In The Name Of ALLAHIn The Name Of ALLAH

The Most Merciful & Benevolent Beyond ReckoningThe Most Merciful & Benevolent Beyond Reckoning

What Is Radiation

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Radiation isRadiation is the the emission and transmissionemission and transmission of of energyenergy through space or through a through space or through a material medium.material medium.Radiation can be in the form of Radiation can be in the form of sub-atomic sub-atomic particles (protons, neutrons and electrons) particles (protons, neutrons and electrons) or or electromagnetic waveselectromagnetic waves

IonizingIonizing RadiationRadiation ( (IRIR)) causes ions to be causes ions to be produced when radiation is absorbed in produced when radiation is absorbed in mattermatter

Non-IonizingNon-Ionizing radiationradiation ( (NIRNIR)) refers to refers to radiation energy that, instead of producing radiation energy that, instead of producing charged ions, when passing through charged ions, when passing through matter, has sufficient energy only for matter, has sufficient energy only for excitation.excitation.

NaturalNatural BackgroundBackground RadiationRadiation

Man-MadeMan-Made RadiationRadiation

Why We MeasureWhy We Measure

Personal Dosimetry vs Regulatory Personal Dosimetry vs Regulatory requirementsrequirementsClassification of safe and dangerous Classification of safe and dangerous zoneszonesSafety vs Protective measuresSafety vs Protective measuresQuality control and Quality assuranceQuality control and Quality assuranceData consistancy and standardizationData consistancy and standardization

Importance of Radiation in Daily Importance of Radiation in Daily LifeLife

MedicineMedicine: : RadiologyRadiology, , RadiationRadiation TherapyTherapyPowerPower Production and Space ExplorationProduction and Space Exploration In Agriculture to improve variety & yield In Agriculture to improve variety & yield productionproductionWeapons of mass destructionWeapons of mass destructionIndustrial applications: Gauges, Industrial applications: Gauges, Radiography, Mineral explorationRadiography, Mineral exploration

Why is Radiation Detection Why is Radiation Detection Difficult?Difficult?

Can’tCan’t seesee ititCan’tCan’t smellsmell ititCan’tCan’t hearhear ititCan’tCan’t feelfeel ititCan’tCan’t tastetaste itit

WeWe taketake advantageadvantage ofof thethe factfact thatthat radiationradiation producesproduces ionizedionized pairspairs toto trytry toto createcreate anan electricalelectrical signalsignal

RADIATION DETECTORS

• Instruments used in the practice of health & Medical physics serve a wide variety of purposes

• one finds instruments designed specifically for the measurement of a certain type of radiation, such as Alpha Particles, Beta Particles low-energy X-rays, high-energy gamma rays. fast neutrons, and so on

• The basic requirement of any such instrument is that its detector interact with the radiation in such a manner that the magnitude of the instrument's response like development of current or voltage pulse, Charging and Discharging of Capacitors is proportional to the radiation effect or radiation property being measured

Radiation Measurement Principles

Detector

SignalPhysicalChemicalBiological

Reader

Calibration

Assessm

ent

Amplification

HowHow aa RadiationRadiation DetectorDetector WorksWorks

TheThe radiationradiation wewe areare interestedinterested inin detectingdetecting allall interactinteract withwith materialsmaterials byby ionizingionizing atomsatomsWhile it is difficult (sometime impossible) to directly While it is difficult (sometime impossible) to directly detect radiation, it is relatively easy to detect (measure) detect radiation, it is relatively easy to detect (measure) the ionization of atoms in the detector material.the ionization of atoms in the detector material. Measure the amount of charge created in a detectorMeasure the amount of charge created in a detector

electron-ion pairs, electron-hole pairselectron-ion pairs, electron-hole pairs Use ionization products to cause a secondary reactionUse ionization products to cause a secondary reaction

use free, energized electrons to produce light photonsuse free, energized electrons to produce light photons ScintillatorsScintillators

We can measure or detect these interactions in many We can measure or detect these interactions in many different ways to get a multitude of informationdifferent ways to get a multitude of information

