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
Proportional scintillation
depends on type of recoil and
applied electric field.
electron recoil → S2 >> S1
nuclear recoil → S2 < S1
Gas
anode
grid
Liquid
but detectable if E large
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
Q
PMT
Anode
Grid
Vg
E
Vc
Bi-207
LXe
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 chargesensitive pre-amplifier.
Charge collection
5 7 0 k e V g a m m a ra ys
80
Charge Collection %
70
5/16/03
60
5/13/03
4/25/03
4/22/03
4/19/03
4/16/03
50
4/11/03
5/22/03
40
30
0
0.5
1
1.5
2
2.5
3
3.5
D rift F ield (kV /cm )
570keV
1064keV
• 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.
Electron lifetime
1064 keV gamma ray
976keV electron
554keV electron
570keV gamma ray
• 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
Vg
Vc
4kV/cm
GXe
LXe E
Bi-207
0.5kV/cm
Proportional light
Vg
GXe
LXe E
Vc
4kV/cm
Bi-207
0.5kV/cm
GXe
4kV/cm
Vg
LXe E
Vc
Bi-207
0.5kV/cm
• 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
• 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
• Liquid level is above the Anode
• Ionization electrons are collected
by the Anode, no GXe => no
proportional light is produced
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.
570keV
• 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.