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Operating Hybrid Photon Detectors in the LHCb RICH counters at high occupancy

Operating Hybrid Photon Detectors in the LHCb RICH counters at high occupancy. Stephan Eisenhardt, University of Edinburgh On behalf of the LHCb experiment. Introduction HPD Benefits for LHCb RICH Operation Experienced from Run 1 Photon Yields Ion Feedback Evolution & HPD Optimisation

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Operating Hybrid Photon Detectors in the LHCb RICH counters at high occupancy

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  1. Operating Hybrid Photon Detectors in theLHCb RICH counters at high occupancy Stephan Eisenhardt, University of Edinburgh On behalf of the LHCb experiment Introduction HPD Benefits for LHCb RICH Operation Experienced from Run 1 Photon Yields Ion Feedback Evolution & HPD Optimisation Conclusions 04.12.2013 RICH 2013, Kanagawa, 04.12.2013

  2. ~1 cm B LHCb RICH Counters b-b angular correlation Dipole Magnet RICH2 RICH1 VELO collision point • for RICH detector description and operation • talk byA. Papanestis • for RICH detector performance • talk by C. Matteuzzi • for RICH upgrade (2019) • talk by S. Easo Stephan Eisenhardt

  3. Hybrid Photon Detectors Anode Vacuum photon detector • Readout: • encapsulated 256x32 pixel silicon sensor • bump-bonded to binary readout chip • low noise of 145 e-→ low background • 8-fold binary OR  effective 3232 pixel array • pixel size 500mm500mm sufficient Anode on carrier RICH1 HPD panel 65% geometric efficiency, incl. m-metal • Pixel HPDs: • developed in collaboration with industry (lead partner: Photonis-DEP) • combines: vacuum photon detector technology with silicon pixel readout • Quartz window with S20photocathode → high QE • QE: increased during production: 25% → 31% • 20kV operating voltage (~5000 e–[eq. Si]) • Factor 5 demagnification @ 20kV → close-packing Stephan Eisenhardt

  4. Quantum Efficiency <QE> (PhotonisData): across delivery batches • QE right where we need it: • increase during production • single most helpful improvement to RICH performance • <QE @ 270nm> = 30.8% >> typical QE = 23.3% <QE> per delivery batch QE [%] QE [%] Wavelength [nm] RMS of batch spread • LHCb QA cross-check: • measured QE on 10% of tubes • confirmed Photonis data Batch number Stephan Eisenhardt

  5. Pixel Chip – Threshold and Noise • excellent signal over noise: specification <measured> • average signal charge @ 20kV: C = 5000 e- • average threshold: T = < 2000 e- 1065 e- • average electronic noise: N = < 250 e- 145 e- • signal over noise: S/N = (C-T)/N > 12 27 (min, max) = (21,33) <noise>: 145 e- <threshold>: 1065 e- <S/N>: 27 Stephan Eisenhardt

  6. Occupancies @ L=4x1032 cm-2s-1 02.09.2012 RICH2 ~2700 photons/event RICH1 ~2400 photons/event RICH1+2: ~500k channels RICH1: 196 HPD RICH2: 288 HPD Stephan Eisenhardt

  7. Image Drifts 14.6 hours 30 hours 30 hours • Observation of image drifts for some HPD • especially in RICH1 with time scale 0.5-1 hour • while most HPDs show stable image, within 0.2 pixels • always the same few HPD show either: • continuous drifts: typically <1.5 pixels, max. <3 pixels • or distinct shifts • without periodicity or correlation to environment • reason not really understood, but looks like charging effect pixel pixel pixel pixel 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 5 10 15 20 25 30 1 pixel 1 pixel 1 pixel 19.5 19.0 18.5 18.0 17.5 17.0 16.5 16.0 1 pixel 19.0 18.5 18.0 17.5 17.0 16.5 16.0 15.5 19.0 18.5 18.0 17.5 17.0 16.5 16.0 15.5 0 400 800 1200 1600 2000 16.6 16.5 16.4 16.3 16.2 16.1 16.0 15.9 15.8 14.6 hours time[hrs] time[hrs] time[hrs] time[min] Stephan Eisenhardt

