Rick Gaitskell Department of Physics Brown University Source at gaitskell.brown

1. XENON Experiment - SAGENAPFactors Affecting Detector Performance Goals and Alternative Photo-detectors Rick Gaitskell Department of PhysicsBrown University Source at http://gaitskell.brown.edu

2. SAGENAP Questions (020313) • 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. • What do you estimate to be the construction costs for the 100 kg experiment? Including shielding, readout, and support equipment costs (Xe purification, cryogenics). • 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? • What is the expected trigger rate and event dead time? • 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? • 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?

3. SAGENAP Q3 • Why 2 years? • 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 Discussions • Japanese • Columbia has close collaboration with Waseda, which will continue. • 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. • UCLA • Collaboration was discussed, but declined. • Future Merger of Xe Efforts • Definitely! - Given scale (manpower resource estimate ~25 scientists) of final projects this will be mandatory at US level • Probable that maximum of 2 Xe experiments will be operated worldwide

4. Q4 Trigger Rate/Dead Time • 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 • Rate above ground - Typical 104 /keV/kg/day, integration over spectrum ~ 20 Hz/kg, 10 kg mass -> 200 Hz - Cosmics (charged) ~few additional Hz • Rate below ground - Depdendent on shield: of order 1 Hz (conservatively) • 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)

5. Q5 Sensitivity Projections

6. Q2 Construction Costs • What do you estimate to be the construction costs for the 100 kg exp? • Include shielding, readout and support equipment costs (Xe, Purification, Cryo) • [ $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) • [ ~$1M ] Xe Purification + Gas System / Handling / Circulation • [ ~$0.5M] Kr Removal • [ ~$1M ] Design + Construction of 1x100 kg module • [ ~$0.2M ] Clean Room Class 1000 • [ ~$1M ] Readout • [ $0.6M ] Shielding • [$4.6M] Total

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

8. Xe Eth=16 keVr gives 1 event/kg/day Very Typical WIMP Signal • Low Thresholds Vital • Graph shows integrated event rates for E>Er for Xe (green), Ge (red) and S (blue) • Large nuclei enhanced by nuclear coherence, however, in reality <<A2 … Xe WIMP rate for Er > 16 keVr is within factor 2 of maximum achievable rate (Er>0) equivalent kg/kg to low threshold Ge detector 5x better kg/kg than light nucleus (e.g. S in CS2) Example cross-section shown is at current (90%) exclusion limits of existing experiments

9. Form Factor Suppression • Form Factor makes very significant modification to naïve ~A2 rate • … due to loss of coherence (since qr>>1) Dashed lines show ~A2 before considering q>0 Form Factor suppression Note Rapidly Falling Rate

10. “Acceptable trade off” Good Performance Must Be Established at “Threshold” • Low threshold vital, since rate falls rapidly with energy • 10% of signal @ Recoil Energy >35 keVr (assuming 100 GeV WIMP) • Assuming 25% Quenching Factor this is equivalent to <8.8 keVee • ~45% of signal @ Recoil Energy >16 keVr • Equivalent to 4 keVee • Factor 2x sacrifice in “effective detector mass” relative to zero threshold rate • Need to maximise performance in low detection signal regime • Ensure that WIMP identification/background discrimination is working well at ~4 keVee

11. e e e e e e Available Signal: UV Photons & Electrons • Focus on two types of messengers from primary interaction site • UV Photons (178 nm) from Xe scintillation • Consider energy required to create photons • Will not consider details of generation mechanism • Note that UVg generated via both Xe* and Xe+ mediated channels • No re-adsorption term to consider • “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

12. Available Signal in Liq. Xe • Summary • The ranges shown reflect spread in existing experimental measurements • Note that the table considers signal from either 1 keV gamma or nuclear recoil event • 60 excitations / keV is equivalent to ~16 eV / excitation • Zero field electron-ion #’s in [ ] are inferred, but are signal is not measured (extracted) directly

13. 40% 90% Available Signal in Liq. Xe (2) • Gamma Event • 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 • 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

14. ~100% 3-8% Available Signal in Liq. Xe (3) • Nuclear Recoil Event • 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 • 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%) ( Note: Bernabei (DAMA) use Quenching Factor of 40% which has not been confirmed elsewhere )

15. Summary - High Field Operation • Detection of primary scintillation light is a challenge • ~12 UV photons / keV recoil energy • Extraction of electron(s) from nuclear recoil densely ionised region is big challenge • We require observation of this signal to ensure correct identification of nuclear recoil event • ~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

16. 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

17. Nuclear Recoil Event ~Threshold 16 keVr • Detection of electrons (drifted) • 0.5-1.0% / (kV applied field) extraction from dense ionised region (avoiding self recombination) • 4-18 electrons drifted toward liquid surface • In high field once electrons start drifting ~100% extraction from liquid • Gas Gain • ~1000 UVg from each electron in gas • Signal is localised to xy position of original interaction • Large signal in PMT • Even considering PMT/geometry efficiency this gives a large signal • Nuclear Recoil of 16 keVr (Threshold) • QF 25% -> 4 keVee • 300 UVg into 4π • Detection in Phototubes • 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% • 300g x 2% = 6 photoelectrons • Generation of electrons in CsI photocathode • 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 • CsI cathode Quantum Efficiency ~30% • 300g x 6-18% = 20-60 photoelectrons These are ball-park numbers - Full simulation actually traces rays and includes all scattering

18. Why is photodetector performance critical? • A factor 2 increase in threshold 16 keVr -> 32 keVr • Factor 5 loss in effective mass of detector for WIMP search • A factor 2 decrease in threshold 16 keVr -> 8 keVr • Factor <2 improvement in effective mass of detector for WIMP search • However, lower threshold will, of course, improve background identification/rejection

19. Existing Photodetector Summary • Hamamatsu Low Temperature/Liquid Tube (6041) • Baseline design for XENON • Metal construction that has been shown to work in liquid Xe • Not Low Background: Could be made low background • Low Quantum Efficiency~10-15% • New Hamamatsu Low Background Tube (R7281) • Being tested by Xmass Collaboration • Room temperature tests only so far • Metal construction, and giving lower backgrounds • ~500 per day (XENON baseline target is 100 per tube per day) • Higher Quantum Efficiency~27-30% • Uses longer optics which give better focusing (could be accommdated in XENON)

20. New Photodetectors • Micro-channel Plate • 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 • 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

21. Effective Quantum Efficiency - LAAPD (Windowless) physics/0203011 demonstrate ~100% QE at 178 nm Advancedphotonix see also recent paper from Coimbra (Portugal) Policarpo Group physics/0203011

22. 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) t~45 ns 150 µs (300 mm)

23. Operation of LAAPD Array in Geiger Mode • Operation of sensor large pixellated array in “binary” mode • 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 • Dark Matter experiment is most concerned with few photon regime • Primary scintillation detection is starved of signal • Investigate Hamamatsu 32-channel APD array (S8550)