Direct Detection of SUSY Cold Dark Matter in Liquid Xenon

Transcription

1 The XENON Project Direct Detection of SUSY Cold Dark Matter in Liquid Xenon One Tonne - Have we got what it takes? Columbia University: E. Aprile (PI), E. Baltz, A. Curioni, K-L. Giboni, C. Hailey, L. Hui, M. Kobayashi, P. Majewski and K.Ni Brown University: Richard Gaitskell Princeton University: Tom Shutt Rice University: Uwe Oberlack LLNL: William Craig

2 The XENON Project: Overview Liquid Xenon is an excellent target material for Cold Dark Matter WIMPs, and likely the only practical one, for a sensitive experiment of the scale required by most SUSY predictions. Driven by the compelling science case and with the confidence in the LXeTPC technology which Columbia has developed for g-ray astrophysics (with NASA support), we submitted a proposal to NSF on Oct 11, 2001, for an accelerated two year research program leading to a demonstration of the XENON design concept with a 10 kg prototype. The 1- tonne XENON experiment would be realized with an array of ten position sensitive LXeTPCs, each with 100 kg Xe target and sourrounded by several cm of LXe as active scintillator shield. Using both light and charge (amplified in gas phase) and the intrinsic imaging capability of a TPC, the goals for XENON are a low energy threshold (~16 kev) and an excellent electron/nuclear recoil discrimination (>99.5%). With an estimated total background rate of 2 x 10-5 counts/kg/kev/day XENON should reach the sensitivity of 1 event /100 kg/year or s~10-10 pb, probing the lowest SUSY parameter space. Following a SAGENAP review on March 12, 2002, NSF has funded the 2-year program, starting on September 1, The outcome of this R&D phase will define the design for the 100 kg unit module, taking into account the low activity materials requirement. For the next phase of construction and underground operation of the XENON array, both the development /support of US National Underground Laboratory, and a strong worldwide collaboration, will be vital for the success of the XENON dark matter experiment.

3 Current & Next Generation Experiments & SUSY Theory Range Edelweiss (June 2002) ~0.25 event/kg/d ~1 event/kg/yr ~ 1 event/100 kg/yr

4 Typical WIMP Signal Ú E r dn de Xe E th =16 kevr gives 1 event/kg/day Example cross-section shown is at current (90%) exclusion limits of existing experiments Experimental Requirements Energy Threshold : as low as possible Target Mass: as high as possible Background: as low as possible

5 Liquid Xenon for WIMPs Direct Detection q High mass Xe nucleus (A ~131) good for WIMPs S.I. Interaction ( s ~A 2 ) q Odd Isotopes with large spin-dependent enhancement factors q High atomic number (Z=54) and density (r=3g/cc) of liquid state good for compact and flexible detector geometry q Production and purification of Xe with << 1ppb O 2 in large quantities for tonne scale experiment. Easy cryogenics at 100 o C q Excellent ionizer and scintillator with distinct charge/light ratio for electron/nuclear energy deposits for high background discrimination q No long-lived radioactive isotopes. 85 Kr contamination reducible

6 and for Solar n and 0nbb Decay 124 Xe (0.10%) 126 Xe (0.09%) 128 Xe 129 Xe 130 Xe (1.92%) (26.4%) (4.07%) Mostly Odd 131 Xe (21.2%) 132 Xe 134 Xe (26.9%) (10.4%) Mostly Even 136 Xe (8.87%) Separation here bb -nucleus Odd enriched Solar neutrino Dark matter Spin dependent Even enriched:containing 136 Xe 2 nbb /0 nbb Dark matter Spin independent XMASS EXO LXe prototype in Kamioka LXe prototype at Stanford

7 Xenon Phase Diagram

8 Properties of LXe vs Ge and Si

9 Ionization and Scintillation in Liquid Xenon Ionization (Xe +, e) Excitation (Xe*) I/S (electron) >> I/S (non relativistic particle) Alpha scintillation Recombination L/L0 or Q/Q0 (%) Electron charge electron scintillation Xe 2 * ( 1 S u, 3 S u ) fi 2Xe+hn (175 nm) Fast Slow Alpha charge Electric Field (kv/cm)

10 Recombination and Attachment reduce electron signal t=1/k s [S] l= t v d = tme e - + S Æ S k s - High drift field High purity gas low-outgassing materials choice of purifiers and materials must be compatible with the low cosmogenics requirements

