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Scintillation Detectors

Scintillation Detectors. Introduction Components Scintillator Light Guides Photomultiplier Tubes Formalism/Electronics Timing Resolution. Elton Smith JLab 2006 Detector/Computer Summer Lecture Series. b p = p/ √p 2 +m p 2 = 0.9957 . b K = p/ √p 2 +m K 2 = 0.9496.

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Scintillation Detectors

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  1. Scintillation Detectors Introduction Components Scintillator Light Guides Photomultiplier Tubes Formalism/Electronics Timing Resolution Elton Smith JLab 2006 Detector/Computer Summer Lecture Series

  2. bp = p/√p2+mp2 = 0.9957 bK = p/√p2+mK2 = 0.9496 tp = L/bpc = 15.77 ns tK = L/bKc = 16.53 ns Particle Identification by time-of-flight (TOF) requires Measurements with accuracies of ~ 0.1 ns DtpK = 0.76 ns Experiment basics p = 0.3 B R = 1.5 GeV/c B field ~ 5/3 T L = ½ p R = 4.71 m R = 3m Elton Smith / Scintillation Detectors

  3. Particle Trajectory Measure the Flight Time between two Scintillators 450 ns Stop Disc 20 cm TDC Start Disc 300 cm 400 cm 100 cm Elton Smith / Scintillation Detectors

  4. c = 30 cm/ns vscint = c/n = 20 cm/ns veff = 16 cm/ns vpmt = 0.6 cm/ns vcable = 20 cm/ns Dt ~ 0.1 ns Dx ~ 3 cm Propagation velocities Elton Smith / Scintillation Detectors

  5. TOF scintillators stacked for shipment Elton Smith / Scintillation Detectors

  6. CLAS detector open for repairs Elton Smith / Scintillation Detectors

  7. CLAS detector with FC pulled apart Elton Smith / Scintillation Detectors

  8. Start counter assembly Elton Smith / Scintillation Detectors

  9. Organic Liquid Economical messy Solid Fast decay time long attenuation length Emission spectra Inorganic Anthracene Unused standard NaI, CsI Excellent g resolution Slow decay time BGO High density, compact Scintillator types Elton Smith / Scintillation Detectors

  10. Photocathode spectral response Elton Smith / Scintillation Detectors

  11. Scintillator thickness • Minimizing material vs. signal/background • CLAS TOF: 5 cm thick • Penetrating particles (e.g. pions) loose 10 MeV • Start counter: 0.3 cm thick • Penetrating particles loose 0.6 MeV • Photons, e+e− backgrounds ~ 1MeV contribute substantially to count rate • Thresholds may eliminate these in TOF Elton Smith / Scintillation Detectors

  12. Light guides • Goals • Match (rectangular) scintillator to (circular) pmt • Optimize light collection for applications • Types • Plastic • Air • None • “Winston” shapes Elton Smith / Scintillation Detectors

  13. acrylic Reflective/Refractive boundaries Scintillator n = 1.58 PMT glass n = 1.5 Elton Smith / Scintillation Detectors

  14. Air with reflective boundary Scintillator n = 1.58 PMT glass n = 1.5 (reflectance at normal incidence) Reflective/Refractive boundaries Elton Smith / Scintillation Detectors

  15. air Scintillator n = 1.58 PMT glass n = 1.5 Reflective/Refractive boundaries Elton Smith / Scintillation Detectors

  16. acrylic Large-angle ray lost Reflective/Refractive boundaries Scintillator n = 1.58 PMT glass n = 1.5 Acceptance of incident rays at fixed angle depends on position at the exit face of the scintillator Elton Smith / Scintillation Detectors

  17. Winston Cones - geometry Elton Smith / Scintillation Detectors

  18. Winston Cone - acceptance Elton Smith / Scintillation Detectors

  19. g e− Photomultiplier tube, sensitive light meter Gain ~ 106 - 107 Electrodes Anode Photocathode Dynodes 56 AVP pmt Elton Smith / Scintillation Detectors

  20. Equal Steps – Max Gain g a k dN-2 dN-1 dN d1 d2 d3 RL 4 2.5 1 1 1 1 1 1 1 1 1 1 16.5 Progressive −HV +HV RL 1.25 6 2.5 1 1.5 1.5 1.75 2.5 3.5 4.5 8 10 44 Linearity Timing Intermediate RL 1.4 1.6 4 2.5 1 1 1 1 1 1 3 2.5 21 Voltage Dividers Elton Smith / Scintillation Detectors

  21. Capacitors for increased linearity in pulsed applications Active components to minimize changes to timing and rate capability with HV VoltageDivider Elton Smith / Scintillation Detectors

  22. High voltage • Positive (cathode at ground) • low noise, capacitative coupling • Negative • Anode at ground (no HV on signal) • No (high) voltage • Cockcroft-Walton bases Elton Smith / Scintillation Detectors

  23. Effect of magnetic field on pmt Elton Smith / Scintillation Detectors

  24. Housing Elton Smith / Scintillation Detectors

  25. Compact UNH divider design Elton Smith / Scintillation Detectors

  26. After-pulsing and Glass radioactivity Thermal Noise Cosmic rays Dark counts Solid : Sea level Dashed: 30 m underground Elton Smith / Scintillation Detectors

  27. Signal for passing tracks Elton Smith / Scintillation Detectors

  28. Single photoelectron signal Elton Smith / Scintillation Detectors

  29. Pulse distortion in cable Elton Smith / Scintillation Detectors

  30. anode dynode trigger Measure time Measure pulse height Electronics Elton Smith / Scintillation Detectors

  31. Formalism: Measure time and position PL PR TL TR X=0 X X=−L/2 X=+L/2 Mean is independent of x! Elton Smith / Scintillation Detectors

  32. Intrinsic timing of electronic circuits Combined scintillator and pmt response Single Photoelectron Response Average path length variations in scintillator From single-photoelectron timing to counter resolution The uncertainty in determining the passage of a particle through a scintillator has a statistical component, depending on the number of photoelectrons Npe that create the pulse. Note: Parameters for CLAS Elton Smith / Scintillation Detectors

  33. Average time resolution CLAS in Hall B Elton Smith / Scintillation Detectors

  34. Formalism: Measure energy loss PL PR TL TR X=0 X X=−L/2 X=+L/2 Geometric mean is independent of x! Elton Smith / Scintillation Detectors

  35. Energy deposited in scintillator Elton Smith / Scintillation Detectors

  36. Uncertainties Timing Assume that one pmt measures a time with uncertainty dt Mass Resolution Elton Smith / Scintillation Detectors

  37. Example: Kaon mass resolution by TOF For a flight path of d = 500 cm, Assume Note: Elton Smith / Scintillation Detectors

  38. Velocity vs. momentum p+ K+ p Elton Smith / Scintillation Detectors

  39. Summary • Scintillator counters have a few simple components • Systems are built out of these counters • Fast response allows for accurate timing • The time resolution required for particle identification is the result of the time response of individual components scaled by √Npe Elton Smith / Scintillation Detectors

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