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Part IIPart II Quantities and MeasurementsQuantities and Measurements

Module 3Module 3 Principles of Radiation Principles of Radiation Detection and MeasurementDetection and Measurement

Session 4,Session 4, Liquid ScintillationLiquid Scintillation 13-15a13-15a Resolution, CoincidenceResolution, Coincidence

Session II.3.4 & 13-15aSession II.3.4 & 13-15a

IAEA Post Graduate Educational CourseIAEA Post Graduate Educational CourseRadiation Protection and Safe Use of Radiation SourcesRadiation Protection and Safe Use of Radiation Sources

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We will describe the liquid scintillation We will describe the liquid scintillation process includes the meaning of:process includes the meaning of: solventsolvent solutesolute scintillation “cocktail”scintillation “cocktail”

We will also explain the purpose of a We will also explain the purpose of a scintillation detector andscintillation detector and

We will describe what is meant by We will describe what is meant by coincidence counting and the significance coincidence counting and the significance and types of quenchingand types of quenching

OverviewOverview

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Liquid Scintillation OverviewLiquid Scintillation Overview

A small sample is taken using a small filter wipe of an A small sample is taken using a small filter wipe of an area suspected of contaminationarea suspected of contamination

The sample is then put in a vial with a given amount The sample is then put in a vial with a given amount of scintillation “cocktail” – a mixture of different of scintillation “cocktail” – a mixture of different agentsagents

If the sample has radioactive contamination, it will If the sample has radioactive contamination, it will cause a reaction with the scintillation cocktail and cause a reaction with the scintillation cocktail and subsequently a flash of light will be emittedsubsequently a flash of light will be emitted

The light signal is proportional to the activity in the The light signal is proportional to the activity in the samplesample

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Liquid ScintillationLiquid Scintillation

The light is not necessarily in a spectrum The light is not necessarily in a spectrum that is visible to the human eyethat is visible to the human eye

A scintillation detection system is used to A scintillation detection system is used to measure the light producedmeasure the light produced

These flashes of light are used to measure These flashes of light are used to measure the amount of activity the sample containsthe amount of activity the sample contains

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Liquid ScintillationLiquid Scintillation

Advantages of liquid scintillation counting Advantages of liquid scintillation counting are:are:

It can detect many different types of It can detect many different types of isotopesisotopes

It provides an means of measuring low It provides an means of measuring low energy beta particlesenergy beta particles

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Liquid ScintillationLiquid Scintillation

Disadvantages of this type of detector are:Disadvantages of this type of detector are:

Expense of equipmentExpense of equipment It is not portable, and It is not portable, and It requires laboratory support and skilled It requires laboratory support and skilled

professionals to operateprofessionals to operate

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Liquid ScintillationLiquid Scintillation

The process of liquid scintillation counting is The process of liquid scintillation counting is relatively simplerelatively simple

The beta decay electron emitted by the radioactive The beta decay electron emitted by the radioactive isotope in the sample excites the solvent molecule, isotope in the sample excites the solvent molecule, which in turn transfers the energy to the solute, or which in turn transfers the energy to the solute, or fluorfluor

The energy emission of the solute (the light photon) The energy emission of the solute (the light photon) is converted into an electrical signal by a is converted into an electrical signal by a photomultiplier tubephotomultiplier tube

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Process OverviewProcess Overview

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Liquid Scintillation ProcessLiquid Scintillation Process

The process of liquid scintillation involves the The process of liquid scintillation involves the detection of beta decay within a sample via capture detection of beta decay within a sample via capture of beta emissions in a system of organic solvents of beta emissions in a system of organic solvents and solutes referred to as the scintillation cocktailand solutes referred to as the scintillation cocktail

This mixture is designed to capture the beta This mixture is designed to capture the beta emission and transform it into a photon emission emission and transform it into a photon emission which can be detected via a photomultiplier tube which can be detected via a photomultiplier tube within a scintillation counterwithin a scintillation counter

The cocktail must also act as a solubilizing agent, The cocktail must also act as a solubilizing agent, keeping a uniform suspension of the samplekeeping a uniform suspension of the sample

