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Dark Matter Search with Noble Liquids: the ArDM experiment

Dark Matter Search with Noble Liquids: the ArDM experiment. Sergio Navas University of Granada, Spain. Sixth International Heidelberg Conference on Dark Matter in Astro and Particle Physics (DARK MATTER 07) University of Sydney , Australia, 23-28 September 2007.

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Dark Matter Search with Noble Liquids: the ArDM experiment

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  1. Dark Matter Search with Noble Liquids: the ArDM experiment Sergio Navas University of Granada, Spain Sixth International Heidelberg Conference on Dark Matter in Astro and ParticlePhysics (DARK MATTER 07) University of Sydney, Australia, 23-28 September 2007

  2. A. Badertscher, A. Baeztner, R. Chandrasekharan, L. Kaufmann, L. Knecht, M. Laffranchi, A. Marchionni, G. Natterer, P. Otiougova, A. Rubbia(contact person), J. Ulbricht ETH Zurich, Switzerland C. Amsler, V. Boccone, S. Horikawa, C. Regenfus, J. Rochet Zurich University, Switzerland A. Bueno, M.C. Carmona-Benitez, J. Lozano, A.J. Melgarejo S. Navas, A.G. Ruiz University of Granada, Spain M. Daniel, M. Del Prado, L. Romero CIEMAT, Spain P. Mijakowski, P. Przewlocki, E. Rondio Soltan Institute Warszawa, Poland H. Chagani, P. Lightfoot, P. Majewski, K. Mavrokoridis, N. Spooner University of Sheffield, England The ArDM collaboration http://neutrino.ethz.ch/ArDM

  3. Argon as target for WIMP detection NIM A 327 (1993) 205 & NIM A 449 (2000) 147 WARP NIM A 574 (2007) 83-88 XENON arXiv:0706.0039 [astro-ph] WIMP High event rate due to high density [1.4 g/cc at 87 K (boiling point at 1atm)], high atomic number Argon WIMP Tmax 2MAr c22 (in therange of tens of KeV) (10-3) ArgonRecoil e-  e- e-  Scintillation from excited Ardimers Escaping ionization electrons  40 000  / MeV  42 000 e- / MeV Our aim is to detect the ionization charge and scintillation light independently 3 S. Navas (U. Granada), DARK07

  4. M. Suzuki et al. NIM 192 (1982) 565 Processes induced by charged particles in Argon • Columnar recombination decreases the secondary electron yield at the favor of scintillation photons. It is affected by an external drift field Edrift • Different ratio of scintillation to ionization for faster electron and slow ion tracks • Observed quenching of triplet (slow) component in high density ionization core Ar+ Ar* Ar* Ar2* tT≈ 1.6 ms tS ≈ 6 ns UV Light (two components) + Charge Ar2+ recombination l ≈ 128 nm Ar** • Various physical processes leading to scintillation & ionization • Yields are particle, energy and drift field dependent • Simulation describes different response to WIMP and MIPs 4 S. Navas (U. Granada), DARK07

  5. Experimental strategy • Ton-scale LAr detector providing self-shielding • Direct detection of ionization charge and primary scintillation light • LAr volume operated as TPC (3D event imaging) • charge readout with fine spatial granularity (transverse coordinate) • longitudinal coordinate from drift time (time difference between primary scintillation light and charge collection time) • Efficient rejection of external  background • Compton processes are source of low energy deposits within the fiducial volume • however, often producing multiple scatters in the active volume • Efficient rejection of neutron background • irreducible genuine nuclear recoils are produced by fast neutrons elastically scattering off target nuclei • however, high probability of multiple scatters within active volume

  6. Prototype layout GAr e- LAr E-field WIMP WLS + Light reflector WLS + Light reflector    • .Cylindrical volume, drift length ≈ 120 cm • 850 kg LAr target 8 Polyethylene pillars as mechanical support. 2x LEM for the electron multiplication and readout (Gain ≈ 103 – 104) Greinacher chain: supplies the right voltages to the field shapers rings and the cathode up to 500kV  ≈4 kV/cm 80cm The field shapers are needed to make an homogeneous 120 cm The aluminized foils reflect the scintillation light(>95%) Cathode: semi-transparent in order to let the scintillation light pass trough … PMTs below the cathode to detect the scintillation light. 6

