Lec 3 Scintillators

7/28/2019 Lec 3 Scintillators

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Pakistan Institute of Engineering & Applied Sciences (PIEAS)

Course: Radiation Interaction & Detection

Dr. Nasir M Mirza

Deputy Chief Scientist,

Room # A114,

Department of Physics & Applied mathematics,

PIEAS, P.O. Nilore, 45650, Islamabad.

Email: [email protected]

Ph: 092 51 9290273 (ext: 3059)
mailto:[email protected]:[email protected]


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Scintillation Detector

Lecture One:


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Text Books for the Course:

1. Glenn F Knoll s Radiation Detection & Measurement (recentedition) (this shall be the main text for the course).

2. W.J. Prices Nuclear Radiation detection, McGraw Hill BookCompany.

By: Dr. Nasir M Mirza

Office: Block A (Room Number, A -144)

Office Phone: extension, 3059Email: [email protected]

Scintillation Detector
mailto:[email protected]:[email protected]


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Scintillation

Scintillation

Flash of light (Visible)

From radiation detection point of view---flash of light

produced from excited atoms as a result of radiationinteraction

Luminescence

Usually occurs at low temperatures and is thus a form

of cold body radiation.


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Scintillation

Scintillation

Detection of ionization radiation by scintillation light

produced in a material is one of the oldest technique

on record (for example ZnS screens were used

initially as detectors);

Common scintillation materials include NaI(Tl),

CsI(Tl), CsI(Na), Li(Eu), ZnS, Ca2F, BaF2, glass-

Scintillators, and plastic Scintillators;


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Scintillation (Contd.)

One of the earliest means of measuring radiation

Rutherford experiments (alpha particle scattering)

used zinc sulphide crystals as the primary detector of

radiation

Used his eye to see the flickers when alpha struck

zinc sulphide

Now-a-days Photo-Multiplier-Tube (PMT) is used;


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Photo-Emissive Processes

Fluorescence: A luminescence that is mostly foundas an optical phenomenon in cold bodies, in whichthe molecular absorption of a photon triggers theemission of another photon with a longer wavelength.

Phosphorescence: A delayed luminescence, that is, aluminescence that persists after removal of theexciting source. It is sometimes called afterglow.Corresponds to light of longer wavelength than

fluorescence Delayed Florescence: Delayed emission than

fluorescence & Light of equal wavelength tofluorescence


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Properties of Good Scintillator

It should convert the Kinetic Energy (K.E.) ofcharged particles into detectable light with a high

scintillation efficiency;

This conversion should be linear the light yieldshould be proportional to deposited energy over as

wide range as possible;

The medium should be transparent to the wavelength

of its own emission for good light collection;


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Properties of Good Scintillator (Contd.)

The decay time of the induced luminescence shouldbe short so that fast signal pulses can be generated;

The material should be of good optical quality and

subject to manufacture in sizes large enough to be ofinterest as a practical detector;

Its index of refraction should be near that of glass (n

~ 1.5) to permit efficient coupling of the scintillation

light to a PMT or other light sensor


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No material simultaneously meets all these criteria----the choice is a compromise

Most widely applied scintillator are

Inorganic Alkali Halides

Organic based Liquids

and Plastics


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Property InorganicScintillator

Organic Scintillator

Light Output yield more Less

Light emission

response

Slow fast

ZNumber high Low

Radiation spectroscopy Gamma rays Beta and neutrons


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Mode of Operation of Scintillator Detectors

Mostly used in pulse mode

Advantages of using in pulse mode are:

Setting small time constant can eliminate the

unwanted phosphorescence and delayed

fluorescence

Current mode scintillator detectors used where

radiation intensity changes rapidly and will sufferfrom after glow effects if long lived decay

components are significant


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Organic Scintillators

Fluorescence in organics is due to transitions in the energy

levels of a single molecule

And it is independent of physical state of the organic

scintillation material

Example is Anthracene where solid or vapor will do the

fluorescence.

However, inorganic Scintillators (such as NaI(Tl) do need a

regular crystal structure as a basis to emit scintillations.


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Organic Scintillators

Large category of organic

Scintillators are based on organicmolecules with some symmetryproperties

and this gives rise to -electronstructure of organic molecules;

Singlet states (spin 0): S0, S1, S2, ...

Triplet states (Spin 1): T1, T2, T3, ...

Energy spacing is generally of theorder of

S1 - S0 ~ 3 - 4 eV

S3 - S2< S2 - S1< S1 - S0


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Organic Scintillators (Contd.)

Vibrational states of molecules

S00, S01, S02 ,

S10, S11, S12

First subscript is for main energy

state and second is for vibrational

state.

S01 - S00 ~ 0.15 eV

However, the thermal energy is

0.025 eV which is much smaller

than 0.15eV so all molecules at

room temperature are at S00 ground

state.


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Types of Organic Scintillators

Pure Organic Crystals

Liquid Organic Solution

Plastic Scintillator

Thin Film Scintillator

Loaded Organic Scintillator


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Types of Organic Scintillators (Contd.)

