How do you measure atmospheric radon?
Before we get into specific techniques, let’s first talk about some measurement units. One that is commonly used is the Working Level, or “WL”.
What is a WL?
It is the energy associated with the decay of radon daughters (progeny) in air.
How is the working level for radon daughters derived?
First, the number of atoms of each of the radon daughters in one liter of air is determined. Each atom is then allowed to undergo “complete” decay and the energy from all the alpha particles released from these decays is added. Dividing this total by 1.3E5 (130,000) MeV gives you an answer in units of WL..
What are some typical radon concentrations in units of WL?
For radon daughters, examples of typical working level concentrations are 0.0016 WL (outdoors), 0.008 WL (indoors, basement), and 0.004 WL (indoors, first floor).
What is the relationship between WL and picocuries per liter (pCi/l)?
Four (4) pCi/l of radon is equivalent to 0.02 WL. A continuous exposure to 0.02 WL is equivalent to a yearly cumulative exposure of approximately one (1) Working Level Month (WLM) , which is derived by multiplying 0.02 WL by 24 hours/day by 365 days/year and dividing this value by 170 hours in a working month. This means that the risks expressed as fatal cancers per WLM/year of continuous exposure are, in fact, the risks associated with 4 pCi/l, the EPA guideline.
Can the working level concept be applied to the thoron daughters?
Yes. The working level concept is applicable to the four (4) thoron daughters, Po-216 , Pb- 212, Bi-212and Po-212. That is, one (1) WL is that combination of thoron daughters in one (1) liter of air which will result in the emission of 1.3E5 MeV of alpha particle energy.
Okay, now that I have been introduced to the measurement units, how do I make measurements of radon concentration or WL in my home?
First, it is important to note that radon levels in in every home should first be determined by at least one short- term measurement.
What does the short term measurement consist of?
A short term test is performed (with a duration of 2 to 90 days) to quickly identify homes/buildings in which high radon levels may exist. Short term measurements showing less than 4 pCi/l (0.02 WL) indicate that the annual average is probably less than the guideline. Measurements above 4 pCi/l indicate that a follow-up measurement is required. Devices used for short-term measurements include charcoal canisters, electrets, alpha track detectors, and continuous monitors.
What other guidance do you have for short-term measurements?
It is recommended that a short term measurement of only two (2) to three (3) days be made under “closed house” conditions. These conditions should be established for at least twelve (12) hours prior to the start of the measurement and maintained throughout the course of the experiment. This requires that doors and windows be kept closed as much as possible and that systems such as fans that exchange indoor and outdoor air not be operated.
It sounds like winter is the most appropriate time of the year for screening measurements.
You’re absolutely correct. Due to the affect of weather on emanation rates, it is important to avoid short term measurements of two (2) to three (3) days during or near periods of unusual weather conditions, e.g. storms or unusually high winds. The “closed house” conditions are used to generate stable radon levels over the measurement period, meaning that the measured radon concentrations will be at their highest.
Where should the measurements be taken?
The measurements should be made in the lowest lived-in area of the building (closest to the underlying soil) in a room that is used regularly (not the kitchen or bathroom) or potentially habitable. Measurement devices should be placed approximately twenty (20) inches above the floor and away from the sources of excessive heat or humidity, unusual air flow patterns, and outside walls. This also means to keep the measuring devices away from fans, doorways, vents, and the like whenever possible. However, usually compromises are required
What about long-term measurements?
It is usually recommended that any followup measurements be for dirations of at least 90 days.
Why so long?
There is evidence that radon levels measured over a few days (eg. charcoal canister) under closed house conditions are likely to be two (2) to five (5) times the actual annual average values. The magnitude of the over-estimate increases as the measured value increases. In other words, it is quite possible that the annual average does not exceed four (4) pCi/l even though the short-term measurement is above twenty (20) pCi/l.
How are measurement and detection techniques classified?
Measurement techniques can be divided into grab sampling, integrating or continuous measurement techniques. Alternatively, they can simply be divided into those that measure radon and those that measure the daughters.
