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C.4.4 Radon detectors and measurement techniques

The following radon detectors and measurement techniques are described:

  • Activated charcoal adsorption;
  • Alpha track detector;
  • Continuous radon monitor;
  • Electret ion chamber;
  • Large area activated charcoal collector;

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System: ACTIVATED CHARCOAL ADSORPTION
Lab/Field: Field
Radiation detected
Primary Radon gas
Secondary None
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Applicability to site surveys: Activated charcoal adsorption is a passive low cost screening method for measuring indoor air radon concentration. The charcoal adsorption method is not designed for outdoor measurements. For contaminated structures, charcoal is a good short-term indicator of radon contamination. Vendors provide measurement services which includes the detector and subsequent readout.
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Operation: For this method, an airtight container with activated charcoal is opened in the area to be sampled and radon in the air adsorbs onto the charcoal. The detector, depending on its design, is deployed for 2 to 7 days. At the end of the sampling period, the container is sealed and sent to a laboratory for analysis. Proper deployment and analysis will yield accurate results.
Two analysis methods are commonly used in activated charcoal adsorption. The first method calculates the radon concentration based on the gamma decay from the radon progeny analyzed on a gamma scintillation or semiconductor detection system. The second method is liquid scintillation which employs a small vial containing activated charcoal for sampling. After exposure, scintillation fluid is added to the vial and the radon concentration is determined by the alpha and beta decay of the radon and progeny when counted in a liquid scintillation spectrometer.
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Specificity/sensitivity: Charcoal absorbers are designed to measure radon concentrations in indoor air. Some charcoal absorbers are sensitive to drafts, temperature and humidity. However, the use of a diffusion barrier over the charcoal reduces these effects. The minimum detectable concentration for this method ranges from 0.007-0.04 Bq/l (0.2-1.0 pCi/l).
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Cost of equipment $10,000 for a liquid scintillation counter, $10,000 for a sodium iodide multi-channel analyzer system, or $30,000+ for a germanium multi-channel analyzer system. The cost of the activated charcoal itself is minimal (year 2002).
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Cost per measurement $5 to $30 including canister (year 2002).

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System: ALPHA TRACK DETECTOR
Lab/Field: Field
Radiation detected
Primary Radon Gas (Alpha Particles)
Secondary None
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Applicability to site surveys: An alpha track detector is a passive, low cost, long term method used for measuring radon. Alpha track detectors can be used for site assessments both indoors and outdoors (with adequate protection from the elements).
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Operation: Alpha track detectors employ a small piece of special plastic or film inside a small container. Air being tested diffuses through a filtering mechanism into the container. When alpha particles from the decay of radon and its progeny strike the detector, they cause damage tracks. At the end of exposure, the container is sealed and returned to the laboratory for analysis.
The plastic or film detector is chemically treated to amplify the damage tracks and then the number of tracks over a predetermined area are counted using a microscope, optical reader, or spark counter. The radon concentration is determined by the number of tracks per unit area. Detectors are usually exposed for 3 to 12 months, although shorter time frames may be used when measuring high radon concentrations.
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Specificity/sensitivity: Alpha track detectors are primarily used for indoor air measurements but specially designed detectors are available for outdoor measurements. Alpha track results are usually expressed as the radon concentration over the exposure period (Bq/l-days). The sensitivity is a function of detector design and exposure duration, and is on the order of 0.04 Bq/l-day (1 pCi/l-day).
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Cost of equipment Not applicable when provided by a vendor (year 2002).
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Cost per measurement $5 to $25 (year 2002).