GeneralGeneral DetectorDetector PropertiesProperties

Characteristics of an “ideal” radiation detectorCharacteristics of an “ideal” radiation detector High probability that radiation will interact with the detector High probability that radiation will interact with the detector

materialmaterial Large amount of charge created in the interaction processLarge amount of charge created in the interaction process

average energy required for creation of ionization pair (W)average energy required for creation of ionization pair (W) Charge must be separated an collected by electrodesCharge must be separated an collected by electrodes

Opposite charges attract, “recombination” must be avoidedOpposite charges attract, “recombination” must be avoided Initial Generated charge in detector (Q) is very small (e.g., Initial Generated charge in detector (Q) is very small (e.g.,

1010-13-13C)C)Signal in detector must be amplifiedSignal in detector must be amplified

Internal Amplification (multiplication in detector)Internal Amplification (multiplication in detector) External Amplification (electronics) External Amplification (electronics)

Want to maximize VWant to maximize V CQV

Detection and Detection and measurement includes measurement includes the following the following components:components:DetectorDetectorPreamplifierPreamplifierAmplifier Amplifier Single channel analyzerSingle channel analyzerMulti-channel analyzerMulti-channel analyzerScalar-Timer Scalar-Timer

a detector a detector produces a signalproduces a signal for every particle for every particle entering in it. entering in it. Every detector works by using some Every detector works by using some interactioninteraction of particles with matter.of particles with matter.Use characteristic effects from interaction of Use characteristic effects from interaction of radiation with matter to detect, identify and/or radiation with matter to detect, identify and/or measure properties of radiation. measure properties of radiation. Respond to radiation by producing various Respond to radiation by producing various physical effects physical effects

Detection ProcessesDetection Processes

Ionization: Gas/Liquid chambers and Ionization: Gas/Liquid chambers and Semiconductor DetectorsSemiconductor DetectorsScintillation: Scintillation counters and TLDsScintillation: Scintillation counters and TLDsSparking: Sparking chamberSparking: Sparking chamberBlackening of photographic film: Nuclear Blackening of photographic film: Nuclear Emulsion detector and Film dosimetryEmulsion detector and Film dosimetryBubbling/Clouding of supersaturated Bubbling/Clouding of supersaturated liquids/vapors: Cloud Chambers and Bubble liquids/vapors: Cloud Chambers and Bubble ChambersChambers

Detection ProcessesDetection Processes

Physical Changes: NMR Dosimetry and Physical Changes: NMR Dosimetry and SSNTDsSSNTDsThemodynamical Changes: Calorimetric Themodynamical Changes: Calorimetric DosimetryDosimetryActivation: Neutron Activation DetectorsActivation: Neutron Activation DetectorsBiological Changes: ESR Detectors and Biological Changes: ESR Detectors and BiosensorsBiosensors

Following is the list of most common types of Following is the list of most common types of detectors:detectors:Gas-filled countersGas-filled counters

ionization chamber ionization chamber Proportional Counter Proportional Counter Geiger-Muller counterGeiger-Muller counter

Scintillation detectors Scintillation detectors Semiconductor detectors Semiconductor detectors

GasGas DetectorsDetectorsMost common form of radiation detectorMost common form of radiation detector Relatively simple constructionRelatively simple construction

Suspended wire or electrode plates in a containerSuspended wire or electrode plates in a containerCan be made in very large volumes (mCan be made in very large volumes (m33))

Mainly used to detect Mainly used to detect -particles and neutrons-particles and neutrons

Ease of useEase of use Mainly used for counting purposes onlyMainly used for counting purposes only

High value for W (20-40 eV / ion pair)High value for W (20-40 eV / ion pair)Can give you some energy informationCan give you some energy information

Inert fill gases (Ar, Xe, He)Inert fill gases (Ar, Xe, He)Low efficiency of detectionLow efficiency of detection Can increase pressure to increase efficiencyCan increase pressure to increase efficiency -rays are virtually invisible-rays are virtually invisible

IonizationIonization ChambersChambersTwo electric plates Two electric plates surrounded by a metal surrounded by a metal casecaseElectric Field (E=V/D) is Electric Field (E=V/D) is applied across electrodesapplied across electrodesElectric Field is lowElectric Field is low

only original ion pairs only original ion pairs created by radiation are created by radiation are collectedcollected

