6. Radiation isRadiation is thethe emission and transmissionemission and transmission
ofof energyenergy through space or through athrough space or through a
material medium.material medium.
Radiation can be in the form ofRadiation can be in the form of sub-atomicsub-atomic
particles (protons, neutrons and electrons)particles (protons, neutrons and electrons)
oror electromagnetic waveselectromagnetic waves
7. • IonizingIonizing RadiationRadiation ((IRIR)) causes ions to becauses ions to be
produced when radiation is absorbed inproduced when radiation is absorbed in
mattermatter
• Non-IonizingNon-Ionizing radiationradiation ((NIRNIR)) refers torefers to
radiation energy that, instead of producingradiation energy that, instead of producing
charged ions, when passing throughcharged ions, when passing through
matter, has sufficient energy only formatter, has sufficient energy only for
excitation.excitation.
10. Why We MeasureWhy We Measure
Personal Dosimetry vs RegulatoryPersonal Dosimetry vs Regulatory
requirementsrequirements
Classification of safe and dangerousClassification of safe and dangerous
zoneszones
Safety vs Protective measuresSafety vs Protective measures
Quality control and Quality assuranceQuality control and Quality assurance
Data consistancy and standardizationData consistancy and standardization
11. Importance of Radiation in DailyImportance of Radiation in Daily
LifeLife
MedicineMedicine:: RadiologyRadiology,, RadiationRadiation TherapyTherapy
PowerPower Production and Space ExplorationProduction and Space Exploration
In Agriculture to improve variety & yieldIn Agriculture to improve variety & yield
productionproduction
Weapons of mass destructionWeapons of mass destruction
Industrial applications: Gauges,Industrial applications: Gauges,
Radiography, Mineral explorationRadiography, Mineral exploration
13. 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
14. • 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
16. HowHow aa RadiationRadiation DetectorDetector WorksWorks
TheThe radiationradiation wewe areare interestedinterested inin detectingdetecting allall interactinteract
withwith materialsmaterials byby ionizingionizing atomsatoms
While it is difficult (sometime impossible) to directlyWhile it is difficult (sometime impossible) to directly
detect radiation, it is relatively easy to detectdetect radiation, it is relatively easy to detect
(measure) the ionization of atoms in the detector(measure) the ionization of atoms in the detector
material.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 manyWe can measure or detect these interactions in many
different ways to get a multitude of informationdifferent ways to get a multitude of information
17. GeneralGeneral DetectorDetector PropertiesProperties
Characteristics of an “ideal” radiation detectorCharacteristics of an “ideal” radiation detector
High probability that radiation will interact with theHigh probability that radiation will interact with the
detector materialdetector material
Large amount of charge created in the interactionLarge amount of charge created in the interaction
processprocess
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 smallInitial Generated charge in detector (Q) is very small
(e.g., 10(e.g., 10-13-13
C)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
C
Q
V =
18. Detection andDetection and
measurement includesmeasurement includes
the followingthe following
components:components:
DetectorDetector
PreamplifierPreamplifier
AmplifierAmplifier
Single channel analyzerSingle channel analyzer
Multi-channel analyzerMulti-channel analyzer
Scalar-TimerScalar-Timer
19. a detectora detector produces a signalproduces a signal for every particlefor every particle
entering in it.entering in it.
Every detector works by using someEvery detector works by using some interactioninteraction
of particles with matter.of particles with matter.
Use characteristic effects from interaction ofUse characteristic effects from interaction of
radiation with matter to detect, identify and/orradiation with matter to detect, identify and/or
measure properties of radiation.measure properties of radiation.
