Slow down fast neutrons Boric acid(5cm) Absorb thermal neutrons Lead(15cm) Absorb γ rays EVOH sheet(30µm) Rn gas barrier Inner Copper(5cm) Absorb γ rays Super Rn free air ~ 3mBq/m 3 (KEK~40Bq/m 3

36 Prototype detector 2 nd run (Aug, 2004) Collimated γ ray source run Test the vertex and energy fitter Self shielding power for γ rays ray Copper γ φ1cm hole C B A Lead Background measurement inside the shield Understand the environmental background Content of radioactive impurities in Xe

37 Energy and vertex fitter Reconstruction is performed by PMT charge pattern using p.e. map made by MC Reconstructe here n exp( µ ) µ Log( L ) = Log( ) n! PMT L: likelihood µ: F(x, y, z) (total p.e./total acceptance) n: observed number of p.e. F(x,y,z,i): p.e. map by MC VUV photon characteristics: L emit =42ph/keV τ abs =60cm τ scat =55cm FADC === Background event sample === QADC, FADC, and TDC information are available for analysis QADC TDC(Hit timing)

38 Performance of the energy reconstruction Arbitrary Unit Collimated γ ray source from the center ( 137 Cs, 662keV) Data All volume 20cm FV 10cm FV Reconstructed Energy (kev) σ = 662keV (σ/e~10%) Energy reconstruction works well!

39 Performance of the vertex reconstruction Collimated γ ray source run from 3 holes ( 137 Cs, 662keV) Hole C Hole B Hole A DATA MC + C + B + A Vertex reconstruction works well!

40 Performance of the self shielding power z position distribution of the collimated γ ray source run 137 Cs(662keV) 60 Co(1173, 1333keV) Saturation cut BG subtracted Good agreement with MC Self shielding works as expected γ Center

41 Estimation of γ ray background Measured γ ray sources for estimation before measurement cpd/kg/kev Outside of the shield 238 U in PMTs 232 Th in PMTs 40 K in PMTs Outside of the shield 0.71 cm -2 s -1 (>500keV RI sources in PMTs 238 U : Bq/PMT 232 Th : Bq/PMT 40 K : Bq/PMT Pb in lead shield kev 210 Pb in the lead shield 250Bq/kg