Pure Organic Crystals

Anthracene

One of the oldest organic material having highest scintillationefficiency

Sti lbene

It has lower scintillation efficiency

Used for pulse shape discrimination

Both Materials are fragile and difficult to obtain in larger sizes

Scintillation efficiency is dependent on orientation of ionizingparticles with respect to crystal axis (20-30% directionalvariation) which spoil energy resolution

They get damaged by exposure to radiation


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Liquid Organic Scintillators Binary (Solvent + Scintillator)

Tertiary (Solvent + Scintillator + Wave-Shifter)

Commercially available in sealed glass containers

Detectors of any size, shape and cost can be made

Sometimes radioactive material is dissolved in L.S.

and counted with 100% efficiency (C14, H3 etc.)

Problems with L.S.-------if there is dissolved O2 then

there is quenching problem (so containers in which

L.S. is to be kept should be purged from O2)


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Plastic Scintillators Organic scintillator dissolved in solvent is

polymerized

Large volume solid Scintillators of low cost can be

made, but large size may cause some problem ofattenuation of light

Available as rods, sheets, cylinders etc.

Small size solid Scintillators---available as single

fiber, group of fibers----as bundles, ribbons etc.

Radiation damage may cause decrease in lightoutput or decrease in the light transmission


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Thin Film Scintillator Films ~ 20 g/cm2 or 10 m can be prepared

Can be used for particles of lowest range (heavy ions)

They act as transmission detectors

Applied in fast timing measurements


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Loaded Organic Scintillators Organic Scintillators are generally used for the

detection of alpha, beta and fast neutrons

For gamma rays detections, high Z materials areadded

By adding high Z material, photo peak efficiency canbe made relatively high, fast response can beattained and also they are of low cost as compared toconventional gamma ray scitillators

But, light output is reduced and resolution is degraded

For detection of neutrons, high neutron cross sectionmaterials (Boron, Lithium, Gadolinium) are added


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NaI(Tl) Scintillator

Pure Crystal NaI

Interaction of radiation results in

UV photons

It can operate at liquid nitrogen

temperature

Activated Crystal NaI(Tl)

Small amount (~ 10-3 mole

fraction) of Thallium is added as

an activator in high purity NaI;

Interaction of radiation------visible photons are emitted;

Operate at room temperature;


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NaI(Tl) Scintillator (Contd.)

Properties

Large ingots can be grown from high purity sodium iodide

Can be machined into different shapes and sizes

Scintillators of usual size or shape can also be fabricated bypressing small crystallites together

Excellent light yield

Small non-proportionality of scintillation response with depositedelectron energy. The departure from proportionality is mostpronounced at low energies

Crystal is fragile----can easily be damaged by mechanical orthermal shocks

Hygroscopic---deteriorate due to water absorption (thereforecanned in air tight containers for normal use);


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Decay time of scintillation pulse is 230 ns which is long for

fast timing or high counting rate applications

Phosphorescence with 0.15 s decay time which contribute9% of overall light yield

High Temperature Operation

Scintillation yield is dropped with increasing temperature,results in poor energy resolution

Decay time decreases with increase in temperature, givefaster response at higher temperatures


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Radiation Damage Effects

Reduction of transparency: caused by the creation ofcolor centers that absorb scintillation light

Interference with processes that give rise to the

emission of the scintillation light itself Radiation exposures can also induce long-lived light

emission in the form of phosphorescence that can betroublesome in some measurements

Radiation damage are often observed to be ratedependent and vary greatly with type of radiationinvolved


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Introduction of PMT

This device is used to convert

weak light signal from scintillator

(few hundred photons) to a

corresponding electrical signal

without adding a large amount of

noise

Simplified structure of a typical

PMT

Outer envelope is made of

glass; It sustain vacuumconditions inside the tube

and serves as pressure

boundary

CrystalPMT Base


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Introduction of PMT (Contd.)

Photocathode

Convert light photon to lowenergy electrons calledphotoelectrons

A few hundred in number

Charge too small to serveas electrical signal

Electron Multiplier Section (Dynode)

Provide an efficient geometryfor photoelectrons

Serve as an amplifier

Multiply the electrons 1071010

electrons per photon Output Stage Anode

Collect electrons

Give output voltage pulse


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Introduction of PMT (Contd.)

Linear Behavior

Output pulse remains

proportional to number of

original photoelectrons

Timing Information

Most of the information of

original light pulse is retained

Electrons are produced within

20 50 ns after light photon


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Photocathode

Photoemission

The conversion of incident light

photon into electrons is called

photo-emission

3 stages of photo-emission process

1.Absorption of incident photon

and transfer of energy to an

electron within the photo

emissive material

2.Migration of the electron tosurface of photocathode

3.Escape of the electron from

the surface of the cathode


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Photocathode (Contd.)

Energy requirements:

Step 1

The energy of a scintillationphoton (~ 3 eV) is absorbed

Step 2

Some of energy is lost in e ecollisions in migration process.

Step 3

The remaining should be greaterthan work function (~ 1.5 2 eV)of the material;

Min. energy of photon must begreater than the potential barrier

Surface barrier must be low tomaximize escape electrons;

Rate of energy loss of migratingelectron must be small tomaximize escape depth;


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Electron Multiplication (Contd.)

Multiplication factor

The overall multiplication factor of asingle dynode is defined as (typical value is 5):

N

gainoverall

incidentelectronsprimaryof#

emittedelectronssecondaryof#

Then the Overall gain of a PM tube having N stages is

Where, is fraction of all photoelectrons collected by the multiplier structure

(typical value is 1) and N is number of stages.


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Various Configurations


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Scintillation Detector & PMT

Lec 3 Scintillators

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