tuukka turtiainen stuk radiation and nuclear safety · pdf filestuk—radiation and...

SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALEN

RADIATION AND NUCLEAR SAFETY AUTHORITY

Integrated radon measurements

Tuukka Turtiainen

STUK—Radiation and Nuclear Safety Authotity



Selecting measurement technique

• Selection of radon measurement technique depends on e.g.

1. Availability of equipment

2. What kind of information about radon is needed

3. Cost of measurement

4. The site of measurement

Examples:

1. An old house where exposure to radon among residents needs to be investigated

2. A tunnel construction site where exposure to workers needs to be investigated

3. An office building where exposure to radon during office hours needs to be investigated

4. Renovation site, where radon leakage from technical lead-ins needs to be measured

5. Water works, where exposure to radon among workers needs to be measured



Classification of radon measurements

Indoor air measurements Other applications

Radon PAEC

Integrated measurement: long-term average concentration

Integrated measurement: long-term average concentration

Radon in soil

Continuous measurement: temporal variation in concentration

Continuous measurement: temporal variation in concentration

Radon exhalation of building materials

Spot measurement: momentary radon concentration

Spot measurement: momentary radon concentration

Radon diffusion through materials

Thoron measurements



Integrated measurement

• Passive sampling is typically applied

– Radon gets into contact with detector by diffusion and free convection

• Open/closed configuration

• The detecor accumulates of measurable quantities (alpha tracks, electric charges, radioactive atoms) when exposed to radon

• The result is average radon concentration during the samplingperiod

• Simple to use and inexpensive

Examples:

Track etch detectors aka SSNTD (solid-state nuclear track detecors)

Electrets, which are discharged by ionization of air caused by radon

Semi conductors and related electronics

Activated charcoal traps radon atoms from air. Subsequent gamma measurement



Integrated measurements

The result is the average radon concentration during the operating (exposure) period. This information in beneficial:

• Residential houses

• Ordinary work places

• Personal radon monitors

• Other special uses (e.g. radon outdoors)



SSNTD

Source: http://www.landauernordic.com/services-and-products/radon-dosimetry/

Source: http://www.radon.fi

http://www.landauernordic.com/wp-content/uploads/radon-dosimetry-product.jpg

http://www.landauernordic.com/wp-content/uploads/radon-dosimetry-product.jpg



Electrets

Source: http://www.biomation.com/radon/s_chamber.htm



Activated charcoal trap

Source: http://kata.fi/radonbox.php



Track etch detecotors (SSNTD)

1. SSNTD

2. Support

3. Filter

4. Accumulation chamber

Source: ISO 11665-4:2012



SSNTD at STUK

1. Base2. Lid3. Mylar

and spring

4. Film

12

3

4



Track etch detectors

• In closed configuration, radon diffuses into the chamber and attains equilibrium with daughters

• Daughters are solids (deposition)

• Alpha particles have random direction theoretical geometric efficiency



Advantages of closed configuration detectors

• Diffusion into chamber is long enough that thoron does not enter the chamber, t½(Rn-220) is 55.6 s

• Radon daughters in ambient air do not irradiate detector knowing equilibrium factor is not necessary

• Other possible alpha active radionuclides in air do not affect the result

• Detector is not exposed to mechanical stress (scratches, etc)



How is it placed?

• 1 – 2 meters above the floor

• 25 cm from walls

• Away from heat sources, ventilation, doors and windows

• Away from children, pets etc.

• General rules for residential buildings:

– One for living room and one for bedroom

– One for each floor

– Normal living conditions, temperature, ventilation

– Measurements during summer months is not recommended

– Measurements in cellars, technical rooms not recommended unless for a specific reason

– Record the time of starting and ending the measurement and send the SSNTD back to the lab asap



The SSNTD

• CR39 and Makrofol™ polycarbonate films

• Alpha particle hits the film

• The energy of the alpha particle is transferred to the medium and changes its structure slightly , so called ”latent” track

• In chemical and/or electrochemical processing the ”latent track” is magnified into a visible ”etched track”

• Etched tracks can be counted with a suitable system, e.g. microscope and software.

