XENON Experiment - SAGENAP
Factors Affecting Detector
Performance Goals
and
Alternative Photo-detectors
Rick Gaitskell
Department of Physics
Brown University
Source at http://gaitskell.brown.edu
Gaitskell
Goal
Why is good detection/discrimination performance required
down to 16 keVrecoil (4 keV electron equivalent)?
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Very Typical WIMP Signal
• Low Thresholds Vital
o Graph
shows integrated event rates for E>Er for Xe (green), Ge (red) and S (blue)
o Large nuclei enhanced by nuclear coherence, however, in reality <<A2 …


Er
Xe Eth=16 keVr gives 1 event/kg/day
dN
dE
Example cross-section
shown is at current (90%)
exclusion limits of existing
experiments
Gaitskell XENON Proposal/SAGENAP 020312
Xe WIMP rate for Er > 16 keVr is
(1) within factor 2 of maximum
achievable rate (Er>0)
(2) equivalent kg/kg to low
threshold Ge detector
(3) 5x better kg/kg than light
nucleus (e.g. S in CS2)
Rick Gaitskell
• Form Factor makes very significant modification to naïve ~A2 rate
o…
due to loss of coherence (since qr>>1)
Dashed lines show ~A2 before considering
q>0 Form Factor suppression
Form Factor Suppression
Note Rapidly Falling Rate
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Good Performance Must Be Established at “Threshold”
• Low threshold vital, since rate falls rapidly with energy
o 10%
of signal @ Recoil Energy >35 keVr (assuming 100 GeV WIMP)
• Assuming 25% Quenching Factor this is equivalent to <8.8 keVee
o ~45%
of signal @ Recoil Energy >16 keVr
• Equivalent to 4 keVee
“Acceptable trade off”
• Factor 2x sacrifice in “effective detector mass” relative to zero threshold rate
o Need
to maximise performance in low detection signal regime
• Ensure that WIMP identification/background discrimination is working well at ~4 keVee
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Available Signal: UV Photons & Electrons
• Focus on two types of messengers from primary interaction site
o UV
Photons (178 nm) from Xe scintillation
• Consider energy required to create photons
e
• Will not consider details of generation mechanism
that UVg generated via both Xe* and Xe+ mediated channels
• No re-adsorption term to consider
e
e
—Note
o “Free”
e
e
e
electrons separated from Xe+ ions
• Consider energy required to create electron-ion pairs
• Need to consider loss due to local recombination in densely ionised region
Summarise existing data from liquid Xe detector studies…
• Electron Recoils from 1 keVee (electron equivalent) Gamma Events
• Nuclear Recoils from 1 keVr (recoil) WIMPs/Neutrons
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Available Signal in Liq. Xe
SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTS
Zero Field
High Field
0 V/cm
8 kV/cm
GAMMA EVENT - 1 keV electron equivalent energy
UV Photons
60-75 UV
Electrons+Ions
[60-75 elec]
NUCLEAR RECOIL EVENT UV Photons
Electrons+Ions
1 keV recoil energy
12-18 UV
[12-18 elec]
EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR"
UV Photons
20-25%
Electrons+Ions
[20-25%]
20-30 UV
50-60 elec
11.6 UV
0.4-1.2 elec
30-50%
0.8-2%
• Summary
The ranges shown reflect spread in existing experimental measurements
o Note that the table considers signal from either 1 keV gamma or nuclear recoil event
o 60 excitations / keV is equivalent to ~16 eV / excitation
o Zero field electron-ion #’s in [ ] are inferred, but are signal is not measured (extracted) directly
o
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Available Signal in Liq. Xe (2)
SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTS
Zero Field
High Field
0 V/cm
8 kV/cm
GAMMA EVENT - 1 keV electron equivalent energy
UV Photons
60-75 UV
40%
Electrons+Ions
[60-75 elec]
90%
NUCLEAR RECOIL EVENT UV Photons
Electrons+Ions
1 keV recoil energy
12-18 UV
[12-18 elec]
EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR"
UV Photons
20-25%
Electrons+Ions
[20-25%]
20-30 UV
50-60 elec
11.6 UV
0.4-1.2 elec
30-50%
0.8-2%
• Gamma Event
o
UV Photons
• w ~13-15 eV / photon for zero field
• As soon as field is applied (>0.2 kV/cm) electron-ions no longer recombine and this route (~50%-60%) for
generation of photons disappears
o
Electrons
• Also w ~13-15 eV / electron, Note that for zero field electrons are not measured directly since no drifting occurs
• >~90% of electrons are extracted in high field
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Available Signal in Liq. Xe (3)
SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTS
Zero Field
High Field
0 V/cm
8 kV/cm
GAMMA EVENT - 1 keV electron equivalent energy
UV Photons
60-75 UV
Electrons+Ions
[60-75 elec]
NUCLEAR RECOIL EVENT UV Photons
Electrons+Ions
1 keV recoil energy
12-18 UV
~100%
[12-18 elec] 3-8%
EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR"
UV Photons
20-25%
Electrons+Ions
[20-25%]
20-30 UV
50-60 elec
11.6 UV
0.4-1.2 elec
30-50%
0.8-2%
• Nuclear Recoil Event
o
UV Photons
( Note: Bernabei (DAMA) use Quenching Factor of
40% which has not been confirmed elsewhere )
• w ~50-70 eV / photon, (Lindhard) Quenching Factor measured as 20-25%
• Ionisation density is very much higher for nuclear recoil so even with high applied field most electron-ions recombine
o
Electrons
• Lindhard Quenching Factor also applies to initial generation of electron-ions
• Extraction of electrons from densely ionised region is very inefficient.