Respond to radiation by producing variousRespond to radiation by producing various
physical effectsphysical effects
20. Detection ProcessesDetection Processes
Ionization: Gas/Liquid chambers andIonization: Gas/Liquid chambers and
Semiconductor DetectorsSemiconductor Detectors
Scintillation: Scintillation counters and TLDsScintillation: Scintillation counters and TLDs
Sparking: Sparking chamberSparking: Sparking chamber
Blackening of photographic film: NuclearBlackening of photographic film: Nuclear
Emulsion detector and Film dosimetryEmulsion detector and Film dosimetry
Bubbling/Clouding of supersaturatedBubbling/Clouding of supersaturated
liquids/vapors: Cloud Chambers and Bubbleliquids/vapors: Cloud Chambers and Bubble
ChambersChambers
22. Following is the list of most common types ofFollowing is the list of most common types of
detectors:detectors:
Gas-filled countersGas-filled counters
ionization chamberionization chamber
Proportional CounterProportional Counter
Geiger-Muller counterGeiger-Muller counter
Scintillation detectorsScintillation detectors
Semiconductor detectorsSemiconductor detectors
23. GasGas DetectorsDetectors
Most 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 container
Can be made in very large volumes (mCan be made in very large volumes (m33
))
Mainly used to detectMainly used to detect ββ-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
γγ-rays are virtually invisible-rays are virtually invisible
24. IonizationIonization ChambersChambers
Two electric platesTwo electric plates
surrounded by a metalsurrounded by a metal
casecase
Electric Field (E=V/D) isElectric Field (E=V/D) is
applied acrossapplied across
electrodeselectrodes
Electric Field is lowElectric Field is low
only original ion pairsonly original ion pairs
created by radiation arecreated by radiation are
collectedcollected
Signal is very smallSignal is very small
Can get some energyCan get some energy
informationinformation
Resolution is poor due toResolution is poor due to
statistics, electronic noise,statistics, electronic noise,
and microphonicsand microphonics
Good for detecting heavy charged
particles, betas
25. ProportionalProportional CountersCounters
Wire suspended in a tubeWire suspended in a tube
Can obtain much higher electricCan obtain much higher electric
fieldfield
EE αα 1/r1/r
Near wire, E is highNear wire, E is high
Electrons are energized to theElectrons are energized to the
point that they can ionizepoint that they can ionize
other atomsother atoms
Detector signal is much largerDetector signal is much larger
than ion chamberthan ion chamber
Can still measure energyCan still measure energy
Same resolution limits as ionSame resolution limits as ion
chamberchamber
Used to detect alphas, betas,Used to detect alphas, betas,
and neutronsand neutrons
26. ScintillatorScintillator DetectorsDetectors
Voltage is not applied to these types ofVoltage is not applied to these types of
detectorsdetectors
Radiation interactions result in the creation ofRadiation interactions result in the creation of
light photonslight photons
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 toTo measure energy, need to convert light to
electrical signalelectrical signal
Photomultiplier tubePhotomultiplier tube
PhotodiodePhotodiode
Two general typesTwo general types
OrganicOrganic
InorganicInorganic
} light → electrons
27. OrganicOrganic ScintillatorsScintillators
Light is generated by fluorescence of moleculesLight is generated by fluorescence of molecules
Organic - 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
))
29. InorganicInorganic ScintillatorsScintillators
Generally, high atomic number and highGenerally, high atomic number and high
density materialsdensity materials
NaI, CsI, BiGeO, Lithium glasses, ZnSNaI, CsI, BiGeO, Lithium glasses, ZnS
Light generated by electron transitions withinLight generated by electron transitions within
the crystalline structure of the detectorthe crystalline 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 (µµs)s)
Commonly used for gamma-ray spectroscopyCommonly used for gamma-ray spectroscopy
W ~ 20 eV (resolution 5% for 1 MeVW ~ 20 eV (resolution 5% for 1 MeV γγ-ray)-ray)
Neutron detection possible with some modificationNeutron detection possible with some modification
Can be made in very large volumes (100s ofCan be made in very large volumes (100s of
33
31. 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 excitedBand 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 cm33
[some even less than 1 cm[some even less than 1 cm33
]]
Efficiency issues for lower Z materialsEfficiency issues for lower Z materials
34. one of theone of the oldest devicesoldest devices used toused to detectdetect andand
measuremeasure ionizing (nuclear) radiationionizing (nuclear) radiation
Named forNamed for Hans GeigerHans Geiger who iwho inventednvented the devicethe device
inin 19081908, and, and Walther MüllerWalther Müller who collaboratedwho collaborated
with Geiger inwith Geiger in developingdeveloping it further init further in 19281928
one of theone of the most sensitivemost sensitive, especially for the, especially for the lowlow
radiationradiation levels typically found in most situations.levels typically found in most situations.
