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3.8.3 Radon detectors and measurement techniques

There are three radon isotopes in nature: 222Rn (radon) in the 238U decay chain, 220Rn (thoron) in the 232Th chain, and 219Rn (actinon) in the 235U chain. 219Rn is the least abundant of these three isotopes, and because of its short half-life of 4 seconds it has the least probability of emanating into the atmosphere before decaying. 220Rn with a 55 second half-life is somewhat more mobile. 222Rn with a 3.8 d half-life is capable of migrating through several decimetres of soil or building material and reaching the atmosphere. Therefore, in most situations, 222Rn should be the predominant airborne radon isotope.

Many techniques have been developed over the years for measuring radon and radon progeny in air [Jenkins]. In addition, considerable attention is given by EPA to measurement of radon and radon progeny in homes [EPA-1992a]. Radon and radon progeny emit alpha and beta particles and gamma rays. Therefore, numerous techniques can and have been developed for measuring these radio-nuclides based on detecting alpha particles, beta particles, or gamma rays, independently or in some combination. It is even difficult to categorize the various techniques that are presently in use. This section contains an overview of information dealing with the measurement of radon and radon progeny and is not claiming 100% complete. The information is focused on the measurement of 222Rn. However, the information may be adapted for the measurement of 219Rn and 220Rn.

Two analytical end points are of interest when performing radon measurements:

  • Most commonly used is radon concentration, which is stated in terms of activity per unit volume (Bq/m2). Although this terminology is consistent with most federal guidance values, it only infers the potential dose equivalent associated with radon.
  • The second analytical end point is the radon progeny working level. Radon progeny usually attach very quickly to charged aerosols in the air following creation. The fraction that remains unattached is usually quite small (i.e., 5 – 10%). Since most aerosol particles carry an electrical charge and are relatively massive (>= 0.1 μm), they are capable of attaching to the surfaces of the lung. Essentially all dose or risk from radon is associated with alpha decays from radon progeny attached to tissues of the respiratory system. If an investigator is interested in accurately determining the potential dose or risk associated with radon in the air of a room, the radon progeny concentration must be known.

Radon progeny concentrations are usually reported in units of working levels (WL), where one working level is equal to the potential alpha energy associated with the radon progeny in secular equilibrium with 3.7 Bq/l (100 pCi/l) of radon. One working level is equivalent to 1.28 × 105 MeV/L of potential alpha energy. Given a known breathing rate and lung attachment probability, the expected mean lung dose from exposure to a known working level of radon progeny can be calculated.

Radon progeny are not usually found in secular equilibrium with radon indoors due to plating out of the charged aerosols onto walls, furniture, etc. The ratio of 222Rn progeny activity to 222Rn activity usually ranges from 0.2 to as high as 0.8 indoors. If only the 222Rn concentration is measured and it is not practical to measure the progeny concentrations, then general practice is to assume a progeny to 222Rn equilibrium ratio of 0.5 for indoor areas. This allows one to estimate the expected dose or risk associated with a given radon concentration.

In general, the following generic guidelines should be followed when performing radon measurements during site investigations:

  • The radon measurement method used should be well understood and documented;
  • Long term measurements are used to determine the true mean radon concentration;
  • The impact of variable environmental conditions (e.g., humidity, temperature, dust loading, and atmospheric pressure) on the measurement process should be accounted for when necessary. Consideration should be given to effects on both the air collection process and the counting system;
  • The background response of the detection system should be accounted for;
  • If the quantity of interest is the working level, then the radon progeny concentrations should be evaluated. If this is not practical, then the progeny activities can be estimated by assuming they are 50% of the measured radon activity.

For a general overview, a list of common radiation detectors with their usual applications during radon surveys is provided in Table 3.46. Descriptions and costs for specific equipment used for the measurement of radon are contained in Appendix C.
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System Description Application Remarks
Large area activated charcoal collector A canister containing activated charcoal is twisted into the surface and left for 24 hours.

Short term radon flux measurements. The LLD is 0.007 Bq m-2s-1.
Continuous radon monitor Air pump and scintillation cell or ionization chamber. Track the real time concentration of radon. Takes 1 to 4 hours for system to equilibrate before starting. The LLD is 0.004-0.04 Bq/l.

Activated charcoal adsorption Activated charcoal is opened to the ambient air, then gamma counted on a gamma scintillator or in a liquid scintillation counter.

Measure radon concentration in indoor air. Detector is deployed for 2 to 7 days. The LLD is 0.007-0.04 Bq/l.
Electret ion chamber This is a charged plastic vessel that can be opened for air to pass through. Measure short term or long term radon concentration in indoor air. Must correct reading for gamma background concentration. Electret is sensitive to extremes of temperature and humidity. LLD is 0.007-0.02 Bq/l.

Alpha track detection A small piece of special plastic or film inside a small container. Damage tracks from alpha particles are chemically etched and tracks counted.

Measure indoor or outdoor radon concentration in air. LLD is 0.04 Bq-ld-1.

Table 3.46 Radiation detectors with applications to radon surveys.
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