11 Spatial Resolution E r Technical limits Physical Limit electron cloud v d transverse longitudinal field line distortions, electronic noise, and effects specific to signal readout scheme Electron Diffusion L r d t d = v d me m = v d spread in electron cloud: D = diffusion = mobility coefficient s = diffusion depends on drift path L d 2Dtd ed 2 ed = k T = < e > T K ev m 3 LXe = 165 Æ ª 0. 3 r m r electron energy depends on E Æ D(E) s a few mm ª L d m

12 Statistical limit Fano Factor limit WF (liquid argon) is 2.54 WF (liquid xenon) is 0.64 Energy Resolution 1/ N N = E / W - value DE E F = 2.35 = 2.35 N DE DE ª 4keV ª 2keV FW 1MeV If all charges are collected and if full energy is absorbed in the liquid, the contributions to the energy resolution of a liquid ionization chamber are: / 2 DE D E T i DE DE DE n s r = 2.35[ DEi + DEn + DEs + DEr Ionization straggling Electronic noise Positive ion effect Rise time effect ]

13 Columbia Experience with LXe Detectors q A 30 kg Liquid Xenon Time Projection Chamber developed and successfully tested at balloon altitude for Compton Imaging and Spectroscopy of Cosmic Gamma- Ray Sources MeV q NASA supported R&D on LXe and development of balloon-borne detector technology LXeGRIT payload. q Road to LXeGRIT: extensive studies of LXe ionization and scintillation properties, purification techniques to achieve long electron drift for large volume application, energy resolution and 3D imaging, electron mobility etc.

14 The Columbia LXeTPC 30 kg 30 kg active Xe mass 20 x 20 cm 2 active area 8 cm drift with 4 kv/cm Charge and Light readout 128 wires/anodes digitizers 4UV PMTs

15 Electron vs Nuclear Recoil Discrimination in XENON Measure both direct scintillation(s1) and charge (proportional scintillation) (S2) Nuclear recoils from WIMPs Neutrons Electron recoils from Gammas Electrons Proportional scintillation depends on type of recoil and applied electric field. electron recoil S2/ S1 >> 1 nuclear recoil S2/ S1 ~0 but detectable if E large Drift Time ~1 s ~40ns Gas e - anode grid Liquid cathode g-ray E

16 The XENON Experiment : Design Overview The XENON design is modular. An array of 10 independent 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass, is used to make the 1-tonne scale experiment. The fiducial LXe volume of each module is self-shielded by additional LXe. Active shield very effective for charged and neutral background rejection. One common vessel of ~ 60 cm diameter and 60 cm height is used to house the TPC teflon and copper rings structure filled with the 100 kg Xe target and the shield LXe (~50 kg ).

17 XENON TPC Signals Time Structure Both Direct and Proportional Scintillation Signals detected by the same PMTs Array t~45 ns 150 µs (300 mm) Three distinct signals associated with typical event. Amplification of primary scintillation light with CsI photocathode important for low threshold and for triggering. Event depth of interaction (Z) from timing and XY-location from center of gravity of secondary light signals on PMTs array. Effective background rejection direct consequence of 3D event localization (TPC)

18 Detection of Xe Light with a CsI Photocathode Stable performance of reflective CsI photocathodes with high QE of 31% in LXe has been demonstrated by the Columbia measurements CsI photocathodes can be made in any size/shape with uniform response, and are inexpensive. LXe negative electron affinity Vo(LXe)= ev and the applied electric field explain the favorable electron extraction at the CsI-liquid interface. Aprile et al. NIMA 338(1994) Aprile et al. NIMA 343(1994)

19 Hamamatsu Low Temperature Tube (R6041) u u u u XENON Baseline Readout: PMTs Developed for LXe detectors. Shown to work reliably at low T and at P< 5 atm Metal construction, compact design, recent tests at Columbia with custom designed HV divider show simultaneous light/charge with good yield Low Background version under study by Hamamatsu Low Quantum Efficiency~10-15% Hamamatsu Low Background Tube (R7281) u u u Being tested by Xmass Collaboration Room temperature tests only so far Metal construction, and giving lower backgrounds ~500 cts/tube/day (XENON baseline goal:~ 100) Higher Quantum Efficiency~27-30% Uses longer optics which give better focusing (could be accommdated in XENON)

