Solid Scintillators

1/6/2010
General
1. Ionizing radiation deposits energy in the scin

2 At least some of the absorbed energy causes 

electrons in the scintillator to go to a higher elevel. 

GeneralThe Scintillation Process

3. At least some of these electrons immediately 

down to a lower energy level. As they do, theemit a photon of light. Collectively, the photonlight form a flash, or scintillation.

4. Each scintillation is converted into an electro

by a photomultiplier tube (PMT). 
One particle of radiation (e.g., a beta particle, gam

neutron) interacting in the scintillator results in one scintillation (flash) of light and, therefore, one pulseThe greater the energy transferred to the scintillato

GeneralTwo Points

The greater the energy transferred to the scintillato

greater the number of excited electrons, and the grnumber of photons of light emitted in the scintillatioother words, the greater the deposition of energy, tbrighter the flash of light and the larger the pulse.
SCINTILLATOR

PHOTOMULTIPLIER

TUBE

GeneralThe Scintillation Process

SCINTILLATOR
SCINTILLATOR

PHOTOMULTIPLIER

TUBE

INCIDENT

RADIATION

GeneralThe Scintillation Process

SCINTILLATOR TUBE
SCINTILLATOR

PHOTOMULTIPLIER

TUBE

LIGHT

PHOTONS

GeneralThe Scintillation Process

SCINTILLATOR TUBE
SCINTILLATOR

PHOTOMULTIPLIER

TUBE

GeneralThe Scintillation Process

SCINTILLATOR TUBE
SCINTILLATOR

PHOTOMULTIPLIER

TUBE

INCIDENT

RADIATION

LIGHT

PHOTONS 

GeneralThe Scintillation Process

SCINTILLATOR TUBE
The decay time (J) is the time required for the emis

63% of the light photons produced in a scintillationIn almost all cases, the shorter the decay time is, th

J

GeneralDecay Time

Light output

Time

63% 37%

1/e = 0.37
1. The probability of the radiation interacting with t

scintillator should be high.

2. A large portion of the absorbed radiation energy 

be emitted as li

ght. 

GeneralProperties of an Ideal Scintillator3. A linear proportionality should exist between the 

deposited in the scintillator and the light produce

4. The emission of light should occur as quickly as 5. The scintillator emission spectrum should overla

photomultiplier tube (or photodiode) absorption 
Chemically, the scintillating material might be:

OrganicInorganic

GeneralTypes of Scintillating materialPhysically, the scintillating material might be a:

GasLiquidSolid
Inorganic scintillators might be a:

Gas (e.g., He, Xe)Solid crystal (e.g., NaI, ZnS, CsI, LiI, LSO, LaB

GeneralThe Scintillation Process

Organic scintillators might be a:

Solid crystal (e.g., anthracene, stilbene, naphtDistributed throughout the volume of a liquid or polymerized plastic (e.g., PPO in polystyrene )
Band Theor

y of Solid
Band Theory of Solids

• The energy levels of electrons in an atom 

referred to as shells.

e M shell

K shellL shell

Energy

e

e

e

e
Band Theory of Solids

• When atoms are brought close to each ot

highest shell occupied by electrons, the vshell, splits.

e

e

e

e
Band Theory of Solids

• The greater the number of atoms, the gre

splitting.

e

e

e

e

e

ee

e
Band Theory of Solids

• When many atoms combine to form a sol

valence shell has split so many times thaa continuum of energies, i.e., a band.

e e Band

e
Band Theory of Solids

• The highest energy band occupied by ele

the valence band.

e Valence Band

e

e Valence Band

e
Band Theory of Solids

• The empty energy band above the valen

is the conduction band.

Valence Band

Conduction Band

e

e

e Valence Band

e
CONDUCTION BAND

Band Theory of Solids• The range of energies between the valen

and the conduction band is called the banforbidden band.

VALENCE BAND e e e e e e e e e 

BAND

GAP
CONDUCTION BAND

Band Theory of Solids• In a pure material, electrons cannot poss

energies in the band gap. If there are impor defects, they can.

