ArDM: Liquid Argon TPC for direct Dark Matter detection Roberto Santorelli (on behalf of the ArDM collaboration) CIEMAT - MADRID 1 DIRECT DETECTION EXPERIMENTS GOAL: Detection of the elastic (inelastic) collisions with atomic nuclei 2 Recoil q 2V 2 Er (1 cos) 50keV Ar,Xe,Ge…Nuclei energy 2mN mN (at rest) q 2 2V 2 (1cos) momentum transfer WIMP ~10-3 6 2 m mN m mN reduced mass (Flux ~10 p/cm /s) Rate R N N V <1 evt/100 kg/day Erecoil<50 keV m N = number of target nuclei in detector Scattering non relativistic : 3 3 coupling spin-dependent / spin-independent = local WIMP density ρ~0.3GeV/cm 3000/m (m=100GeV) 7 <N> = scattering cross section (10 pb) Experimental signatures: Backgrounds: • Nuclear recoils • : higher energy depositions • Featureless recoil spectrum (no bump) • : underground + veto • Single scatters •, : ER shielding + discrimination (uniform throughout the detector volume) • n : NR shielding + multiple interactions Extremely low background technique required and good discrimination NR-ER 6 (ER bkg rate > 10 higher) 2 Roberto Santorelli – MultiDark Canfranc 13/4/2012 COUPP PICASSO PHONONS CDMS, CRESST EDELWEISS ROSEBUD CHARGE LIGHT DRIFT DEAP/CLEAN GERDA ArDM DAMA,KIMS, LUX,ZEPLIN, XMASS XENON.. 3 Roberto Santorelli – MultiDark Canfranc 13/4/2012 NOBLE LIQUIDS FOR DARK MATTER DETECTION • low energy threshold Experimental requirements for DM experiments: • large mass • ultra low background • event by event discrimination High atomic number compact detectors good stopping power (i.e. self shielding active volume) High scintillation and ionization yield Nuclear recoil discrimination different charge/light ratios pulse shape discrimination Event imaging (fiducialization, multiple hits) “Easy” to purify with a variety of methods (long drift) Available in large quantity (affordable) 4 Roberto Santorelli – MultiDark Canfranc 13/4/2012 NOBLE LIQUIDS AS DETECTOR MEDIUM Element Z(A) Boiling Liquid Energy Radiation Collision Ionization Scintillation Cost point (Tb) density loss length length [e-/keV] [/keV] @Tb dE/dx (cm) @1bar [k] X0(cm) [g/cm3] (MeV/cm) Ne 10(20) 27.1 1.21 1.4 24 80 46 7 Ar 18(40) 87.3 1.40 2.1 14 80 42 40 Kr 36(84) 119.8 2.41 3.0 4.9 29 49 25 Xe 54(131) 165.0 3.06 3.8 2.8 34 64 46 5 Roberto Santorelli – MultiDark Canfranc 13/4/2012 LXe vs LAr LXe: Density Price ~50% odd isotopes ( 129Xe, 131Xe ) for spin dependent interactionsBOXING No long-lived radioactiveTHE “NOBLE” isotopes SPORT LAr: Background discrimination Radioactive isotopes Available in large quantity 39Ar 1.01 Bq/kg (NIM-A 574 (2007) 83–88) MultiDark Canfranc 13/4/2012 CanfrancMultiDark – 6 Roberto SantorelliRoberto LAr TPC Charge readout In Gas Ar Gas Ar Eg Ed Ionization charge Primary scintillation LAr interaction in LAr xy position reconstructed through the charge PMTs Z position reconstructed through the Drift time 7 Roberto Santorelli – MultiDark Canfranc 13/4/2012 ArDM-1t BASIC CONCEPTS • ~2000 kg Ar mass, ~850 kg instrumented • Cylindrical volume 80 cm diameter, ~120 cm max drift Services • 0.53 kV drift field Charge • Cryogenic low radioactivity PMT (Hamamatsu R5912) readout 14 8” in liquid (LEM/EL) 14 8” in gas (first phase) HV multiplier Field rings 2 m PMT array LAr recic. 8 Roberto Santorelli – MultiDark Canfranc 13/4/2012 NUCLEAR RECOIL DISCRIMINATION I Applying a drift field fewer and fewer electrons recombine Nuclear recoil with the parent ions : recombination light suppressed. Scintillation Charge Different track structures of recoiling electron and nuclei two different amount of quenching Different ionization/scintillation ratio for electron Electron recoil and nuclear recoil Scintillation Charge Event by Event discrimination 9 Ultra pure liquid necessary to preserve small electron signal (~10 el) Roberto Santorelli – MultiDark Canfranc 13/4/2012 NUCLEAR RECOIL DISCRIMINATION II Very large difference in decay times between fast Argon excimer (1, 3) (≈ 7 ns) and slow (1.6 µs) components of the emitted UV light. * Ar 2 128 nm The ionization density strongly affects the relative intensity of the singlet and triplet state de- excitation light . Ar Ar Unbound state Pulse shape discrimination of primary scintillation. 