20 Light Collection Efficiency for XENON Hamamatsu R6041 Assumptions W ph : 13 ev l ph : 1.7 m Quenching Factor: 25% Q.E. of PMTs: 26% Q.E. of CsI : 31% R.E of Teflon Wall: 90% Xe Mass: 100 kg 37 PMTs (2 inch) array

21 Baseline - Simulation Results 16 kev recoil threshold event Assumes 25% QE for 37 phototubes, and 31% for CsI photocathode With a W ph = 13 ev, a 16 kev (true) nuclear recoil gives ~ 24 photoelectrons. The CsI readout contributes the largest fraction of them. Multiplication in the gas phase gives a strong secondary scintillation pulse for triggering on 2-3 PMTs. We trigger on this amplified ( UV photons/electron) CsI signal. Coincidence of direct PMTs sum signal and amplified light signal from CsI. Trigger being the last signal in time sequence post-triggered digitizer read out. Trigger threshold can be set very low because of low event rate and small number of signals to digitize. PMTs at low temperature low noise Even w/o CsI (replaced by reflector) we still expect ~6 pe. Several ways to improve light collection possible

22 Scintillation Efficiency for Nuclear Recoils in LXe F. Arneodo et al. NIMA 449(2000) Si Lindhard theory no LXe data at low energy LXe

23 Neutrons Induced Background Soft neutrons from muons not important if deep underground. (a,n) neutrons from rock Readily reduced by 20 cm moderator. Very high energy neutrons from muons Needs further study. Depends on site. From U/Th from materials inside shield Low cosmogenic materials selection -2 y Muon Intensity, m Muon flux vs overburden Proposed NUSL Homestake Current Laboratories WIPP Soudan Kamioka Homestake (Chlorine) Depth, meters water equivalent Gran Sasso NUSL - Homestake Baksan Mont Blanc Sudbury

24 Radioactive impurities in Xe. No long lived Xe isotopes. 85 Kr t 1/2 =10.7y, b KeV. g and b induced background Commercial research grade Xe: 10 ppm Kr -> 200 counts/kg/kev/day Need 0.1 ppb for 1x10-5 counts/kg/kev/day (after discrimination). 42 Ar More readily removed than Kr. Rn Emanation from components, welds. Typical values: 0.1 mbq -> 10-4 counts/kg/kev/day Probably adsorbed on cryostat surfaces. U, Th, K Very low solubility for ionic impurities in Xe. Particulates removed by filtering.

25 Background from PMTs Standard PMTs very hot. Example: Borexino 8 Ø ultra-low-background PMTs: U 1.4 Bq; Th 0.2 Bq; K 1.9 Bq Problems: Glass, Ceramic, Dynodes, Components New PMTs. Quartz windows, metal cans. Hamamatsu R7281Q (being tested for XMASS) activity: 4.5 (tube) (base) mbq Nearly a 1000 fold-improvement! Burle microchannel-plate based PMT. R&D for low background, low temperature PMT for XENON started Goal: Cu+sapphire 1g glass MCP. Monte Carlo simulations With 20 PMTs, each at 6 mbq, 5 cm fiducial volume cut background after discrimination = 2 x 10-5 counts/kg/kev/day.

26 GEMs Charge readout : a promising alternative to PMTs High gain in pure Xe with 3GEMs demonstrated Coating of GEMs with CsI 2D readout for mm resolution R&D for XENON- CERN/Rice/Princeton Bondar et al.,vienna01

27 XENON Phase 1 Study: 10 kg Chamber Demonstrate electron drift over 30 cm (Columbia) Measure nuclear recoil efficiency in LXe (Columbia) Demonstrate HV multiplier design (Columbia) Measure gain in Xe with multi GEMs (Rice and Princeton ) Test alternative to PMTs, i.e. LAAPDs (Brown) Selection and test of detector materials (LLNL) Monte Carlo simulations for detector design and background studies (Columbia /Princeton/Brown) Study Kr removal techniques (Princeton) Characterize 10 kg detector response with g and neutron sources (Collaboration)

28 Construction Costs What do you estimate to be the construction costs for the 100 kg exp? u u u u u Include shielding, readout and support equipment costs (Xe, Purification, Cryo) [ $0.32M ] Xe: 100 kg Active Target + ~100 kg Active Shield $1.6/g ($6/g CDMS cryo-detector grade) 1 module $320k of Xe (1 tonne active Xe -> $1.6m) [ ~$1M ] Xe Purification + Gas System / Handling / Circulation [ ~$0.5M] Kr Removal [ ~$1M ] Design + Construction of 1x100 kg module u [ ~$0.2M ] Clean Room Class 1000 u [ ~$1.6 M ] Readout, DAQ, Shield [$4.6M] Total

