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The LHCb Pixel Hybrid Photon Detectors

This presentation discusses the hybrid photon detectors used in the LHCb experiment, including their manufacture, testing, and installation progress. It also highlights the requirements and performance of these detectors.

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The LHCb Pixel Hybrid Photon Detectors

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  1. The LHCb Pixel Hybrid Photon Detectors Robert W. Lambert, University of Edinburgh On behalf of the LHCb RICH collaboration PD07, 28th June 2007

  2. Outline • Introduction: LHCb and Particle ID • Hybrid Photon Detectors (HPDs) • HPD Manufacture • Testing and Results • RICH Installation Progress • Summary PD07, 28th June 2007

  3. LHCb • LHCb will examine CP-violation in B-mesons [1] • Why is there a matter-antimatter asymmetry in the universe? • What are the reasons for the parameters in the Standard Model? • Is there physics beyond the Standard Model? PD07, 28th June 2007

  4. LHCb @-100m RICH 2 Magnet RICH 1 PD07, 28th June 2007

  5. RICH 1 and RICH 2 • Ring Imaging CHerenkov (RICH) detectors [2] • Relativistic charged particles in a medium radiate light • Characteristic cone angle, cos  = 1/bn RICH 1 (Vertical) RICH 2 (Horizontal) TT MAGNET T1-T3 PD07, 28th June 2007

  6. Cherenkov Imaging • Rings reconstructed • Velocity  Cone angle ≡ Ring radius • Combine with momentum to get Particle ID RICH 1 for 1 < p < 60 GeV/c RICH 2 for p < 100 GeV/c ~20 hits/ring ~4 hits/ring ~25 hits/ring PD07, 28th June 2007

  7. Requirements • Stringent requirements for RICH photodetectors • 2.6 m2 detector plane • Single-photon sensitive • 65% active area overall • 80% for cylindrical devices • Cherenkov Spectrum • 2.5 mm x 2.5 mm granularity • 25 ns Clock <25ns response • 40 MHz Clock 40 MHz read out • Trigger decision 4ms data buffer • Photon yield  High Signal:Noise • 5-10 year lifetime • Radiation tolerant  30 krad • Fringe magnetic field C4F10 200-600nm PD07, 28th June 2007

  8. Photocathode (S20) at -20kV Si Sensor 8192 pixels Ceramic Carrier 8192 Vacuum Photoelectrons Photon 120 mm Electrode Solder bump bonds 87 mm Binary electronics chip Quartz Window Hybrid Photon Detectors • 484Hybrid Photon Detectors HPDs required [3] • Photocathode (S20) • Silicon sensor • Binary read-out chip PD07, 28th June 2007

  9. HPD Manufacture (1) • Production at IBM, Canberra, Kyocera, VTT, HCM, DEP-Photonis • Full testing and gold plating by LHCb at CERN Silicon sensor Bump-bonding Packaging Readout chip Ceramic carrier Brazing and gold-plating PD07, 28th June 2007

  10. HPD Manufacture (2) • Encapsulated by DEP-Photonis • Testing by LHCb Photo-cathode deposition and vacuum sealing Tube body assembly Packaging PD07, 28th June 2007

  11. HPD Production 536 / 550 HPDs Produced 519 / 536 HPDs Tested 18th June 2007 PD07, 28th June 2007

  12. PDTF • Photo-Detector Test Facilities • 2 centres (2 stations each) • Test 1 HPD/site/day • 506 of 519 HPDs pass  Failures Replaced. PD07, 28th June 2007

  13. PDTF Tests • Check out every function of the HPD, from the ground up Photocathode Dark Count Response to light Quantum Efficiency Electron Optics Image Size Image Centre HV Stability Field Distortions Readout Chip Connections Communications DAQ Readout Dead Channels Noisy Channels Masking Responses Threshold Noise HPD Body Dimensions HV Stability Vacuum Quality Silicon Sensor IV Curve Efficiency (Backpulse) PD07, 28th June 2007

