PH0008 Quantum Mechanics and Special Relativity Lecture 08
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Transcript PH0008 Quantum Mechanics and Special Relativity Lecture 08
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
SUMMA RY OF PA RA METERS FROM EXISTING MEA SUREMENTS
Zero Field
H igh Field
0 V /c m
8 kV /c m
GA MMA EVENT - 1 keV elect ron equivalent energy
U V P hotons
60-75 U V
E lec trons +I ons
[6 0 - 7 5 elec ]
NUCLEA R RECOIL EVENT - 1 keV recoil energy
U V P hotons
12-18 U V
E lec trons +I ons
[1 2 - 1 8 elec ]
EFFECTIVE (NR/GA MMA ) "QUENCHING FA CTOR"
U V P hotons
20-25%
E lec trons +I ons
[2 0 - 2 5 % ]
20-30 U V
5 0 - 6 0 elec
1 1 .6 U V
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)
SUMMA RY OF PA RA METERS FROM EXISTING MEA SUREMENTS
Zero Field
H igh Field
0 V /c m
8 kV /c m
GA MMA EVENT - 1 keV elect ron equivalent energy
U V P hotons
60-75 U V
40%
E lec trons +I ons
[6 0 - 7 5 elec ] 90%
NUCLEA R RECOIL EVENT - 1 keV recoil energy
U V P hotons
12-18 U V
E lec trons +I ons
[1 2 - 1 8 elec ]
EFFECTIVE (NR/GA MMA ) "QUENCHING FA CTOR"
U V P hotons
20-25%
E lec trons +I ons
[2 0 - 2 5 % ]
20-30 U V
5 0 - 6 0 elec
1 1 .6 U V
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)
SUMMA RY OF PA RA METERS FROM EXISTING MEA SUREMENTS
Zero Field
H igh Field
0 V /c m
8 kV /c m
GA MMA EVENT - 1 keV elect ron equivalent energy
U V P hotons
60-75 U V
E lec trons +I ons
[6 0 - 7 5 elec ]
20-30 U V
5 0 - 6 0 elec
NUCLEA R RECOIL EVENT - 1 keV recoil energy
U V P hotons
1 2 - 1 8 U V ~100%
E lec trons +I ons
[1 2 - 1 8 elec ] 3-8%
1 1 .6 U V
0 .4 - 1 .2 elec
EFFECTIVE (NR/GA MMA ) "QUENCHING FA CTOR"
U V P hotons
20-25%
E lec trons +I ons
[2 0 - 2 5 % ]
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