VALENCE BAND e e e e e e e e e 

BAND

GAP

Impurity/defect 
Band Theory of Solids

• Insulator. The valence band in an insu

full. As such, the electrons are immobile amaterial does not conduct electricity.

• Conductor. The valence band in a con

not full As such the electrons are mobile not full. As such, the electrons are mobile 

material conducts electricity.

• Semiconductor. The valence band in 

semiconductor is full. As such, the electroimmobile and the material does not condelectricity.
Band Theory of Solids

• The difference between a semiconductor 

insulator is the width of the band gap. 

• In an insulator, the band gap is greater th

electron volts. In a semiconductor, the ba

less than 5 electron volts (one electron voless than

5 electron volts (one electron vo

typical).Inorganic Scintillators are insulators. This that the band gap is greater than 5 eV.
The Scintillation Proce

and the

Importance of Activator
CONDUCTION BAND

BAND

The Scintillation Process – no activ

VALENCE BAND e e e e e e e e e e 

BAND

GAP
CONDUCTION BAND

BAND

e e

The Scintillation Process – no activ

VALENCE BAND e + e e e + e e e 

BAND

GAP
CONDUCTION BAND

BAND

The Scintillation Process – no activ

VALENCE BAND e e e e e e e e e 

BAND

GAP LIGHT PH
• Unfortunately, the light that is emitted by a 

insulating material is of too high an energ

• The short wavelength of the light is not re

detected b

y the 

photomulti

plier tube.

The Scintillation Process - with act

yp p

• To reduce the energy (lengthen the wavel

an activator impurity is added to the crysta
CONDUCTION BAND

BAND

ACTIVATOR

EXCITED STATE

The Scintillation Process - with act

VALENCE BAND e e e e e e e e e e 

BAND

GAP

ACTIVATOR 

GROUND STATE

e
CONDUCTION BAND

BAND

ACTIVATOR

EXCITED STATE

e

The Scintillation Process - with act

VALENCE BAND e + e e e e e e e e 

BAND

GAP

ACTIVATOR 

GROUND STATE

e
CONDUCTION BAND

BAND

ACTIVATOR

EXCITED STATE

e

The Scintillation Process - with act

VALENCE BAND e e e e e e e e e e 

BAND

GAP

ACTIVATOR 

GROUND STATE

+
CONDUCTION BAND

BAND

ACTIVATOR

EXCITED STATE

e

The Scintillation Process - with act

VALENCE BAND e e e e e e e e e e 

BAND

GAP

ACTIVATOR 

GROUND STATE

+
CONDUCTION BAND

BAND

ACTIVATOR

EXCITED STATE 

The Scintillation Process - with act

VALENCE BAND e e e e e e e e e e 

BAND

GAP

ACTIVATOR 

GROUND STATE

e
Inorganic Scintillators
Applications of NaI scintillation detectors inc

• Detection of gamma rays and x-rays

Inorganic ScintillatorsSodium Iodide (NaI)

• Gamma spectroscopy• Exposure rate measurements (e.g., uR 
• NaI crystal activated with thallium

• Density 3.67 g/cm

3

• Clear colorless crystal• Sodium’s atomic number : 11Inorganic ScintillatorsSodium Iodide (NaI)• Iodine’s atomic number: 53• Extremely high light yield – ca. 12% of en

deposited by gamma ray or x-ray is emitt

• 52 – 56 x 10

3 photons/MeV

• Large afterglow: 4% after 2 ms 

0.3-0.5% after 6 ms
• Long decay time (230 ns) which limits the

effectiveness at high count rates

• Maximum emission wavelength: 415 nm• NaI crystals are fragile, susceptible to boInorganic ScintillatorsSodium Iodide (NaI)

thermal and mechanical shock

• Hygroscopic therefore hermetically seale• Can be grown very large (e.g., 4” x4” x 16
• Typical crystal sizes for survey work (loca

gamma emitters) are 1” to 2.” The backgcount rates increase with the crystal size might range from 1000 to 10,000 cpm 

Inorganic ScintillatorsSodium Iodide (NaI)• Typical crystal size for gamma spectrosc

the lab: 3” x 3”

• Resolution: 7 to 8%
Inorganic Scintillators

Sodium Iodide (NaI)

Cracked NaI Damaged NaI 

Crystal
Inorganic Scintillators

Sodium Iodide (NaI)

Probability of interactions (linear attenuation coefficient) as a function of energy for NaI.

ff

The NaI counting e iciency 

(e.g., survey meter) follows the Total probability curve.The photopeak counting efficiency (gamma spectroscopy) follows the photoelectric effect probability.
Inorganic Scintillators

Sodium Iodide (NaI)

Counting efficiency (total efficiency) for different sized NaI crystals. 

ff

The e iciency 

decreases below 100 keV (not shown) due to interactions in the detector housing.