Let’s start with grab samples. What are those?
Grab samples are samples taken over a very short time period. This could include samples collected over an hour as well as those collected instantaneously.
Why do grab sampling?
In general, grab samples are simple measurements to perform and they require fairly inexpensive equipment. They are therefore useful when you have a lot of measurements to make in a short period of time.
Are there any problems with grab sampling that I should be aware of?
Grab samples are not suitable for measuring long term average concentrations unless the concentration over that time period is known to be constant. This is rarely the case for either radon or radon daughters, which can exhibit pretty large spatial and temporal variations.
Can you provide some examples of grab sampling devices?
Yes. Examples include the Kusnetz and Tsivoglou methods, Lucas cells (scintillation chambers) and alpha spectrometry systems.
What about long-term? Is that when I might use a continuous sampler?
Yes. These devices perform a continuous collection and concurrent analysis. The data, therefore, is provided in real time. This often means that a concentration is calculated each hour or so.
What advantages are associated with these devices?
The main advantages of these instruments include the instantaneous readout and the information they provide regarding short term variations in the radon and radon daughter levels.
What disadvantages are there?
Continuous samplers are somewhat expensive and not generally suitable for long term use in a single location.
Can you provide some examples of continuous samplers?
Many different types are available. A notable example includes the Wrenn Chamber. Commercially available units employ a wide variety of detection systems ranging from flow through scintillation chambers to solid state detectors analyzing the alpha spectrum of the collected daughters.
You mentioned integrating systems. What are those?
Integrating systems make measurements over periods of time longer than a few hours, i.e. days to years. They do not provide a real time analysis, however – the detector/sample is analyzed after the collection period and the average concentration for that time is determined.
When should I use an integrating system?
As a rule, integrating devices are the most practical way of obtaining a long-term average radon concentration, which is usually the information that interests the homeowner the most.
And the problems?
Well, it takes a long time to obtain the results. Also, the most sensitive of these devices tend to be pretty expensive.
What are some examples of integrating devices?
Examples include alpha track devices, PERMs, RPISUs, and charcoal canisters. I’ll tell you more about these devices, and what all the initials mean, later on.
So, should I be measuring radon or radon daughter (WL) concentrations?
This is not an easy question to answer. Consider that: a) The daughters, not the radon, are responsible for the dose to the lung; b) The radon concentration in a building is less dependent than the working level concentration on how the building is used or occupied. As a result, radon may give a better estimate of the potential, rather than the current hazard; c) The possibly significant contribution to the lung dose from the unattached RaA fraction is not reflected in a working level measurement, but is included (at least indirectly) by a radon measurement; and c) Radon calibration chambers are readily available; similar calibration facilities for the radon daughters are not. Therefore, the choice of measurement method is often not straightforward.
Okay. How can radon daughter concentrations be measured?
The principal methods are known as the Kusnetz, Tsivoglou, and Radon Progeny Integrating Sampling Units (RPISU) methods. In addition, there is also alpha spectrometry and alpha track detection.
Explain the Kusnetz Method.
An air sample is collected on a surface loading filter over a five (5) minute sampling time. Between forty (40) and ninety (90) minutes after sampling, the total alpha activity on the filter is measured with a ten (10) minute count. The working level concentration is determined using a formula established for this method. This technique is widely used because of its simplicity. It is not necessary to calibrate this system in a radon/radon daughter chamber. All that is needed is a counting efficiency for the alpha detector.
What is the Tsivoglou Method?
This is a somewhat more sensitive technique than the Kusnetz method and it provides information about the concentrations of the individual radon daughters. Like the Kusnetz Method, a five (5) minute sampling time is typically used. However, three (3) alpha counts, rather than one (1) , are performed. The first count is taken from 2-5 minutes, the second from 6-20 minutes and the third from 21-30 minutes after sampling. The concentrations (pCi/l) of the individual radon daughters (Po-218, Pb-214, and Bi-214) and the working level concentration are calculated using pre-established formulas.