  8. Image Drifts • Observation of image drifts for some HPD • correlation in time between x- and y-movement, but not linear • Solution: automated monitoring of movement • fit image position from beam data • using Sobel algorithm for edge detection • online correction time [min] time [min] 14.6 hours 14.6 hours 2000 1600 1200 800 400 0 2000 1600 1200 800 400 0 15.0 15.4 15.8 16.2 16.6 17.0 15.6 16.0 16.4 16.8 17.2 y [pixel] x [pixel] 18.0 17.6 17.2 16.8 16.4 16.0 x [pixel] 20.0 19.0 18.0 17.0 16.0 15.0 y [pixel] photo cathode image on anode with edge from Sobel fit Stephan Eisenhardt

  9. HPD Gas Atmosphere • 2011: during period of increase of data rate • HPD saw “Beam Induced Light Events” – corona • 2011: during period of increase of data rate • HPD saw “Beam Induced Light Events” • spreading to other HPD • CO2 reported to better suppress corona than N2 • changed atmosphere in HPD box from N2 to CO2 • changed HV: 1816kV, at negligible efficiency loss • result: stable ever since bias current of RICH1 rows vs. time RICH1 panels N2 CO2 corona light from corona in opposite panel N2 CO2 11.5.2011 26.5.2011 5.6.2011 Stephan Eisenhardt

  10. Photon Yield - Method • Choose the cleanest data set: Tracks RICH1: ppppm+m- event RICH1: typical 2012 event • fit shape of Cherenkov angle resolution: Dq = qrec - qexp • get photon yield from area of fit to each track (using all photons, fixed shape & flat background) • get average track yield over sample (long run) Stephan Eisenhardt

  11. Photon Yield - Results drop is rate dependent, no QE degradation • Npe from data slightly lower than MC prediction from D*D0p+ optimised HPD chip settings RICH1 C4F10 2011 2012 2010 RICH2 CF4 Stephan Eisenhardt

  12. Ion Feedback – Monitoring high IFB low IFB • Process: • photoelectron ionises residual gas atom • drift of ion to photo cathode (200-300ns) • release of secondary photo electrons (~10-40 e-) • impact of ph.e. cluster on sensor (cluster size) • Three measurement methods: • 1) measure gas gain in QE setup – very sensitive • 2) scan delay of DAQ gate – standard lab tool • 3) cluster size – RICH in-situ measurement example dark count hit maps HPD response to 15ns LED pulses with varied delay 50ns strobe signal Very low IFB <<1% hits / event photoelectron current vs. bias voltage Delay [ns] Stephan Eisenhardt

  13. IFB – in-situ Monitoring • In-situ monitoring: • cw-laser (635nm) • record IFB from cluster size • evolution in time cut typical cluster size distributions • In physics data: • IFB removed by clustering • to first order: no effect on PID photon yields /event /HPD with cw-laser RICH1 hit map: cw-laser illumination Stephan Eisenhardt

  14. IFB – Evolution • Evolution: without beam • residual gas increases linear in time • typically: DIFB <0.5% / year • illumination anneals IFB • fraction of HPD evolve more quickly • Threshold: self-sustained IFB • from IFB > 5% • where photocathodes degrade quickly  exchange & repair H602003: IFB rate monitored in RICH2 Ion Feedback [%] Ion Feedback [%] 2008 2009 2008 2009 2010 2011 2012 2010 2011 2012 Date Date • Evolution: with increasing data rate • IFB increases stronger • correlated with heat (data rate) • the fraction of HPD evolving more quickly increased • a stretch for the exchange & repair programme IFB distribution 03/2010 Stephan Eisenhardt

  15. IFB – Evolution – Long Shutdown 1 • Evolution: during shutdown • some HPD show significant increase when operation conditions are not well defined • Strategy for LS1: • LV off – to keep HPDs cool • HV on – to allow photoelectron production • cw-laser on – to cause annealing • Si-Bias on – to monitor tubes • ~monthly IFB runs (needs LV on) • Strategy pays off: • 163 HPD show negative DIFB • 144 HPD show reduced DIFB • 97 HPD show continuous DIFB • 9 HPD show increased DIFB • O(50) special cases H638003: IFB rate monitored in RICH2 Ion Feedback [%] 2008 2009 2010 2011 2012 Date H721002: IFB rate annealed during LS1 Ion Feedback [%] 2008 2009 LS1 2013 2010 2011 2012 Date Stephan Eisenhardt

