Scintillation Detectors Introduction Components Scintillator Light Guides Photomultiplier

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

Experiment basics p = 0. 3 B R = 1. 5 Ge. V/c B field ~ 5/3 T L = ½ p R = 4. 71 m bp = p/√p 2+mp 2 = 0. 9957 b. K = p/√p 2+m. K 2 = 0. 9496 R = 3 m tp = L/bpc = 15. 77 ns t. K = L/b. Kc = 16. 53 ns Dtp. K = 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 450 ns Particle Trajectory Start Disc 20 c m TDC Stop Disc 300 cm 0 0 4 100 cm Elton Smith / Scintillation Detectors cm

Propagation velocities n c = 30 cm/ns n vscint = c/n = 20 cm/ns n veff = 16 cm/ns n vpmt = 0. 6 cm/ns n 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

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

Photocathode spectral response Elton Smith / Scintillation Detectors

Scintillator thickness n Minimizing material vs. signal/background n CLAS TOF: 5 cm thick q n Start counter: 0. 3 cm thick q Ø Ø Penetrating particles (e. g. pions) loose 10 Me. V Penetrating particles loose 0. 6 Me. V Photons, e+e− backgrounds ~ 1 Me. V contribute substantially to count rate Thresholds may eliminate these in TOF Elton Smith / Scintillation Detectors

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

Reflective/Refractive boundaries Scintillator n = 1. 58 acrylic Elton Smith / Scintillation Detectors PMT glass n = 1. 5

Reflective/Refractive boundaries Scintillator n = 1. 58 Air with reflective boundary PMT glass n = 1. 5 (reflectance at normal incidence) Elton Smith / Scintillation Detectors

Reflective/Refractive boundaries Scintillator n = 1. 58 air PMT glass n = 1. 5 Elton Smith / Scintillation Detectors

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 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 Photocathode e− Dynodes 56 AVP pmt Elton Smith / Scintillation Detectors

Voltage Dividers k g d 1 4 d 2 2. 5 Equal Steps – Max Gain d 3 1 1 1 d. N-2 1 1 a d. N-1 d. N 1 1 RL 1 16. 5 −HV +HV Progressive 6 2. 5 1 1. 25 1. 75 2. 5 3. 5 44 Timing 4. 5 8 10 RL 2. 5 RL Linearity Intermediate 4 2. 5 1 1 1 21 Elton Smith / Scintillation Detectors 1 1. 4 1. 6 3

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

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

Effect of magnetic field on pmt Elton Smith / Scintillation Detectors

Housing Elton Smith / Scintillation Detectors

Compact UNH divider design 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

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

Formalism: Measure time and position PL PR TL TR X=−L/2 X=0 X 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 Single Photoelectron Response Combined scintillator and pmt response Average path length variations in scintillator Elton Smith / Scintillation Detectors Note: Parameters for CLAS

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

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

Energy deposited in scintillator Elton Smith / Scintillation Detectors

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

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

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

Summary n Scintillator counters have a few simple components q q n 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