Radiation Safety CourseRadiation Monitoring

Heath de la GirodayDispensing Chemist

Radiation Safety Officer

RADIATION MONITORING

• Dose rate instruments– Personal dosimetry TLDs– Survey meters

• Contamination instruments– Liquid scintillation counter– Survey meters

Dose Rate Instruments

• Personal dosimetry– Thermoluminescent

dosimeters (TLDs)– β, γ and x-ray– Landauer

Dose Rate Instruments

• TLD proper use and care– Wear badge inside or outside lab coat at chest

level– Wear ring on dominant hand with text facing

radioactive material• Best practice to wear on inside of gloves to match

actual exposure and prevent contamination

– Be careful not to contaminate TLDs– Return TLDs to storage box after use

Dose Rate Instruments

• Dose rate meters– Displays µSv/hr or mR/h – Can be used to calculate dose– RSO uses dose rate meters when receiving

radioisotopes to measure radiation field

Liquid Scintillation Counter

• Uses– Alpha, beta, and gamma– Only method able to detect H-3– Only detect loose surface contamination

• Extremely sensitive

Liquid Scintillation Counter

• Principles of operation– Sample added to vial of liquid scintillation cocktail– Particle or photon energy is transferred to a solvent

molecule, causing it to enter an excited state– Energy is eventually transferred to a fluor molecule– Fluor decays to ground state with emission of

photon– Light is detected by photomultiplier tube– Photomultiplier tube generates electrical signal

Liquid Scintillation Counter• LSC operation, e.g. wipe test

– Map wipe spots– Wet absorbent wipe, e.g. filter paper soaked w/ethanol– Wipe 100cm2 area– Label all vials and include a control vial– Fill vials with scintillation cocktail to cover wipes

Liquid Scintillation Counter

• Technical issues– Chemiluminescence

• Chemical reactions generate photon emissions

– Photoluminescence• Excitation of fluors by ambient like results in photon

emission

– Quenching• Molecules absorb energy, preventing photon emissions

• Give false readings

Liquid Scintillation Counter

• Heat adapting—37C for 1-2 hours– Speeds chemical reactions

• Dark adapting—Put in dark for 5-10 minutes– Stops absorption of external photons

• Quenching—address if it becomes a problem

Contamination Instruments

• Survey meters– Ludlum model 3 survey meter detector– Geiger-Mueller probe

• Thin end window or pancake

– NaI probe

Survey Meters

• Geiger-Mueller probe– Thin end window or pancake– Principles of operation

• Particle causes ionization of gas inside tube• Potential causes ion to accelerate to inner wire or

outer wall• Succession of ionizations occur• This causes meter to register a count

Survey Meters

• Geiger-Mueller probe– Use

• Nearly all beta emitters– Including beta/gamma emitters

– Not H-3 because the beta particles do not have sufficient energy to penetrate the window

• Some ‘pure’ gamma emitters– Does not detect low-energy gamma rays (I-125) efficiently

– Does not detect low-energy x-rays efficiently

– Both CPM and mR/h scales are calibrated

Survey Meters

• NaI probe– Solid scintillator– Principles of operation

• Particle or photon interacts with a NaI crystal– A solid crystal is better than a gas-filled tube at

absorbing low-energy gamma rays and x-rays (density)

• Light is emitted by crystal• Emitted light falls on photomultiplier tube• Photomultiplier tube generates a signal

Survey Probe

• NaI probe– Use

• Low-energy gamma radiation (e.g. I-125)• Low-energy x-rays (e.g., Bramsstrahlung)

– CPM scale is calibrated– Dose rate (mR/h) scale is not calibrated

Survey Meters-Ludlum Model 3

• Audio always on

• F for rapid monitoring

• S for accurate monitoring

• Always start at most sensitive

Master dial

Battery Compartment

Calibration dials (normally covered)

Probe plug

Reset

Audible/mute

Sampling rateF/S

Display

Ludlum Pre-Operation Checks

• Four Checks– Calibration

• Verify calibration from sticker• 1 year in Canada

– Battery• Rotate master knob from ‘Off’ to ‘Bat’• Verify needle is in ‘Bat Test’ region of scale

– Background count rate– Check source (if available)

Survey Meter Operation

• Scanning for contamination– Move at a steady pace and be systematic-1”/s– If a hot spot is found ,use a spiral pattern to

find the edges of a hot spot

Monitoring surfaceFinding edges of contamination

Ludlum Operation

• Contamination Check– If meter chirp rate increases above background rate,

find location of highest count rate• If necessary, slowly rotate master knob counterclockwise

until needle is on-scale• Push reset button to speed equilibration after switching to

lower sensitivity

– When site of contamination is located, flip response fast/slow switch to ‘slow’ for more accurate readings

Keep in Mind

• CPM and dose rate of survey meters is variable between radioisotopes– Detection efficiency

• Do not contaminate the probe or meter

Survey Meter Operation-Summary1. Check calibration sticker2. Ensure unit is in good condition3. Turn unit on and check battery4. Turn switch to lowest multiplication scale5. Check background6. Perform positive check7. Measure radioactivity8. Note highest count rate in CPM9. Turn unit off when finished (Be green-use less

batteries)