Activated Carbon (AC)
Sometimes referred to as ‘charcoal devices’, ACs come in the form of devices, trays and pouches and utilize activated carbon to adsorb radon gas by molecular diffusion into carbon grains, where it decays into the short-lived RDPs (polonium-218) Po-218 lead-214 (pb-214) bismuth-214 (Bi-214) and polonium-214. As we learned in Chapter 3, Bi-214 and Pb-214 are gamma-ray emitters. The radon concentration to which the devices are exposed is determined by counting the gamma-ray emissions of these two RDPs.
Radioactive decay from the midpoint of the exposure period to the time of analysis is calculated at the laboratory, but if too much time elapses before the device is analyzed, the radon and subsequent RDPs in the carbon bed may decay beyond detection.
At the laboratory, most AC devices are analyzed by placing directly on a gamma spectroscopy system. Such a system usually consists of a sodium iodide detector and photomultiplier tube, shielded by lead to reduce background radiation. The interaction of the gamma rays produced by the RDPs causes the sodium iodide crystal to emit light pulses.
Each light emission is called a scintillation and is sensed by the photomultiplier tube, which produces an electrical pulse whose amplitude is proportional to the gamma-ray energy. The pulse is further amplified by electronics and fed to an analyzer that displays a count rate. The counting efficiency of the gamma spectroscopy system is determined by counting a calibration standard and dividing the net counts of the detector by the known activity of the calibration standard. In addition, the laboratory must perform a batch calibration to determine the collection efficiency of the devices themselves.
Some Activate Carbon devices are measured by weight in the laboratory then calculated by time integrated over the exposure period and a decay factor accounted for. This Decay Factor accounts for the time elapsed from the stop time of the test to the analysis after it has arrived at the lab. This can be several days to a week so accurate recording of start and stop times is extremely important.
Collection efficiency will vary between device types and sizes, different batches of charcoal, and the amount of water vapor that can get to the carbon bed. Water vapor in the air will compete with the radon for a place on the carbon grains.
The more water absorbed, the less room there is for radon. Therefore, the higher the relative humidity of the air being sampled, the less sensitive to radon adsorption the charcoal becomes. In some devices, radon will be adsorbed quickly at first and then more slowly as the moisture takes up more space on the carbon bed. Since the addition of water weight in the carbon changes the rate of radon collection, calibration factors vary depending on the humidity of the air being sampled. To determine these different calibration factors, the laboratory exposes a number of devices at different humidity and time periods to know radon concentrations in a calibration chamber.
Since the weight gain is indicative of the humidity to which the device was exposed, measuring the water weight that a device gains during routine exposures is often necessary determine which calibration factor must be utilized for calculating the radon concentration
The 4″ open-faced device pictured in Figure 4-1, with its charcoal openly exposed, is quite sensitive to radon collection for a two-day exposure. Exposures beyond two days in a high humidity environment rapidly slow the radon adsorption rate, and correction during analysis becomes increasingly more difficult. The optimum exposure period for this style device is two days. If the device is placed in areas of extremely high humidity or is exposed longer than the optimum time, so much moisture could be added that the laboratory would be unable to adjust the calibration factor properly. Do not place charcoal devices of any type in bathrooms kitchens spa rooms or other areas of high humidity.
Many other device styles have been designed to make them less sensitive to both humidity and to air velocities across the device face. Significant air movement across an open face device causes it to over respond by increasing the adsorption rate. In 1989, the 4″ EPA-style open-faced device was modified by inserting a diffusion barrier membrane over the top of the carbon bed. Other AC devices have also adopted a diffusion barrier membrane. Although the membrane makes this style device less sensitive to excessive airflows than the open faced style, never place any device near drafts caused by ceiling fans, forced air ducts, etc.
The diffusion membrane works remarkably well in retarding moisture uptake in the carbon bed, making laboratory adjustments for moisture gain less critical. However, the decreased sensitivity to radon collection caused by the diffusion barrier means this style device should be exposed for periods of five to seven days in order to collect an adequate sample. For this reason, some laboratories still prefer the open-faced device, because it works well for a shorter, two-day exposure period.
|Diffusion Barrier Charcoal Canisters should be exposed a minimum of 5 days. The Diffusion barrier protects the device against excessive moisture gain but in doing so also prevents adequate exposure to normal airflow.|
The passive nature of activated charcoal allows continual adsorption and desorption of radon, while the adsorbed radon undergoes radioactive decay during the exposure period. Charcoal devices do not truly integrate over time therefore. Open-faced devices will be biased to the radon concentration of the last 12-24 hours of the exposure period. A diffusion barrier reduces the adsorption/desorption rate of the carbon bed, thus improving integration ability.
|Advantages of Charcoal Devices||Disadvantages of Charcoal Devices|
|Economical||Results biased toward the last 12-24 hours of exposure.|
|Convenient to handle and install.||Charcoal sensitive to temperature, humidity and airflow extremes.|
|Easy to mail.||Sampling periods limited to a few days.|
|Simple to use – skilled operators are not required to place and retrieve device||Sampling conditions that might adversely affect the measurement may be unknown.|
|Measurement periods are short.||Inability to provide graphical data relative to diurnal cycles.|
|Excellent precision and relatively accurate.||Susceptible to over adsorption of radon when exposed to excessive airflow.|
|Notes regarding ACs: ACs measure gamma radiation.|
Alpha Track Detectors (AT)
Alpha track detectors consist of a small alpha sensitive plastic chip or cellulose film positioned in a small container (decay chamber) with a membrane filter. The filter allows only radon (not the RDPs) to enter the chamber. However, as radon gas passively diffuses through the filter, radiation damage (“alpha tracks”) to the plastic or film results from the subsequent decay of radon and its RDPs.
