Scintillation Detectors Introduction Components Scintillator Light Guides Photomultiplier Tubes Formalism/Electronics Timing Resolution Elton Smith JLab 2009 Detecto Summer Lecture Series
Elton Smith / Scintillation Detectors Experiment basics B field ~ 5/3 T p = 0.3 B R = 1.5 GeV/c L = ½ p R = 4.71 m bp = p/√p2+mp2 = 0.9957 bK = p/√p2+mK2 = 0.9496 R = 3m tp = L/bpc = 15.77 ns tK = L/bKc = 16.53 ns DtpK = 0.76 ns Particle Identification by time-of-flight (TOF) requires Measurements with accuracies of ~ 0.1 ns Elton Smith / Scintillation Detectors
Measure the Flight Time between two Scintillators Particle Trajectory 450 ns Stop Disc 20 cm TDC Start Disc 300 cm 400 cm 100 cm Elton Smith / Scintillation Detectors
Propagation velocities 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 Elton Smith / Scintillation Detectors
TOF scintillators stacked for shipment Elton Smith / Scintillation Detectors
CLAS detector with FC pulled apart Elton Smith / Scintillation Detectors
Start counter assembly Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Scintillator types 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 Elton Smith / Scintillation Detectors
Photocathode spectral response Elton Smith / Scintillation Detectors
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
Elton Smith / Scintillation Detectors Light guides Goals Match (rectangular) scintillator to (circular) pmt Optimize light collection for applications Types Plastic Air None “Winston” shapes Elton Smith / Scintillation Detectors
Reflective/Refractive boundaries Scintillator n = 1.58 acrylic PMT glass n = 1.5 Elton Smith / Scintillation Detectors
Reflective/Refractive boundaries Air with reflective boundary Scintillator n = 1.58 PMT glass n = 1.5 (reflectance at normal incidence) Elton Smith / Scintillation Detectors
Reflective/Refractive boundaries air Scintillator n = 1.58 PMT glass n = 1.5 Elton Smith / Scintillation Detectors
Reflective/Refractive boundaries Scintillator n = 1.58 acrylic Large-angle ray lost 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
Winston Cones - geometry Elton Smith / Scintillation Detectors
Winston Cone - acceptance Elton Smith / Scintillation Detectors
Photomultiplier tube, sensitive light meter Gain ~ VN ~ 106 - 107 Electrodes Anode g e− Photocathode N Dynodes 56 AVP pmt Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Voltage Dividers d1 d2 d3 dN dN-1 dN-2 a k g 4 2.5 1 16.5 RL +HV −HV Equal Steps – Max Gain 6 2.5 1 1.25 1.5 1.75 3.5 4.5 8 10 44 RL Progressive Timing Linearity 4 2.5 1 1.4 1.6 3 21 RL Intermediate Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Voltage Divider Active components to minimize changes to timing and rate capability with HV Capacitors for increased linearity in pulsed applications Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors 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
Effect of magnetic field on pmt Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Housing Elton Smith / Scintillation Detectors
Compact UNH divider design Elton Smith / Scintillation Detectors
Signal for passing tracks Elton Smith / Scintillation Detectors
Single photoelectron signal Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Dark counts Solid : Sea level Dashed: 30 m underground After-pulsing and Glass radioactivity Thermal Noise Cosmic rays Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Electronics Measure time Measure pulse height anode trigger dynode Elton Smith / Scintillation Detectors
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
Measure the Flight Time between two Scintillators Particle Trajectory 450 ns Stop Disc 20 cm TDC Start Disc 300 cm 400 cm 100 cm Elton Smith / Scintillation Detectors
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 Intrinsic timing of electronic circuits Combined scintillator and pmt response Average path length variations in scintillator Single Photoelectron Response Elton Smith / Scintillation Detectors
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
Energy deposited in scintillator Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors Velocity vs. momentum p+ K+ p Elton Smith / Scintillation Detectors
Example: Kaon and pion time differences Momentum P = 1 GeV Kaons 1.116 GeV 0.896 2.26 18.60 ns pions 1.010 GeV 0.990 7.21 16.84 ns Flight path d = 500 cm tK – tp = 1.76 ns Difference observable with dt~0.15 ns Elton Smith / Scintillation Detectors
Elton Smith / Scintillation Detectors 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
Elton Smith / Scintillation Detectors Backup slides Elton Smith / Scintillation Detectors