35. Following is theFollowing is the
assembly of theassembly of the
components of GMcomponents of GM
counter:counter:
Power-supplyPower-supply
GM tubeGM tube
DiscriminatorDiscriminator
Scaler/timerScaler/timer
36. GM Counter AssemblyGM Counter Assembly
Variable voltage sourceVariable voltage source
Gas-filled counting chamberGas-filled counting chamber
Two coaxial electrodes well insulated fromTwo coaxial electrodes well insulated from
each othereach other
Electron-pairsElectron-pairs
produced by radiation in fill gasproduced 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
37. GM Tube In ActionGM Tube In Action
wall
fill gas
R
Output
Aor
Anode (+)
Cathode (-)
End
window
Or wall
41. Voltage versus Ions CollectedVoltage versus Ions Collected
Voltage
Number
of Ion
Pairs
collected
Ionization region
Saturation Voltage
100 % of initial
ions are collected
Recombination
region
42. The characteristics curveThe characteristics curve
depends on three factorsdepends on three factors
Plateau:Plateau: the part of thethe part of the
curve where the numbercurve where the number
of counts per second isof counts per second is
(almost) independent of(almost) independent of
the voltage.the voltage.
Threshold voltage:Threshold voltage:
always lies in the plateaualways lies in the plateau
region and is a functionregion and is a function
of the gas pressure andof the gas pressure and
the anode diameterthe anode diameter
Figure of merit:Figure of merit: is alwaysis always
less than 1%less than 1%
43. A process causing theA process causing the dischargedischarge toto
terminateterminate
Two methods used for quenching are:Two methods used for quenching are:
ExternalExternal quenchingquenching
InternalInternal quenchingquenching
44. Used toUsed to restorerestore the counterthe counter
to its quiescent state afterto its quiescent state after
the passage of ionizingthe passage of ionizing
radiationradiation
AnAn RC circuitRC circuit is used foris used for
reducing the high voltagereducing the high voltage
applied to the tube, for aapplied to the tube, for a
fixed time after each pulse,fixed time after each pulse,
to a value that is too low toto a value that is too low to
support further gassupport further gas
multiplicationmultiplication
The voltage must beThe voltage must be reducedreduced
for afor a few hundred secsfew hundred secs whichwhich
is greater than the transitis greater than the transit
time of the positive ionstime of the positive ions
A counter withA counter with 98% pure98% pure
argonargon is usedis used
45. TheThe advantageadvantage of external quenching isof external quenching is
that it gives long life time to GM tubethat it gives long life time to GM tube
TheThe disadvantagedisadvantage is that it has longis that it has long
recovery timerecovery time
46. TheThe quenchingquenching agent gasagent gas in the Geiger counterin the Geiger counter
stops the flow ofstops the flow of electrical currentelectrical current after a fewafter 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 itare preferably used because it
increases the lifeincreases the life of GM tubeof GM tube
OrganicOrganic quenched tubes usually have aquenched tubes usually have a flatterflatter
plateauplateau than halogen quenched tubesthan halogen quenched tubes
47. The purpose of the quenching additive to theThe purpose of the quenching additive to the
gas is to effectivelygas is to effectively absorb UV-photonsabsorb UV-photons emittedemitted
from the electrodes when the ions produced infrom the electrodes when the ions produced in
the multiplication process impact on thethe multiplication process impact on the
electrodes.electrodes.
Such photons otherwise liberate secondarySuch photons otherwise liberate secondary
electrons (via the photo-electric effect) whichelectrons (via the photo-electric effect) which
may initiate the avalanche process all overmay initiate the avalanche process all over
again, thereby leading toagain, thereby leading to catastrophiccatastrophic
breakdownbreakdown of the tube (i.e. a spark).of the tube (i.e. a spark).