Which method is better – the Kusnetz or Tsivoglou?
The Tsivoglou method is considered the “better” of the two, but it is less widely used because the math and counting are both simpler in the Kusnetz method and because of the inconvenience of having to perform the first count two (2) minutes after sampling.
What are Radon Progeny Integrating Sampling Units?
RPISUs are integrating devices typically operated over a period of three to seven (3-7) days. They consist of a filter (a 0.8 µm membrane filter is commonly used), a sampling head which holds two (2) or more TLDs in addition to the filter, a sampling pump, a flowmeter and an elapsed timer.
How do RPISU’s work?
The pump, running at 0.1 – 4 liters per minute (lpm), collects radon daughters on the filter. As the collected daughters decay, some of the released alpha energy is deposited in a thermoluminescent dosimeter (TLD) placed near the filter in the devices’ sampling head. One (1) or two (2) other TLDs are also located in the sampling head but shielded from the alpha particles. Their function is to account for the gamma ray background. The total amount of alpha energy deposited in the TLD over a period of time is related to the time-averaged radon daughter concentration using an established formula.
What are the advantages and disadvantages of using RPISUs?
RPISUs are sensitive devices that can provide very accurate measurements of the radon daughter concentrations. They have the disadvantages that they require an electrical outlet near the sampling location, they can be noisy, it is easy for a homeowner to interfere with their proper operation, and a substantial drop in flowrate during the sampling time can result from filter loading in dusty atmospheres.
What is alpha spectrometry?
Alpha spectrometry is a technique that both identifies the energy of the alpha particles emitted from a collection surface and quantifies the amount of radioactivity present. Several methods are available for determining radon daughter (working level) concentrations via alpha spectrometry. Some involve the analysis of single grab samples, while others operate as continuous monitors.
Can I combine grab sampling and alpha spectrometry.
Sure. For example, the radon daughters associated with airborne dust can be collected on a membrane filter. Typical sampling times and rates are ten (10) minutes at fifteen (15) lpm. Two (2) minutes after collection, the filter is then analyzed with an alpha spectrometer (e.g., a surface barrier or diffused junction detector) in two (2) consecutive counts. The first of these counting intervals might run from two (2) to twelve (12) minutes and the second from fifteen (15) to thirty (30) minutes after collection.
What does this particular methodology tell me?
The computer analyzing the data will tell you the energy of the alpha particles and the half lives of the isotopes that are present.
What is a major advantage of the alpha spectrometry method?
The ability to evaluate interference from thoron and/or actinon (Rn-219) daughters is a major advantage.
Sadly, yes. A major disadvantage is that a rather sophisticated detection system needs to be located at or near the point of sampling.
Remind me again of some other methods exist for the measurement of radon concentrations.
There are several. These include the Lucas Chamber or Scintillation Cell, Passive Environmental Radon Monitor (PERM), Charcoal Canisters, Liquid Scintillation Counting, Wrenn Chamber, Alpha Track, and Electrets. I can go through each, if you like.
Yes, thank you. Let’s start first with a Lucas Chamber/Scintillation Cell. What’s that?
Sometimes called “Lucas Cells”, these are grab sampling devices for measuring radon concentrations. They consist of empty chambers coated on the inside with zinc sulfide (ZnS) and equipped with one (1) or two (2) valves. One end of the cell may or may not be left uncoated. Sizes typically range from 50 – 500 milliliters (mls) but can be made considerably larger. At the sampling location, air can be collected by opening the valve on the chamber. If the chamber has two (2) valves, the ambient air may be pulled into the chamber by a pump. In most cases the air is filtered as it is collected in order to keep the chamber clean and the background low. The cell is not counted until at least three (3) or four (4) hours after collection (by which time the radon daughters will have come into equilibrium with the radon). Light pulses produced in the ZnS by the decay of the alpha emitting daughters are counted via a photomultiplier tube assembly. Count times are variable but typically on the order of thirty (30) minutes. The radon concentration is then determined via a pre- established formula.