  7. Cryogenics Detector vessel Liquid Argon should be kept free from electronegative impurities (O2 contamination < 1 ppb) LN2 out Secondary LAr out DEWAR Primary LAr out Insulation vacuum connection top Vacuum Insulation Liquefying system Insulation vacuum connection bottom Purification cartridge Secondary LAr Liquid N2 bath Vacuum superinsulation External wall Cryogenic connections bottom LN2 in / Secondary Argon in / Primary Argon in

  8. DEWAR at CERN

  9. High Voltage system for drift field generation • A cascade of rectifier cells (Greinacher/Cockroft-Walton circuit) • The total voltage we aim to reach is Vtot = 500 kV (≈ 4 kV/cm) • Tests in liquid nitrogen have been performed Cathode Top view Polypropylene capacitors 82 nF 2.5 kV/stage 210 stages

  10. Layout of the charge readout system 30 kV/cm Etransf = 3 kV/cm GARFIELD simulations indicate an expected single electron gain ≈ 104 Stable Measured Gain ≈ 104 measured Eextract = 5 kV/cm GAr LAr Gain 103 • Distance between stages: 3 mm • Avalanche spreads into several holes at second stage • Higher Gain reached as with one stage, with good stability • Hole dimension: 500 m diameter, 800 m distance. • Thickness of PCB: 1.6 mm 10 V/d (kV/cm)

  11. Segmented LEM • Final LEM charge readout system will be • segmented • Orthogonal strips readout • Number of channels:1024 • Strip width:1.5mm Prototype of a segmented LEM. • Kapton flex-prints are used for signal transfers to the readout electronics • The flex-prints, connected on one side to the LEM board, exit the dewar through a slot, sealed with epoxy-resin, in a vacuum tight feed-through flange Custom-made front-end charge preamp + shaper G ~15mV/fC 32 channels/cable to ZIF connectors on the LEM board to front-end preamplifers

  12. Layout of Light Readout system and PMT Photomultiplier tube: potential PMTs ETL 9357KFL (low background) R5912-MOD and R5912-02MOfrom Hamamatsu Wavelength shifter (WLS): Tetra-Phenyl-Butadiene (TPB) evaporated on reflector Reflectivity @430nm ~97% Shifting eff. 128 nm  430nm >97% 14 low background PMTs at the bottom of the detector immersed in LAr Tetratex reflecting foil

  13. GEANT4 simulation detector top view WLS on wall WLS on PMT • On average, 50% of the produced photons hit PMTs Average incident angle of photons on PMTs: 40º

  14. Light measurements in Liquid Argon (preliminary) real data Event separation in liquid argon 210Po radioactive source:  (5.4 MeV) +  (Q = 1.163 MeV) Scintillation light from  in 1200 mbar liquid argon   100 nVs PM Amplitude Por un lado hay más luz de centelleo DETECTADA de alphas que de betas events L50/Ltot 10-4 0 6000 Time (ns) •  events separate well from ,e events • Fast and slow light components distinguishable nVs ,e events 1u+ (singlet) t=6 ns (fast) Ar2* ~ 0.3 for  particles t=1.7 s (slow) 3u+(triplet) ~ 1.3 for  particles [Phys.Rev.B27 (1983) 5279]

  15. PMT tests in LAr PMT after LAr immersion test LAr purificationstation from241Am source Radioactivesource test TPB coatingtests No source

  16. Slow Control Devices • A series of custom designed Slow Control devices have been built, tested and installed to monitor temp., level, pressure … PT10K resistors Temperature sensor Levelmeters 10 K at 0ºC Range: -200 to 400ºC Capacity levelmeter Electronic circuits  0.7 pF/mm precission of  0.03 mm

