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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 1Session 1 Ionization ChambersIonization Chambers

Session II.3.1Session II.3.1

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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OverviewOverview

In this session we will discuss the principle In this session we will discuss the principle of measuring radiation using ionization of measuring radiation using ionization chambers.chambers.

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As humans, we are virtually incapable of detecting As humans, we are virtually incapable of detecting the presence of radiationthe presence of radiation

Individuals who have been exposed to high levels of Individuals who have been exposed to high levels of radiation have reported feeling “a burning radiation have reported feeling “a burning sensation” in their eyes, nausea, dizziness and sensation” in their eyes, nausea, dizziness and other symptoms, however, these symptoms have all other symptoms, however, these symptoms have all resulted from exposure to lethal or near lethal resulted from exposure to lethal or near lethal dosesdoses

Exposure to radiation doses within the occupational Exposure to radiation doses within the occupational limits produce no apparent indications of exposurelimits produce no apparent indications of exposure

As a result, we must employ devices to detect and As a result, we must employ devices to detect and measure radiationmeasure radiation

Human PerceptionHuman Perception

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Types of Interactions:Types of Interactions:

ExcitationExcitation IonizationIonization

Media in which the interactions occur:Media in which the interactions occur:

SolidSolid LiquidLiquid GasGas

Interaction with MatterInteraction with Matter

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Ion chambers are the simplest of all gas filled Ion chambers are the simplest of all gas filled detectorsdetectors

An electric field is used to collect all the An electric field is used to collect all the ionizations (positive and negative charged ionizations (positive and negative charged particles) produced by the incident radiation in particles) produced by the incident radiation in the gas volumethe gas volume

Ion ChamberIon Chamber

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W value (eV/ion pair)W value (eV/ion pair)

GasGas Fast ElectronsFast Electrons AlphasAlphas

CHCH44 30.230.2 29.029.0

OO22 32.232.2 32.232.2

HeHe 32.532.5 31.731.7

AirAir 35.035.0 35.235.2

NN22 35.835.8 36.036.0

HH22 38.038.0 37.037.0

““W” FactorW” Factor

In most gases, the In most gases, the energy needed to energy needed to ionize the least ionize the least tightly bound tightly bound electrons is 10-20 eV electrons is 10-20 eV however, the however, the “average” energy is “average” energy is higher since some higher since some energy is lost in energy is lost in “non-ionizing” “non-ionizing” encountersencounters (typically 30-35 eV/ion pair)(typically 30-35 eV/ion pair)

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The current (flow of free electrons) is directly The current (flow of free electrons) is directly related to the radiation intensityrelated to the radiation intensity

If recombination of free electrons and positively If recombination of free electrons and positively charged atoms is eliminated, then the current charged atoms is eliminated, then the current flow is a direct measure of the amount of flow is a direct measure of the amount of radiation incident on the chamberradiation incident on the chamber

Free ElectronsFree Electrons

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PULSEHEIGHT

VOLTAGE APPLIED

IION CHAMBER

REGION

IIPROPORTIONAL

REGION

IIILIMITEDPROPOR-TIONALREGION

IVG-M

REGION

VCONTINUOUSDISCHARGEREGION

(100 X B)

a

(100 X Y)

a

B

Y

VS VP VL VG VD

RecombinationRegion

Regions of a Gas Filled ChamberRegions of a Gas Filled Chamber

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RecombinationRecombination

IonizationIonizationProportionalProportional

Limited ProportionalLimited ProportionalGeiger MuellerGeiger Mueller

Continuous DischargeContinuous Discharge

Operating Regions of aOperating Regions of aGas Filled DetectorGas Filled Detector

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Free Air ChamberFree Air Chamber

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Electronic EquilibriumElectronic Equilibrium

This is what we see; This is what we see; a thin walled a thin walled

chamber full of airchamber full of air

Based on the densities of Based on the densities of the materials, this is what the materials, this is what the photon radiation sees; the photon radiation sees;

a massive block of a massive block of material surrounding a material surrounding a

small volume of airsmall volume of air

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Photon Energy Photon Energy (MeV)(MeV)

Density ThicknessDensity Thickness

(g/cm(g/cm22))

0.020.02 0.00080.0008

0.050.05 0.00420.0042

0.10.1 0.0140.014

0.20.2 0.0440.044

0.50.5 0.170.17

11 0.430.43

22 0.960.96

55 2.52.5

1010 4.94.9

Electronic EquilibriumElectronic Equilibrium

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Free Air ChamberFree Air Chamber

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Dose rate meters measure the operational Dose rate meters measure the operational quantity of ambient dose equivalent ratequantity of ambient dose equivalent rate