Signal is very smallSignal is very small

Can get some energy Can get some energy informationinformation

Resolution is poor due to Resolution is poor due to statistics, electronic noise, statistics, electronic noise, and microphonicsand microphonics

Good for detecting heavy charged particles, betas

ProportionalProportional CountersCountersWire suspended in a tubeWire suspended in a tube

Can obtain much higher electric Can obtain much higher electric fieldfield

E E 1/r 1/r

Near wire, E is highNear wire, E is highElectrons are energized to the Electrons are energized to the point that they can ionize other point that they can ionize other atomsatoms

Detector signal is much larger than Detector signal is much larger than ion chamberion chamber

Can still measure energyCan still measure energy Same resolution limits as ion Same resolution limits as ion

chamberchamber

Used to detect alphas, betas, Used to detect alphas, betas, and neutronsand neutrons

ScintillatorScintillator DetectorsDetectorsVoltage is not applied to these types of detectorsVoltage is not applied to these types of detectorsRadiation interactions result in the creation of light Radiation interactions result in the creation of light photonsphotons Goal is to measure the amount of light createdGoal is to measure the amount of light created Light created is proportion to radiation energyLight created is proportion to radiation energy

To measure energy, need to convert light to To measure energy, need to convert light to electrical signalelectrical signal Photomultiplier tubePhotomultiplier tube PhotodiodePhotodiode

Two general typesTwo general types OrganicOrganic InorganicInorganic

} light electrons

OrganicOrganic ScintillatorsScintillatorsLight is generated by fluorescence of moleculesLight is generated by fluorescence of moleculesOrganic - low atomic numbers, relatively low densityOrganic - low atomic numbers, relatively low density Low detection efficiency for gamma-raysLow detection efficiency for gamma-rays

Low light yield (1000 photons/MeV) - poor signalLow light yield (1000 photons/MeV) - poor signal Light response different for different types of radiationLight response different for different types of radiation

Light is created quicklyLight is created quickly Can be used in situations where speed (ns) is necessaryCan be used in situations where speed (ns) is necessary

Can be used in both solid and liquid formCan be used in both solid and liquid form LiquidLiquid formform forfor lowlow energyenergy, , lowlow activityactivity betabeta monitoringmonitoring, ,

neutrinoneutrino detectiondetection VeryVery largelarge volumesvolumes ( (mm33))

OrganicOrganic ScintillatorsScintillators ComeCome inin ManyMany FormsForms

InorganicInorganic ScintillatorsScintillators

Generally, high atomic number and high density Generally, high atomic number and high density materialsmaterials NaI, CsI, BiGeO, Lithium glasses, ZnSNaI, CsI, BiGeO, Lithium glasses, ZnS

Light generated by electron transitions within the Light generated by electron transitions within the crystalline structure of the detectorcrystalline structure of the detector Cannot be used in liquid form!Cannot be used in liquid form!

High light yield (~60,000 photons / MeV)High light yield (~60,000 photons / MeV) light yield in inorganics is slow (light yield in inorganics is slow (s)s)

Commonly used for gamma-ray spectroscopyCommonly used for gamma-ray spectroscopy W ~ 20 eV (resolution 5% for 1 MeV W ~ 20 eV (resolution 5% for 1 MeV -ray)-ray) Neutron detection possible with some modificationNeutron detection possible with some modification

Can be made in very large volumes (100s of cmCan be made in very large volumes (100s of cm33))

Inorganic ScintillatorsInorganic Scintillators

SolidSolid StateState ( (SemiconductorSemiconductor) ) DetectorsDetectors

Radiation interactions yield electron-hole pairsRadiation interactions yield electron-hole pairs analogous to ion pairs in gas detectorsanalogous to ion pairs in gas detectors

Very low W-value (1-5 eV)Very low W-value (1-5 eV) High resolution gamma-ray spectroscopyHigh resolution gamma-ray spectroscopy

Energy resolution << 1% for 1 MeV gamma-raysEnergy resolution << 1% for 1 MeV gamma-rays

Some types must be cooled using cryogenicsSome types must be cooled using cryogenics Band structure is such that electrons can be excited Band structure is such that electrons can be excited

at thermal temperaturesat thermal temperatures

Variety of materialsVariety of materials Si, Ge, CdZnTe, HgISi, Ge, CdZnTe, HgI22, TlBr, TlBr