The efficiency increases at the higher energies becthe increased probability of pair production.
The primary application of lanthanum bromi

gamma spectroscopy - especially the handspectroscopy systems often used by field personnel. In a lab situation, germanium deInorganic ScintillatorsLanthanum Halide (LaBr

3 and LaCl

3

)

p ,gare much preferred.The cost of lanthanum halide detectors is tofor them to be just used to detect (i.e., locategamma emitters or measure exposure rates
• Relatively new category of scintillator mat

very expensive. Lanthanum bromide is mwidely used than lanthanum chloride.

• Activated with cerium 3

Inorganic ScintillatorsLanthanum Halide (LaBr

3 and LaCl

3

)

• Density: 5.06 g/cm

3

• Lanthanum atomic number : 57• Bromine atomic number: 35• Higher light output than NaI• Better resolution than NaI: LaBr3 2.8

LaCl3 3.

NaI 7-8
• Much shorter decay time (26-28 ns) than 

hence it performs better at high count rate

• Higher detection efficiency than NaI due 

greater density of the lanthanum halides.

Inorganic ScintillatorsLanthanum Halide (LaBr

3 and LaCl

3

)

• Hygrosopic• The major disadvantage of lanthanum ha

scintillators is the presence of the naturaloccurring radionuclide La-138 (ca. 1 Bq/c

This can compromise their performance flevel counting.
Inorganic Scintillators

Lanthanum Halide (LaBr

3 and LaCl

3

)

• Approximately one third of the La-138 dec

involve the emission of beta particles. In a background spectrum, this results in what like bremsstrahlung below 255 keV. 

• The 788.7 keV gamma is emitted in coinc

with the beta particles. The 1435.8 keV gaemitted in coincidence with barium x-rays
• Primarily application is for the detection o

rays and x-rays detection.

• It is not used for gamma spectroscopy du

poor resolution. 

Inorganic ScintillatorsCesium Iodide (CsI)

• Used as the detection device in some Ra

Pagers.

• CsI (Tl) used in x-ray digital imaging syste
• Density: 4.51 g/cm3

• Cesium atomic number : 55• Iodine atomic number: 53• Can be grown into fairly large crystalsInorganic ScintillatorsCesium Iodide (CsI)• Pure CsI crystals can be employed, but it 

common for CsI to be activated with thallsodium (Na). CsI (Tl) has a peak light emnear 550 nm which makes it feasible to esilicon photodiodes as the light collection 

• CsI (Tl) typically used with photodiodes. • Low afterglow possible (<0.1% after 5 ms
• Very efficient due to relatively high Z and 

• Resistant to thermal and mechanical sho• CsI (Na) hygroscopic; CSI and CsI (TL) leInorganic ScintillatorsCesium Iodide (CsI)

CsI CsI(Na) C

Peak Emission 

Wavelength

ca. 300 nm 420 nm 5

Decay Time 16 ns 630 ns 1

Photoelectron 

yield (% of NaI)

4 – 6 % 85% 
• Primary application is in x-ray detection (

imaging systems)

• Density: 7.9 g/cm

3

• Atomic number of cadmium: 48Inorganic ScintillatorsCadmium Tungstate (CdWO

4

)

•

Atomic number of tungsten: 74

• Peak light emission: 475 nm• Light yield (photons/MeV): ca 12,000 – 1• Photocathode yield (electrons/MeV): ca 3

that of NaI

• Decay time: ca. 5000 - 14000 ns• Not hygroscopic• Resolution: ca. 10%
• Primary application is in PET imaging sys