  17. Background rejection e/g-like WIMP-like Visible energy e/g-like Amplitude Time Amplitude WIMP-like Time Charge/Light: Neutrons and WIMPs interact with the argon nucleus e-/interact with shell electrons Light shape: • Background rejection tools: • Different light/charge ratios • Different shape of the scintillation light (ratio fast/slow components) • Exploit 3D imaging capabilities of the detector

  18. Neutron Background from detector components Geant4 simulation No. of events 10 Neutron energy (MeV) 0 No. of recoils 300 Argon recoil energy (keV) 0 Neutron sources: •  Uranium and Thorium contamination (spontaneous fission) of the detector components and the surronding rock: • flux about 3.8  10-6 cm-2 s-1 (at 2450 m.w.e.) • can be shielded, e.g. by a hydrocarbon shield •  Muon-induced neutrons from surrounding rock, shielding and detector components High energy neutrons penetrate shielding, are thereby moderated and can cause WIMP-like events. Event numbers per year • Nuclear recoils: • 70% scatter more than once within the fiducial volume  advantage of large detectors • 10% produce a WIMP-like event (single scattering, recoil energy  [30,100] keV) Compared with ~ 3500 WIMP events at  = 10-43 cm-2 Low Background Materials are crucial

  19. Intrinsic background from Argon 39 isotope Natural argon from liquefaction of air contains small fractions of 39Ar radioactive isotope (well known to geophysicists) • Induced in atmospheric argon by cosmic rays • Concentration in natural Ar: 8.1x10–1639Ar/Ar [H.H. Loosli, Earth and Planetary Science Letters, 63 (1983) 51 and “Nachweis von 39Ar in atmosphärischem Argon” PhD thesis University Bern 1968] • T1/2 = 269 years, Q=565 KeV , <E>= 218 keV • Integrated rate in 1 ton LAr  1kHz [WARP Coll.] astro-ph/0603131 Energy(MeV) To suppress 39Ar fraction we consider using Ar extracted from well gases (extracted from underground natural gas). On the other hand, this source, evenly distributed in the target, provides precise calibration and monitoring of the detector response.

  20. Assembly at CERN Top flange Access platform Detector insertion Reflector foil

  21. The ArDM schedule for near future • Test of detector in vacuum, at CERN: • High voltage system, purity • Currently in preparation • Test with gaseous argon, at CERN: • PMTs, high voltage system and small version of LEM plates • Within a month • Test in liquid argon, at CERN: • Recirculation and purification system • Before end of 2007 • Test underground at shallow depth • 2008? 21

  22. Conclusions • Construction and first tests of the ArDM detector are ongoing. • Three technical key points: • High drift field • Charge readout with LEM • Light readout with PMTs • After tests at CERN and possibly at shallow depth, the detector will be moved underground(presumably to the Canfranc underground laboratory in Spain). • Depending on the rejection power, the present ArDM detector can reachsensitivities of the order of 10-8 pb. • The technique of ArDM is scalable. Larger detectors of 10 tons or more are a realistic perspective. 22

  23. Estimated event rates on Argon With true recoil energy threshold ≈ 30 keVr CREST Edelweiss I ZEPLIN III WARP CDMS ≈100 event/ton/day XENON10 ≈1 event/ton/day ≈1 event/ton/100 day • Assumptions for simulation: • • Cross-section normalized to nucleon   = 10–42 cm2 =10–6pb  MWIMP = 100 GeV • • Halo Model • WIMP Density = 0.5 GeV/cm3  vesc = 600 km/s • Interaction  Spin independent  Engel Form factor

  24. WIMPs vs. 39Ar background discrimination This is MONTE CARLO, thisrelies heavily on MC, there is no reason to belive this is OK, this is exactly what the 1 ton test at CERN should prove. Full GEANT4 simulation E = 5 kV/cm LEM ampli= 103 PMTQE = 10 % CUTS: True recoil energy > 30 keV Q > 2000 electrons light  phe phe = 2 : ~91 WIMP evts/day phe = 4 : ~85 WIMP evts/day (If Quenching = 0.28) phe = 2 : ~39 WIMP evts/day phe = 4 : ~9 WIMP evts/day 0.7 phe / keV

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