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

2. 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

3. 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
Funded Sept
Currently - R&D towards 10 kg prototype.
Proposal for XENON100 submitted Oct. 2003

4. XENON Dark Matter Sensitivity
Edelweiss (June 2002)
~0.25 event/kg/d
~1 event/kg/yr
~ 1 event/100 kg/yr

5. 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

6. 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

7. 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

8. 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.

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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.

15. 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)

16. 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