Scintillation Counters and Photomultiplier Tubes
Learning Objectives
• Understand the basic operation of CROP scintillation
counters and photomultiplier tubes (PMTs) and their
use in measuring cosmic ray air showers
• Understand how light is generated in a scintillator
• Understand how light is transmitted to a PMT
• Understand how a PMT generates an electric signal
• Be able to hook up a scintillation counter to its high
voltage and an oscilloscope for viewing signals
• Be able to identify light leaks in a scintillation counter
• Be able to observe scintillation counter signals using
an oscilloscope and identify cosmic ray muons
• Be able to discuss scintillation counter performance
in terms of gain, efficiency and attenuation length
Scintillation Counters and Photomultiplier Tubes
Outline
• Introduction
• Light Generation in Scintillators
• Light Collection
• Optical Interfaces and Connections
• Photodetectors and photomultiplier tubes
• Performance and Exercises
• References
Scintillation Counters and Photomultiplier Tubes
Introduction
• Scintillation counters are multi-purpose particle detectors
used in many experimental physics applications
• Used for charged particle detection (positive or negative),
but also neutral particles (photons, neutrons),
although light-generation mechanisms are different for
charged and neutral particles
• Basic sequence -- light generation by particle passing
through scintillator material, light collection,
photodetector turns light into electric signal
Scintillation Counter Properties
• Fast time response -- light generated almost immediately
after particle passes through scintillator, photodetectors
give fast electric signal
• Can count number of particles using pulse height.
• The larger the signal size, the greater the number of
particles
• Position information
• Based on size of active scintillator material
Scintillation Counters and Photomultiplier Tubes
Basic principles of operation
Passage of
charged particle
generates light
in scintillator
Charged particle
Light guide
transmits light
to photodetector
Photomultiplier tube (PM or PMT)
generates electric signal
Scintillation Counters and Photomultiplier Tubes
Introduction
• Examples from High Energy Physics experiments
at particle accelerators
• Hodoscope -- an array of several counters covering
a large area
• Veto counters -- for particles you don’t want to
measure
• Calorimetry -- measuring a particle’s total energy
• Triggering -- a fast signal which indicates an
interesting event to record
Examples from cosmic ray experiments
• CASA
• KASCADE
Scintillation counters in High-Energy
Physics Experiments
Fermilab, Batavia, Illinois
Protons
Anti-protons
CERN, Geneva, Switzerland
Scintillation Counters and Photomultiplier Tubes
Scintillation counter hodoscope
Photomultiplier
tube
Scintillator wedge
Foil wrapping
Counters arranged
as pizza slices
Chicago Air Shower Array (CASA)
Dugway Proving Grounds, Utah
• University of Chicago and University of Utah collaboration
to study extended cosmic ray air showers
• 1089 boxes in a rectangular grid, 15 meter spacing, each with
• 4 scintillator planes and 4 photomultplier tubes
• 1 low voltage and 1 high voltage supply
• 1 electronics card for data triggering and data acquisition
• CASA collected data in the 1990’s and is now complete
• CROP will use retired scintillation counters recovered
from CASA
Scintillation Counters and Photomultiplier Tubes
Contents of a CASA detector station
Weatherproof box top
Electronics card
4 scintillators and PMTs
Box bottom
The KASCADE experiment
in Karlsruhe, Germany
KASCADE = KArlsruhe Shower Core and Array DEtector
• 252 detector stations
• Rectangular grid with 13 m spacing
• Array of 200 x 200 m2
The KASCADE experiment
Scintillation Counters and Photomultiplier Tubes
Introduction
Other uses of scintillation counters -- biological research,
medical applications (PET scans)
Use of scintillation counters in CROP
• Several counters firing at once indicates extended air
shower -- on one school or inter-school
• Pulse heights related to number of particles in shower and
energy of primary cosmic ray