10 Roberto Santorelli – MultiDark Canfranc 13/4/2012 ArDM-1t TPC 11 Roberto Santorelli – MultiDark Canfranc 13/4/2012 HV system Greinacher (Cockcroft-Walton) circuit • 210 stages (1520 capacitors, 1260 diodes) in LAr • 30 field shapers (4 cm spacing) • Alternating current input (50 Hz, Vpp~2.5 kV) • Up to 400 kV (3 kV/cm field) at cathode S. Horikawa et al., J. Phys. 12 Conf. Ser 308 (2011) 012027 13 Roberto Santorelli – MultiDark Canfranc 13/4/2012 ArDM Phases • Surface operation: build and commission1-ton two-phase LArTPC at CERN [done!] • Underground operation Phase I: development of underground infrastructure (shielding etc) + science run using natural Ar [starts in 2012] • Underground operation Phase II: science run using depleted Ar [starts when sensitivity of science run with natural Ar is 39Ar limited AND a sufficient amount of depleted 39Ar is available. 2014?] 14 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Tests @ CERN • Detector successfully operated in stable conditions • Several calibration runs with external sources taken (22Na, AmBe) • New DAQ tested • Light collection and particle discrimination studies 22Na 15 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Recent upgrades • The cooling system of ArDM has been upgraded with a recondenser, with 600 W of cooling power provided by two GM cryocoolers [Cryomech AL300]. In this way we have a closed loop, zero loss system • The new system has been operated for over 7 weeks, providing a full characterization together with a MultiDark Canfranc 13/4/2012 CanfrancMultiDark stability test – • The control of the cryocoolers/cryogenic system is fully integrated in the existing PLC • Reduced manpower requirements in operating ArDM • The system is safe: in case of failure of the cryocoolers, the LAr evaporates slowly [less than 100 L/day SantorelliRoberto for the cooling bath and much less for the inner vessel] through the overpressure valve 16 2011 Tests 2011 17 Roberto Santorelli – MultiDark Canfranc 13/4/2012 18 Roberto Santorelli – MultiDark Canfranc 13/4/2012 19 Roberto Santorelli – MultiDark Canfranc 13/4/2012 ArDM @ Canfranc • ArDM under vacuum • Pumps are on MultiDark Canfranc 13/4/2012 CanfrancMultiDark – • Controlled remotely Roberto SantorelliRoberto 20 Neutron shield Three structural parts: lateral shield, top shield, bottom shield CIEMAT –ArDM team Maria De Prado Barbara Montes Luciano Romero Roberto Santorelli + technicians and “experts” 21 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Neutron shield • The lateral sides of the slab are bent in order to avoid straight paths to the heart of the ArDM detector. • The thickness of the slab is not precise: 50.5±0.5mm. • 4 slabs are glued together with silicone to make a block of 203 mm, with controlled height. • The top slab in the block has mechanical fixations. 2425 mm 22 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Slab= 20Kg Blocks (4 slabs)= 80Kg Rings (8 blocks)= 640Kg Lateral shield (600 slabs)= 12Tons 23 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Top and bottom shields Bottom shield weigth: 2.5 tons Top shield weigth: 2.5 tons 24 Roberto Santorelli – MultiDark Canfranc 13/4/2012 25 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Gamma and neutron calibration Under discussion Currently studied 26 Roberto Santorelli – MultiDark Canfranc 13/4/2012 • Simulations of the energy deposited by different sources currently carried out • Event discimination 27 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Signal and background rates Assuming: • 500 Underkg fiducial discussion mass • 107 Electron recoil rejection (103 from charge/light and 104 from pulse shape) • Shield fully in place • Recoil energy range of 30-100 keV • 50% nuclear recoil acceptance • WIMP mass 100 GeV and x-sec 10-44 cm2 39Ar Gamma Neutrons Bkg WIMP (evt/day) (evt/day) (evt/day) (evt/day) (evt/day) 1.5 106 47500 0.07 0.22 0.25 28 Before suppression After bkg suppression Roberto Santorelli – MultiDark Canfranc 13/4/2012 29 Roberto Santorelli – MultiDark Canfranc 13/4/2012 Conclusions • LAr offers very good prospects for DM direct search •ArDM-1t successfully tested on surface • Underground operations @ Canfranc started • Start of the data taking with atmospheric Ar in 2012 30 Roberto Santorelli – MultiDark Canfranc 13/4/2012 .