48. Device serving asDevice serving as pulse height selectorpulse height selector
able to makeable to make selectionselection fromfrom output analogicaloutput analogical
pulsespulses,, rejectingrejecting the impulses with voltagethe impulses with voltage
amplitude inferior to a certainamplitude inferior to a certain threshold voltagethreshold voltage
Threshold voltageThreshold voltage should neithershould neither too lowtoo low nornor tootoo
highhigh to avoid noise and data-loss respectivelyto avoid noise and data-loss respectively
Its function is double:Its function is double:
ToTo eliminateeliminate the noisethe noise
ToTo provideprovide a standard shaped pulse to scalera standard shaped pulse to scaler
49.
50. Scalar:Scalar: counts thecounts the number of pulsesnumber of pulses
Timer:Timer: measures themeasures the length of countinglength of counting
timetime in a given measurementin a given measurement
Collectively used to:Collectively used to:
MakeMake measurement of pulsesmeasurement of pulses for a presetfor a preset
length of time (set on timer) recording thelength of time (set on timer) recording the
number of counts by the scalernumber of counts by the scaler
Determine theDetermine the count-ratecount-rate by measuringby measuring
duration with timer for a preset number ofduration with timer for a preset number of
countscounts
51. Objective:Objective: toto extractextract thethe amplitude or timing informationamplitude or timing information thethe
electrical signal is coupled to an amplifier, sent through gainelectrical signal is coupled to an amplifier, sent through gain
and filtering stages, and finally digitized to allow data storageand filtering stages, and finally digitized to allow data storage
and analysis.and analysis.
amplitude or timing information include the differentamplitude or timing information include the different
characteristics of the radiation, such as thecharacteristics of the radiation, such as the typetype, the, the intensityintensity
andand energyenergy of the radiationof the radiation
The signal can be either processed entirely through analogThe signal can be either processed entirely through analog
circuit or can be converted into digital formcircuit or can be converted into digital form
52. The signal can be aThe signal can be a continuously varyingcontinuously varying
signalsignal
aa sequence of pulsessequence of pulses, occurring, occurring
periodicallyperiodically
at known timesat known times
randomlyrandomly
All of these affect the choice of signalAll of these affect the choice of signal
processing techniques.processing techniques.
53. First steps in signal processing:First steps in signal processing:
FormationFormation of the signal in the detectorof 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
54. 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 requiresTo extract any kind of information requires
amplification of detector signalamplification of detector signal
PreamplifierPreamplifier is a simple and efficient amplifieris a simple and efficient amplifier
directly connected to detectordirectly connected to detector
55. A preamplifier, in effect, acts as aA preamplifier, in effect, acts as a capacitancecapacitance
terminatorterminator thus preventing deterioration ofthus preventing deterioration of
detector.detector.
Matches the high electric impedance of detectorMatches the high electric impedance of detector
with low impedance of the coaxial cablewith low impedance of the coaxial cable
connected to subsequent signal processingconnected to subsequent signal processing
circuitcircuit
Basically plays a role as anBasically 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
56. The function is to amplify the pulses fromThe function is to amplify the pulses from
detector via a preamplifierdetector via a preamplifier
Also used to shape a pulse for furtherAlso used to shape a pulse for further
detectiondetection
57. High-voltage power supply typicallyHigh-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 ofRaising voltage increases magnitude of
voltage pulses from PMTvoltage pulses from PMT
Preamp connected to PMT using veryPreamp connected to PMT using very
short cableshort cable
Amplifies voltage pulses to minimize distortionAmplifies voltage pulses to minimize distortion
and attenuation of signal during transmissionand attenuation of signal during transmission
to remainder of systemto remainder of system
58.
59. SCA Contd.SCA Contd.
Amplifier further amplifies the pulses andAmplifier further amplifies the pulses and
modifies their shapes – gain typically adjustablemodifies their shapes – gain typically adjustable
SCA allows user to set two voltage levels, aSCA 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, outputIf input pulse has voltage within this range, output
from SCA is a single logic pulse (fixed amplitude andfrom SCA is a single logic pulse (fixed amplitude and
duration)duration)
Counter counts the logic pulses from the SCACounter counts the logic pulses from the SCA
for a time interval set by the timerfor a time interval set by the timer
60.
61. 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 level
Window modeWindow mode – one knob (often labeled E) sets– one knob (often labeled E) sets
the midpoint of the range of acceptable pulsethe midpoint of the range of acceptable pulse
heights and the other knob (often labeledheights and the other knob (often labeled ∆∆E orE or
window) sets a range of voltages around thiswindow) sets a range of voltages around this
value.value.