• Relative arrival times related to primary cosmic ray
incident direction
Scintillation Counters and Photomultiplier Tubes
PET Scans
(Positron Emission Tomography)
3-D image
Scintillating crystal detector
and photomultiplier
Cross
Section
Scintillation Counters and Photomultiplier Tubes
2. Light generation in scintillators
• Different scintillator materials
• Plastic scintillator -- good for large areas
• Sodium Iodide (NaI)
Inorganic crystals
• BGO (Bi4Ge2O12)
• Lead Tungstanate (PbWO4)
• Focus on plastic scintillator
• Composition
• Polystyrene (plexiglass)
• Doped with small admixture of a fluor
• Fluor is organic macro-molecule like
POPOP: 1,4-Bis-[2-(5-phenyloxazolyl)]-benzene
C24H16N2O2
• Light generated by fluorescence process
• One of energy loss mechanisms when charged
particles pass through matter
• Similar to television screen or computer monitor
• Quantum mechanical process
• Light (photons) emitted isotropically
• Emission spectrum from typical scintillator
• Relation to visible light spectrum
Energy absorption and emission diagram
Electrons excited
to higher energy
levels when a
charged particle
passes, absorbing
part of its energy
Electron ground
state
Electrons drop back
to ground state,
emitting fluorescence
or scintillation light
Scintillation Counters and Photomultiplier Tubes
Typical plastic scintillator emission spectrum
Wavelength of emitted light
• 1 nm = 1 nanometer = 1  10-9 meter
• For reference, 1 nm = 10 Angstroms,
where 1 Angstrom is approximate size of an atom
• Maximum emission at about 425 nm
Scintillation Counters and Photomultiplier Tubes
The wavelengths of visible light
Scintillation Counters and Photomultiplier Tubes
Scintillation Counters and Photomultiplier Tubes
3. Light Collection
• Purpose -- Direct as much generated light as possible to
the photodetector
• Need for making counters light tight
• Light transmission within scintillator
• Reflections from surfaces, total internal reflection
• Transmission through surfaces
• Critical angle
• Importance of smooth polished surfaces
• Use of reflective coverings
(foil, white paint, white paper, black paper)
• Multiple bounces (many!)
• Ray-tracing simulation programs
• Attenuation of light in scintillator
Scintillation Counters and Photomultiplier Tubes
Light transmission within scintillator
Charged particle
passes through
here
Scintillator
Light
rays
Photomultiplier tubes
Scintillation Counters and Photomultiplier Tubes
Reflection and transmission at surfaces
Air
Scintillator material
Light totally internally reflected for incident angle
greater than critical which depends on optical
properties of scintillator and air
Scintillator
Air
Refraction (i.e. transmission) outside scintillator for
incident angle less than critical
Scintillation Counters and Photomultiplier Tubes
3. Light Collection
• Different light collection schemes
• Different types of plastic light guides
• Air light guides (KASCADE)
• CASA scheme
• Not optimal, PMT glued onto surface
• Wavelength-shifting side bars
• Embedded wavelength-shifting optical fibers
• Connected to clear optical fibers
• Can transport light over long distance
• Other fiber optics applications
• Laproscopic surgery
• Telecommunications
Scintillation Counters and Photomultiplier Tubes
Laproscopic surgery
• Optical fibers transmit image to surgeon
• Less instrusive technique
Light collection in the KASCADE experiment
Electron and photon detector
Photomultiplier
33 kg of liquid scintillator
Argon-filled space
(better light transmission than air)
Light emitted from scintillator is guided by conical reflecting
surfaces to photomultiplier tube above
Light collection in the KASCADE experiment
Muon detector
Wavelength-shifting bars around
perimeter of planes guide light to
photomultiplier tubes
4 plastic scintillator planes
Scintillation Counters and Photomultiplier Tubes
Optical Fibers
• Fiber core and cladding optimized to
prevent leakage of light out of the fiber
• 95% transmission over 1 km
• If this were true for ocean water, you could
clearly see ocean bottom
Transmission modes within optical fibers
Scintillation Counters and Photomultiplier Tubes
What’s wrong with
this picture?
