SAGENAP Review of the XENON Project March 12-13, 2002 The XENON Project A 1 tonne Liquid Xenon experiment for a sensitive Dark Matter Search Elena Aprile Physics Department, Columbia University
SAGENAP Review of the XENON Project March 12-13, 2002 The XENON Project Overview Outline Science Motivation and Goals Overview Dark Matter Direct Searches Worldwide LXe Properties relevant to WIMP Detection XENON Instrument Design Overview Comparison with other LXe Projects XENON Team Presentations XENON Organization and Management
The XENON Collaboration Columbia University:E. Aprile (Principal Investigator) T. Baltz, A. Curioni, K-L. Giboni, C. Hailey, L. Hui, M. Kobayashi and K. Ni Brown University:R. Gaitskell Princeton University:T.Shutt Rice University:U. Oberlack LLNL:W. Craig
Why should NSF support XENON • Because a WIMP experiment with discovery potential will have enormous scientific impact in particle physics and astrophysics. Need to validate discovery with different targets and technology. • Because the timing is right and the proposed XENON concept is based on a relatively simple technology with unique suitability for the 1-tonne scale required by the science. • Because the proposing team combines extensive experience with large scale LXe detectors with complementary experience in other key areas required for a successful realization of the XENON dark matter project!
The Case for Non-Baryonic Dark Matter • Standard BBN calculations + 4He and D primordial abundance Ωbh2 = 0.020 ± 0.001 (APJ, 552, L1, 2001) • Measurements of the matter density Ωm = 0.2 ~ 0.4 h=H0/100 kms -1Mpc-1 (h = 0.6 ~ 0.8) • Cluster velocity dispersion (Mass to Light ratio) • Galactic rotation curves • Cluster baryon fraction from X-ray gas • CMB anisotropies give Ωmh2 = 0.15 ± 0.05 (APJ, 549, 669, 2001) and also confirms Ωbh2 ~ 0.02 Ωm >> Ωb
Non-baryonic Dark Matter Candidates Neutrinos:hard to make up a significant fraction of mass density with neutrinos, unless much more massive than observed m < 0.1 eV ( PRL 81(1998)1562) Axions:strong CP, m ~ 10-5eV, search is in progress using microwave cavities ( PRL 80(1998)2043) Massive Compact Halo Objects (MACHO):with 10-7 - 10 Mocannot account for a large fraction of the DM in the Milky Way halo (ApJ 550(2001)L169) Weakly Interacting Massive Particles (WIMPS): Stable (or long lived) particles left over from the BB, decoupling when non-relativistic: their relic density ΩXh2~ 1/<Xv> (X~weak) ΩXh2 ~ 1
Supersymmetry • Stabilizes MPL and MZ hierarchy • Unification of coupling constants • Lightest Super Particle is stable • Neutralino SUSY offers the favorite WIMP candidate Superposition of photino, zino and higgsinos • SUSY particles were not invented to solve the dark matter problem. • Particles with several 100 GeV/c2 actively being pursued at accelerators. • Direct WIMP searches can probe mass values impossible to reach at colliders. • Typical WIMP nucleon cross sections in the range 10-5 and 10-11 pb
Muon g-2 Measurement • BNL results on muon anomalous magnetic moment disagree with Standard Model at 1.6 level (PRL 86(2001)2227) • If discrepancy is due to SUSY, a large neutralino-nucleon cross section (10-9 pb) and a low mass (<500 GeV) are favored • World eagerly awaiting for new results from last run!
