1. Gaitskell 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

2. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Goal Why is good detection/discrimination performance required down to 16 keVrecoil (4 keV electron equivalent)?

3. 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 <<A 2 … Example cross-section shown is at current (90%) exclusion limits of existing experiments Xe E th =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 CS 2 )

4. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Form Factor makes very significant modification to naïve ~A 2 rate o … due to loss of coherence (since qr>>1) Form Factor Suppression Dashed lines show ~A 2 before considering q>0 Form Factor suppression Note Rapidly Falling Rate

5. 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 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”

6. 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 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 e e e e e e

7. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Graphic Xe * +Xe Xe 2 * Triplet 27ns Singlet 3ns 2Xe 175nm Xe ** + Xe Xe 2 + +e - (recombination) Xe + +Xe

8. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Xe Photon & Electron Yield (v3) GAMMA depositing 1 keVee or for high field UV ~0.4 keVvis_zero_field Zero FieldHigh Field (8 kV/cm) 60^ UV + loss^^20-30^& UV + 50-60^ eion + smaller loss^^ NUCLEAR RECOIL 1 keVrecoil or for high field UV ~0.2 keVvis_zero_field Zero FieldHigh Field (8 kV/cm) 12 UV11.6 UV + 0.4 eion EFFECTIVE (NR / GAMMA) "QUENCHING FACTOR" Zero FieldHigh Field (8 kV/cm) 20%50% UV + 0.8% eion ^ Empirical measurement ^^ Loss such as non-rad or electrons that escape ^& Some literature uncertainty here && Above the (0.4 eion/1 keVrecoil) is calculated as follows Gamma 1keVee generates 50 eion * 20% QF for Lindhard estimate of nuclear recoil coupling to ionisation state would give 10 eion * 4% survive after self recombination of dense ionisation cloud (estimate based on rule of thumb ~0.5% per kV/cm field) equals 0.4 eion/1 keVr or 2 electrons for 5 keVrecoil A 5 keVr would generate same UV light as a 1 keVee gamma in zero field case. or as a ~2.7 keVee gamma in high field case

9. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Available Signal in Liq. Xe Summary o 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

10. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Available Signal in Liq. Xe (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 90% 40%

11. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Available Signal in Liq. Xe (3) Nuclear Recoil Event o 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 )

12. 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 UV  / electron makes signal easy to observe

13. 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  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

14. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Nuclear Recoil Event ~Threshold 16 keVr Nuclear Recoil of 16 keVr (Threshold) o QF 25% -> 4 keVee o 300 UV  into 4π 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. o Tube photocathode Quantum Efficiency ~30% o 300  x 2% = 6 photoelectrons 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 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 Gain o ~1000 UV  from each electron in gas o Signal is localised to xy position of original interaction Large signal in PMT o Even considering PMT/geometry efficiency this gives a large signal

15. 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

16. 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)

17. 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 cm 2 ) 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 cm 2 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/cm 2 ) (i dark ) 170K ~ 10 -4 (i dark ) RT

18. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Effective Quantum Efficiency - LAAPD (Windowless) Advancedphotonix see also recent paper from Coimbra (Portugal) Policarpo Group physics/0203011 physics/0203011 demonstrate ~100% QE at 178 nm

19. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell 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

20. 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)

21. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Large Area APD Advanced Photonix o 16 mm diameter active area () Xe tests at Coimbra on 5 mm (Area 0.2 cm 2 )

22. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell LAAPD Testing Simple Experimental Rig

23. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Time Structure of Event direct proportional drift time  ~45 ns

24. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Avalanche Photodiode o How does it actually work? o Avalanche Silicon Photodiodes (APDs) have built in gain due to impact ionization. This gives high responsivity and high gain - bandwidth product with superior sensitivity compared to PIN photodiode/amplifier combinations for high bandwidth applications. Silicon is especially suited for making good avalanche photodiodes as the ionization coefficient for electrons are much higher than for holes.

25. Gaitskell XENON Proposal/SAGENAP 020312 Rick Gaitskell Hamamatsu o They are looking for large orders (CMS PbWO4 Calorimeter >25,000 0.5 cm 2 units) o Hamamatsu 32-channel APD array (S8550) — Unfortunately, ceramic packaging.