What’s so good about a Lucas Cell?
Advantages of this technique include its sensitivity and the rapidity with which the results can be obtained.
Yes. Some disadvantages include the fact that it is a grab sample and radon levels can show large spatial and temporal variations. Furthermore, quality assurance and quality controls (QA/QC) have to be tight since the cells (single valve) can leak causing them to lose their vacuum or collected radon. It is also necessary to keep a close watch on the background of each cell since they can easily become contaminated. For example, contamination can be introduced when cells are calibrated using standard radium solutions.
What is a Passive Environmental Radon Monitor (PERM)?
This is an integrating device for determining radon concentrations. It consists of a conical chamber into which air passively diffuses through the bottom. Before reaching the chamber, the air first passes through a filter and a bed of silica gel. This simultaneously dries the air and removes any radon daughters. Inside the chamber, the radon decays to RaA which, due to its positive charge, is drawn to a negative electrode at the chamber top. Near this electrode is located a TLD which will absorb a given fraction of the alpha energy produced from the RaA and RaC’ decay. A second “background” TLD chip is housed in the unit but shielded from the electrode and the radon daughters. The radon concentration is then calculated using a pre-established formula.
What are the advantages of this method?
These units are sensitive, reliable, and since they require no external power supply, suitable for environmental measurements.
And the disadvantages?
A PERM is a labor intensive device. Aside from all the problems associated with reading TLDs, its response is greatly affected by humidity. This means the silica gel in these units must be replaced periodically, usually on a weekly basis. New designs are currently being evaluated that may reduce the need for such frequent changes of the desiccant.
Where did the charcoal canister technique originate and what is it used for?
In 1981, a gentleman by the name of Andy George, located at the DOE’s Environmental Measurements Laboratory, described a simple method employing charcoal canisters to measure indoor radon concentrations. Today it is probably the most widely used technique for this purpose. The canister is essentially a small pan of varying depth which contains charcoal granules of various grain sizes. In its typical design, the charcoal is covered with a wire mesh screen which is glued to the inside of the pan.
How does it work?
In brief, the method requires that a charcoal canister be “deployed” (opened and exposed to the atmosphere) for the specific number of days (usually 2-6) and then sealed. After deployment, the activity of the radon daughters in the canister is determined by gamma spectrometry (a method which identifies the gamma rays emitted from a particular radionuclide and quantifies the amount of radioactivity). The atmospheric radon concentration is calculated from the daughter activity with the aid of a predetermined calibration factor.
Tell me more.
During deployment, radon is passively adsorbed onto the surface of the charcoal granules. (The word “passive” means that the canister just sits in its assigned location and collects radon gas – no sampling pump or other “active” type of equipment is used.) At the same time as the charcoal is adsorbed, the radon decays as well as desorbing back to the atmosphere. If the exposure period is long enough, the adsorption and desorption/decay rates will become equal. When this happens, extending the duration of the deployment will not increase the total amount of adsorbed radon. In other words, there is a law of diminishing returns with respect to increasing exposure times. Therefore, charcoal canisters are rarely deployed for more than six (6) days. The EPA standard protocol employs a two (2) day deployment.
What affects the radon adsorption rate?
Although the radon adsorption is relatively unaffected by temperature, it is affected by humidity. For a given atmospheric radon concentration, the greater the humidity, the less radon will be adsorbed onto the charcoal. Therefore, the calibration factor depends on the humidity as well as on the length of deployment. The humidity conditions are evaluated by weighing the canister before and after exposure. Any increase in weight is due to adsorbed moisture.
What happens next?
At the end of deployment, the canister is sealed to allow the radon daughters to achieve secular equilibrium, a process that will take three (3) – four (4) hours. Following this, the canister is counted on a sensitive gamma ray detector, known as a sodium iodide detector (NaI) detector, and the radon concentration calculated.
