rick gaitskell department of physics brown university source at gaitskell.brown

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

SAGENAP Questions (020313)

(1) What are the FTE commitments of each of the of the people listed in the final table over the two-year R&D? For each person, estimate also the FTE commitment to each of the other projects, and note faculty teaching loads as necessary.

(2) What do you estimate to be the construction costs for the 100 kg experiment? Including shielding, readout, and support equipment costs (Xe purification, cryogenics).

(3) We’d like to understand better the nature of the competition. Specifically, what is driving your two-year timescale? Have there been discussions for the collaboration with the other Xe TPC groups in the world, and what were the results? Would a merging of efforts be possible after the first phase of this project?

(4) What is the expected trigger rate and event dead time?(5) The sensitivity plots on p14 of the presentation and the Zeplin IV Feb Status

Report Presentation appear different by a factor ~50 (both are 1 tonne). Can you explain the reasons?

(6) Please make a list of the deliverables from this 2-year R&D program and the person responsible for each. What design decisions will be made at the end of year 1 and year 2?


SAGENAP Q3

• Why 2 years?o LXeGRIT is a prototype “-1” that already demonstrates much of base engineering. In this

proposal we are focusing on a number of key technologies that will enable us to realise radioactive background, signal-to-noise and discrimination goals. The timeline of 2 years to construct 10 kg dark matter chamber is necessary, in order to contribute meaningfully to “world-wide” Xe beauty contest (100 kg+1 tonne) that we expect will occur in ~2004.

• Collaboration Discussionso Japanese

• Columbia has close collaboration with Waseda, which will continue. o UK

• Collaboration discussed. They requested our direct commitment of 100 kg and 1 tonne phases to Boulby site, which is not necessarily consistent with large US involvement.

o UCLA• Collaboration was discussed, but declined.

• Future Merger of Xe Effortso Definitely! - Given scale (manpower resource estimate ~25 scientists) of final projects this

will be mandatory at US levelo Probable that maximum of 2 Xe experiments will be operated worldwide


Q4 Trigger Rate/Dead Time

(1) We trigger on “large” light signal:- Direct light on CsI photocathode (>20 p.e.) amplified by Proportional Scintillation (~1000).- Requirement ≥ 2 PMT- Coincident with direct light on PMTs (>4 p.e.) exactly 150µs earlier

(2) Rate above ground- Typical 104 /keV/kg/day, integration over spectrum ~ 20 Hz/kg, 10 kg mass -> 200 Hz- Cosmics (charged) ~few additional Hz

(3) Rate below ground- Depdendent on shield: of order 1 Hz (conservatively)

(4) Dead Time- Gas drift time: 150 µs- Post triggered FADC read out with buffer- Signals ~1 µs => FADC 5-10 Msample/sec- 37 FADC channels (for 100 kg)- 32 ADC for PMT in Xe shield

150 µs (300 mm)


Q5 Sensitivity Projections

Projected

Sensitivities (cm2)

XENON Experiment

Change From Baseline Specs

ZEP–AXE-like ~10-46 Use GEMS instead of PMTs, 85Kr limits performance

~2x10-46 *** BASELINE ***

~3x10-46 Use conservative U/Th from proposal

~5x10-46 Use 85Kr 1 ppb limits performance

XMASS-like scaled to 1 tonne ~8x10-46 Remove multiple hit cut, or PMT

has 10x higher backround

~3x10-45 Realized U/Th background, or large area PMT background

ZEPLIN IV-like ~10-44

XMASS 1 tonne ~8x10-44 Eth= 20 keVee vs 5 keVee

XENON ~2x10-46 if all works great; ~10-45 is a minimal (?) expectation, if it works at all like envisaged

Note: Correction to proposal: Table # on 85Kr should be 0.1 ppb (all calcs are correct)Note: 99.5% muon veto for tertiary neutron only, not spallation


Q2 Construction Costs

• What do you estimate to be the construction costs for the 100 kg exp?o Include shielding, readout and support equipment costs (Xe, Purification, Cryo)o [ $0.32M ] Xe: 100 kg Active Target + ~100 kg Active Shield

• $1.6/g ($6/g CDMS cryo-detector grade) • 1 module $320k of Xe• (1 tonne active Xe -> $1.6m)

o [ ~$1M ] Xe Purification + Gas System / Handling / Circulationo [ ~$0.5M] Kr Removalo [ ~$1M ] Design + Construction of 1x100 kg moduleo [ ~$0.2M ] Clean Room Class 1000o [ ~$1M ] Readouto [ $0.6M ] Shielding

• [$4.6M] Total


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

ee

ee

e e


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)

rick gaitskell department of physics brown university source at gaitskell.brown

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