  14. Silicon Sensor • PDTF perform a bias scan of each sensor • Measures sensor quality H527009, 0.46 mA leakage at 80V Contract Typical 1mA Ramp-up Operating Point, 80V Ramp-down PD07, 28th June 2007

  15. Readout Chip • Low number of faulty channels • Average 0.15% dead channels << 5% specification • Average 0.02% noisy channels << 5% specification Specification < 400 Specification < 400 PD07, 28th June 2007

  16. Sensor + Readout • Thresholds and noise • Threshold scan performed on all 8192 pixels • ~85% sensor efficiency, Typical signal is 5000 e- <T> = 1063 e- <N> = 145 e- PD07, 28th June 2007

  17. HV Stability • PDTF perform a HV scan of each HPD • Measures HV stability • Pulsed LED used at each voltage step H527009, a typical HPD H527009, 200k events, LED run Reflections Operating Point, 20kV PD07, 28th June 2007 Backscatter

  18. Vacuum Quality • Ion-feedback (IFB), afterpulse • Ionisation of residual gas atoms, particularly He, produces afterpulse • At 20 kV, IFB measures the vacuum quality 1 Residual Gas Ionised <IFB> = 0.03% Specification < 1% Ion liberates many secondary electrons 2 3 Secondaries measured after characteristic delay PD07, 28th June 2007

  19. Dark Count • Thermionic emission, noise, and IR-sensitivity produce Dark Count • Specification 5 kHz cm-2 • Average 2.6 kHz cm-2 ≡ 0.003 hit / event / HPD in LHCb H520009, 5M events, 2.0 kHz cm-2 Specification < 5kHz cm-2 PD07, 28th June 2007

  20. Quantum Efficiency • QE is a function of wavelength, large improvement seen • Independent measurements: photocurrent from known light level • DEP improved the QE with each batch (i.e. with time) Increased over time Agreement across measurements Decreased over time Expectation from preseries PD07, 28th June 2007

  21. SQE dE • S QE dE, integrate improvement in QE across energy • Cherenkov light has flat energy spectrum • 24% relative increase in S QE dE over expectations from preseries  Expectation from preseries PD07, 28th June 2007

  22. HPDs in use • HPDs fulfil or exceed all requirements for the LHCb RICH • Excellent performance demonstrated in testbeam scenarios Ring from Pions, over 3 HPDs 124k events, with C4F10 Hit spectrum, 124k events In agreement with expected yields PRELIMINARY pedestal and noise Frequency, thousands of events signal Hit Count in expected region of ring for 1 HPD PD07, 28th June 2007

  23. HPD Integration • HPDs -> Columns • Magnetic Sheilds • Level-0 Data Processing • LV power distribution • HV power distribution HPD L0 LV HV PD07, 28th June 2007

  24. RICH 2 Installation PD07, 28th June 2007

  25. Summary • 484 HPDs are required for the LHCb RICH • 536 HPDs have now been produced • 519 tested at PDTF with 506 passes • Excellent results overall • 24% relative improvement in QE will directly improve photon yields • RICH is now under installation and commissioning • RICH 2 fully populated with HPDs • LHCb is getting ready for data…. PD07, 28th June 2007

  26. References • LHCb collaboration, LHCb Technical Proposal, CERN-LHCC-98-004 LHCb, 20th February 1998 • LHCb collaboration, LHCb RICH, Technical Design Report 3, CERN-LHCC-2000-037 LHCb, 7th September 2000 • T. Gys, LHCb RICH, “Production of 500 pixel hybrid photon detectors for the RICH counters of the LHCb,” NIM A 567 (2006), pp. 176-179 PD07, 28th June 2007

  27. Backup • Additional slides hereafter PD07, 28th June 2007

  28. Physics and Photons • RICH crucial to separate Kaons and Pions [1] • Similar hadrons, different in mass • Contribute to different physics • Important to separate Signal Bd p+p- PD07, 28th June 2007

  29. HPDs Realised • Hybrid Photon Detectors Quartz window thin metal Photocathode (S20) Photoelectric effect produces electrons 120 mm 20kV accelerating potential Pixelated anode 8192 pixels 500 mm x 62.5 mm Amplifier, Thresholder, Buffer, Read out 87 mm PD07, 28th June 2007

