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

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 open for repairs 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 ~ 106 - 107 Electrodes Anode g e− Photocathode Dynodes 56 AVP pmt Elton Smith / Scintillation Detectors

Elton Smith / Scintillation Detectors Window Transmittance Elton Smith / Scintillation Detectors

Elton Smith / Scintillation Detectors Voltage dividers Equal voltage steps Maximum gain Progressive, higher voltage near anode Excellent linearity, limited gain Time optimized, higher voltage at cathode Good gain, fast response Zeners Stabilize voltages independent of gain Decoupling capacitors “reservoirs” of charge during pulsed operation 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

Electrostatics near cathode at −HV Stable performance with negative high voltage is achieved by Eliminating potential gradients in the vicinity of the photocathode. The electrostatic shielding and the can of the crystal are both Maintained at cathode potential by this arrangement. 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

Signal for passing tracks Elton Smith / Scintillation Detectors

Single photoelectron signal Elton Smith / Scintillation Detectors

Pulse distortion in cable Elton Smith / Scintillation Detectors

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

Time-walk corrections 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

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. Intrinsic timing of electronic circuits Combined scintillator and pmt response Average path length variations in scintillator Single Photoelectron Response Note: Parameters for CLAS Elton Smith / Scintillation Detectors

Average time resolution CLAS in Hall B 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 Uncertainties Timing Assume that one pmt measures a time with uncertainty dt Mass Resolution Elton Smith / Scintillation Detectors

Integral magnetic shield Elton Smith / Scintillation Detectors

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

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