Nuclear Radiation Detection and Instrumentation

(NSE 236)

SCINTILLATION DETECTORS

Professor Abi Farsoni

Department of Nuclear Engineering and Radiation Health Physics
Oregon State University

Scintillation Detectors 1
Principles of Scintillation Detectors

• Scintillators are detectors that operate on the basis of light production as the
result of energy deposition by the radiation.

• Then a photon detector such as photomultiplier tube (PMT) is required to

convert the scintillation photons to electric signal pulse.

Scintillation Signal
Radioactive photons
Photon Detector
Source (PMT)
Scintillator
HV

• They are generally categorized as either organic or inorganic.

• The two general categories of scintillators emit light by different mechanisms.

Scintillation Detectors 2
Ideal Scintillation Materials

• The ideal scintillation material have the following properties:

1. High density for high intrinsic efficiency (gamma detection)

2. High Z for high photoelectric efficiency (gamma detection)
3. High scintillation efficiency or light yield (conversion of radiation energy to
light)
4. The conversion from radiation energy to light should be linear
5. Crystal should be transparent to its emission wavelength
6. Short light decay time (fast pulses less pile-up events)
7. Good optical quality and of appropriate size for efficient detection
8. Index of refraction should be near that of glass for optimal coupling to
photon detector (PMT)

Scintillation Detectors 3
Scintillation Efficiency

• Scintillation efficiency of any scintillator is defined as the fraction of all incident

particle energy that is converted into visible light:

𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 𝑡𝑜 𝑙𝑖𝑔ℎ𝑡

𝜀𝑠𝑐𝑖𝑛𝑡 =
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑒𝑛𝑒𝑟𝑔𝑦

• This efficiency isn't always as high as we'd like because there are forms of de-
excitation that don't result in the emission of visible light (simply converted to
heat with no light emission).

• The scintillation efficiency of a given scintillator

is dependent on particle type and energy
(sometimes may be independent of energy).

• Particles of lower ionization density (electrons)

result in higher scintillation efficiency (because
of essential saturation of light output with high
density ionizations), contrary to what we'd first
think.
Scintillation Detectors 4
Scintillation Efficiency: Example

• For NaI(Tl), about 20 eV/i.p is required to create an electron-hole pair, scintillation

efficiency is about 13%, and the average output photon light energy (Ep) is ~3 eV.

• For 1 MeV of particle energy absorbed in NaI(Tl) detector, calculate (1) # of

electron-hole pairs created, (2) amount of energy converted to light, (3) # of
scintillation photons created.

𝐸 106 (𝑒𝑣)
1) = 𝑒𝑣 = 50,000 𝑖𝑝
𝐸0 20(𝑖𝑝 )

2) 𝐸𝑡𝑝 = 𝜀. 𝐸 = 0.13 ∗ 106 𝑒𝑣 = 1.3 ∗ 105 (𝑒𝑣)

𝐸𝑡𝑝 1.3∗105 𝑒𝑣
3) # 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛 𝑙𝑖𝑔ℎ𝑡𝑠 = = = 4.3 ∗ 104 𝑝ℎ𝑜𝑡𝑜𝑛𝑠
𝐸𝑝 3 𝑒𝑣/𝑝ℎ𝑜𝑡𝑜𝑛

• Therefore, from 1 MeV, 4.3 ∗ 104 photons result from 5 ∗ 104 electron-hole pairs.

• So, in NaI(Tl), approximately 1 photon produced per electron-hole pair.

Scintillation Detectors 5
Electromagnetic Spectrum: Energy, Frequency, Wavelength
Scintillation photons

Scintillation Detectors 6
Organic Scintillators
1. Fluorescence
• The organic scintillator is based on excitation 2. Phosphorescence
at the molecular level (single molecules). 3. Delayed fluorescence

• Just like atoms have excitation within the

nucleus of the orbital electrons, a molecule has
excitation states.

• There are various mechanisms of light

production:

1. Fluorescence: prompt emission

2. Phosphorescence: different wavelength and

usually much slower than fluorescence
Energy levels of an organic molecule with
𝜋 structure.
3. Delayed fluorescence: same wavelength but
slower than fluorescence

Scintillation Detectors 7
Organic Scintillators

• A good scintillation detector will produce a lot of

Short lifetime Long lifetime
light as fluorescence, with very little contribution
from delayed fluorescence or phosphorescence.

• The molecule de-excites after absorption of energy

by emitting prompt fluorescent light.

• The lifetime of the singlet states are very short,

however, the lifetime of triplet states is longer thus
resulting in phosphorescence (with different
wavelength).

• Delayed fluorescence would occur if some of the

energy in the triplet states was transferred back
(after some time) to the singlet states and lost as
Energy levels of an organic molecule with
fluorescence. 𝜋 structure.