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Liquid Scintillation ProcessLiquid Scintillation Process

The scintillation counting system The scintillation counting system consists of three primary components:consists of three primary components:

the radioactive substancethe radioactive substance the solvent, andthe solvent, and the solute (or fluor)the solute (or fluor)

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SolventSolvent

The solvent is the first compound in the scintillation The solvent is the first compound in the scintillation cocktail to capture the energy of the beta particlecocktail to capture the energy of the beta particle

The solvent molecule achieves an excited state, and The solvent molecule achieves an excited state, and the excess energy is transferred from solvent the excess energy is transferred from solvent molecule to solvent moleculemolecule to solvent molecule

The solvent tends to remain in an excited stated for The solvent tends to remain in an excited stated for an extended period of time, decaying into the an extended period of time, decaying into the ground state without the emission of light; thus, ground state without the emission of light; thus, solvents tend to have a low quantum fluorescent solvents tend to have a low quantum fluorescent yieldyield

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SolventSolvent

List of solvents and their characteristicsList of solvents and their characteristics

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SoluteSolute

Solutes (or fluors) exhibit properties which in many Solutes (or fluors) exhibit properties which in many respects are just the opposite of those of solventsrespects are just the opposite of those of solvents

They tend to decay rapidly mainly through the They tend to decay rapidly mainly through the emission of light photons, thus having a high emission of light photons, thus having a high quantum fluorescent yieldquantum fluorescent yield

Solutes that directly absorb the excitation energy of Solutes that directly absorb the excitation energy of the solvent are also known as primary solutesthe solvent are also known as primary solutes

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SoluteSolute

Early scintillation counters were sometimes unable Early scintillation counters were sometimes unable to detect the short wavelengths emitted by primary to detect the short wavelengths emitted by primary solutes; as a result, secondary solutes were added solutes; as a result, secondary solutes were added to ampilify the primary emissionsto ampilify the primary emissions

Secondary solutes were also complex organic Secondary solutes were also complex organic compounds with the ability to absorb the decay compounds with the ability to absorb the decay energy of the primary solute and rapidly emitting it energy of the primary solute and rapidly emitting it at a longer wavelength, shifting the overall signal to at a longer wavelength, shifting the overall signal to a wavelength more easily detectable by a wavelength more easily detectable by photomultiplier tubesphotomultiplier tubes

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SoluteSolute

As more sensitive photomultiplier tubes As more sensitive photomultiplier tubes were constructed, secondary solutes were constructed, secondary solutes became unnecessarybecame unnecessary

However, they may still be used to improve However, they may still be used to improve counting efficiency, as both the shorter and counting efficiency, as both the shorter and longer wavelengths can be detected longer wavelengths can be detected

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Sample PreparationSample Preparation

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ProcessProcess

The solvent is the first compound in the The solvent is the first compound in the scintillation cocktail to capture the energy of the scintillation cocktail to capture the energy of the beta particlebeta particle

The solvent molecule achieves an excited state, The solvent molecule achieves an excited state, and the excess energy is transferred from solvent and the excess energy is transferred from solvent molecule to solvent moleculemolecule to solvent molecule

The solvent remains in the excited stated for an The solvent remains in the excited stated for an extended period of time, decaying into the extended period of time, decaying into the ground state without the emission of light. ground state without the emission of light.

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ProcessProcess

The solute then absorbs the excitation The solute then absorbs the excitation energy of the solvent, and quickly returns to energy of the solvent, and quickly returns to the ground state by emitting lightthe ground state by emitting light

If a secondary solute is used, that solute If a secondary solute is used, that solute absorbs the signal of the first solute and absorbs the signal of the first solute and emits a second burst of light at a longer emits a second burst of light at a longer wavelengthwavelength

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CountingCounting

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Sample AnalysisSample Analysis

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Process DevelopmentProcess Development

Soon after the discovery of the basic principles of Soon after the discovery of the basic principles of liquid scintillation in 1950, instruments designed liquid scintillation in 1950, instruments designed for counting began appearing, with the first for counting began appearing, with the first commercial model becoming available in 1954commercial model becoming available in 1954