WIMP Direct Detection • Elastic scattering off nuclei in laboratory target measure nuclear recoil energy • Spin-independent interactions are coherent ( A2) at low energy dominate for most models. Target with odd isotopes needed for spin-dependent interactions • Energy spectrum and rate depend on local dark matter density 0: measured galactic rotation curve : flat out to 50 kpc with vcir220 km/s spherical halo with 0 0.3-0.5 GeV/cm3 and M-B velocity distribution with v220 km/s
Experimental Challenges • Recoil energy is small few keV detectors with low threshold • Event rates are low << radioactive background detectors with low radioactivity, deep underground and with active background rejection With E0 = 1/2MX(0c)2 r = 4 MX MA /(MX+MA )2 R0 = T0c0 c10.78 and c20.58 F=form factor (see Phys.Rept.267(1996)195
BackgroundRejection Methods • Reject events more likely to be due to g, e, a radioactivities multiple-scatters (WIMPs interact too weakly) HDMS single-scatters localized near detector walls (WIMPs interact anywhere) CDMS ZIP detectors electron recoils (WIMPs more likely interact with nucleus) CDMS, EDELWEISS (CRESST, ZEPLINs, DRIFT) A 3D LXeTPC like XENON will combine all these rejection capabilities • Use motion of Earth/Sun through WIMP halo direction of recoil DRIFT annual modulation DAMA, NAIAD
Expected rates for various targets For a heavy target nucleus such as Xe, a very low recoil energy threshold is crucial. The expected rate, integrated above threshold of ~16 keV is 1 events/ kg/day
Current and Projected Limits of Spin-Independent WIMP Searches • Projection for CDMS Soudan (7kg Ge+Si) and competing experiments in Europe, including LXe projects of the UKDM program is~1 event / kg / yr • It will take a target mass at 1 tonne scale and similar background discrimination power to reach a sensitivity of ~1 event / 100kg / yr or s ~ 10-46 cm2 • LXe attractive target for scale-up. Projection for XENON based on Homestake, 99.5% recoil discrimination, 16 keV true recoil energy threshold and an overall 3.9x 10-5 cts /kg /d /keV background rate.
Why is Liquid Xenon Attractive for Dark Matter • High mass Xe nucleus good for scalar interaction of WIMPs • High atomic number (Z=54) and density (r=3g/cc) good for compact and flexible detector geometry. “Easy” cryogenics at –100C • High ionization (W=15.6eV) yield and small Fano factor for good DE/E • High electron drift velocity (v=2 mm/ms) and low diffusion for excellent spatial resolution. Calorimetry and 3D event localization powerful for background rejection based on fiducial volume cuts and event multiplicity • High scintillation (W~13 eV) yield with fast response and strong dependence on ionizing particle for event trigger and background discrimination with PSD • Distinct charge/light ratio for electron/nuclear energy deposits for high background discrimination • Available in large quantity and “easy” to purify with a variety of methods. Demonstrated electron lifetime before trapping of order 1 millisecond for long drift. No long-lived radioactive isotopes. 85Kr contamination reducible to ppb level
Ionization and Scintillation in Liquid Xenon I/S (electron) >> I/S (non relativistic particle) Alpha scintillation Electron charge L/L0 or Q/Q0 (%) electron scintillation Alpha charge Electric Field (kV/cm)
Electron vs Nuclear Recoil Discrimination (Direct & Proportional Scintillation ) Measure both direct scintillation(S1) and charge (proportional scintillation) (S2) Dual Phase Detection Principle Common to All LXe DM Projects • Nuclear recoil from • WIMP • Neutron • Electron recoil from • gamma • Electron • Alpha Gas ~1μs anode grid Drift Time Proportional scintillation depends on type of recoil and applied electric field. electron recoil → S2 >> S1 nuclear recoil→ S2 < S1 but detectable if E large e- E Liquid ~40ns cathode
The XENON Experiment : Design Overview • The XENON design is modular. An array of 10 independent 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass, is used to make the 1-tonne scale experiment. • The fiducial LXe volume of each module is self-shielded by additional LXe. The thickness of the active shield will be optimized for effective charged and neutral background rejection. • One common vessel of ~ 60 cm diameter and 60 cm height is used to house the TPC teflon and copper rings structure filled with the 100 kg Xe target and the shield LXe (~50 kg ).