• Bi

4Ge

3

O12

• Density: 7.13 g/cm

3

• Atomic number of bismuth: 83• Peak light emission: 480 nm

Inorganic ScintillatorsBismuth Germanate (BGO)

Peak light emission: 480 nm

• Light yield (photons/MeV): ca 25% that of • Photocathode yield (electrons/MeV): ca 1

that of NaI

• Decay time: ca. 300 ns• Almost no afterglow (0.005% after 3 ms)• Not hygroscopic• Resolution: ca. 10%
• Primary application is in PET imaging sys

• Cerium activated• Density: 7.4 g/cm

3

• Atomic number of lutetium: 71Inorganic ScintillatorsLutetium Oxyorthosilicate (LSO)• Effective atomic number of LSO: 65• Peak light emission at 420 nm• Light yield: ca. 30,000 photons/MeV (75% • Photocathode yield (electrons/MeV) • Decay time ca. 41- 46 ns • Resolution: 10%• Not hygroscopic
• Primary application is for counting alpha 

particles. Has also been used in fast neudetectors.

• Only available as very fine (powder) cryst

Inorganic ScintillatorsZinc Sulfide (ZnS)

• Silver (Ag) used as activator• For all practical purposes, ZnS only respo

to alpha particles although it does have a response to neutrons.

• Typical background is 0 – 1 cpm• Light leaks can be a problem.
PMT

Light pipe 

(reflector)

Inorganic ScintillatorsZinc Sulfide (ZnS)

Alpha Source Aluminiz

La

Clear 

sup

(reflector)
Inorganic Scintillators

Zinc Sulfide (ZnS)
• Li-6 containing glass activated with ceriu

• Primarily for neutrons. Thermal neutron a

by Li-6 to produce alpha particle and recatom. Some of the kinetic energy of the a

t iti it th i Th d it t

Inorganic ScintillatorsGlass

t

ritium excites the cerium. The 

de-excit

at

cerium results in the emission of light (ca

• Can be manufactured in many shapes a• Very rugged• Shorter decay time than NaI
Organic Scintillators
Organic Scintillators

General• Are aromatic hydrocarbons• The individual molecules emit light – as s

organic scintillators can be solids, liquids 

O

N

PPO 2,5-diphenyloxazole
S20

S21

Singlet

Triplet

Excitation levels of typical organic molecOrganic Scintillators

Absorption

Fluorescence Phosphorescence

S00

S01S02S03S10S11S12S13

T1T2
• Only two have been used to any extent: 

and anthracene.

• Naphthalene has been used to a lesser eOrganic ScintillatorsOrganic Crystals

Naphthalene has been used to

a lesser e

• They are fragile, not available in large siz

have poor resolution

• As such, they are rarely used today
Organic Scintillators

Organic Crystals vs Inorganic Crystals

Calcium Fluoride

Inorganic

Anthracene

Organic
• The organic scintillator is mixed with a pla

solvent that is then polymerized

• The most commonly used scintillator is POrganic ScintillatorsPlastic Scintillators

The most commonly used scintillator is P

(ca. 1%) along with POPOP (ca. 0.4%) 

• The most commonly employed plastics a

polyvinyl toluene and polystyrene

• Detection of gamma rays, x-rays, beta pa
• Plastic can be manufactured in almost an

and shape

• RuggedOrganic ScintillatorsPlastic Scintillators - advantages• Light• Non-hygroscopic – this means they don’t 

be housed in a hermetically sealed conta

• Flat energy response when used for gam

and x-ray detection.
• Plastic scintillators are not very suitable f

ray or x-ray spectroscopy since only a frathe photon energy is absorbed in the scin

Pl ti ’ l t i b h t

Organic ScintillatorsPlastic Scintillators - disadvantages

Plasti

c’s low a

tomic num

ber means p

h

o

to

much more likely to interact via compton than the photoelectric effect.The spectrum shape, especially the comconveys some information about photon is possible to determine the general enerof the photons, e.g., high, medium or low
Scintillator Housing 

and Optical Couplingand Optical Coupling
• The scintillator must be protected from am

light. In some cases it must also be shieldfrom specific types of radiation (e.g., beta 

particles

).