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System: CONTINUOUS RADON MONITOR
Lab/Field: Field
Radiation detected
Primary Radon gas
Secondary None
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Applicability to site surveys: Continuous radon monitors are devices that measure and record real-time measurements of radon gas or variations in radon concentration on an hourly basis. Since continuous monitors display real-time hourly radon measurements, they are useful for short-term site investigation.
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Operation: Continuous radon monitors are precision devices that track and record real-time measurements and variations in radon gas concentration on an hourly basis. Air either diffuses or is pumped into a counting chamber. The counting chamber is typically a scintillation cell or ionization chamber. Using a calibration factor, the counts are processed electronically, and radon concentrations for predetermined intervals are stored in memory or directly transmitted to a printer.
Most continuous monitors are used for a relatively short measurement period, usually 1 to 7 days. These devices do require some operator skills and often have a ramp-up period to equilibrate with the surrounding atmosphere. This ramp-up time can range from 1 to 4 hours depending on the size of the counting chamber and rate of air movement into the chamber.
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Specificity/sensitivity: Most continuous monitors are designed for both indoor and outdoor radon measurements. The limiting factor for outdoor usage is the need for electrical power. In locations where external power is unavailable, the available operating time depends on the battery lifetime of the monitor. The minimum detectable concentration for these detectors ranges from 0.004-0.04 Bq/l (0.1-1.0 pCi/l).
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Cost of equipment $1,000 to $5,000 (year 2002).
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Cost per measurement $80 + based on duration of survey (year 2002).

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System: ELECTRET ION CHAMBER
Lab/Field: Field
Radiation detected
Primary Radon gas (alpha, beta)
Secondary Gamma
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Applicability to site surveys: Electrets are used to measure radon concentration in indoor environments. For contaminated structures, the electret ion chamber is a good indicator of short-term and long-term radon concentrations.
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Operation: For this method, an electro-statically charged disk (electret) is situated within a small container (ion chamber). During the measurement period, radon diffuses through a filter into the ion chamber, where the ionization produced by the decay of radon and its progeny reduces the charge on the electret. A calibration factor relates the voltage drop, due to the charge reduction, to the radon concentration. Variations in electret design enable the detector to make long term or short term measurements. Short term detectors are deployed for 2 to 7 days, whereas long term detectors may be deployed from 1 to 12 months.
Electrets are relatively inexpensive, passive, and can be used several times before discarding or recharging, except in areas of extreme radon concentrations. These detectors need to be corrected for the background gamma radiation during exposure since this ionization also discharges the electret.
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Specificity/sensitivity: Electrets are designed to make radon measurements primarily in indoor environments. Care must be taken to measure the background gamma radiation at the site during the exposure period. Extreme temperatures and humidity encountered outdoors may affect electret voltage. The minimum detectable concentration ranges from 0.007-0.02 Bq/l (0.2 to 0.5 pCi/l).
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Cost of equipment Included in rental price (year 2002).
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Cost per measurement $8 to $25 rental for an electret supplied by a vendor (year 2002).

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System: LARGE AREA ACTIVATED CHARCOAL COLLECTOR
Lab/Field: Field
Radiation detected
Primary Radon gas
Secondary None
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Applicability to site surveys: This method is used to make radon flux measurements (the surface emanation rate of radon gas) and involves the adsorption of radon on activated carbon in a large area collector.
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Operation: The collector consists of a 10 inch diameter PVC end cap, spacer pads, charcoal distribution grid, retainer pad with screen, and a steel retainer spring. Between 170 and 200 grams of activated charcoal is spread in the distribution grid and held in place by the retainer pad and spring.
The collector is deployed by firmly twisting the end cap into the surface of the material to be measured. After 24 hours of exposure, the activated charcoal is removed and transferred to plastic containers. The amount of radon adsorbed on the activated charcoal is determined by gamma spectroscopy. This data is used to calculate the radon flux in units of Bq m-2s-1.
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Specificity/sensitivity: These collectors give an accurate short-term assessment of the radon gas surface emanation rate from a material. The minimum detectable concentration of this method is 0.007 Bq m-2s-1. (0.2 pCi m-2s-1.).
Exposures greater than 24 hours are not recommended due to atmospheric and surface moisture and temperature extremes which may affect charcoal efficiency.
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Cost of equipment Not applicable (year 2002).
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Cost per measurement $20 – $50 including canister (year 2002).