Sizes < 100 cmSizes < 100 cm3 3 [some even less than 1 cm[some even less than 1 cm33]] Efficiency issues for lower Z materialsEfficiency issues for lower Z materials

NaI Scintillator

Ge Detector

Ideal Detector for Detection of Radiation

Radiation Ideal Detector Thin Semiconductor Detectors

Proportional Counters Organic Scintillators

Geiger CountersProportional Counters

Inorganic ScintillatorsThick Semiconductor Detectors

neutrons Plastic ScintillatorsProportional Counters (He, BF3)

Lithium Glass Scintillators

one of the one of the oldest devicesoldest devices used to used to detectdetect and and measuremeasure ionizing (nuclear) radiation ionizing (nuclear) radiationNamed for Named for Hans GeigerHans Geiger who i who inventednvented the device the device in in 19081908, and , and Walther MüllerWalther Müller who collaborated who collaborated with Geiger in with Geiger in developingdeveloping it further in it further in 19281928one of the one of the most sensitivemost sensitive, especially for the , especially for the low low radiationradiation levels typically found in most situations. levels typically found in most situations.

Following is the Following is the assembly of the assembly of the components of GM components of GM counter:counter:Power-supplyPower-supplyGM tubeGM tubeDiscriminator Discriminator Scaler/timerScaler/timer

GM Counter AssemblyGM Counter Assembly

Variable voltage sourceVariable voltage sourceGas-filled counting chamberGas-filled counting chamberTwo coaxial electrodes well insulated from Two coaxial electrodes well insulated from each othereach otherElectron-pairs Electron-pairs produced by radiation in fill gas produced by radiation in fill gas move under influence of electric fieldmove under influence of electric field produce measurable current on electrodes, produce measurable current on electrodes, oror transformed into pulsetransformed into pulse

GM Tube In ActionGM Tube In Action

wall

fill gas

R

Output

Aor

Anode (+)

Cathode (-)

End windowOr wall

Indirect Ionization ProcessIndirect Ionization Process

wall

Incident gamma photon

e -e - e -e -

e -

e -

e -

e -

Direct Ionization ProcessDirect Ionization Processwall

Incident charged particle

e -

e -e -

e -

e -e -e -

e -

beta (β-)

Competing Processes - Competing Processes - recombinationrecombination

R

Outpute -

e -

+

+

Voltage versus Ions CollectedVoltage versus Ions Collected

Voltage

Number of Ion Pairs collected

Ionization region

Saturation Voltage

100 % of initialions are collected

Recombinationregion

The characteristics curve The characteristics curve depends on three factors depends on three factors Plateau:Plateau: the part of the the part of the curve where the number curve where the number of counts per second is of counts per second is (almost) independent of (almost) independent of the voltage.the voltage.Threshold voltage:Threshold voltage: always lies in the plateau always lies in the plateau region and is a function region and is a function of the gas pressure and of the gas pressure and the anode diameter the anode diameter Figure of merit:Figure of merit: is always is always less than 1%less than 1%

A process causing the A process causing the dischargedischarge to to terminateterminate

Two methods used for quenching are:Two methods used for quenching are:External External quenchingquenchingInternalInternal quenching quenching

Used to Used to restorerestore the counter the counter to its quiescent state after to its quiescent state after the passage of ionizing the passage of ionizing radiationradiationAn An RC circuitRC circuit is used for is used for reducing the high voltage reducing the high voltage applied to the tube, for a applied to the tube, for a fixed time after each pulse, fixed time after each pulse, to a value that is too low to to a value that is too low to support further gas support further gas multiplication multiplication The voltage must be The voltage must be reducedreduced for a for a few hundred secsfew hundred secs which which is greater than the transit is greater than the transit time of the positive ionstime of the positive ionsA counter with A counter with 98% pure98% pure argonargon is used is used

The The advantageadvantage of external quenching is of external quenching is that it gives long life time to GM tubethat it gives long life time to GM tube

The The disadvantagedisadvantage is that it has long is that it has long recovery timerecovery time