Scintillator Housing and Optical CouScintillator Housing

p )

• If the scintillator is not hygroscopic (e.g., 

plastic scintillator), it does not need to be protected in a moisture proof container. TPMT might be coupled directly to the scin
• If the scintillator is hygroscopic, it must be 

housed in a hermetically sealed container a NaI crystal in an aluminum “can”).

• When the scintillator and PMT cannot be Scintillator Housing and Optical CouScintillator Housing

When the scintillator and PMT cannot be 

physical contact because the former is hermetically sealed, the housing must hatransparent window to transmit the emitteSuch a container is usually lined with a whpaint or powder to reflect light through the window.
Quartz or Glass Window

Refle

Scintillator Housing and Optical CouHermetically Sealed NaI Crystal

Crystal

Alum

Hou
• If it is desirable to detect beta particles, a 

aluminized mylar window might be incorpinto the scintillator housing/covering.

• If it is necessary to detect low energy phoScintillator Housing and Optical CouScintillator Housing

If it is necessary to detect low energy pho

the housing might incorporate a thin windmade of low Z material (e.g., beryllium).
• To maximize the detector’s efficiency for s

samples, a hole might be drilled all or parthrough the scintillator. During the count, sample is positioned in the hole.

Scintillator Housing and Optical CouScintillator Housing• Although the detection efficiency would be 

the analysis is limited to small samples. Athe sample count rate might still be low.

• Such a configuration can be unsuitable fo

gamma spectroscopy because surroundinsample with the scintillator can dramaticaincrease coincident (summation) losses. 
Crystal

Housing

Window

Scintillator Housing and Optical CouWell Detector

Well
• In most cases, the window of the scintilla

housing is directly coupled (in direct contathe photocathode of the photomultiplier tu

Scintillator Housing and Optical CouOptical Coupling

An optical coupling grease or gel is almos

applied to the scintillator window - PMT into minimize reflection.
• All sorts of scintillator-PMT arrangements 

possible. For example:

One 

photomulti

plier tube mi

ght be cou

Scintillator Housing and Optical CouOptical Coupling

pp g

one or more scintillating crystals.Multiple photomultiplier tubes might be coupled to a single crystal
Typical arrange

a single NaI crycoupled directly single photomutube.

Scintillator Housing and Optical CouOptical Coupling

Photomultipli

NaI Crystal
PMT

Scintillator Housing and Optical CouOptical Coupling

CRYSTAL

Multiple PMTs con

single large c
• A scintillator can also be coupled to a “re

photomultiplier tube via a light pipe (typicaacrylic plastic).

Scintillator Housing and Optical CouLight Pipes• One end of the light pipe is connected to t

scintillator and the other end is connected photomultiplier tube.

• Optical cement is often used to connect t

scintillator to the light pipe.
Scintillator Housing and Optical Cou

Light Pipes

Example light pipes (guides) for transmitting light from the scintillator to a remote PMT.

Teledyne
Photomultiplier Tubes
Photomultiplier Tubes

General

• The photomultiplier tube (PMT) converts 

of light (scintillation) from the detector into electronic pulse

• Photodiodes are less frequently used for 

purpose. For photodiodes to be employeemitted by the scintillator should be in the 600 nm range.

• PMTs are fragile
Photomultiplier Tubes

General

• PMTs are affected by magnetic fields• The pins (electrical contacts) on the end 

should be kept clean

– do not touch with 

should be kept clean do not touch with 

hands. 

• PMT’s are usually round in cross section 

be square if multiple tubes must be packetogether.
• The photocathode is a thin metallic depos

Sb-Cs, Sb-Rb-Cs) on the inside of the winthe PMT.