29 XMASS experiment at Kamioka -- Double Phase Xe detector for dark matter searchfor XMASS collaboration M. Yamashita (Waseda university) Contents Introduction to XMASS Double phase detector and shield setup Preliminary result and Background Summary 23/Feb/2002 NDM02 at IISAS (Kyoto)

30 The Japanese Dark Matter Program at Kamioka

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32 Tokyo University ICRR,Kamioka observatory Y. Suzuki, M. Nakahata, Y. Itow, M. Shiozawa, Y. Takeuchi, S. Moriyama, T. Namba, M. Miura, Y. Koshio, Y. Fukuda, S. Fukuda ICRR, RCNN T. Kajita, K. Kaneyuki, A. Okada, M. Ishituka Saga University T. Tsukamoto, H. Ohsumi, Y. Iimori Niigata University K. Miyano, K. Ito Tokai University K. Nishijima, T. Hashimoto Gifu University S. Tasaka Waseda University S. Suzuki, M. Yamashita, T. Doke, J. Kikuchi, K. Kawasaki TIT Y. Watanabe, K. Ishino Seoul National University Soo-Bong Kim, In-Seok Kang INR-Kiev Y. Zdesenko, O. Ponkratenko UCI H. Sobel, M. Smy, M. Vagins XMASS Collaboration XMASS Xenon MASSive detector for Solar neutrino (pp/ 7 Be) Xenon detector for Weakly Interacting MASSive Particles (Dark Matter search) Xenon neutrino MASS detector (double beta decay)

33 Idea for detector Now, we develop two type of detector for low background experiment. Self shielding Surrounded by 30cm Xenon 10t scale Double phase Particle id for rejection BG

34 Set up (shield) Detector is cooled and kept by cold finger contact from Liq. N 2 at 170K. Detector OFHC(5cm) OFHC(5cm) H.V. and signal feed through is out side of the shield Liq. N 2 Dewar Boric acid (5g/cm 2 ) Lead (15cm) Polyethylene(15cm)

35 Set up (detector) Cold finger gas filling line Wire set (Grid1,Anode Grid2) PTFE Teflon (Reflector) MgF 2 Window with Ni mesh (cathode) Gas Xe Liq. Xe(1kg) OFHC vessel (5cm) PMT

36 1kg Double Phase Xe Detector Teflon Field shaping ring surrounded by 5cm OFHC PMT

37 ucalibration counts 57 Co (122keV) /E = 15 % 2.4 [p.e./kev] at 250[V/cm] 137 Cs 662keV p.e. with R7281MgF 2 (Q.E.30%) (HAMAMATSU(prototype)) counts p.e.

38 Background Spectrum flow Super Radon free air (3mBq/m 3 ) around detector Low energy part clean room in Kamioka mine Kr free Xe(10ppb Kr) #normal Xe( 10ppm

39 Preliminary urejection Preliminary gamma region Recoil region Rejected Recoil Need to define Recoil band of this detector by neutron source Performance was not good in low energy part ( _ )

40 ubackground rate in Z-axis normalized in radius PMT 0 Z-axis PMT 9.5cm Background from Outside. need to reduce it! Z-axis Z-axis Z-axis Z-axis

41 ubackground(monte Carlo) Test Run Total PMT steel(shaping ring) These material was main component of background. (using value of 1 upper limit) PMT Base 85 Kr steel vessel DECAY4 has been used for the simulation: Y. Zdesenko, O. Ponkratenko Kr 10ppb

42 Background(Improvement) Material Total (Bq/PMT) Material Total (Bq/PMT) PMT(prototype) HAMAMATSU PMT(R7281Q) HAMAMATSU PMT base HAMAMATSU PMT(with R7281Q) HAMAMATSU Steel(Vessel, shaping ring ) (Bq/kg) OFHC < (Bq/kg)

43 Expected results (Spin Independent case) Quench factor = 0.2 Test run preliminary Kr free natural Xenon (Kr 10ppb) no rejection low back PMT (expected) 99% rejection expected EDELWEISS astro-ph/ CDMS Phys. Rev. Lett. 84 (2000) p.5699 DAMA/NaI-1 to 4 combined, Phys. Lett. B480 (2000) 23-31

44 XMASS: Next Step (2003)