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Process SchematicProcess Schematic

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Photomultiplier TubePhotomultiplier Tube

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Coincidence CountingCoincidence Counting

Most commercial scintillation counters Most commercial scintillation counters are coincidence systems utilizing PMTs in are coincidence systems utilizing PMTs in tandem to monitor for a photon event tandem to monitor for a photon event

A pulse is not registered unless both A pulse is not registered unless both PMTs view the incident photons within PMTs view the incident photons within the predetermined time interval usually the predetermined time interval usually 20-30 nsec20-30 nsec

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Coincidence CountingCoincidence Counting

If a pulse is recorded by the two PMTs within If a pulse is recorded by the two PMTs within the 20-30 nanosecond window, a the 20-30 nanosecond window, a coincidence pulse is recorded that is a coincidence pulse is recorded that is a measure of the number of single events measure of the number of single events which occurred during the window. This is which occurred during the window. This is called coincidence countingcalled coincidence counting

If an event occurs within only one of the If an event occurs within only one of the PMTs, a coincidence pulse will not be PMTs, a coincidence pulse will not be recordedrecorded

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Pulse SummationPulse Summation

Early liquid scintillation counter models only Early liquid scintillation counter models only monitored the pulses from one PMT, using the other monitored the pulses from one PMT, using the other to detect for coincidence onlyto detect for coincidence only

Current models uses a pulse summation circuit, Current models uses a pulse summation circuit, which adds together all of the pulses from the which adds together all of the pulses from the output of the PMTs within a given time windowoutput of the PMTs within a given time window

This technique is particularly helpful for dual This technique is particularly helpful for dual isotope counting of isotope counting of 33H and H and 1414CC

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Pulse AmplificationPulse Amplification

There are two types of pulse amplification:There are two types of pulse amplification:

linearlinear logarithmiclogarithmic

A linear amplifier generates a pulse proportional to A linear amplifier generates a pulse proportional to the energy obtained from the scintillation solutionthe energy obtained from the scintillation solution

A logarithmic amplifier processes the summed A logarithmic amplifier processes the summed output of the 2 PMTs into a single pulse equal to the output of the 2 PMTs into a single pulse equal to the log of the summed PMT pulseslog of the summed PMT pulses

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QuenchingQuenching

Quenching is the reduction in the efficiency in the Quenching is the reduction in the efficiency in the energy transfer process in the scintillation solutionenergy transfer process in the scintillation solution

This presents a problem in liquid scintillation This presents a problem in liquid scintillation counting due to a reduced signalcounting due to a reduced signal

Types of quenching are:Types of quenching are:

chemicalchemical dilutiondilution self-quenchingself-quenching colorcolor opticaloptical

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QuenchingQuenching

ChemicalChemical – scintillation solution components – scintillation solution components interact with excited molecules before they can emit interact with excited molecules before they can emit a light photona light photon

DilutionDilution – dilution reduces the probability of – dilution reduces the probability of scintillation eventsscintillation events

Self-quenchingSelf-quenching – the concentration of the solute is – the concentration of the solute is too high which interact with excited molecules and too high which interact with excited molecules and dissipate the energy before a photon can be dissipate the energy before a photon can be producedproduced

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QuenchingQuenching

ColorColor – color in the sample materials absorb some – color in the sample materials absorb some of the fluorescent photons before they can leave the of the fluorescent photons before they can leave the counting vialcounting vial

OpticalOptical – fogging on the outside of the scintillation – fogging on the outside of the scintillation sample vial impedes transmission of photons.sample vial impedes transmission of photons.

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Where to Get More InformationWhere to Get More Information

Cember, H., Introduction to Health Physics, 3Cember, H., Introduction to Health Physics, 3rdrd Edition, McGraw-Hill, New York (2000)Edition, McGraw-Hill, New York (2000)

Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8Table of Isotopes (8thth Edition, 1999 update), Wiley, Edition, 1999 update), Wiley, New York (1999)New York (1999)

International Atomic Energy Agency, The Safe Use International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)IAEA, Vienna (1995)