Proportional Light in a Dual Phase Xenon Chamber Elena Aprile for the XENON Collaboration Physics Department and Columbia Astrophysics Laboratory Columbia University, New York

The XENON Collaboration Columbia University Elena Aprile (PI),Karl-Ludwig Giboni ,Chuck Hailey ,Pawel Majewski, Kaixuan Ni and Masaki Yamashita Rice University Uwe Oberlack ,Omar Vargas Yale University Daniel McKinsey Princeton University John Kwong, Tom Hartmann, Kirk McDonald, Nathaniel Ross, Tom Shutt Brown University Richard Gaitskell, Peter Sorensen, Luiz DeViveiros Lawrence Livermore National Laboratory William Craig, Norm Madden University of Florida Laura Baudis

The XENON Dark Matter Experiment Dual Phase Liquid/Gas Xe The XENON design is modular Multiple 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass --> 1-tonne scale experiment. The 100 kg fiducial LXe volume of each module is shielded by additional 50 kg LXe. Active shield very effective for charged and neutral background rejection Proposed Sept. 2001. Funded Sept. 2002. Currently - R&D towards 10 kg prototype. Proposal for XENON100 submitted Oct. 2003

XENON Dark Matter Sensitivity http://dmtools.berkeley.edu Edelweiss (June 2002) ~0.25 event/kg/d ~1 event/kg/yr ~ 1 event/100 kg/yr

Liquid Xenon for Dark Matter WIMPs High mass Xe nucleus (A ~131) good for WIMPs S.I. Int. ( s ~A2 ) Odd Isotopes with large spin-dependent enhancement factors High atomic number (Z=54) and density (r=3g/cc) of liquid state good for compact and flexible detector geometry Production and purification of Xe with << 1ppb O2 in large quantities for tonne scale experiment. “Easy” cryogenics at – 100 C. Excellent ionizer and scintillator with distinct charge/light ratio for electron/nuclear energy deposits for background rejection No long-lived radioactive isotopes. 85Kr reducible to ppt level

Electron vs Nuclear Recoil Discrimination in XENON Measure both direct scintillation(S1) and charge (proportional scintillation) (S2) Nuclear recoil from WIMP Neutron Electron recoil from gamma Electron Alpha Gas ~1μs anode Proportional scintillation depends on type of recoil and applied electric field. electron recoil → S2 >> S1 nuclear recoil → S2 < S1 but detectable if E large grid Drift Time e- E Liquid ~40ns cathode

Outline Operation of a single phase xenon chamber with PMT in LXe Chamber description, experiment setups Charge collection and electron lifetime Light and charge correlation Light collection improvement with PTFE Operation of a dual phase xenon chamber Operation technique, chamber parameters Direct and proportional light waveforms, method of analysis Proportional light spectrum Properties of electron emission and proportional scintillation Electron emission yield with extraction field Proportional light yield as a function of field and pressure Ratio between direct and proportional light

Single phase LXe Detector with Charge and Light PMT Q Anode Vg Grid E LXe Bi-207 Vc Cathode 1cm between Grid and Cathode, 5mm between Anode and Grid. Gamma rays (570keV and 1064keV) and electrons (554keV and 976keV) from Bi-207 deposited on the center of Cathode. Direct scintillation light read out by PMT (Hamamatsu R6041) immersed in LXe. Ionization electrons read out by charge-sensitive pre-amplifier.

Charge collection 570keV Charge collection calculated assuming W- value of 15.6 eV for LXe. With PMT and its HV divider in the LXe, a good charge collection was achieved after several cycles of purification and baking of chamber. 1064keV

Electron lifetime 1064 keV gamma ray 976keV electron 554keV electron Event drift time defined with respect to the scintillation light trigger. A linear fit of the 570keV Gamma ray line shows a lifetime of about 1.5ms. Electron drift velocity at 1kV/cm is about 2mm/μs

Light and charge correlation There is clear anti-correlation between ionization (charge) and scintillation (light) in liquid xenon. Energy resolution can be improved by combining charge and light signals. 570keV Gamma Rays

Light collection improvement with PTFE Adding PTFE wall and PTFE piece on the bottom improved the light collection efficiency. The purity level of liquid xenon is not affected by PTFE. Light spectrum of Bi-207 at zero field was obtained with the PTFE structure. Light Spectrum, Bi-207 570keV PTFE 1064keV

Operation of a dual phase xenon chamber Liquid level is below the Grid No ionization electron can escape from LXe to GXe at low extraction field (0.5kV/cm) => No proportional light is produced GXe 4kV/cm Vg LXe E 0.5kV/cm Bi-207 Vc Proportional light Liquid level is above the Grid and below the Anode Ionization electron can be extracted from LXe to GXe at high extraction field (4kV/cm) => proportional light is abundantly produced GXe Vg 4kV/cm LXe E 0.5kV/cm Bi-207 Vc GXe Liquid level is above the Anode Ionization electrons are collected by the Anode, no GXe => no proportional light is produced Vg 4kV/cm LXe E 0.5kV/cm Bi-207 Vc

Method of analysis of waveforms Low energy x-ray from Bi-207 Bi-207 energy spectrum, readout from proportional light signals Direct light Proportional light Event waveform shows both direct and proportional light signals The area of the proportional light pulse is proportional to the ionization electrons. Spectrum of Bi-207 from the proportional light is compared with charge spectrum in a single phase operation (right) Lower energy threshold can be reached from proportional light Bi-207 energy spectrum, readout from charge signals directly.

Electron emission and proportional scintillation The negative ground state energy of quasi-free electron in liquid xenon requires an electric field to extract electron from LXe to GXe. For a full extraction of ionization electrons to gas phase, a field of 10kV/cm in the gas xenon is needed from our data. The proportional light yield is related to the field in the gas, the gas gap and the gas pressure [Bolozdynya, NIM A 99] The proportional light yield has been measured as a function of reduced field (field/pressure)

Ratio between direct and proportional light The ratio between proportional and direct light for Bi-207 is measured to be about 500 at 1kV/cm drift field and 4kV/cm in the gas phase. The gas pressure is around 2atm. With improved geometry and control of the liquid level, we will improve light collection efficiency and energy resolution, and will lower the energy threshold. The ratio of proportional and direct light will be measured for low energy electron recoils and nuclear recoils with improved chamber. 570keV