• Organic scintillators are produced in several different forms: 1) pure organics, 2)

liquid organics, 3) plastics, and 4) loaded organics (high Z additives).
Scintillation Detectors 8
Organic Scintillators: Properties

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Organic Scintillators: Time Response

• Timing of light output can be described by exponential decays (can be used for
any type of scintillator):
𝑡 𝑡
−𝜏 −𝜏
𝐼 = 𝐼0 (𝑒 − 𝑒 1)

where the first exponential describes light decay and the second exponential
describes light increase.

• The time required to excite the scintillator is often very fast (ns). The decay
time quoted (τ) usually refers to the prompt fluorescence time.

• If delayed fluorescence or phosphorescence is

Depending on light
present, it can be discriminated out by pulse- detector, the shape of
shape analysis, because these mechanisms signal pulse might be
decay over hundreds of nanoseconds. different

• The slow decay component is typically greatest

for particles with high ionization density
(useful for particle identification).
Scintillation Detectors 10
Inorganic Scintillators: Pure Crystal

• In contrast to organic scintillators, the mechanism for scintillation in inorganics

depends on the energy states of the crystal lattice. In a typical pure crystal:

Conduction Band
Band Scintillation photon
Gap electron
Valence Band

• Energy absorbed creates electron-hole pairs. Then electrons move to conduction

band. Electron returns to valence band by emitting visible photons.

• Notice by the conduction/valence drawing that in the pure crystal, the energy
emitted is equal to the energy required to excite.

• This leads to great self absorption which is not desirable.

• Generally it takes an average of about 3 times the band-gap energy to create an

electron-hole pair (for NaI(Tl): Ebg~7ev, E0~20 ev).
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Scintillation Detectors
Inorganic Scintillators: Adding Activator

• Adding small amounts of an impurity, called activator, creates special sites in the
lattice at which the normal energy structure is modified from that of pure crystal.

• When activated sites are added, the emission

photon is of lower energy (longer wavelength)
than that required to excite, thus making the
crystal transparent to its emitted light

• Also, impurities are added (such as Tl in

NaI(Tl)) to create sites for photon emissions
that are in the visible range (with lower
energies).

• There are two reasons to add an activator:

1. Minimizing self absorption,
2. Matching the emission spectra to the
spectral response of common photon
sensors (PMT, SiPM, …).
Scintillation Detectors 12
Inorganic Scintillators: Properties
A comprehensive listing of scintillator properties: http://scintillator.lbl.gov/

Scintillation Detectors 13
Inorganic Scintillators: Properties

• Interaction of gamma rays

1. Photoelectric absorption

2. Compton scattering

3. Pair-production

• All these processes lead to the

partial or complete transfer of
gamma-ray energy to electron
energy.

Energy dependence of the various gamma-

ray interaction processes in sodium iodide.

Scintillation Detectors 14
Inorganic Scintillators: Different Types of Inorganics

• NaI(Tl)
 Thallium is the activator
 This type of detector must be "canned" or it will take on water (hygroscopic)
 Excellent light yield (good resolution)
 Fairly linear response
 High density and high Z (high efficiency)
 Gamma-spectroscopy standard (efficiency)
 Easily damaged by mechanical or thermal shock

• CsI(Tl or Na)
 High gamma absorption coefficient
 Less brittle and commonly used in environmental settings
 Variable decay times for various particles (good for PSD)
 CsI(Tl): slightly hygroscopic, CsI(Na): highly hygroscopic
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Scintillation Detectors
Inorganic Scintillators: Different Types of Inorganics
• LiI(Eu)
 Neutron detection
 High cross section for 63𝐿𝑖(𝑛, 𝛼) 31𝐻 reaction
• ZnS(Ag)
 Only as powder
 Used primarily for alpha or heavy ion detection
 Thin, because thick layers are opaque
• CaF2(Eu)
 Non-hygroscopic and inert (not chemically active)
 For environmental conditions
• BGO (Bi4Ge3O12) bismuth germanate
 High density, high Z (high efficiency), non-hygroscopic
 But poor light output (bad resolution)
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Scintillation Detectors
Light Collection

• One of the most important characteristics of scintillators is light collection.

• Light collection (optimum) depends on (1) low internal absorption and (2) good
efficiency at the surfaces (either losses or reflection)

• Internal absorption for most scintillators is generally not a problem with common
units.

• Efficiency at the surfaces deals with both internal reflection at the edges of the
crystal and light transmission out of the crystal to the light quantifying device.

• We're always concerned with collecting the light efficiently, and we want the best
resolution that we can achieve (more photons collected better resolution).

