The ArDM Project a Direct Detection Experiment, based on Liquid Argon, for the Search of Dark Matter

Transcription

1 2nd Workshop on TeV Particle Astrophysics University of Wisconsin, Madison The ArDM Project a Direct Detection Experiment, based on Liquid Argon, for the Search of Dark Matter August 29, 2006 L. Kaufmann, ETH Zurich 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 1

2 The ArDM group (6 institutes, 31 members) A. Badertscher, A. Baetzner, R. Chandrasekharan, L. Kaufmann, L. Knecht, M. Laffranchi, M. Messina, G. Natterer, P. Otiougova, A. Rubbia, J. Ulbricht ETH Zurich, Switzerland C. Amsler, V. Boccone, A. Buechler-Germann, C. Regenfus Zurich University, Switzerland A. Bueno, M.C. Carmona-Benitez, J. Lozano, S. Navas-Concha University of Granada, Spain M. Daniel, P. Ladron de Guevara, L. Romero CIEMAT, Spain P. Mijakowski, P. Przewlocki, E. Rondio Soltan Institute Warszawa, Poland H. Chagani, E. Daw, V. Kudryavtsev, P. Lightfoot, P. Majewski, N. Spooner University of Sheffield, England We acknowledge informal contribution from LNF, Italy. Interest from JINR, Russia. 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 2

3 Argon as a target for Dark Matter Direct Detection Dark Energy 70% Dark Matter 25% Over 80% of the matter in the universe is unknown. It presumably consists of a sea of Weakly Interacting Massive Particles (WIMPs). One candidate: the Lightest Supersymmetric Particle (LSP) The light is seen by photomultipliers A WIMP collides with argon inside the detector WIMP Ar the Argon nucleus recoils Ar Light and free electrons are produced from interaction with neighbouring argon atoms transmitting its kinetic energy to the nucleus e - The electrons are seen by electron multipliers 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 3

4 Preliminary remarks It has been shown that liquid xenon or argon can act as a target for WIMP detection (NIM A 327 (1993) 205 & NIM A 449 (2000) 147) We aim to detect the ionization charge and scintillation light independently (hep-ph/ ): Event identification with characteristic light/charge ratio The time distribution of the scintillation light will be used to further discriminate between heavy recoils and other backgrounds Charge readout allows for localization of events (in space), important for γ-ray and neutron background rejection from surrounding elements 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 4

5 WIMP - Argon elastic scattering Galactic Halo WIMP Elastic Scattering Argon nucleus WIMP E kin ~10-100keV Event rate in argon is less sensitive to threshold on recoil energy than for xenon (form factors) Recoil spectra in xenon and argon are different, providing an important cross-check in case of positive signal Assumptions for simulation: Cross-section normalized to nucleon σ = cm 2 =10 6 pb M WIMP = 100 GeV Halo Model WIMP Density = 0.5 GeV/cm 3 v esc = 600 km/s Interaction Spin independent Engel Form factor Event Rate per Day and per Ton 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 5

6 Estimated event rates on argon With true recoil energy threshold 30 kevr 100 events/ton/day CDMS (TAUP05) 1 event/ton/day 0.01 events/ton/day 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 6

7 ArDM bi-phase detection principle E-field WIMP GAr LAr Charge extraction from liquid argon to gaseous argon, amplification and readout with Large Electron Multiplier (LEM) Field shaping + immersed HV multiplier Reflecting mirrors Light readout Assumed baseline parameters: Cylindrical volume, drift length 120 cm 850 kg target Drift field 1 to 5 kv/cm Charge LEM readout: Single electron gain 10 3 to 10 4 Global light readout collection efficiency 5% Single photon detection 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 7

8 Prototype layout Two-stage LEM for electron multiplication and readout Greinacher chain: supplies the right voltages to the field shaper rings and the cathode up to 500 kv Field shapers are needed to provide a homogeneous electric field, but are thin enough to permit the scintillation light to be reflected from the container walls Transparent cathode ~85 PMTs below the cathode to detect the scintillation light 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 8

9 Assembly at CERN Rack for the HV electronics Turbo pump Slow control for the vacuum Computer monitoring HV and vacuum 1-ton prototype 1 ton PROTOTYPE 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 9

10 High voltage system for drift field generation A cascade of rectifier cells (Greinacher/Cockroft-Walton circuit) is used The total voltage we aim to reach is V tot = 500 kv, i.e. 4 kv/cm Tests in liquid nitrogen have been performed The largest system successfully operated consists of 80 stages and reached stable operation at up to 120 kv Mounted on field shaper rings Polypropylene capacitors 82 nf 2.5 kv/stage 200 stages Top view 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 10

11 Charge readout: Thick Large Electron Multiplier (LEM) Thick-LEM: Vetronite with holes, coated with copper macroscopic GEM easier to operate at cryogenic temperatures hole dimensions: 500 µm diameter, 800 µm distance High gain operation of LEM in pure argon at high pressure LEM thickness 1.6mm GAIN 2.3bar GAIN 2.9bar GAIN 2.5bar GAIN 2.7bar GAIN 3.21bar GAIN 3.41bar GAIN 3.54bar 800 Gain GAIN Voltage (V) 6000 Voltage (V) The level of the liquid argon is placed just below a LEM readout system Each extracted electron creates an avalanche which is detected on the anode. Gain of up to 800 possible even at high pressure (good prospects for operation in cold) The segmented LEM readout facilitates event localization 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 11