The XENON TPC: Principle of Operation • 30 cm drift gap to maximize active target long electron lifetime in LXe demonstrated • 5 kV/cm drift field to detect small charge from nuclear recoils internal HV multiplier (Cockroft Walton type) • Electrons extraction into gas phase to detect charge via proportional scintillation (~1000 UV g/e/cm) demonstrated • Internal CsI photocathode with QE~31% (Aprile et al. NIMA 338,1994) to enhance direct light signal and thus lower threshold demonstrated • PMTs readout inside the TPC for direct and secondary light need PMTs with low activity from U/Th/K
The XENON TPC Signals • Three distinct signals associated with typical event. Amplification of primary scintillation light with CsI photocathode important for low threshold and for triggering. • Event depth of interaction (Z) from timing and XY-location from center of gravity of secondary light signals on PMTs array. • Effective background rejection direct consequence of 3D event localization (TPC)
Detection of LXe Light with a CsI Photocathode • Stable performance of reflective CsI photocathodes with high QE of 31% in LXe has been demonstrated by the Columbia measurements • CsI photocathodes can be made in any size/shape with uniform response, and are inexpensive. • LXe negative electron affinity Vo(LXe)= - 0.67 eV and the applied electric field explain the favorable electron extraction at the CsI-liquid interface. Aprile et al. NIMA 338(1994) Aprile et al. NIMA 343(1994)
Assumptions Wph : 13 eV lph: 1.7 m Quenching Factor: 25% Q.E. of PMTs: 26% Q.E. of CsI : 31% R.E of Teflon Wall: 90% Mass of Liquid Xe: 100 kg 37 PMTs (2 inch) array Light Collection Efficiency: MonteCarlo
Simulation Results • A 16 keV (true) nuclear recoil gives ~ 24 photoelectrons. The CsI readout contributes the largest fraction of them. • Multiplication in the gas phase gives a strong secondary scintillation pulse for triggering on 2-3 PMTs. • Coincidence of direct PMTs sum signal and amplified light signal from CsI • Main Trigger is the last signal in time sequence post-triggered digitizer read out Trigger threshold can be set very low because of low event rate and small number of signals to digitize. PMTs at low temperature low noise. • Even w/o CsI (replaced by reflector) we still expect ~6 pe . Several possible ways to improve light collection.
Summary of Previous Nuclear Recoil Measurements (Quenching Factor) previous measurements have wide scatter no measurements at all at low energies results consistent with Lindhard theory
We have experience measuring neutron-nuclear recoil efficiency typical setup for measurement of nuclear recoil scintillation efficiency at University of Sheffield measured low energy nuclear recoil efficiency of liquid scintillator Hong, Hailey et. al., J. AstroParticle Physics 2001 2.9 MeV neutron beam
Why Do Nuclear Recoil Scintillation Efficiency Measurements? • Confirm that measured efficiency at higher energies extends down to lowest energies of interest to a WIMP search • Confirm result in our particular experimental configuration. • Results can vary with Xe purity, light collection efficiency etc. • Measure true nuclear recoil scintillation pulse shapes
High gain in pure Xe with 3GEMs demonstrated Coating of GEMs with CsI 2D readout for mm resolution Charge readout with GEMs: a promising alternative See Bondar et al.,Vienna01
A 30 kg Liquid Xenon Time Projection Chamber developed with NASA support. 3D imaging detector with good spectroscopy is the basis of the balloon-borne LXeGRIT, a novel Compton Telescope for MeV Gamma- Ray Astrophysics. The LXeTPC operation and response to gamma-rays successfully tested in the lab and in the harsh conditions of a near space environment. Road to LXeGRIT: extensive R&D to study LXe ionization and scintillation properties, purification techniques to achieve long electron drift for large volume application, energy resolution and 3D imaging resolution studies, electron mobility etc. XENON Technical Heritage: LXeGRIT
A Liquid Xenon Time Projection Chamber for Gamma-Ray Astrophysics
The Columbia 10 liter LXeTPC • 30 kg active Xe mass • 20 x 20 cm2 active area • 8 cm drift with 4 kV/cm • Charge and Light readout • 128 wires/anodes digitizers • 4UV PMTs