Scintillation Counters and Photomultiplier Tubes
Several scintillators tied together
optically with optical fibers
To photo-detector
Wavelehgth-shifting
optical fiber
Scintillator planes
Scintillation Counters and Photomultiplier Tubes
• Advantages and limitations of each type of light
read-out scheme
• Definition of efficiency of light collection
Number of photons arriving at the photo-detector
Number of photons generated by charged particle
• About 10% for light guide attached to side
• A few percent for CASA counters
Scintillation Counters and Photomultiplier Tubes
4. Optical Interfaces and Connections
Purpose -- transmit light with high efficiency,
sometimes provide mechanical stability of detector
as well (should decouple the two tasks if possible)
• Interface between scintillator material and
• Light guide
• Optical fiber
• Wavelength-shifting bar
• Interface between light guide or fiber and
photodetector
• Commonly used
• Optical cements and epoxies
• Optical grease
• Air gap
Scintillation Counters and Photomultiplier Tubes
5. Photodetectors and Photomultiplier Tubes
Purpose -- transform light into electric signal for
further processing of particle information
• Examples
• Photomultiplier tube (CROP focus)
• Photodiode
• Charged-coupled device
• Avalanche photodiode (APD)
• Visible Light Photon Counter (cryogenics)
Photomultiplier tube details
• Entrance window
• Must be transparent for light wavelengths which
need to enter tube
• Common: glass
• Fused silicate -- transmits ultraviolet as well
Scintillation Counters and Photomultiplier Tubes
Schematic drawing of a photomultiplier tube
(from scintillator)
Photocathode
Photons eject
electrons via
photoelectric effect
Each incident
electron ejects
about 4 new
electrons at each
dynode stage
“Multiplied” signal
comes out here
Vacuum inside
tube
An applied voltage
difference between
dynodes makes
electrons accelerate
from stage to stage
Scintillation Counters and Photomultiplier Tubes
Different types of dynode chain geometries
Scintillation Counters and Photomultiplier Tubes
Definition of Photomultiplier Tube Gain
•  = average number of electrons generated at each dynode
stage
• Typically,  = 4 , but this depends on dynode material
and the voltage difference between dynodes
• n = number of multiplication stages
• Photomultiplier tube gain =  n
• For n = 10 stages and  = 4 , gain = 410 = 1  107
• This means that one electron emitted from the
photocathode (these are called “photoelectrons”)
yields 1  107 electrons at the signal output
Scintillation Counters and Photomultiplier Tubes
The Photocathode
• Incoming photons expel electrons from the metallic
surface of the photocathode via the photoelectric effect.
• The effect was discovered by
Heinrich Hertz in 1887 and
explained by Albert Einstein
in 1905.
• According to Einstein's theory,
light is composed of discrete
particles of energy, or quanta,
called PHOTONS. When photons with enough energy
strike the photocathode, they liberate electrons that have
a kinetic energy equal to the energy of the photons less
the “work function” (the energy required to free the
electrons from a particular material).
• Einstein received the Nobel Prize for his 1905 paper
explaining the photoelectric effect. What were the other
two famous Einstein papers from 1905?
• Theory of special relativity
• Explanation of Brownian motion
Scintillation Counters and Photomultiplier Tubes
The Photocathode
• Photocathode composition
• Semiconductor material made of antimony (Sb) and
one or more alkalai metals (Cs, Na, K)
• Thin, so ejected electrons can escape
• Definition of photocathode quantum efficiency, h(l)
h(l) =
Number of photoelectrons released
Number of incident photons (l) on cathode
• Typical photocathode quantum efficiency is 10 - 30%
• Photocathode response spectrum
• Need for matching scintillator light output spectrum with
photocathode response spectrum
Scintillation Counters and Photomultiplier Tubes
Typical photocathode response curve
200 nm
Wavelength of light
1 nm = 1 nanometer = 1  10-9 meter
Note: Quantum efficiency > 20% in range 300 - 475 nm
Peak response for light wavelengths near 400 nm
700 nm
Scintillation Counters and Photomultiplier Tubes
The dynode chain
• High voltage applied to dynodes creates electric fields
which guide electrons between from stage to stage
• Process of secondary emission yields more electrons
at each stage
• This is the “multiplication” in “photomultiplier”
• Process is similar to photoelectric effect, with incident
photon replaced by incident electron
• Composition of dynodes
• Ag - Mg
• Cu - Be
Deposited in thin layer on