Does the charcoal itself contain natural radioactivity even before deployment?
Yes, it does. Therefore, to determine the real radon contribution, a “net” count rate is calculated by establishing the count rate for a background canister and then subtracting that count rate from the deployed canister rate.
What advantages are are there in using charcoal canisters?
The charcoal canister technique is simple, quick, inexpensive and makes it possible for the homeowner to deploy the canister and mail it back to the company for analysis.
And the down side?
While the canister is sufficiently sensitive for measuring concentrations at or above the EPA guideline level of four (4) pCi/l, large errors are associated with measurements below one (1) pCi/l. This, and its sensitivity to humidity and air currents, make the method unsuitable for environmental use. It should also be kept in mind that for all practical purposes, it is only measuring the radon concentration for the last day or so of deployment.
Do other canister designs exist?
Yes. Certain charcoal canister designs employ a “diffusion barrier”. In its typical design, the air enters through a three-quarter-inch hole on the top of the can. Before it reaches the charcoal, the air must diffuse across a nylon/filter paper barrier. Such systems are not as sensitive as the “open” canisters but they provide a better average measurement since the effective collection time is two to four times as long as with the open type. A dessicant is often added to the canister to reduce the influence of humidity.
You mentioned liquid scintillation counting as another method. What’s that?
Liquid scintillation counting is a technique which assays (quantifies) a radioactive sample by dissolving that sample in an organic solution (cocktail) that fluoresces (emits light) when the solution adsorbs the energy of the ionizing radiation. The light flashes are converted to electronic pulses which are proportional to the energy of the radiation. The pulses can then be analyzed and the sample assayed.
How does it work for radon measurements?
A few grams of charcoal are placed in a liquid scintillation counting vial. The vial may or may not include a diffusion barrier and/or dessicant. After the vial has been deployed for the appropriate period (a few days), it is analyzed by liquid scintillation counting. This involves adding the cocktail directly into the vial and counting. Instead of counting gamma rays, this approach measures total alpha and beta activity.
What advantages are associated with this method?
The advantages are a lower background and a higher efficiency than with gamma spectroscopy. The result is a technique of superior sensitivity. Another major advantage is that liquid scintillation counters employ automatic sample changers which can analyze up to several hundred samples without the need for a system operator.
What is a Wrenn Chamber?
The Wrenn Chamber is a continuous device for measuring radon concentrations. Radon passively diffuses into a hemispherical chamber that is about ten (10) inches or so in diameter. The walls of this chamber consist of foam rubber supported on a wire screen. The foam acts to filter out the radon daughters. In the center of the chamber is a small hemispherical plexiglass dome coated with scintillation detector material and covered with aluminized mylar. A negative charge on the latter attracts the positively charged radon daughter produced from the decay of radon. Subsequent decays of radon daughters emit alpha particles which produce scintillations in the detector. The light pulses are then converted to electronic pulses by a photomultiplier assembly and, in turn, are analyzed by a microprocessor to provide hourly determinations of radon concentrations. The calibration must be performed in an enclosure containing radon of a known concentration.
Sounds like a nifty device.
Yes, but the Wrenn Chamber isn’t used much any more. There is a modern variation that is available, which is known as the “At Ease Radon Monitor”. The major difference between it and the old Wrenn Chamber is that the modern version uses a semiconductor detector instead of a scintillator.
What is the alpha track method?
The alpha track method is an integrating technique for the evaluation of radon (or radon daughter) concentrations. The detector itself is a small piece of plastic located inside a plastic cup/container. The opening of the container is usually covered with a filter/membrane that is permeable to radon but not its daughters. During use, radon passively enters the unit. When the radon decays, the daughters plate out on the container walls. Alpha particles from the decay of radon and the daughters themselves produce damage tracks in the detector.
How are they read?
After deployment, which may be anywhere from one to twelve (1 to 12) months, the damage tracks are chemically enhanced (made visible) by etching in a solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH). The tracks over a specified area of the detector are then counted manually with a light microscope or, more commonly, with an automatic system employing a computer.