The The quenching quenching agent gas agent gas in the Geiger counter in the Geiger counter stops the flow of stops the flow of electrical currentelectrical current after a few after a few microsecondsmicroseconds.. the quenching gas is of low ionization potentialthe quenching gas is of low ionization potential (halogens or organic vapors)(halogens or organic vapors) HalogensHalogens are preferably used because it are preferably used because it increases the lifeincreases the life of GM tube of GM tube OrganicOrganic quenched tubes usually have a quenched tubes usually have a flatter flatter plateauplateau than halogen quenched tubes than halogen quenched tubes

The purpose of the quenching additive to the The purpose of the quenching additive to the gas is to effectively gas is to effectively absorb UV-photonsabsorb UV-photons emitted emitted from the electrodes when the ions produced in from the electrodes when the ions produced in the multiplication process impact on the the multiplication process impact on the electrodes. electrodes. Such photons otherwise liberate secondary Such photons otherwise liberate secondary electrons (via the photo-electric effect) which electrons (via the photo-electric effect) which may initiate the avalanche process all over may initiate the avalanche process all over again, thereby leading to again, thereby leading to catastrophic catastrophic breakdownbreakdown of the tube (i.e. a spark). of the tube (i.e. a spark).

Device serving as Device serving as pulse height selectorpulse height selector able to make able to make selection selection from from output analogical output analogical pulsespulses, , rejecting rejecting the impulses with voltage the impulses with voltage amplitude inferior to a certain amplitude inferior to a certain threshold voltagethreshold voltageThreshold voltageThreshold voltage should neither should neither too lowtoo low nor nor too too highhigh to avoid noise and data-loss respectively to avoid noise and data-loss respectively Its function is double:Its function is double:

To To eliminateeliminate the noise the noise To To provideprovide a standard shaped pulse to scaler a standard shaped pulse to scaler

Scalar:Scalar: counts the counts the number of pulsesnumber of pulsesTimer:Timer: measures the measures the length of counting length of counting timetime in a given measurement in a given measurementCollectively used to:Collectively used to:Make Make measurement of pulsesmeasurement of pulses for a preset for a preset length of time (set on timer) recording the length of time (set on timer) recording the number of counts by the scalernumber of counts by the scalerDetermine the Determine the count-ratecount-rate by measuring by measuring duration with timer for a preset number of duration with timer for a preset number of countscounts

Objective:Objective: toto extractextract thethe amplitude or timing informationamplitude or timing information thethe electrical signal is coupled to an amplifier, sent through gain electrical signal is coupled to an amplifier, sent through gain and filtering stages, and finally digitized to allow data storage and filtering stages, and finally digitized to allow data storage and analysis.and analysis.amplitude or timing information include the different amplitude or timing information include the different characteristics of the radiation, such as the characteristics of the radiation, such as the typetype, the , the intensityintensity and and energy energy of the radiation of the radiation The signal can be either processed entirely through analog The signal can be either processed entirely through analog circuit or can be converted into digital formcircuit or can be converted into digital form

The signal can be aThe signal can be a continuously varyingcontinuously varying signalsignalaa sequence of pulsessequence of pulses, occurring, occurring

periodicallyperiodicallyat known timesat known timesrandomlyrandomly

All of these affect the choice of signal All of these affect the choice of signal processing techniques.processing techniques.

First steps in signal processing:First steps in signal processing: FormationFormation of the signal in the detector of the signal in the detector (sensor)(sensor) CouplingCoupling the sensor to thethe sensor to the amplifieramplifier

Detectors use eitherDetectors use either direct detectiondirect detection oror indirect detectionindirect detection

The detector pulse has a very low amplitude & The detector pulse has a very low amplitude & time duration i.e. narrow band widthtime duration i.e. narrow band width

To extract any kind of information requires To extract any kind of information requires amplification of detector signal amplification of detector signal

PreamplifierPreamplifier is a simple and efficient amplifier is a simple and efficient amplifier directly connected to detectordirectly connected to detector

A preamplifier, in effect, acts as aA preamplifier, in effect, acts as a capacitance capacitance terminatorterminator thus preventing deterioration of thus preventing deterioration of detector.detector.Matches the high electric impedance of detector Matches the high electric impedance of detector with low impedance of the coaxial cable with low impedance of the coaxial cable connected to subsequent signal processing connected to subsequent signal processing circuitcircuitBasically plays a role as an Basically plays a role as an impedance matcherimpedance matcher between the detector and the rest of the circuitbetween the detector and the rest of the circuit