Photomultiplier TubesPhotocathode

• It is usually flat and circular• The photo cathode is usually on one end o

PMT (end-on) but some PMTs have the photocathode on the side (side-on).
• The absorption of light photons from the s

knocks electrons off the photocathode

• 20 to 30% of the light photons incident on 

Photomultiplier TubesPhotocathode

20 to 30% of the light photons incident on 

photocathode result in the freeing of electrThis is known as the “quantum efficiency”

• The quantum efficiency is usually greatest 

photons around 400 nm. Below a typical c350 nm, light photons (UV), can’t penetraglass.
• Electrons from the photocathode are direc

towards the first of several (usually 10) dyby a focusing grid

Photomultiplier TubesDynodes• The dynodes are curved metal plates with 

positive potential that attracts and accelerelectrons
Window

PhotocathodFocusing G

AnodeGlass EnvelDynodes
+ 150 V + 220 V

+ 290 V + 360 V

Each succedynode has increasingly 

+ 430 V + 500 V+ 570 V + 640 V+ 710 V

+ 780 V

+ 850 V

potential vo
• Each electron striking a dynode frees 4-6 

electrons

• For each electron leaving the photocathodPhotomultiplier TubesDynodes

For each electron leaving the photocathod

so) leave the first dynode, 25 leave the sedynode, 125 leave the third dynode etc. 

• For every electron freed from the photoca

over a million will eventually be collected anode.
Light Photons
Electron ejected 

from photocathode

e
Each electron striki

dynode causes 4 to ejected
Each electron strik

second dynode res4 to 5 bein

g ejecteg j
Each electron freed the photocathode rin approximately 10,000,000 electroreaching the anode
Photocathode

Curved dynode 
Photomultiplier Tubes

Various Dynode Configurations

Venetian Blind 

Dynodes

Circular Grid 

Dynodes

Box an

Dyn
Focusing 

Grid

Photomultiplier TubesFocusing Grid
• The voltage divider is a series of resisters 

possibly some capacitors) usually located tube socket/base.

Photomultiplier TubesVoltage Divider• The divider ensures that the photocathode

focusing grid, dynodes and the anode all appropriate potential
Photomultiplier Tubes

Voltage Divider
Two 5” 

tubes. the left 

Photomultiplier TubesVoltage Divider

the left 

directly connethe vodivider
An

Focusing Grid Photocathode

R R R 

C C
Potential Problems wit

Photomultiplier TubesPhotomultiplier Tubes
• Noise

• Generally consists of small pulses that ca

discriminated against

Potential Problems with PMTsDark Current• A variety of sources: thermionic emission, 

emission, ionization of residual gas, ohmileakage, radioactive components in the tuluminescent effects
• Magnetic fields can change the paths of th

electrons

• The result is a decreased 

gain and 

possib

Potential Problems with PMTsMagnetic Fields

g p

complete loss of signal

• PMTs often shielded by “mu-metal”
• A gain shift means that the pulse amplifica

(gain) has increased or decreased

• As a result, the pulses are larger or smallePotential Problems with PMTsGain Shifts

they were before the shift - not good.

• In gamma spectroscopy, this can result in 

photopeak being shifted to the right or left
Photopeak Gain Decrease

Potential Problems with PMTsGain Shifts

662 keV 
Photopeak Gain Increase

Potential Problems with PMTsGain Shifts

662 keV 662 ke
Gain shifts can occur because of a change 

potential difference between the last few dya PMT (often a 70 volt difference).This potential is affected by the number of ePotential Problems with PMTsGain ShiftsThis potential is affected by the number of e

moving across the dynodes per unit time. When very large numbers of electrons (whicnegative charge) move across the last few dthe resulting negative electric field reduces tpotential difference between the dynodes. 
When the potential is reduced, the pulses be

smaller and the spectrum shifts to the left.Potential Problems with PMTsGain ShiftsOne method that has been 

l d t t thi

employe

d

to preven

t this 

change in potential is to run capacitors in parallel with the last few resisters in the voltage divider. 
In addition to changes in count rate, change

gamma ray energies being counted can caushifts.Potential Problems with PMTsGain ShiftsThis shift occur because the number of elecmoving through the PMT is directly related tnumber of photons striking the photocathodthe latter is directly related to gamma ray enThe higher the count rate and the gamma raenergy, the greater the spectrum shift to the 
Other possible causes of gain shifts include

- bad connection between the tube bas

and socket

Potential Problems with PMTsGain Shifts

- changes in the line current- failure of HV power supply- changes in temperature- changes in count rate