Factors Affecting Detector Performance Goals and Alternative Photo-detectors

Transcription

1 XENON Experiment - SAGENAP Factors Affecting Detector Performance Goals and Alternative Photo-detectors Department of Physics Brown University Source at Gaitskell

2 Review WIMP Detection Signal owhy is good detection/discrimination performance required down to 16 kevrecoil (4 kev electron equivalent)?

3 Low Thresholds Vital Very Typical WIMP Signal 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, <<A 2 Ú E r dn de Xe E th =16 kevr gives 1 event/kg/day Example cross-section shown is at current (90%) exclusion limits of existing experiments 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 Form Factor makes very significant modification to naïve ~A 2 rate o due to loss of coherence (since qr>>1) Dashed lines show ~A 2 before considering q>0 Form Factor suppression Form Factor Suppression Note Rapidly Falling Rate

5 Good Performance Must Be Established at Threshold Low threshold vital, since rate falls rapidly with energy o 10% of Recoil Energy >35 kevr (assuming 100 GeV WIMP) Assuming 25% Quenching Factor this is equivalent to <8.8 kevee o ~45% of Recoil Energy >16 kevr Acceptable trade off 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

6 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 UVg generated via both Xe* and Xe+ mediated channels No re-adsorption term to consider e e e e e e 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

7 Available Signal in Liq. Xe Summary SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTS Zero Field High Field 0 V/cm 8 kv/cm GAMMA EVENT - 1 kev electron equivalent energy UV Photons UV UV Electrons+Ions [60-75 elec] elec NUCLEAR RECOIL EVENT - 1 kev recoil energy UV Photons UV 11.6 UV Electrons+Ions [12-18 elec] elec EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR" UV Photons 20-25% 30-50% Electrons+Ions [20-25%] 0.8-2% 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

8 Available Signal in Liq. Xe (2) SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTS Zero Field High Field 0 V/cm 8 kv/cm GAMMA EVENT - 1 kev electron equivalent energy UV Photons UV 40% UV Electrons+Ions [60-75 elec] elec 90% NUCLEAR RECOIL EVENT - 1 kev recoil energy UV Photons UV 11.6 UV Electrons+Ions [12-18 elec] elec EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR" UV Photons 20-25% 30-50% Electrons+Ions [20-25%] 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

9 Available Signal in Liq. Xe (3) SUMMARY OF PARAMETERS FROM EXISTING MEASUREMENTS Zero Field High Field 0 V/cm 8 kv/cm GAMMA EVENT - 1 kev electron equivalent energy UV Photons UV UV Electrons+Ions [60-75 elec] elec NUCLEAR RECOIL EVENT - 1 kev recoil energy UV Photons UV ~100% 11.6 UV Electrons+Ions [12-18 elec] elec EFFECTIVE (NR/GAMMA) "QUENCHING FACTOR" UV Photons 20-25% 30-50% Electrons+Ions [20-25%] 0.8-2% Nuclear Recoil Event o UV Photons w ~50-70 ev / photon, (Lindhard) Quenching Factor measured as 20-25% ( Note: Bernabei (DAMA) use Quenching Factor of 40% which has not been confirmed elsewhere ) 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 ( %)/kV of applied field (in this case use 8 kv/cm so 4-8%) 3-8%

10 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 ~ electrons / kev recoil energy Note once electron extracted from liquid to gas, significant gain ~1000 UVg / electron makes signal easy to observe

11 Baseline - Simulation Results 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

12 Nuclear Recoil Event ~Threshold 16 kevr Nuclear Recoil of 16 kevr (Threshold) o QF 25% -> 4 kevee o 300 UVg into 4ϖ These are ball-park numbers - Full simulation actually ray-traces and includes multiple scattering Detection of electrons (drifted) o % / (kv applied field) extraction from dense ionised region (avoiding self recombination) o 4-18 electrons drifted toward liquid surface 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 300g 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 300g x 6-18% = photoelectrons 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 Large signal in PMT o Even considering PMT/geometry efficiency this gives a large signal

13 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, this will improve background identification/rejection

14 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)

15 New Photodetectors Micro-channel Plate o Burle ~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 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

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

17 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)

18 Operation of LAAPD Array in Geiger Mode Operation of sensor array in binary mode o High voltage bias regime Single photon causes flip - readout hit time (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)