The function is to amplify the pulses from The function is to amplify the pulses from detector via a preamplifierdetector via a preamplifier

Also used to shape a pulse for further Also used to shape a pulse for further detectiondetection

High-voltage power supply typically High-voltage power supply typically provides 800 to 1,200 volts to the PMTprovides 800 to 1,200 volts to the PMT Raising voltage increases magnitude of Raising voltage increases magnitude of

voltage pulses from PMTvoltage pulses from PMT

Preamp connected to PMT using very Preamp connected to PMT using very short cableshort cable Amplifies voltage pulses to minimize distortion Amplifies voltage pulses to minimize distortion

and attenuation of signal during transmission and attenuation of signal during transmission to remainder of systemto remainder of system

SCA Contd.SCA Contd.

Amplifier further amplifies the pulses and Amplifier further amplifies the pulses and modifies their shapes – gain typically adjustablemodifies their shapes – gain typically adjustableSCA allows user to set two voltage levels, a SCA allows user to set two voltage levels, a lower level and an upper levellower level and an upper level If input pulse has voltage within this range, output If input pulse has voltage within this range, output

from SCA is a single logic pulse (fixed amplitude and from SCA is a single logic pulse (fixed amplitude and duration)duration)

Counter counts the logic pulses from the SCA Counter counts the logic pulses from the SCA for a time interval set by the timerfor a time interval set by the timer

SCA energy modesSCA energy modes

LL/UL modeLL/UL mode – one knob directly sets the lower – one knob directly sets the lower level and the other sets the upper levellevel and the other sets the upper levelWindow modeWindow mode – one knob (often labeled E) sets – one knob (often labeled E) sets the midpoint of the range of acceptable pulse the midpoint of the range of acceptable pulse heights and the other knob (often labeled heights and the other knob (often labeled E or E or window) sets a range of voltages around this window) sets a range of voltages around this value.value. Lower-level voltage is E - Lower-level voltage is E - E/2 and upper-level E/2 and upper-level

voltage is E + voltage is E + E/2E/2

An MCA system permits an energy spectrum to An MCA system permits an energy spectrum to be automatically acquired much more quickly be automatically acquired much more quickly and easily than does a SCA systemand easily than does a SCA systemThe detector, HV power supply, preamp, and The detector, HV power supply, preamp, and amplifier are the same as for SCA systemsamplifier are the same as for SCA systemsThe MCA consists of an analog-to-digital The MCA consists of an analog-to-digital converter, a memory containing many storage converter, a memory containing many storage locations called locations called channelschannels, control circuitry, a , control circuitry, a timer, and a displaytimer, and a display

Personnel Monitor DevicesPersonnel Monitor Devices

The most common monitor devices to determine the personal exposureThe most common monitor devices to determine the personal exposurehistory are: history are:

Radiation Film Badges Radiation Film Badges

Pocket Dosimeter Pocket Dosimeter

Radiation film Radiation film badges badges are composed of two pieces of film, are composed of two pieces of film, covered by light tight paper in a compact plastic container. Various covered by light tight paper in a compact plastic container. Various filters in the badge holder allow areas to be restricted to X-ray, filters in the badge holder allow areas to be restricted to X-ray, -ray, -ray, -rays only. -rays only.

Radiation causes a blackening (silver) of the film material Radiation causes a blackening (silver) of the film material (mostly a silver bromide emulsion) The sensitivity of the film material is (mostly a silver bromide emulsion) The sensitivity of the film material is limitedlimited

For For -radiation the sensitivity is in the range of 10 - 1800 mrem-radiation the sensitivity is in the range of 10 - 1800 mrem. .

For For -radiation the sensitivity is in the range of 50 - 1000 mrem-radiation the sensitivity is in the range of 50 - 1000 mrem. .

Special film material is used for neutron monitoring. Special film material is used for neutron monitoring. The badge is usually not sensitive for The badge is usually not sensitive for radiation because the radiation because the -particles are absorbed in the light-tight paper. -particles are absorbed in the light-tight paper.