• Cs - Sb
conducting support
• Sensitivity to earth’s magnetic field
• Earth’s magnetic field is typically 0.5 - 1.0 Gauss
• Trajectories of charged particles moving in a magnetic
field will curve, depending on field orientation
• Can cause photoelectrons and secondary-emitted
electrons not to reach next stage
• First few stages, when there are few electrons,
most vulnerable
• Use of magnetic shields
• Should extend shield beyond front of tube
Scintillation Counters and Photomultiplier Tubes
The phototube base and high voltage supply
Purpose -- provide an electric field between
• photocathode and first dynode
• successive dynodes
to accelerate electrons from stage to stage
• About 100 V voltage difference needed between stages
• Chain of resistors forms voltage divider to split up
high voltage into small steps
• Capacitors store readily-available charge for electron
multiplication
• Typical base draws 1 - 2 milliamperes of current
Scintillation Counters and Photomultiplier Tubes
The electric field between successive dynodes
A simplified view
Represents a dynode
- - -
- - Electric field between plates
100 Volts
+
+ + +
+ + +
Represents the next
dynode
An electron (negative charge) released from the negative
plate will be accelerated toward the positive plate
Scintillation Counters and Photomultiplier Tubes
Typical phototube base schematic
Output signal
to oscilloscope
Photocathode
Dynodes
Tube body
Ground
High voltage
supply
Positive
Capacitors
(which store
charge) needed
for final stages
when there are
many electrons
Current flows
through resistor
chain for voltage
division
Output signal flows out of tube
Scintillation Counters and Photomultiplier Tubes
A simple voltage divider
Greek omega for
resistance unit, Ohms
Current, I
(amperes)
Battery
Vbatt = 9 Volts
+
-
4 W = R1
a
2W=R2
Voltmeter
here
b
V
R
Vbatt
9 Volts
Current in circuit : I =
=
= 1.5 Amps
R1  R 2
6W
Vacross R 2 = I  R 2 = (1.5 Amps )( 2 W) = 3 Volts
Ohm' s law : V = I  R or I =
Measured with voltmeter between points (a) and (b)
You have “divided” the 9 Volt battery: 3 Volts and
6 Volts are now accessible with this circuit.
Scintillation Counters and Photomultiplier Tubes
Vacuum inside tube body
Purpose -- minimize collisions of electrons with gas
molecules during transit
• Requires strong tube body
• Pins for electrical connections pierce through glass
at bottom of tube (leak-tight)
• Damage to tube by helium or hydrogen
• “Small” gas molecules can leak into tube, even
through glass
Scintillation Counters and Photomultiplier Tubes
Variation of PMT gain with high voltage
• Increasing high voltage increases electron transmission
efficiency from stage to stage
• Especially important in first 1-2 dynodes
• Increasing high voltage increases kinetic energy of
electrons impacting dynodes
• Increases amplification factor 
Scintillation Counters and Photomultiplier Tubes
Oscilloscope traces from scintillation counters
Plastic scintillator
10 nsec / division
Inorganic crystal, NaI
5000 nsec / division
(Longer time scale for
fluorescence to occur)
Scintillation Counters and Photomultiplier Tubes
Close-up of photoelectron trajectories to first dynode
Scintillation Counters and Photomultiplier Tubes
References
1. Introduction to Experimental Particle Physics by
Richard Fernow, Cambridge University Press, 1986,
ISBN 0-521-30170-7 (paperback), Chapter 7, pages 148-177
(includes exercises)
2. Photomultiplier Manual, Technical Series PT-61, 1970,
RCA Corporation
3. Techniques for Nuclear and Particle Physics by
W. R. Leo, Springer-Verlag, Germany, 1994,
ISBN 3-540-57280-5, Chapters 7-9, pages 157-214
4. Radiation Detection and Measurement, 3rd Edition,
by Glenn F.Knoll, Wiley 2000, ISBN 0-417-07338-5,
Chapters 8-10, pages 219 - 306
Scintillation Counters and Photomultiplier Tubes
Light transmission through entrance wnidow
Percent of light which passes
Different
window
materials
200 nm
Wavelength of light
700 nm
• Observe:
• 20% transmission typical for 400 nm light
• Fused silica extends transmission into lower wavelengths
• Less than 400 nm is ultraviolet light
Scintillation Counters and Photomultiplier Tubes
Scintillation Counters and Photomultiplier Tubes
6. Performance and exercises
Signal shape, pulse height and duration
Pulse height distributions
Linearity
Attenuation length
Oscilloscope examples and exercises with changing
high voltage, radioactive source, attenuation length
Scintillation Counters and Photomultiplier Tubes
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Scintillation Counters and Photomultiplier Tubes
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