gaitskell xenon experiment - sagenap factors affecting detector performance goals and alternative...

Gaitskell

XENON Experiment - SAGENAP

Factors Affecting Detector Performance Goals

and Alternative Photo-detectors

Rick Gaitskell

Department of PhysicsBrown University

Source at http://gaitskell.brown.edu

Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell

Goal

Why is good detection/discrimination performance required down to 16 keVrecoil (4 keV electron equivalent)?


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 …

€

dN

dEEr

∞

∫

Example cross-section shown is at current (90%) exclusion limits of existing experiments

Xe Eth=16 keVr gives 1 event/kg/day

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)


• Form Factor makes very significant modification to naïve ~A2 rateo … due to loss of coherence (since qr>>1)

Form Factor Suppression

Dashed lines show ~A2 before considering q>0 Form Factor suppression

Note Rapidly Falling Rate


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 • 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

“Acceptable trade off”


Available Signal: UV Photons & Electrons

• Focus on two types of messengers from primary interaction siteo UV Photons (178 nm) from Xe scintillation

• Consider energy required to create photons• Will not consider details of generation mechanism

—Note that UV generated via both Xe* and Xe+ mediated channels• No re-adsorption term to consider

o “Free” 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

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SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTSZero Field High Field

0 V/cm 8 kV/cm

GAMMA EVENT - 1 keV electron equivalent energyUV Photons 60-75 UV 20-30 UVElectrons+Ions [60-75 elec] 50-60 elec

NUCLEAR RECOIL EVENT - 1 keV recoil energyUV Photons 12-18 UV 11.6 UVElectrons+Ions [12-18 elec] 0.4-1.2 elec

EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR"UV Photons 20-25% 30-50%Electrons+Ions [20-25%] 0.8-2%

Available Signal in Liq. Xe

• Summaryo The ranges shown reflect spread in existing experimental measurementso Note that the table considers signal from either 1 keV gamma or nuclear recoil evento 60 excitations / keV is equivalent to ~16 eV / excitationo Zero field electron-ion #’s in [ ] are inferred, but are signal is not measured (extracted) directly



0 V/cm 8 kV/cm




Available Signal in Liq. Xe (2)

• Gamma Evento 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

disappearso 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

90%40%



0 V/cm 8 kV/cm




Available Signal in Liq. Xe (3)

• Nuclear Recoil Evento UV Photons

• 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%)

3-8%~100%

( Note: Bernabei (DAMA) use Quenching Factor of 40% which has not been confirmed elsewhere )


Summary - High Field Operation

• Detection of primary scintillation light is a challengeo ~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 UV / electron

makes signal easy to observe


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 post-triggered 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


Nuclear Recoil Event ~Threshold 16 keVr

• Nuclear Recoil of 16 keVr (Threshold)o QF 25% -> 4 keVee o 300 UV into 4π

• Detection in Phototubeso 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.o Tube photocathode Quantum Efficiency ~30%o 300 x 2% = 6 photoelectrons

• Generation of electrons in CsI photocathodeo 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

o CsI cathode Quantum Efficiency ~30%o 300 x 6-18% = 20-60 photoelectrons

These are ball-park numbers - Full simulation actually traces rays and includes all scattering

• Detection of electrons (drifted)o 0.5-1.0% / (kV applied field) extraction from dense

ionised region (avoiding self recombination)o 4-18 electrons drifted toward liquid surface

• In high field once electrons start drifting ~100% extraction from liquid

• Gas Gaino ~1000 UV from each electron in gaso Signal is localised to xy position of original

interaction

• Large signal in PMTo Even considering PMT/geometry efficiency this

gives a large signal


Why is photodetector performance critical?

• A factor 2 increase in threshold 16 keVr -> 32 keVro Factor 5 loss in effective mass of detector for WIMP search

• A factor 2 decrease in threshold 16 keVr -> 8 keVro Factor <2 improvement in effective mass of detector for WIMP searcho However, lower threshold will, of course, improve background identification/rejection


Existing Photodetector Summary

• Hamamatsu Low Temperature/Liquid Tube (6041)o Baseline design for XENONo Metal construction that has been shown to work in liquid Xe

• Not Low Background: Could be made low backgroundo Low Quantum Efficiency~10-15%

• New Hamamatsu Low Background Tube (R7281)o Being tested by Xmass Collaboration

• Room temperature tests only so faro 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)


New Photodetectors

• Micro-channel Plateo 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 Photodiodeso 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


Effective Quantum Efficiency - LAAPD (Windowless)

Advancedphotonixsee also recent paper from Coimbra (Portugal) Policarpo Group physics/0203011

physics/0203011 demonstrate ~100% QE at 178 nm


XENON TPC Signals Time Structure

• 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)

150 µs (300 mm)

~45 ns


Operation of LAAPD Array in Geiger Mode

• Operation of sensor large pixellated array in “binary” modeo 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 experiment - sagenap factors affecting detector performance goals and alternative...

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