Pocket dosimeterPocket dosimeter

The pocket dosimeter or pen dosimeter is a common small sized The pocket dosimeter or pen dosimeter is a common small sized ion chamber which measures the originated charge by direct collection on a ion chamber which measures the originated charge by direct collection on a quartz fiber electroscope.quartz fiber electroscope.

The U-shaped fiber is close to a U-shaped wire. If the fiber is The U-shaped fiber is close to a U-shaped wire. If the fiber is charged it will be deflected away from the wire. The position of charged it will be deflected away from the wire. The position of deflection is a measure of the accumulated radiation dose.deflection is a measure of the accumulated radiation dose.

The dosimeter records total exposure from the initial The dosimeter records total exposure from the initial charging to the time of reading. charging to the time of reading.

It is an active device as the radiation exposure can be It is an active device as the radiation exposure can be read immediately as opposed to the passive film badge which is read immediately as opposed to the passive film badge which is only read after approximately six months.only read after approximately six months.

DosimetersDosimeters, , which are also available in high or low ranges, can be in the which are also available in high or low ranges, can be in the form of a badge, pen/tube type, or even a digital readout and all form of a badge, pen/tube type, or even a digital readout and all measure exposuremeasure exposure or the total accumulated amount of radiation to which you were exposed. (The Civil or the total accumulated amount of radiation to which you were exposed. (The Civil Defense pen/tube tube would show a reading like below when looking through it.) It's Defense pen/tube tube would show a reading like below when looking through it.) It's also similar to the odometer of a car; where both measure an accumulation of units. also similar to the odometer of a car; where both measure an accumulation of units. The dosimeter will indicate a certain total number of R or mR exposure received, just The dosimeter will indicate a certain total number of R or mR exposure received, just as the car odometer will register a certain number of miles traveled. as the car odometer will register a certain number of miles traveled.

ThermoluminescenceThermoluminescence

(TL) is the ability to convert energy from (TL) is the ability to convert energy from radiation to a radiation of a different wavelength, radiation to a radiation of a different wavelength, normally in the visible light range.normally in the visible light range.Two categories Two categories Fluorescence - emission of light during or immediately Fluorescence - emission of light during or immediately

after irradiationafter irradiation Not a particularly useful reaction for TLD useNot a particularly useful reaction for TLD use Phosphorescence - emission of light after the Phosphorescence - emission of light after the

irradiation period. Delay can be seconds to months. irradiation period. Delay can be seconds to months. 

TLDs use phosphorescence to detect radiation. TLDs use phosphorescence to detect radiation. 

ThermoluminescenceThermoluminescence

Radiation moves electrons into “traps”Radiation moves electrons into “traps”Heating moves them outHeating moves them outEnergy released is proportional to Energy released is proportional to radiationradiationResponse is ~ linearResponse is ~ linearHigh energy trap data is stored in TLD for High energy trap data is stored in TLD for a long timea long time

TL ProcessTL Process

Valence Band (outermost electron shell)

Conduction Band (unfilled shell)

Phosphor atom

Incident radiation

Electron trap (meta stable

state)-

TL Process, continuedTL Process, continued

Valence Band (outermost electron shell)

Conduction Band

Phosphor atom

Thermo luminescent photon Heat Applied-

Output – Glow CurvesOutput – Glow CurvesA glow curve is obtained from heating A glow curve is obtained from heating Light output from TL is not easily interpretedLight output from TL is not easily interpretedMultiple peaks result from electrons in "shallow" traps Multiple peaks result from electrons in "shallow" traps Peak results as traps are emptied. Peak results as traps are emptied. Light output drops off as these traps are depleted. Light output drops off as these traps are depleted. Heating continuesHeating continuesElectrons in deeper traps are released. Electrons in deeper traps are released. Highest peak is typically used to calculate doseHighest peak is typically used to calculate doseArea under represents the radiation energy deposited in Area under represents the radiation energy deposited in the TLDthe TLD

Trap Depths - Equate to Long Trap Depths - Equate to Long Term Stability of InformationTerm Stability of Information

Time or temperature

TLD Reader ConstructionTLD Reader Construction

Power Supply

PMT

DC Amp

Filter

Heated Cup

TL material

To High Voltage To ground

Recorder or meter

QUESTIONS?

ANY QUESTION PLEASE?ANY QUESTION PLEASE?

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