Scintillation Detectors Introduction Components Scintillator Light Guides Photomultiplier Tubes Formalism/Electronics Timing Resolution Elton Smith JLab 2006 Detector/Computer Summer Lecture Series
Experiment basics B field ~ 5/3 T p = 0.3 B R = 1.5 GeV/c β π = p/ p 2 +m π2 = 0.9957 L = ½ π R = 4.71 m β Κ = p/ p 2 +m Κ2 = 0.9496 R = 3m t π = L/β π c = 15.77 ns t Κ = L/β Κ c = 16.53 ns t πk = 0.76 ns Particle Identification by time-of-flight (TOF) requires Measurements with accuracies of ~ 0.1 ns
Measure the Flight Time between two Scintillators 450 ns Particle Trajectory Stop Start Disc Disc TDC 20 cm 300 cm 400 cm 100 cm
Measure the Flight Time between two Scintillators 450 ns Particle Trajectory Stop Start Disc Disc TDC 20 cm 300 cm 400 cm 100 cm
Propagation velocities c = 30 cm/ns v scint = c/n = 20 cm/ns v eff = 16 cm/ns t ~ 0.1 ns x ~ 3 cm v pmt = 0.6 cm/ns v cable = 20 cm/ns
TOF scintillators stacked for shipment
CLAS detector open for repairs
CLAS detector with FC pulled apart
Start counter assembly
Scintillator types Organic Inorganic Liquid Economical messy Solid Fast decay time long attenuation length Emission spectra Anthracene Unused standard NaI, CsI Excellent γ resolution Slow decay time BGO High density, compact
Photocathode spectral response
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
Light guides Goals Match (rectangular) scintillator to (circular) pmt Optimize light collection for applications Types Plastic Air None Winston shapes
Reflective/Refractive boundaries Scintillator n = 1.58 acrylic PMT glass n = 1.5
Reflective/Refractive boundaries Scintillator n = 1.58 Air with reflective boundary PMT glass n = 1.5 R air = 1 1+ n n 2 4 5% (reflectance at normal incidence)
Reflective/Refractive boundaries Scintillator n = 1.58 air PMT glass n = 1.5
Reflective/Refractive boundaries Scintillator n = 1.58 acrylic PMT glass n = 1.5 Large-angle ray lost Acceptance of incident rays at fixed angle depends on position at the exit face of the scintillator
Winston Cones - geometry
Winston Cone - acceptance
Photomultiplier tube, sensitive light meter Gain ~ 10 6-10 7 Electrodes Anode γ e Photocathode Dynodes 56 AVP pmt
Window Transmittance
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
Voltage Dividers k g Equal Steps Max Gain d 1 d 2 d 3 d N-2 d N-1 d N a HV 4 2.5 1 1 1 1 1 1 1 1 1 1 16.5 Progressive 6 2.5 1 1.25 1.5 1.5 1.75 2.5 3.5 4.5 8 10 +HV R L R L Timing 44 Intermediate Linearity 4 2.5 1 1 1 1 1 1 1.4 1.6 3 2.5 R L 21
Voltage Divider Capacitors for increased linearity in pulsed applications Active components to minimize changes to timing and rate capability with HV
High voltage Positive (cathode at ground) low noise, capacitative coupling Negative Anode at ground (no HV on signal) No (high) voltage Cockcroft-Walton bases
Effect of magnetic field on pmt
Housing
Compact UNH divider design
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.
Dark counts Solid : Sea level Dashed: 30 m underground After-pulsing and Glass radioactivity Thermal Noise Cosmic rays
Signal for passing tracks
Single photoelectron signal
Pulse distortion in cable
Electronics anode dynode trigger Measure pulse height Measure time
Time-walk corrections
Formalism: Measure time and position P L P R L T L T R X= L/2 X=0 X X=+L/2 = TL 0 + x veff T R = TR 0 x / veff T / 1 2 1 2 0 L 0 R T = ( T + T ) = ( T + T ) Mean is independent of x! ave x = v eff 2 L R [ ] 0 0 eff ( T T ) ( T T ) ( T T ) L R L R v 2 L R
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 N pe that create the pulse. σ TOF ( ns) 2 2 σ1 + ( σ P L / 2) = σ 0 + N 1000 N exp( L / 2λ) pe pe 2 σ 0 = 0.062 ns Single } σ 1 = 2.1 ns Photoelectron Response σ P = 0.0118 ns / cm λ = 134cm + 0.36 L (15cm counters) λ = 430cm (22cm counters) Intrinsic timing of electronic circuits Combined scintillator and pmt response Average path length variations in scintillator Note: Parameters for CLAS
Average time resolution CLAS in Hall B
Formalism: Measure energy loss P L P R T L X= L/2 X=0 X T R X=+L/2 P L = P 0 x / λ L e P = R P 0 x / λ R e Energy = P L P R = P 0 L P 0 R Geometric mean is independent of x!
Energy deposited in scintillator
Uncertainties Timing Assume that one pmt measures a time with uncertainty δt Mass Resolution 2 ~ 2 1 2 2 t t t t R L ave δ δ δ δ + = 2 ~ ) 2 1 ( 2 2 t v t t v x eff R L eff δ δ δ δ + = γ E m = 2 2 2 2 2 2 1 ) 1 ( p E m = = β β β 2 2 4 2 + = p p m m δ β δβ γ δ
Integral magnetic shield
Example: Kaon mass resolution by TOF P K = 1 GeV/ c E K = 0.495 + 1 = 1. 116 GeV P E K β K = = 0.896 K γ K = = 2. 26 EK mk 500 cm For a flight path of d = 500 cm, t = = 18. 6ns 0.896 30 cm/ ns Assume δm m Note: 2 δ p p δt = 0. 15ns = 0. 01 2 4 0.15 2 2 = 2.26 + ( 0.01) = 0.042 18.6 δm δβ 2 for fixed γ m β 2 δm K ~ 21MeV
Velocity vs. momentum π + K + p
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 N pe
Magnetic fields