Latent track 0.01 µm chemical etching 1–2 µm electrochemical etching 30–200 µm



Etched tracks and track density

MC simulated film: 50 tracks/cm2 MC simulated film: 450 tracks/cm2



Maginification of etched tracks

MC simulated film 450 cm–2: overlapping

Real film 450 cm–2: overlapping



Detector response is not linear

• The more and larger the tracks are recorded on the detector, the higher the probability that two separate tracks overlap increases

0

100000

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0 1000 2000 3000 4000 5000

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osu

re(B

q d

m–3

)

Track density (cm–2)

Datasovite



Maximum detectable concentration

• At a certain point, counting of etched tracks becomes difficult and detector is saturated

0

200000

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600000

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0 1000 2000 3000 4000 5000

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sovite



Minimum detectable concentration

Background

Signal

SC

Track density



Sending SSNTD back to the lab

• Some labs want the customer to seal the detector into a bag with a tape:

– Air tight or not? – air tight sealing with normal tape is difficult

– Indoor air (momentary concentration) is sealed into the bag

– The time between sealing and opening must be taken into consideration when calculating the result

– If the bag is not airtight, too much may be subtracted

• Some labs prefer sending without sealing

– A large number of unexposed detectors should be sent back from different addresses in order to assess the ”postal” background which is subtracted from the gross track density



Calibration of SSNTDs

• Calibration against primary standard / against secondary standard (STAR)

• If the response is not linear, several exposure levels must be investigated before the method is commissioned/authorized

• Every new batch of film (detector) should be calibrated (>10 films, or 1% of films) lighter procedure

• Background track density of every new batch of film should be investigated (10 films minimum)

• Postal background (if detector is not sealed air tight for the return)

• Reference (standard) condition during calibration:

– Temperature 20C (18–22 C)

– Rel. humidity 50% (40–60%)

– Atmospheric pressure 1013 hPa (860–1060 hPa)

– Ambient dose rate <0.25 µSv/h



Activated charcoal traps

reported




1. Lid to seal the coal air-tight

2. Diffusion barrier

3. Mesh to keep the coal in constant geometry

4. Coal



Indirect detection of radon

• As adsorbed radon in coal reaches equilibrium with ambient air, it also reaches equilibrium with its daughters (Po-218, Pb-214, Bi-214 and Po-214)

• Radon daughters Pb-214 and Bi-214 are gamma emitters and can be detected with e.g. gamma spectrometers (scintillation, semi-conductors) or GM-tubes

• When measurement is done, activated coal device is sealed air tight and brought to laboratory/measured on site

• It is important to measure background gamma radiation separately



Activated charcoal traps• Humidity and exposure time affects the results (weighing of carbon

trap)Source: ISO 11665-4:2012




• Radon concentration is not constant in air and activity concentration in coal follows these changes with a delay

• If there is no diffusion barrier on the coal, the final concentration in activated charcoal represents only the last day’s radon concentration

• Calibration factor depends thus on exposure time

• Calibration can be tricky: large activated charcoal boxes absorb radon ”too efficiently” resulting in declining radon concentration in calibration facility (especially if ventilation rate and the chamber volume are small)

• Calibration must be carried out with different humidities

• Calibration with different exposure times (if required)



Integrated semi conductors

• Integrated results (1 day, 2 days, 7 days, loger)

• Electromagnetic signals may interfere (cell phones)

Source: http://www.radiansa.com/detectors-radiation/radon-gas-detectors.htm



Recap: integrated measurements

• Best options for residential measurements: inexpensive, simple to use, long-term average radon concentration

• Good options for screening workplaces

• Can be used as personal radon monitors at workplaces where exposure takes place in many different locations

• All measurement methods need regular calibration checks

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