  30. HPD Production 536 / 550 HPDs Produced 18th June 2007 PD07, 28th June 2007

  31. PDTF Progress 519 / 550 HPDs tested 15th May 2007 PD07, 28th June 2007

  32. Shutter/iris combination bandpass filter (+- 10 nm FWHM) Calibrated photodiode (Newport 818-UV unbiased) HPD Quartz-tungsten halogen lamp (6V, 50 W) LOT Oriel lamp housing ND filter (where required) Fused silica lens f = 50 mm, diam. = 25.4 mm Shutter/iris combination RL IR-blocking filter (Schott KG-5) 100V q(HPD) = q(pd)* I(HPD)/ I(pd) I HPD (pA) I pd (pA) QE at PDTF • Uses existing Darkbox • PC at -100V, focussing cathodes at -100V, Anode at ground PD07, 28th June 2007

  33. Results Summary • HPD Quality Assurance • 506 of 519 tubes pass  HPDs with higher darkcount A+: Exceeds key specifications. A specifically recommended HPD A: Pass all aspects of tests B: Falls beneath contracted specifications, but still recommended for use in the RICH E: HPD qualified for use in the RICH, but is flagged with an issue F: Clear failure of HPD, such that it is unusable in the RICH. HPD returned to DEP if possible for replacement HPDs with high leakage current and with >1% dead pixels PD07, 28th June 2007

  34. HPD electron optics • Reliable manufacture • 73% of centres within 1 pixel of chip centre • standard deviation of image size ~ ¼ pixel 1 Pixel PD07, 28th June 2007

  35. Peak QE • QE peaks at ~270 nm • Consistent improvement of QE with batch, • All production HPDs are over specifications <QE> = 30.9% Contract Minimum 20.0% PD07, 28th June 2007

  36. Backpulse • Efficiency of hit detection, hSi • Pixel chip efficiency important for reconstruction • Probability that the chip registers a hit, given a photoelectron has struck • Comparing the number of photoelectrons seen by the chip (via normal chip readout) to the number arriving at the backplane of the Si sensor. • We measure: hsi = (87±2)%. 3 electrons 2 electrons 4 electrons 5 electrons 1 electron Fit to charge spectrum at backplane PD07, 28th June 2007

  37. Afterpulse • Ion Feedback from Strobe Scan • Consistently low, indicating excellent vacuum in all tubes • Single HPD, H546002, displayed IFB and dark-count anomalies <IFB> = 0.03% Specification < 1% Very low IFB <<1% PD07, 28th June 2007

  38. Source Sites • HPD sourced from around the world ! PD07, 28th June 2007

  39. QW Reflections • As predicted by naïve CAD approximations • 75% of light reflected off Chromium coating • TIR at QW-Air interface • ~20% reflection at QW-PC interface PD07, 28th June 2007

  40. Reflective Effects • Activating QW reflections and Chromium reflections • 8.0% more hits (naïve estimate would predict ~11%) Improved Geometrical Description Reflections Activated 1.5 M events, 4,992,419 Hits 1.5 M events, 5,393,100 Hits PD07, 28th June 2007

  41. Backscatter • Only ~85% of all real photoelectrons produce digital hits • Thermal effects • Thresholding effects • Backscatter effects - - + - + + - + - + - + Si Sensor + - Charge Sharing. <5000 e-h pairs per pixel Thermal absorption v. few e-h pairs, Damage to lattice Normal Case ~5000 e-h pairs Backscatter. Smaller amount of energy deposited Electron “may” Fall back onto Si sensor PD07, 28th June 2007

  42. RICH in UV • Below 200 nm photon yield is limited by absorption of air PD07, 28th June 2007

  43. Expected Spectra • Folding in the expected QE Cherenkov spectrum for RICH radiators RICH 1, C4F10 RICH 2, CF4 RICH 1, Aerogel Cherenkov Photons Wavelength [nm] PD07, 28th June 2007

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