• For the best resolution:

1) we need greatest light collection efficiency and

2) we'd like for that collection to be uniform over the volume of the detector

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Scintillation Detectors
Light Collection

• If we had non-uniformity in light collection we'd get signals with different

amplitudes for equal energy deposits, based on where the reaction took place, this
can be tested with thin beams.

• In ordinary scintillators, uniformity isn't a problem, but in large crystals or well-

type detectors it may be significant.

• Once light collection is efficient and uniformity is established, the next task is to
get the light out of the crystal.

• Crystals typically will be made with an "index of refraction" that is similar to that
of glass (because of the photocathode). For glass, index of refraction is 1.6.

• If the index of refraction is too large, the light will never escape the crystal.

• The crystal should be wrapped with high-quality reflecting materials: Teflon tape
or reflecting films such as Enhanced Specular Reflector (ESR)

• By the way, scintillators must be shielded from ambient light and should be made
with low-background materials.
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Scintillation Detectors
Light Guides

• Sometimes, because of the application or the matching of scintillator to light

measuring equipment, it is necessary to use light guides.

Scintillator PMT

Light Guide
• Light guides are essentially:

1. transparent to the photon wavelength,

2. have similar index of refraction,
3. and have good reflective surfaces (to keep light contained).

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Scintillation Detectors
 Photomultiplier Tube (PMT)

• To turn the scintillation light into a detectable electrical pulse, we should use a light-
detection device or sensor such as photomultiplier tube (PMT).

• Two steps are performed by the PMT:

1. the light is converted to electrons (through photoelectric) in photocathode and

2. the electrons are then multiplied in a vacuum chamber (because there are not
enough from the conversion alone).

A typical PMT Inside a PMT

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Scintillation Detectors
Photomultiplier Tube: Photocathode

• First, the conversion from light to electrons occurs in the “photocathode”

• The light energy is absorbed by the material and transferred to an electron, the
electron migrates, and is emitted from the surface of photocathode.

• The electron generally has a few eV energy when it escapes (and it needs more to
migrate and overcome the potential barrier to escape), although all it needs to do
is become free outside the photocathode.

• The photocathode’s sensitivity is quantified in terms of the quantum efficiency:

# 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑐𝑟𝑒𝑎𝑡𝑒𝑑
𝑄𝐸 =
# 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠
• QE for typical photocathodes is 20-30% - ideal if
an electron is created by each photon (100%).

• the QE of a photocathode is dependent on

incident energy (wavelength) so that matching a
photocathode to the emission wavelength of the
scintillator is important. 21
Scintillation Detectors
Photomultiplier Tube: Electron Multiplication

• Electron multiplication occurs in the second compartment of a PMT (in vacuum)

• This multiplication process is dependent on the fact that in some materials it is

possible to liberate 2 or more electrons from a surface (conductors) after having
been struck by a single energetic electron.

• Therefore, in a PMT, a series of “dynodes” are strategically placed with increasing

potential difference so that electrons are accelerated toward them, they strike the
material and liberate more electrons.

• The dynodes are in steps and the

anode is the final collecting material
for all multiplied electrons.

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Scintillation Detectors
Photomultiplier Tube: Multiplication Factor

• The multiplication factor in a single dynode is:

# 𝑜𝑓 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
𝛿=
# 𝑜𝑓 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
• Values of 𝜹 reach 10, but more typically 4-6 with a few hundred volts between
dynodes.

• Then, the overall gain is: 𝐺 = 𝛼𝛿 𝑛

where: G = overall gain
𝛼 = fraction of electrons that stay within the PMT structure
n = number of stages

• A typical PMT: 𝛼=0.8, 𝛿=5, n=10 then the overall gain is 𝐺 = 7.8 ∗ 106

• 𝛼 can be near 1 for good designs, also remember that 𝛿 is not constant, it does have
some variability.

• This variability is driven by what happens in the first few stages because of the
very low # of electrons in those stages.
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Scintillation Detectors
Photomultiplier Tube: Voltage Divider

• There is basically a voltage divider internal to the PMT to allow subsequently

increasing potential to be placed on each dynode.

• Determined by the type of the voltage divider, a positive (a) or negative (b)
polarity high voltage is used.

Positive HV

Negative HV

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Scintillation Detectors
Photodiodes for Scintillators

• Recent advances in the development of semiconductor photodiodes have led to the

replacement of solid-state devices for PMTs in some applications.

• In general, photodiodes offer several advantages over PMTs:

 Higher quantum efficiency Avalanche Photodiode PIN Photodiode

 Lower operating voltage
 Lower cost
 More compact size
 Improved ruggedness
 Insensitive to magnetic fields
 Better time response
 Tileable (SiPM) Silicon Photomultiplier (SiPM)

• Three general designs:

1. PIN photodiodes
2. Avalanche photodiodes (APD)
3. Silicon photomultipliers (SiPM)
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Scintillation Detectors
PIN Photodiodes

• Conventional photodiodes or PIN photodiodes have no internal gain and operate

by directly converting the optical photons from the scintillator to electron-hole
pairs that are simply collected.