High Purity Xenon for Long Electron Drift and Energy Resolution And the power of Compton Imaging
From the Lab to the Sky: The Balloon-Borne Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) LXeGRIT inflight energy spectra Atm/Cosmic Diffuse MC simulation and Data Compton Imaging Events
Background Considerations for XENON • and induced background 85Kr (1/2=10.7y): 85Kr/Kr 2 x 10-11 in air giving ~1Bq/m3 Standard Xe gas contains ~ 10ppm of Kr10 Hz from 85Kr decays in 1 liter of LXe. Allowing <1 85Kr decay/day i n XENON energy band <1 ppb level of Kr in Xe 136Xe 2 decay (1/2=8 x 1021y): with Q= 2.48 MeV expected rate in XENON is 1 x 10-6 cts/kg/d/keV before any rejection • Neutron induced background Muon induced neutrons: spallation of 136Xe and 134Xe take 10 mb and Homestake 4.4 kmwe estimate 6 x 10-5 cts/kg/d before any rejection reduce by muon veto with 99% efficiency (,n) neutrons from rock: 1000/n/m2/d from (,n) reactions from U/Th of rock appropriate shield reduces this background to 1 x 10-6 cts/kg/d/keV Neutrons from U/Th of detector materials: within shield, neutrons from U/Th of detector components and vessel give 5 x 10-5 cts/kg/d/keV lower it by x10 with materials selection
Background Considerations for XENON • -rays from U/Th/K contamination in PMTs and detector components dominate the background rate. For the PMTs contribution we have assumed a low activity version of the Hamamatsu R6041 ( 100 cts/d ) consistent with recent measurements in Japan with a Hamamatsu R7281Q developed for the XMASS group (Moriyama et al., Xenon01 Workshop). Numbers are based on Homestake location and reflect 99.5% background rejection but no reduction due to 3D imaging and active LXe shield.
ZEPLIN II ZEPLIN IV 30 kg 1000 kg The latest design as at DM2002
The LXe Program at Kamioka XMASS present Cold finger with new PMTs no rejec. gas filling line Wire set (Grid1,Anode Grid2) with 99% rejection PTFE Teflon (Reflector) Gas Xe MgF2 Window with Ni mesh (cathode) Liq. Xe(1kg) 9.5 cm Drift OFHC vessel (5cm) PMT
Signals from 1kg XMASS Prototype 42000photon/MeV Decay time 45nsec direct direct direct proportional drift time drift time proportional
XMASS Recoil /γ ray Separation >99% γ ray rejection 22 keV gamma ray Proportional scintillation(S2) Recoil Xenon (neutron source) Direct scintillation(S1) (Ref. JPS vol.53,No 3,1998, S.Suzuki)
XMASS: low activity PMT development 57 Co (122keV) σ/E = 15 % 2.4 [p.e./keV] at 250[V/cm] counts with R7281MgF2 (Q.E.30%) (HAMAMATSU(prototype) A low activity version of this tube shows ~4.5× 10-3 Bq! p.e. 137Cs 662keV Towards a 20 kg Detector counts p.e.
Answer to Question • LXe long recognized as promising WIMP target for a large scale experiment with relatively simple technology. So far however development effort has been subcritical. • Low energy threshold and background rejection capability yet to be fully demonstrated. • Recent move to an underground lab - 1 kg XMASS detector in Kamioka- an important milestone. Scale up to a 20 kg detector of same design (7 PMTs vs 1) started. • UCLA ZEPLIN II is similar in size and design to XMASS: drift in LXe over ~ 10 cm with low electric. Secondary lightpulsefrom low energy nuclear recoilshard to detect. Scale up to 1 tonne with a monolithic detector (ZEPLIN IV) too risky and unpractical. • UKDM ZEPLIN III better discrimination power and lower threshold due to high electric field. Design does not present an easy scale up from 6 kg to sizable modules of order 100 kg. • XENONcombines the best of the techniques with a design which can be easily scaled. Strength of experience with a 30 kg LXeTPC for gamma ray astrophysics + critical mass at Columbia with collaborators key experiences in DM searches.
XENON Phase 1 Study: 10 kg Chamber • Demonstrate electron drift over 30 cm (Columbia) • Measure nuclear recoil efficiency in LXe (Columbia) • Demonstrate HV multiplier design (Columbia) • Measure gain in Xe with multi GEMs (Rice and Princeton) • Test alternative to PMTs, i.e. LAAPDs (Brown) • Selection and test of detector materials (LLNL) • Monte Carlo simulations for detector design and background studies (Columbia /Princeton/Brown) • Study Kr removal techniques (Princeton) • Characterize 10 kg detector response and with g and neutron sources (Entire Collaboration)