  16. HPD Repair&Replacement Program • Pre-Run1 Repair & Replacement program: • 2009: 35 HPD – catching up on old HPD which were most affected • 2010 prediction: • need to exchange O(13) HPD/year • Run1 Repair & Replacement program: • 2010: 6 HPD • 2011: 38 HPD • 2012: 39 HPD • 2012/13: R&D to improve stability of tube vacuum • see next slide • Long Shutdown 1 Repair & Replacement program: • with optimised production parameters • 2013: 40 HPD • 2014: O(40) HPD (planned) • Procedure: • remove HPD from RICH and return to Photonis • recuperate anode, body and Quartz window • build new tube • full Quality Assurance • re-introduce to RICH RICH1 duringbuilding HPD volumes Stephan Eisenhardt

  17. HPD Production Improvement • Standard production procedure: • yielded a rather large variation of DIFB • bulk: 0.1%/yr < DIFB < 0.2%/yr • tail: < 0.5%/yr (which is tolerable) • repair reset the clock and ‘threw dice’ again • Improvement: • introducing getters • optimised production recipe • they integrate now very well • dimensioned to last 10 years • New production procedure: • reliably gives very low initial IFB • and gives DIFB <<0.01%/yr • used for repair & replace in LS1 0.5%/yr without getters with getters 0.2%/yr IFB vs. time: R&D sample IFB scale: x100 zoom near sensitivity limit Stephan Eisenhardt

  18. RICH: the LHCb PID Workhorse • Used by (virtually) every analysis in LHCb to do positive ID Without RICH With RICH JHEP 10 (2012) 037 Without RICH PID, the B0 p+p- is completely dominated by B0  K+p- Stephan Eisenhardt

  19. Conclusions • HPD benefits for LHCb RICH • high QE • low noise  low background • Got operational challenges under control quickly or well maintained • Developed reliable tools and measures to deal with IFB • beautiful PID properties of RICH are maintained • Developed now long-term fix to suppress IFB in the HPDs • HPD repair for Run2 (2015-18) is under way • RICH is the reliable PID workhorse for LHCb • most (student) members these days just know it from their PID selection code… Stephan Eisenhardt

  20. Spare Slides • t Stephan Eisenhardt

  21. Flat mirrors Spherical Mirrors Support Structure 7.2 m Central Tube Photon Funnel + Shielding RICH1 and RICH2 Layout RICH1 RICH2 reversible magnetic field Interaction Point Stephan Eisenhardt

  22. LHCb Operation 2010-2012 • Excellent running: 2010 2011 2012 • Beam energy 3.5TeV 3.5TeV4.0TeV • Luminosity [cm-2 s-1] 2x10322-4x10324x1032 successful test: 6x1032 • Visible interactions/crossing m = 0.4m = 0.4-1.4 m = 1.6 • Data taking efficiency >90% >91% >94% • High Level Trigger output to tape 3kHz 4.5kHz • bunch spacing 50ns 50ns50ns • Recorded luminosity 0.037fb-1>1.0 fb-1>2.0 fb-1 bb cross- section +15% design values (25ns from 2015) design lumi 2012 2011 LHCb lumi levelling by beam adjustment 2010 Stephan Eisenhardt

  23. Occupancies @ L=6x1032 cm-2s-1 30.11.2012 RICH2 ~3200 photons/event RICH1 ~2800 photons/event RICH1+2: ~500k channels RICH1: 196 HPD RICH2: 288 HPD Stephan Eisenhardt

  24. Magnetic Distortions • Imaging in HPDs is distorted by external magnetic fields • used projected test pattern with and without field to extract corrections • done for both filed orientations • produced maps for online correction Before After RICH1 Before After hits RICH2 Dx = 0.18 pixel 0 mT 3 mT 0 mT 3 mT transversal field axial field pixels Stephan Eisenhardt

  25. t Stephan Eisenhardt

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