Lower-level voltage is E -Lower-level voltage is E - ∆∆E/2 and upper-levelE/2 and upper-level
voltage is E +voltage is E + ∆∆E/2E/2
62.
63. An MCA system permits an energy spectrum toAn MCA system permits an energy spectrum to
be automatically acquired much more quicklybe automatically acquired much more quickly
and easily than does a SCA systemand easily than does a SCA system
The detector, HV power supply, preamp, andThe detector, HV power supply, preamp, and
amplifier are the same as for SCA systemsamplifier are the same as for SCA systems
The MCA consists of an analog-to-digitalThe MCA consists of an analog-to-digital
converter, a memory containing many storageconverter, a memory containing many storage
locations calledlocations called channelschannels, control circuitry, a, control circuitry, a
timer, and a displaytimer, and a display
64.
65.
66.
67. Personnel Monitor DevicesPersonnel Monitor Devices
The most common monitor devices to determine the personal exposureThe most common monitor devices to determine the personal exposure
history are:history are:
Radiation Film BadgesRadiation Film Badges
Pocket DosimeterPocket Dosimeter
68. Radiation filmRadiation film badgesbadges are composed of two pieces of film,are composed of two pieces of film,
covered by light tight paper in a compact plastic container. Variouscovered 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, γγ-ray,-ray, ββ--
rays only.rays only.
Radiation causes a blackening (silver) of the film materialRadiation 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
ForFor γγ-radiation the sensitivity is in the range of 10 - 1800 mrem-radiation the sensitivity is in the range of 10 - 1800 mrem..
ForFor ββ-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 forThe badge is usually not sensitive for αα radiation because theradiation because the
αα-particles are absorbed in the light-tight paper.-particles are absorbed in the light-tight paper.
69. Pocket dosimeterPocket dosimeter
The pocket dosimeter or pen dosimeter is a common small sizedThe pocket dosimeter or pen dosimeter is a common small sized
ion chamber which measures the originated charge by direct collection on aion 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 isThe 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 ofcharged 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.
70. The dosimeter records total exposure from the initialThe 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 beIt is an active device as the radiation exposure can be
read immediately as opposed to the passive film badge which isread immediately as opposed to the passive film badge which is
only read after approximately six months.only read after approximately six months.
71. DosimetersDosimeters,, which are also available in high or low ranges, can be in thewhich 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 allform 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 Civilor 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'sDefense 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, justThe 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.
72. 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 categoriesTwo categories
Fluorescence - emission of light during or immediatelyFluorescence - 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 thePhosphorescence - 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.
73. ThermoluminescenceThermoluminescence
Radiation moves electrons into “traps”Radiation moves electrons into “traps”
Heating moves them outHeating moves them out
Energy released is proportional toEnergy released is proportional to
radiationradiation
Response is ~ linearResponse is ~ linear
High energy trap data is stored in TLD forHigh energy trap data is stored in TLD for
a long timea long time
74. TL ProcessTL Process
Valence Band (outermost electron shell)
Conduction Band (unfilled shell)
Phosphor atom
Incident
radiation
Electron trap
(meta stable state)
-
75. TL Process, continuedTL Process, continued
Valence Band (outermost electron shell)
Conduction Band
Phosphor atom
Thermo luminescent
photon Heat Applied-
76. Output – Glow CurvesOutput – Glow Curves
A glow curve is obtained from heatingA glow curve is obtained from heating
Light output from TL is not easily interpretedLight output from TL is not easily interpreted
Multiple peaks result from electrons in "shallow" trapsMultiple 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 continues
Electrons 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 dose
Area under represents the radiation energy deposited inArea under represents the radiation energy deposited in
the TLDthe TLD
77. Trap Depths - Equate to LongTrap Depths - Equate to Long
Term Stability of InformationTerm Stability of Information
Time or temperature
78. TLD Reader ConstructionTLD Reader Construction
Power Supply
PMT
DC Amp
Filter
Heated Cup
TL material
To High
Voltage To ground
Recorder or meter