For most applications, this measurement gives For most applications, this measurement gives a good approximation of the effective dose rate a good approximation of the effective dose rate to our bodiesto our bodies

Ion Chamber MeasurementsIon Chamber Measurements

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Condenser R ChamberCondenser R Chamber

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Condenser R ChamberCondenser R Chamber

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Ion ChamberIon Chamber

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Ion ChamberIon Chamber

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Ion ChamberIon Chamber

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Ion ChamberIon Chamber

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Ion ChamberIon Chamber

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Pressurized Ion ChamberPressurized Ion Chamber

Model 450PModel 450Ppressurizedpressurized

Model 450Model 450non-pressurizednon-pressurized

Model PIC-6Model PIC-6pressurizedpressurized

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Extrapolation ChamberExtrapolation Chamber

BohmBohm

Model 2535Model 2535

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Extrapolation ChamberExtrapolation Chamber

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Five linear ranges: 0-5 mR/h to 0-50 R/hFive linear ranges: 0-5 mR/h to 0-50 R/hEnergy range 12 keV to 7 MeVEnergy range 12 keV to 7 MeVWeight only 1.4 kgWeight only 1.4 kg

A compact, rugged, lightweight ion A compact, rugged, lightweight ion chamber dosemeter for beta, gamma chamber dosemeter for beta, gamma and X-ray detection and and X-ray detection and measurements of superficial or deep measurements of superficial or deep doses in pulsed or static fields. doses in pulsed or static fields.

Sample Ionization ChambersSample Ionization ChambersBicron RSO 50EBicron RSO 50E

True electronic range switching is included, avoiding True electronic range switching is included, avoiding reed switches and their susceptibility to magnetic fields. reed switches and their susceptibility to magnetic fields. The 1000 mg/cmThe 1000 mg/cm22 shutter makes deep dose versus shutter makes deep dose versus shallow dose measurements routinely possible. shallow dose measurements routinely possible.

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DETECTORDETECTORFive linear ranges: 0-5, 0-50, 0-500 mR/hr; 0-5, 0-50 R/hFive linear ranges: 0-5, 0-50, 0-500 mR/hr; 0-5, 0-50 R/hAir filled ionization chamber vented to atmosphereAir filled ionization chamber vented to atmosphereDetector Volume 13.4 inDetector Volume 13.4 in33

1,000 mg/cm1,000 mg/cm22 walls wallsChamber window 7 mg/cmChamber window 7 mg/cm22

Beta shield 1,000 mg/cmBeta shield 1,000 mg/cm22

30% from 8 keV to 6 MeV30% from 8 keV to 6 MeV15% from 33 keV to 6 MeV15% from 33 keV to 6 MeVEXTERNAL CONTROLSEXTERNAL CONTROLSRotary switch for Off, Battery check, Zero, 5, 50, 500 Rotary switch for Off, Battery check, Zero, 5, 50, 500 mR/h; 5-50 R/hmR/h; 5-50 R/hZero knobZero knobPHYSICALPHYSICALSize: 20.1 x 10.7 x 19.6 cmSize: 20.1 x 10.7 x 19.6 cmWeight: 1.6 kgWeight: 1.6 kg

Sample Ionization Chambers Sample Ionization Chambers Eberline Model RO2Eberline Model RO2

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Sample Ionization Chambers Sample Ionization Chambers Eberline Model RO2Eberline Model RO2

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Sample Ionization Chambers Sample Ionization Chambers Eberline Model RO2Eberline Model RO2

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The 440RF/D is a highly sensitive, low energy, RF The 440RF/D is a highly sensitive, low energy, RF shielded survey meter suited for fast, accurate shielded survey meter suited for fast, accurate measurements of background and other low measurements of background and other low radiation levels. radiation levels.

It is the basis of measurement for radiation It is the basis of measurement for radiation exposure in the color television industry. This exposure in the color television industry. This instrument is also used to measure radiation from instrument is also used to measure radiation from radar and transmission towers where RF may be radar and transmission towers where RF may be present.present.

Internal software-controlled pressure transducer Internal software-controlled pressure transducer and temperature sensor automatically apply and temperature sensor automatically apply standard air density correction factors from 70 to standard air density correction factors from 70 to 106 kPa to the unsealed ion chamber106 kPa to the unsealed ion chamber

Sample Ionization Chambers Sample Ionization Chambers Victoreen Model 440RF/DVictoreen Model 440RF/D

Range Range ((Sv/hr)Sv/hr)

ResponseResponse(sec)(sec)

0-100-10 77

0-300-30 77

1-1001-100 55

0-3000-300 55

0-10000-1000 55

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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)