12 Two-stage Large Electron Multiplier Distance between stages: 3 mm E transf = 3 kv/cm Avalanche spreads into several holes at second stage Higher gain reached as with one stage, with good stability Simulation of avalanche GAr LAr E drift = 5 kv/cm A stable gain of 10 4 has been measured /29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 12

13 Two-stage LEM test setup Custom-made front-end charge preamplifier (4.5 mv/fc) 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 13

14 Measurements of the Gain At room temperature At liquid argon temperature GAIN /29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 14

15 Two-stage LEM: measurements (preliminary) Shapes of the signals from double-stage LEM system Average signal rise time: 12 µs. Signals from Fe 55 radioactive source (5.8 kev), event rate about 1 khz. Custom-made front-end charge preamplifier (4.5 mv/fc) 200 mv 50 µs 200 mv 2 ms 200 mv 1 ms 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 15

16 Photon collection in the detector Geant4 simulation: photon angle with respect to the direction perpendicular to PMT plane investigated The distribution is strongly influenced by Rayleigh scattering of 128 nm scintillation light 1.35 A(ϕ)/A 0 Measurement of reflectivity Incident photon angles on PMTs 130 No. of photons 0 0 Angle (deg.) Angle (deg.) 90 Reflectivity of 85% to 90% measured for mylar foils coated with thin Al+MgF 2 layers (CERN) 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 16

17 WLS-coated PMT Hamamatsu 6237mod Scintillation light detection via WLScoated PMTs: Polymer and Tetra-Phenyl-Butadiene (TPB) compound coated on PMT window shifts the DUV light (128 nm) to 430 nm Efficiency of wavelength shifting: 30% PMTs Type: R MOD Pt underlay QE ~ 20% Bi-Alkali type 7.6 x 7.6 cm 2 8 dynodes, G~3x10 5 open leads 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 17

18 WLS reflector Significantly increases geometrical collection efficiency Shifting the scintillation light at walls instead of PMT surface Dewar WLS Field shapers PMTs at walls 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 18

19 Light measurements in liquid argon (preliminary) Event separation in liquid argon Radioactive source: α (5.4 MeV) + β (Q = MeV) Scintillation light from α in 1200 mbar liquid argon 10 0 L 50 /L tot 5.4 MeV nvs α events PM Amplitude Time (ns) 6000 γ,e events α events separate well from γ,e events Fast and slow light components distinguishable 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 19

20 Background sources 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 20

21 Intrinsic background from Argon 39 isotope Natural argon from liquefaction of air contains small fractions of 39 Ar radioactive isotope Induced in atmospheric argon by cosmic rays Half life: 269 years, Q=565 kev Mean Energy: 218 kev Integrated rate in 1 ton LAr ~1kHz Energy(MeV) To suppress 39 Ar fraction we also consider using argon 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 as a function of time and position. 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 21

22 Event distinction Charge readout GAr Charge/light: e/γ-like Nuclear recoil-like E-field LAr Light shape: Visible energy WLS+light reflector WIMP WLS+light reflector Amplitude e/γ-like ( 39 Ar) Time Light readout Amplitude Nuclear recoil-like (WIMP, n) Time 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 22

23 Neutron background estimation Neutron background can produce nuclear recoils that are hardly distinguishable from WIMP events, background γ and e - events have different light/charge ratios Neutron sources: Muon-induced neutrons from surrounding rock, shielding and detector components GO UNDERGROUND Uranium and Thorium contamination of the detector components and the surrounding rock Muon-induced neutrons: high energy neutrons penetrate the shielding, are thereby moderated and can cause WIMP-like events U/Th neutrons from rock: flux about cm -2 s -1 (at 2450 m.w.e.) can be shielded, e.g. by a hydrocarbon shield 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 23

24 Neutron background from detector components Neutrons from detector components Uranium spontaneous fission (α,n) reactions Uncertainties: U/Th contamination precise value of (α,n) cross sections Component Container LEM (std. mat.) LEM (low bg. mat.) 85 PMTs (std. mat.) Event numbers per year 85 PMTs (low bg. mat.) n per year ~ 400 ~ < 20 ~ ~ 600 WIMP-like recoils Compared with ~ 3500 WIMP events at σ = cm -2 low background materials important ~ 30 ~ 900 < 2 ~ 1000 ~ 50 No. of events No. of recoils Geant4 simulation 0 Neutron energy (MeV) 10 0 Argon recoil energy (kev) 300 Nuclear recoils: 55% scatter more than once within the fiducial volume (threshold 30 kev) advantage of large detectors 5-10% produce a WIMP-like event (single scattering, recoil energy [30,100] kev)

25 Neutron background: energy spectra Neutron energy spectra simulated with SOURCES 4a, modified by M.C. Carson et al. The average neutron energy varies between 1.5 MeV and 3.5 MeV, depending on the material The number of argon recoils in the critical region ([30,100] kev) depends on the neutron energies Neutron number 0 Vetronite Neutron energy (MeV) 10 Polyethylene Stainless steel 304L Neutron number Neutron number 0 Neutron energy (MeV) 10 0 Neutron energy (MeV) 10 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 25

26 Outlook A 1-ton prototype is being assembled at CERN to be run in a first phase above ground ( ). Key components: high drift field device LEM-based charge readout: encouraging results argon scintillation light detection system First goal: to demonstrate that large dark matter detectors can be built and operated. Following the operation above ground, a deep underground operation (2nd phase, 2007?) is considered. The expected sensitivity of the prototype should be cm 2 to cm 2 SI, depending on the background rejection power. This technology could provide the means to develop larger detectors to reach sensitivities below cm 2 SI cross-section. 8/29/2006 L. Kaufmann, ETH Zurich, TeV P. A. II 26