• Literature quotes extraction in range (0.5-1.0%)/kV of applied field (in this case use 8 kV/cm so 4-8%)
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Summary - High Field Operation
• Detection of primary scintillation light is a challenge
o ~12
UV photons / keV recoil energy
• Extraction of electron(s) from nuclear recoil densely ionised region is
big challenge
o We
require observation of this signal to ensure correct identification of nuclear
recoil event
o ~0.4-1.2 electrons / keV recoil energy
• Note once electron extracted from liquid to gas, significant gain ~1000 UVg / electron
makes signal easy to observe
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Baseline - Simulation Results
16 keV recoil threshold event
•
Assumes 25% QE for 37 phototubes, and 31% for CsI
cathode
•
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 posttriggered 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 ways to improve light
collection possible
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Nuclear Recoil Event ~Threshold 16 keVr
• Nuclear Recoil of 16 keVr (Threshold)
QF 25% -> 4 keVee
o 300 UVg into 4π
o
These are ball-park numbers Full simulation actually traces
rays and includes all scattering
• Detection in Phototubes
o
Nominal Geometric Efficiency ~6%
• Tubes have a active fill factor of ~50% at top of
detector
• Photons lost in windows (T=80%) and by wires
(T=80%) giving ~60%
• Total Internal Reflection(TIR) at liquid surface
(n~1.65), acceptance ~20%
• Ignore Teflon losses for this calc.
Tube photocathode Quantum Efficiency ~30%
o 300g x 2% = 6 photoelectrons
• Detection of electrons (drifted)
0.5-1.0% / (kV applied field) extraction from dense
ionised region (avoiding self recombination)
o 4-18 electrons drifted toward liquid surface
o
• In high field once electrons start drifting
~100% extraction from liquid
• Gas Gain
o ~1000 UVg from each electron in gas
o Signal is localised to xy position of original
interaction
o
• Generation of electrons in CsI photocathode
o
Nominal Geometric Efficiency ~20-60%
• CsI covers entire bottom surface
• Due to TIR and Teflon this value is high
• Strong position sensitivity, poor energy resolution
• Large signal in PMT
o
Even considering PMT/geometry efficiency this
gives a large signal
CsI cathode Quantum Efficiency ~30%
o 300g x 6-18% = 20-60 photoelectrons
o
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Why is photodetector performance critical?
• A factor 2 increase in threshold 16 keVr -> 32 keVr
o Factor
5 loss in effective mass of detector for WIMP search
• A factor 2 decrease in threshold 16 keVr -> 8 keVr
o Factor
<2 improvement in effective mass of detector for WIMP search
o However, lower threshold will, of course, improve background
identification/rejection
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Existing Photodetector Summary
• Hamamatsu Low Temperature/Liquid Tube (6041)
o Baseline
design for XENON
o Metal construction that has been shown to work in liquid Xe
• Not Low Background: Could be made low background
o Low
Quantum Efficiency~10-15%
• New Hamamatsu Low Background Tube (R7281)
o Being
tested by Xmass Collaboration
• Room temperature tests only so far
o Metal
construction, and giving lower backgrounds
• ~500 per day (XENON baseline target is 100 per tube per day)
o Higher
Quantum Efficiency~27-30%
• Uses longer optics which give better focusing (could be accommdated in XENON)
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
New Photodetectors
• Micro-channel Plate
o Burle
85001
• ~30% Quantum Efficiency (since photocathode can be selected separately)
• Promising for low temperature operation
• Large area (5x5 cm2) and compact design (few feed-throughs)
• Investigate radioactive background situation
• Large Area Avalanche Photodiodes
o Advance
Photonix / Hamamatsu
• 100% Quantum Efficiency demonstrated at UV 178 nm (windowless)
• Operation in liquid Xe has been demonstrated
• “Large Area” 0.5-2 cm2 device available
• Silicon construction is intrinsically low background/investigate packaging
• Recent progress in device fabrication
—leakage
currents (dark noise) has been reduced significantly & benefits considerably from low
temperature operation (<1 pA/cm2) (idark)170K~ 10-4 (idark)RT
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Effective Quantum Efficiency - LAAPD (Windowless)
physics/0203011 demonstrate ~100% QE at 178 nm
see also recent paper from Coimbra (Portugal) Policarpo Group physics/0203011
Gaitskell XENON Proposal/SAGENAP 020312
Advancedphotonix
Rick Gaitskell
XENON TPC Signals Time Structure
t~45 ns
150 µs (300 mm)
• 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)
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell
Operation of LAAPD Array in Geiger Mode
• Operation of sensor large pixellated array in “binary” mode
o High
voltage bias regime
• Single photon causes flip - readout hit time only (not proportional mode)
• Device recovery based on either passive (resistor) or active control of bias voltage
o Dark
Matter experiment is most concerned with few photon regime
• Primary scintillation detection is starved of signal
o Investigate
Hamamatsu 32-channel APD array (S8550)
Gaitskell XENON Proposal/SAGENAP 020312
Rick Gaitskell