Slide1: Dark Matter Search with Noble Liquids: the ArDM experiment Sixth International Heidelberg Conference on Dark Matter in Astro and Particle Physics (DARK MATTER 07) University of Sydney, Australia, 23-28 September 2007 Sergio Navas University of Granada, Spain


Slide2: S. Navas (U. Granada), DARK07 2 The ArDM collaboration http://neutrino.ethz.ch/ArDM


Slide3: ( 10-3) Argon Argon Recoil Scintillation from excited Ar dimers   e- e- e- Escaping ionization electrons High event rate due to high density [1.4 g/cc at 87 K (boiling point at 1atm)], high atomic number  42 000 e- / MeV  40 000  / MeV Argon as target for WIMP detection WIMP Tmax 2MAr c2 2 (in the range of tens of KeV) WIMP Our aim is to detect the ionization charge and scintillation light independently NIM A 327 (1993) 205 & NIM A 449 (2000) 147 WARP NIM A 574 (2007) 83-88 XENON arXiv:0706.0039 [astro-ph] S. Navas (U. Granada), DARK07 3


Slide4: 4 S. Navas (U. Granada), DARK07 4 tS ≈ 6 ns tT≈ 1.6 ms Ar+ Ar2+ Ar** Ar* Ar2* M. Suzuki et al. NIM 192 (1982) 565 Ar* recombination Processes induced by charged particles in Argon UV Light (two components) + Charge l ≈ 128 nm Various physical processes leading to scintillation & ionization Yields are particle, energy and drift field dependent Simulation describes different response to WIMP and MIPs 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


Slide5: S. Navas (U. Granada), DARK07 5 Ton-scale LAr detector providing self-shielding Direct detection of ionization charge and primary scintillation light Experimental strategy 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


Slide6: 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 The field shapers are needed to make an homogeneous PMTs below the cathode to detect the scintillation light. Cathode: semi-transparent in order to let the scintillation light pass trough … The aluminized foils reflect the scintillation light (>95%) Prototype layout . Cylindrical volume, drift length ≈ 120 cm 850 kg LAr target 6 S. Navas (U. Granada), DARK07 80cm 120 cm


Slide7: 7 Liquid Argon should be kept free from electronegative impurities (O2 contamination < 1 ppb) Cryogenics S. Navas (U. Granada), DARK07


Slide8: 8 DEWAR at CERN S. Navas (U. Granada), DARK07


Slide9: 9 Polypropylene capacitors 82 nF 2.5 kV/stage 210 stages Top view 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 High Voltage system for drift field generation S. Navas (U. Granada), DARK07


Slide10: Layout of the charge readout system GARFIELD simulations indicate an expected single electron gain ≈ 104 Etransf = 3 kV/cm Eextract = 5 kV/cm GAr LAr 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 S. Navas (U. Granada), DARK07 30 kV/cm 10


Slide11: S. Navas (U. Granada), DARK07 11 Final LEM charge readout system will be segmented Orthogonal strips readout Number of channels: 1024 Strip width: 1.5mm Prototype of a segmented LEM. to ZIF connectors on the LEM board to front-end preamplifers 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 32 channels/cable Segmented LEM Custom-made front-end charge preamp + shaper G ~15mV/fC


Slide12: S. Navas (U. Granada), DARK07 12 Photomultiplier tube: potential PMTs ETL 9357KFL (low background) R5912-MOD and R5912-02MO from 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 Layout of Light Readout system and PMT


Slide13: 13 S. Navas (U. Granada), DARK07 GEANT4 simulation detector top view Average incident angle of photons on PMTs: 40º


Slide14: 14 ,e events  events nVs Scintillation light from  in 1200 mbar liquid argon Event separation in liquid argon  events separate well from ,e events Fast and slow light components distinguishable PM Amplitude 10-4 100 Time (ns) 0 6000 210Po radioactive source:  (5.4 MeV) +  (Q = 1.163 MeV) L50/Ltot Light measurements in Liquid Argon (preliminary) nVs Por un lado hay más luz de centelleo DETECTADA de alphas que de betas ~ 0.3 for  particles ~ 1.3 for  particles   real data [Phys.Rev.B27 (1983) 5279] Ar2* 1u+ (singlet) 3u+(triplet) t=6 ns (fast) t=1.7 s (slow)


Slide15: 15 PMT tests in LAr PMT after LAr immersion test TPB coating tests  from 241Am source No  source Radioactive source test LAr purification station S. Navas (U. Granada), DARK07


Slide16: 16 Levelmeters Temperature sensor Slow Control Devices A series of custom designed Slow Control devices have been built, tested and installed to monitor temp., level, pressure … S. Navas (U. Granada), DARK07


Slide17: Charge/Light: Light shape: Neutrons and WIMPs interact with the argon nucleus e-/ interact with shell electrons Background rejection tools: Different light/charge ratios Different shape of the scintillation light (ratio fast/slow components) Exploit 3D imaging capabilities of the detector Background rejection


Slide18: 18 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 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 Neutron Background from detector components High energy neutrons penetrate shielding, are thereby moderated and can cause WIMP-like events. Low Background Materials are crucial S. Navas (U. Granada), DARK07


Slide19: 19 Induced in atmospheric argon by cosmic rays Concentration in natural Ar: 8.1x10–16 39Ar/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 , = 218 keV Integrated rate in 1 ton LAr  1kHz Natural argon from liquefaction of air contains small fractions of 39Ar radioactive isotope (well known to geophysicists) 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. Intrinsic background from Argon 39 isotope Energy(MeV) [WARP Coll.] astro-ph/0603131 S. Navas (U. Granada), DARK07


Slide20: S. Navas (U. Granada), DARK07 20 Access platform Detector insertion Reflector foil Top flange Assembly at CERN


Slide21: The ArDM schedule for near future S. Navas (U. Granada), DARK07 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


Slide22: Conclusions S. Navas (U. Granada), DARK07 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 reach sensitivities 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


Slide23: S. Navas (U. Granada), DARK07 23


Slide24: 24 ≈100 event/ton/day ≈1 event/ton/day With true recoil energy threshold ≈ 30 keVr ≈1 event/ton/100 day Estimated event rates on Argon CREST Edelweiss I ZEPLIN III WARP CDMS XENON10 Assumptions for simulation: • Cross-section normalized to nucleon   = 10–42 cm2 =10–6 pb  MWIMP = 100 GeV • Halo Model WIMP Density = 0.5 GeV/cm3  vesc = 600 km/s • Interaction  Spin independent  Engel Form factor S. Navas (U. Granada), DARK07


Slide25: E = 5 kV/cm LEM ampli= 103 PMTQE = 10 % Full GEANT4 simulation 25 (If Quenching = 0.28) phe = 2 : ~39 WIMP evts/day phe = 4 : ~9 WIMP evts/day 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. CUTS: True recoil energy > 30 keV Q > 2000 electrons light  phe phe = 2 : ~91 WIMP evts/day phe = 4 : ~85 WIMP evts/day 0.7 phe / keV S. Navas (U. Granada), DARK07