radiation safety instruments

RR ADIATION ADIATION

SS AFETY AFETY

II NSTRUMENTSNSTRUMENTS

Pawitra Masa-at Pawitra Masa-at 4937092 SIRS/M 4937092 SIRS/M

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Dose-rate monitors and Survey meters.Dose-rate monitors and Survey meters. Gas-Filled Detectors

Ionization chambers Proportional counter Geiger-Muller (GM) tubes

Integrated dose indicatorsIntegrated dose indicators Thermoluminescence Dosimeter (TLD) Optically Stimulated Luminescence (OSL)

Gas-Filled DetectorsGas-Filled Detectors

Gas-Filled Detectors-ComponentsGas-Filled Detectors-Components Variable voltage source Gas-filled counting chamber Two coaxial electrodes well insulated from each

other Electron-pairs

produced by radiation in fill gas move under influence of electric field produce measurable current on electrodes, or transformed into pulse

Gas-Filled DetectorsGas-Filled Detectors

wall

fill gas

R

Output

Aor

Anode (+)

Cathode (-)

End windowOr wall

I ; Ionization chamber regionP ; Proportional regionGM ; Geigur-Mueller region

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In the ionization chamber region The number of ion pairs collected by the

electrodes is equal to the number of ion pair produced by the radiation in the detector.

There is no change in the number of ion pairs collected as the voltage increase.

Ionization Ionization ChamberChamber

Ionization Ionization ChamberChamber

HV

+

-

Negative ion

Positive ion

1234

Electrometer

The response is proportional toionization rate (activity, exposure rate)

Ionization Ionization Chamber ChamberMain properties

High accuracy Stable Relatively low sensitivity Slow response (used as integrator) Wide range (uGy/hr – several thousand) Portable

Used for Monitoring instrument : Survey for radiation level >1 mR/hr Main x-ray QC. Instrument

Portable ion chamber survey instrument

Contamination Survey meter

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Proportional counterProportional counterIn the proportional region

The number of ion pairs collected is greater than the number of ion pairs produced in the detector by the radiation.

There is gas amplification.

The amplification factor at specific voltage is the same for any type of radiation or energy of radiation.

The number of ion pairs collected is proportional to the number of ion pairs originally produced.

produced pairsion

collected pairsion factor ion amplificat The =

Operates in pulse mode Most common general-purpose : 90% argon + 10%

methane

Proportional counterProportional counter

Main properties Laboratory instrument. Accuracy. A little higher sensitivity than the ion chamber. Used for particles and low energy photon.

Used for Monitoring instrument ; Assay of small quantities of

radionuclides spectrometer

Proportional counterProportional counter

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Geiger-MüllerGeiger-Müller tube tubeIn the Geiger-Muller region

Any single ionizing event will produce so many secondary ions that very large pulse is produce.

These ion pairs produce more ion pairs, until literally millions of ion pairs are produced. This effect called “Avalanching”

Because of the avalanche, It’s possible to tell that the radiation is present, but it isn’t possible to determine the type of radiation.

To stop the continual avalanche, another gas, called “Quenching gas” is mixed with argon

Knoll

-

+

-

A single incident particle cause full ionization

Geiger Müller-tube principle

Geiger Müller counter

Main properties High sensitivity. Lower accuracy Limited to <100 mR/hr Portable

Used for Contamination monitor Survey for low radiation level Dosimeter (if calibrated)

pocket dosimeter

survey meter

Area Alarm Monitor

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Thermo-luminescence Thermo-luminescence Dosimeter (TLD)Dosimeter (TLD)

ThermoluminescenceThermoluminescence (TL) is the ability to convert energy from radiation

to a radiation of a different wavelength, normally in the visible light range.

Two categories Fluorescence - emission of light during or

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

irradiation period. Delay can be seconds to months.

TLDs use phosphorescence to detect radiation.

ThermoluminescenceThermoluminescence Radiation moves electrons into “traps” Heating moves them out Energy released is proportional to radiation Response is ~ linear High energy trap data is stored in TLD for a long

time

TL ProcessTL Process

Valence Band (outermost electron shell)

Conduction Band (unfilled shell)

Phosphor atom

Incident radiation

Electron trap (metastable state)

-

TL Process, continuedTL Process, continued

Valence Band (outermost electron shell)

Conduction Band

Phosphor atom

Thermoluminescent photon - Heat Applied

Recorder or meter

Power Supply

PMT

DC Amp

Filter

Heated Cup

TL material

To High Voltage To ground

TLD Reader ConstructionTLD Reader Construction

•Output as “glow curve”

•Area under represents the radiation energy deposited in the TLD

ThermoluminescenceThermoluminescenceMain properties

High sensitivity. Accuracy Wild range (uGy-Gy) Size-so small, can be taped to finger. Versatility ; can use for long period. Reusable - economical Readout convenience - rapid (<30 sec) No wet chemical ; data won’t be lost if it wet.

Used for personnel monitoring stationary area monitoring

Thermoluminescence dosimeter : area monitoring , Personnel radiation monitoring (Courtesy Bicron.)

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Optically stimulated Optically stimulated luminescence (OSL)luminescence (OSL)

Optically stimulated luminescenceOptically stimulated luminescence (OSL)(OSL)

Minimum detectable dose 1 mRem for gamma and x-ray radiation, 10 mRem for beta radiation.

Uses thin layer of aluminum oxide Has a TL sensitivity 50 times greater than TLD-100

(LiF:Mg,Ti) Almost tissue equivalent. Strong sensitivity to light Readout stimulated using laser Intensity luminescence in proportion to radiation

dose.

( )COAl :32

Optically Stimulated Luminescence dosimeters (Courtesy Laudauer.)

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