• Photons from a typical scintillator carry about

3-4 eV, sufficient to create electron-hole pairs
in a semiconductor with a band gap of 1-2 eV.

• Since the conversion is not limited by the need for electrons to escape from the
surface (as in a photocathode), so the maximum QE can be as high as 60-80%.

• In typical scintillation events, only a few thousand photons are created, so the size
of the charge pulse is limited at best to no more than the same number of
electronic charges

• Because of the small signal amplitude, electronic noise is a major problem in

pulse mode operation.

• PIN photodiodes are the light detector of choice for current mode scintillators in
X-ray computed (CT) scanners for medical imaging.
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Scintillation Detectors
Avalanche Photodiodes (APD)

• At higher values of the applied voltage, the small amount of charge that is
produced in a PIN photodiode can be increased through an avalanche process.

• The charge carriers are accelerated sufficiently between collision to create

additional electron-hole pairs (the same way that gas multiplication occurs in a
proportional counter).

• The internal gain helps pull the

signal up from the electronic noise
and permits good energy
resolution in pulse mode. The gain
is very sensitive to temperature
changes (~-2% per oC increase).

• Gain factors of a few hundred are typical under normal conditions.

• Position-sensitive APD for readout of arrays of small scintillator elements.

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Scintillation Detectors
Silicon Photomultiplier (SiPM)

• Just as in gas detectors, by increasing applied voltage, a point is reached when the
diode (APD) enters the Geiger mode.

• The Geiger-mode avalanche photodiode can produce a large output pulse from as
little as a single incident photon.

• The Silicon Photomultiplier (SiPM) or Multi-Pixel Photon Counter (MPPC) is an

array of small avalanche photodiode cells (~tens of microns). The number of cells
producing an avalanche is proportional to the number of incident scintillation
photons.

• Energy dynamic range depends on the # of pixels, sensitive to temperature change.

Scintillation Detectors 28
Silicon Photomultiplier
Applications
Very small spectrometers
can be built using
scintillators and SiPM
devices.

• CsI(Tl) as the scintillator (6 x 6 x 10 mm3)

• SiPM (MicroSL, SensL) as the photon detector

• Active area = 6 x 6 mm2

• Number of pixels = 19,096
• Pixel fill factor = 65%
• Peak wavelength = 500nm
• Operating voltage = 29.5V

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Scintillation Detectors
Silicon Photomultiplier, Applications

Low-cost and Compact Wireless Digital Spectrometer

using SiPM (a research project at OSU)

First Prototype

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Scintillation Detectors
YouTube Link Here

Scintillation Detectors 31
Silicon Photomultiplier, Applications

Low-cost and Compact Wireless Digital Spectrometer

using SiPM

• First Measurement Results

 Energy Resolution

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Scintillation Detectors
Silicon Photomultiplier, Applications

Low-cost and Compact Wireless Digital Spectrometer

using SiPM

• First Measurement Results

 Linearity

• Non-linearity at high
energies due to a limited
dynamic range in the
SiPM.

• Can be compensated in
the pulse processing
algorithm.

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Scintillation Detectors
 Gamma point source
Silicon Photomultiplier, Applications

Radiation Compass, a directional gamma-ray

detector
• Sixteen-element direction-sensitive radiation
detector for low-altitude UAS-based search

16 Detector Panels
(BGO + SiPM)

MCNP Modeling

articulation
platform

34 Scintillation Detectors
Silicon Photomultiplier, Applications

Radiation Compass, a directional gamma-ray detector

• Demonstration with a 137Cs source

Scintillation Detectors 35
Phoswich Detectors γ β

Phoswich Detector
• A phoswich (“phosphor sandwich”) is a combination of
several scintillators with dissimilar pulse shape
characteristics optically coupled to each other and to a
common photon sensor such as PMT.
PMT

• Pulse shape analysis distinguishes the signals from the

two scintillators, identifying in which scintillator the
event occurred.
• Radioxenon detectors based
on β-γ coincidence
• Well-type Actively Shielded Beta Energy Spectrum

Phoswich Detector (WASPD)

Scintillator BC-400 CsI(Tl) BGO
Decay time (ns) 2.4 ~1000 300
Light output Gamma Energy Spectrum
13,000 65,000 8200
(photon/MeV)
Peak emission (nm) 423 540 480
Refractive Index 1.58 1.8 2.15
Density (g/cm3) 1.032 4.51 7.13

Scintillation Detectors 36