ArDM: Liquid TPC for direct 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 (1cos) 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 (ER bkg rate > 106 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..

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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 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

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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)

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6 Roberto Santorelli Roberto 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 • 0.53 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 2m

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

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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.

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Roberto Santorelli – MultiDark Canfranc 13/4/2012 ArDM-1t TPC

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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?]

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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

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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 Santorelli Roberto for the cooling bath and much less for the inner vessel] through the overpressure valve

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2011 Tests

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– Roberto Santorelli Roberto

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18 Roberto Santorelli Roberto 19

Roberto Santorelli – MultiDark Canfranc 13/4/2012

ArDM @ Canfranc

• ArDM under vacuum

• Pumps are on

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• Controlled remotely Roberto Santorelli Roberto

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”

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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

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Roberto Santorelli – MultiDark Canfranc 13/4/2012 Top and bottom shields

Bottom shield weigth: 2.5 tons Top shield weigth: 2.5 tons

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25 Roberto Santorelli Roberto Gamma and neutron calibration

Under discussion Currently studied

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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

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Roberto Santorelli – MultiDark Canfranc 13/4/2012