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

2. 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 =
bK = p/√p2+mK2 =
R = 3m
tp = L/bpc = ns
tK = L/bKc = ns
DtpK = 0.76 ns
Particle Identification by time-of-flight (TOF) requires
Measurements with accuracies of ~ 0.1 ns
Elton Smith / Scintillation Detectors

3. 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

4. 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

5. 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

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

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

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

9. Start counter assembly
Elton Smith / Scintillation Detectors

10. 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

11. Photocathode spectral response
Elton Smith / Scintillation Detectors

12. 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

13. 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

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

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

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

17. 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

18. Winston Cones - geometry
Elton Smith / Scintillation Detectors

19. Winston Cone - acceptance
Elton Smith / Scintillation Detectors

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

21. Elton Smith / Scintillation Detectors
Window Transmittance
Elton Smith / Scintillation Detectors

22. 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

23. 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

24. 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

25. 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

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

27. Elton Smith / Scintillation Detectors
Housing
Elton Smith / Scintillation Detectors

28. Compact UNH divider design
Elton Smith / Scintillation Detectors

29. 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

30. 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

31. Signal for passing tracks
Elton Smith / Scintillation Detectors

32. Single photoelectron signal
Elton Smith / Scintillation Detectors

33. Pulse distortion in cable
Elton Smith / Scintillation Detectors

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

35. Time-walk corrections
Elton Smith / Scintillation Detectors

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

37. 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

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

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

40. Energy deposited in scintillator
Elton Smith / Scintillation Detectors

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

42. Integral magnetic shield
Elton Smith / Scintillation Detectors

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

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

45. 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

46. Elton Smith / Scintillation Detectors
Magnetic fields
Elton Smith / Scintillation Detectors