Are these long-term devices?
Yes. Since relatively few tracks are produced, it is necessary to deploy the devices for periods greater than one (1) month. The precision can also be improved by counting the tracks over a larger area of the detector. Since the technique is insensitive to beta or gamma radiation, background measurements do not have to be subtracted.
What are the advantages of alpha track measurements?
A major advantage of these systems is that they are passive, simple and can be deployed for long periods of time. This allows a good determination of yearly averages.
What disadvantages are associated with this method?
This technique requires careful QA/QC procedures and it is best to employ the services of a company experienced in performing this sort of work. In addition, the alpha track device should be deployed as soon after receipt as possible since the exposure to radon leaking through the detector package during storage is always a concern. In some facilities, these devices are actually stored inside charcoal “shields”. Several studies have indicated that a high degree of variability is sometimes associated with alpha track measurements. In view of this, it may be advisable to deploy more than one at a given location. The radon or working level concentration is calculated using a pre-established formula. The accumulated exposure is determined from a calibration curve.
What are electrets?
The electret operates in the same fashion as an electroscope. The intensity of the radiation present is measured by the latter’s ability to reduce an electric charge. The electret itself is a dielectric material (e.g. teflon) that is given an electric charge and sealed from the air in a chamber made from low atomic number (low Z) material. During deployment, the device is opened allowing air to passively enter the chamber through a filter (which effectively removes the radon daughters). Air inside the chamber will consist of radon as well as its subsequent decay products. The ions created (of the appropriate sign) are attracted to the electret where they reduce the collected charge.
By measuring the difference in the charge (or voltage) before and after deployment and with the aid of a suitable calibration, it is possible to calculate the radon concentration. For a typical unit, a one (1) day exposure to a one (1) pCi/liter concentration results in a voltage drop of two (2) volts. In the absence of radon, the ionization created by background gamma rays results in a similar drop; this should always be subtracted from the measured value.
Is there some additional information on these measurement methods that I can read?
Absolutely. For example you might want to check out the 1986 EPA publication Interim Indoor Radon and Radon Decay Product Measurement Protocols (PB86-2215258). Also, be sure to read the following Department of Energy (DOE) publications: Protocol for the Estimation of Average Indoor Radon Daughter Concentrations (GJ/TMS-09), Procedure Manual for the Estimation of Average Indoor Radon Daughter Concentrations by the Radon Grab Sampling Technique (GJ/TMC- 11), and Procedure Manual for the Estimation of Average Indoor Radon Daughter Concentrations by the Radon Progeny Integrating Sampling Unit (RPISU) Method (GJ/TMC-12). In addition, P. Jenkins’ Radon Measurement Methods: An Overview is an excellent review of the subject, as is NCRP Report No. 97, Measurement of Radon and Radon Daughters in Air.
By the way, I’ve heard about the Indoor Radon Abatement Act. What is it and what is its basic goal?
This act was an amendment to the Toxic Substances Control Act in 1988. It begins by stating that the national long term goal is that “air within buildings in the United States should be as free of radon as the ambient air outside of buildings”.
What does the act require?
The Indoor Radon Abatement Act requires: a) Specific changes in the EPA Citizens Guide; b) EPA to develop construction standards and techniques for controlling radon in new buildings and to incorporate these standards and techniques in the national building codes; c) EPA to provide technical assistance to the states in the form of information, proficiency programs, training, demonstration programs, data base development, etc.; d) Federal financial aid to the states for radon program; e) A national study of radon in schools; f) A national study of radon in federal buildings; and g) Establishing three regional radon training centers to provide training and information regarding radon mitigation and measurement.
Where can I obtain more information about radon measurements?
There are a number of excellent references that discuss radon measurement methods in great detail. Quite a few of them are listed in the “Bibliography” that is located in this web page’s “Tool Box”. If you don’t find the information you need there, please don’t hesitate to “Ask a CHP”.