Current XENON Collaboration Columbia University Elena Aprile (PI), Karl-Ludwig Giboni, Sharmila Kamat+, Pawel Majewski+, Kaixuan Ni*, Bhartendu Singh+ and Masaki Yamashita+ Brown University Richard Gaitskell, Peter Sorensen*, Luiz De Viveiros* University of Florida Laura Baudis, David Day* Lawrence Livermore National Laboratory Adam Bernstein, Chris Hagmann and Celeste Winant+ Princeton University (move CWRU Jan 2005->) Tom Shutt, John Kwong* Rice University Uwe Oberlack ,Omar Vargas* +PostDoc *Grad Yale University Daniel McKinsey, Richard Hasty+, Angel Manzur* Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <1> XENON Event Discrimination: Electron or Nuclear Recoil? Within the xenon target: • Neutrons, WIMPs => Slow nuclear recoils => PMT Array strong columnar recombination (not all tubes shown) => Primary Scintillation (S1) preserved, but Ionization (S2) strongly suppressed Time • γ, e-, µ, (etc) => Fast electron recoils => Gas phase => Weaker S1, Stronger S2 - - Anode e - e e - Proportional e E Liq. Surface Ionization signal from nuclear recoil too small to be directly AG - - detected => extract charges from liquid to gas and detect e - e Grid ~1 µs width e - much larger proportional scintillation signal => dual phase e 0–150 µs depending on depth Electron Drift Simultaneously detect (array of UV PMTs) primary - - e - e ~2 mm/µs (S1) and proportional (S2) light => e e- EGC Distinctly different S2 / S1 ratio for e / n recoils Primary provide basis for event-by-event discrimination. ~40 ns width Liquid phase Cathode Challenge: ultra pure liquid and high drift field to Light Signal preserve small electron signal (~20 electrons) ; UV ~175 nm efficient extraction into gas; efficient detection of photons E > E small primary light signal AG GC (~ 200 photons) associated with 16 keVr Interaction (WIMP or Electron) Gaitskell Addition of CsI Photocathode at base CsI is possible option being A tertiary signal can be generated evaluated for prototype design from absorbing primary photons by CsI photocathode in LXe: PMT Array (not all tubes shown) - No transmission loss Time - high QE >20% (3 kV/cm) CsI Tertiary Gas phase Note: 16 keV nuclear recoil: Anode EAG Liq. Surface ≈ 200 photons 150 µs (if 30 cm Grid before applying efficiencies for chamber) geometry and PMT QE. EGC Also ionization signal Primary ≈ 7-20 electrons ~40 ns width (assumes high field 8 kV/cm) e- e- CsI Light Signal UV ~175 nm Cathode photons EAG > EGC Interaction (WIMP or Electron) Gaitskell WIMP Recoil Spectrum o Calculated differential and integrated event rates for E>Er for Xe , Ar and Ge o Total event rate per mass in a Xe detector with a threshold of 16 keVr is identical to that of a Ge detector with a lower threshold of 10 keVr o Ar detector with a threshold of 40 keVr would have a total event rate per mass which is 1/5 of Xe Xe Eth=16 keVr gives 0.1 event/kg/day (50% of zero thresh. sig.) Xe Rate enhanced by " high A, but low threshold dN Xe Eth=36 keVr gives (10% necessary to avoid Form of zero thresh. sig.) # dE Factor suppression Er Example cross-section shown is at current (90%) ! exclusion limits of existing experiments Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <4> XENON R&D: Dual Phase 3D XeTPC Prototype XENON Set-up at Columbia Nevis Lab CsI PC in LXe. • Pulse Tube Refrigerator used to liquefy and maintain LXe at –95.1 ± 0.05 C • Array of 7 PMTs (Hamamatsu R9288) directly coupled to the Xe active volume • Fast and Slow digitizers for direct and proportional light waveforms • Drift Field > 1kV/cm; Extraction Field > 10 kV/cm • Calibration with gamma (Co-57), alpha (Po-210) and neutronSept. (AmBe '03) sources. Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <5> XENON10:Cryogenic System and Xe Purification ~ 120 liters of LXe for target and shield. Pulse Tube Refrigerator with ~ 100 W cooling power (heat load on the XENON100 detector estimated at ~ 50W), used for keeping LXe at – 100 C within 0.1 degree. Pre-cooling (~1 day) and Xe liquefaction (~2 days) with same PTR. Hot Getter: Ba/Ti/V Pellets 350 degC - Remove H2O+O2, but not hot enough for more Gaitsckoelml pBrleowxn m Unoilveecrsuitlye s . T i S p a r k P u rXifEieNrO Nw Dilal rbk eM aettvear lCuoalltaebodr.ation CfCP workshop, Dec 2004 v02 <7> > 1m Drift Electron Attenuation Length XENON goal of 30 cm drift and ~few kV/cm E-field necessary for detection of ~ 20 e- signal (from 16 keV Xe recoil); requires purity < 1 ppb O2 equivalent. Have built & tested a dedicated Gas Purification System with continuous circulation through High-temp Getter. Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <8> R&D Milestone: QF Measurement Current data are inconsistent & do not extend to the low-energy CU RARAF 2.2 MeV neutrons (~10-30 keV) region, of interest to next-generation DM searches: p(t,3He)n LXe Recoil (keV) Measured QF Authors Borated Akimov, et. al. Polyethylene 40 - 70 0.22 ± 0.01 2002 Arneodo, et. al. Lead 45 - 110 ~ 0.2 2000 Bernabei, et. al. 35 - 70 0.45 ± 0.10 2001 LXe L ~ 20 cm θ BC501A scintillator tags recoil events: BC501A Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <9> QF: Preliminary Results 56.5 keV NR 15.5 keV NR - Electron equivalent Energy scale calibrated with 57Co spectrum & compared with MC - Multiple scattering makes low-E data more difficult to interpret Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <10> Dual-Phase Discrimination Primary/Secondary Discrimination Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <13> XENON10 Schematic of Detector and Shield Design 16 Outer PMTs Polyethylene (30cm, 2.2 tonnes ) Pb (23 cm, 31 tonnes ) Stainless Steel Cryostat (100 kg) Teflon 7 Inner PMTs (7 x R9288) Xe Gas Liquid Xe – Inner Region (ø18 cm, h 15 cm, 12 kg) CsI Photo Cathode Liquid Xe – Veto Region Copper (2.5cm) (thickness 5cm, 50 kg) (parameters used in Monte Carlo) Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <14> XENON10 – Neutron Background Event Rates • Neutron Background Event Rates for XENON10 Module o XENON10 Goal is 1.3 evts/10kg/month => 360 µdrur (100GeV WIMP) o Assumes LNGS 24 µ/m2/day (No muon veto required) Inner Event Rate (no cuts) Source (@ 2 keVr) [ µdrur ] PMT/Stainless Internal (α,n) Neutrons 0.01 (α,n)/Fission Neutrons from Cavern 15 Muon-Induced Neutrons from Pb Shield 10 * Muon-Induced Neutrons from Poly Shield 6 * High Energy Muon-Induced Neutrons 3 ** from Rock Total 34 µdrur XENON10 Goal is 360 µdrur * factor 2 uncertainty ALSO Alpha decays inner surfaces -> daughter (nuclear) recoils ** factor 4 uncertainty - Control Rn plating / Edge fiducial cuts µdrur = 10-6 evts/keVr/kg/day Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <16> XENON10 – Gamma/Electron Background Event Rates • Gamma Background Event Rates (8 < E < 16keVee) for XENON10 Module o XENON10 Goal is 140 mdruee gammas before electron recoil rejection (x200) o Assumes using 5 cm outer LXe active veto and inner multiple scatters cut Inner Event Rate with LXe outer veto Source (8 < E <16 keVee) [ mdruee ] 7 Inner PMTs 9 (5 *) 16 Outer PMTs 0.64 HV Shaping Ring Resistors 1.6 Stainless Steel Cryostat 12 Polyethylene Shield 9 External/Pb shield Gammas < 5 Teflon Walls < 1 85Kr (< 0.1 ppb) < 6 210Pb Brem (Pb shield 30 Bq/kg) < 5 Tritium (removed by Gas sep./getter) Total ~< 40 mdru XENON10 Goal is 140 mdruee before 200x rejection * if a 1 cm depth cut is made at top of inner LXe mdruee = 10-3 evts/keVee/kg/day Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <17> TPC and PMTs Details Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <21> Light Collection • Fraction of photons from light source hitting photo-sensors Photon Hit PMTs CsI Fractions * 7 x ø5 cm Photocathode Liquid Evt α1 = 3.3% α3 = 69.1% (±0.8) (±8) “Anode” ~0.5 cm above liquid surface Gas Electro- α = 13.3% α = 41.5% 2 4 “Grid” just below liquid surface luminescence (±2) (±3) ( ) indicate characteristic variation of values 3.1 cm drift of α as position of source is moved around “Cathode” 9 x 9 cm CsI Photocathode • The PMTs suffer reduced collection of photons from liquid because of Total Internal Reflection at liquid-gas interface (~20% solid angle acceptance) • CsI covers entire lower surface of chamber * Photon Monte Carlo (Kaixuan Ni, Columbia) Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <22> Signals (Photoelectron # in PMTs) • Factors determining initial scintillation & ionization conversion to signals in PMTs “Anode” ~ +4 kV SIGNAL Sn is the signal, type n, in phe (photoelectrons) “Grid” - 0 V SOURCE ne number of electrons from event site np number of photons from event site “Cathode” ~ -10 kV PHOTON- > ELECTRON QPMT PMT efficiency = (QEPMT ~ 20%) " (CEPMT ~ 60%) ~ 12%) QCsI CsI quantum efficiency (~ 20% at 3 kV/cm) S1 = (QPMT"1)np GAS ELECTROLUMINESENCE S2 = Q " # $ $ n #gas photons per electron in gas (~100 -250, EAnode & P dep.) ( PMT 2)( gas ex d ) e S3 = (QPMT" 2) #gas$ex$ d% (QCsI" 3)np ELECTRON DRIFT ( ) S4 = Q " # $ $ % Q " # $ $ n $d electron drift efficiency (Xe clean ~ 100%), ( PMT 2)( gas ex d )( CsI 4 )( gas ex d ) e $ electron liquid-gas extraction efficiency (~ 85%-100%) ex S2 S3 # # " = 2 3 $ 7 S1 S4 #1# 4 ! Gaitskell Brown University XENON Dark Matter Collaboration CfCP workshop, Dec 2004 v02 <23> ! ! Cathode Feedback 4kV Anode Voltage 0V “Anode” ~ +4 kV “Grid” - 0 V “Cathode” ~ -10 kV • Present mode of operation … to prevent total saturation of feedback o Anode Voltage - is triggered on initial light (n>2 phe) in PMTs o and then shut down ~2 drift times (27 us) later.