Alpha track detectors are packaged in an airtight foil bag to prevent exposure during shipping. To collect a sample, the bag is removed to allow air to diffuse through the membrane into the container. After exposure, the detector is resealed and returned to the laboratory for analysis. There, the film is removed from the container and etched by a caustic solution of sodium hydroxide to enhance the tracks. The tracks or damage scars can then be counted over predetermined fields, either by a trained technician using a microscope or by a computer image analyzer.
The density (number per area) of tracks is proportional to the radon concentration and is linear over a wide range of exposure durations and concentrations. The average number of tracks per field (unit area) is used to calculate the integrated concentration to which it was exposed. That integrated concentration is expressed in pCi/L Day and is divided by the total number of exposure days to compute the average radon concentration.
Since the number of tracks produced per field per unit of time is proportional to the radon concentration AT’s are true integrating detectors. The lower limit of detection as well as measurement certainty is dependent on the total number of tracks counted. Laboratories will generally analyze enough fields to count at least 100 net tracks. Naturally, the fewer tracks counted, the higher the relative counting error.
The collection efficiency or sensitivity of alpha track detectors is relatively low, requiring exposure for long periods. Typically such measurements are made for at least 90 days and often up to a year, if the annual average concentration is to be determined. Of course, if the expected radon concentrations are sufficiently high, enough activity may be recorded in shorter periods to provide good counting statistics.
The AT has many of the same advantages as the charcoal device plus a longer term integrating ability. The relatively low cost and other advantages of the AT make it a very popular detector for making long-term measurements.
|Advantages of Alpha Track Detectors||Disadvantages of Alpha Track Detectors|
|Low cost, easy to use, easy to mail, convenient.||Inability to measure for short time periods unless concentrations are high.|
|No need for external power (passive).||Relatively large precision error at low concentrations if only a small detector area is counted.|
|Unobtrusiveness – no need for closed-building conditions.||Long term devices.|
|Ability to measure integrated (average) radon concentrations over long periods.||Must be analyzed at a lab.|
|True time integration (not biased towards most recent exposure).||Study Notes: ATs measure Alpha radiation. ATs are considered true time integrating devices|
Electret Ion Chamber (EIC)
Electret Ion Chambers detect ions produced by the decay of radon as a method of measuring the gas concentration (Figure 4-8). Radon in the air is sampled by a small (200 cc) bottle-like collection chamber made of electrically conductive plastic. The chamber is coupled with a charged Teflon disk called an electret to create and electrostatic field. A special voltmeter is used to measure the voltage depletion on the surface of the electret caused by the collection of ions and electrons produced during radon decay. Prior to deployment the electret is kept covered by a spring loaded screw cap. Once the cap is released the disk is exposed to the inside of the collection chamber through several small filtered holes that prohibit the entry of airborne RDPs.
When radon atoms and its RDPs disintegrate, their nuclei emit alpha, beta and gamma particles that pass a short distance through the air. These high energy particles collide with many atoms of oxygen and nitrogen knocking electrons free from their orbits A “cloud” of free electrons and positively charged oxygen and nitrogen atoms is left in the path of each alpha particle. The charged atoms and electrons (now called positive and negative ions, respectively) are attracted in the electric field established by the electret.
Since the face of the electret is positively charged, it attracts the negatively charged free electrons. The shell of the chamber is negative and attracts the positive ions. Every ion that reaches the electret surface depletes the electrical charge of the electret by a small amount.
The electrostatic charge of the disk is measured both before and after the deployment period. The determination of the specific charge loss for each detector, in terms of the radon concentration to which it was exposed, is the fundamental calibration factor. Care must be taken by the technician never to touch the surface of the electret disk itself, to avoid inadvertently depleting its electrostatic charge.
The electrets can be adversely affected by background gamma radiation at the test site (gamma rays also ionize the air and will penetrate the shell). Average background gamma readings for both lower and higher elevations are available for every state and corresponding correction factors can be subtracted from the measurement calculations. An alternate way to account for this would be actually to take a background gamma reading with a micro-R meter at the test site prior to placement of the EIC.
An electret chamber exposed in a radon environment will typically lose approximately 2 volts of charge per every 1 pCi/L per day of exposure.
There are two types of electrets available: short-term (ES) disks and long-term (EL) disks. The more sensitive short-term disk will be depleted of a charge more readily than a less sensitive one that is used for long-term exposures. By utilizing the appropriate electret, EICs can make integrated measurements from 2 days to 1 year.
|Electret discs are fully charged at 700 volts and considered stable to 100 volts, at which time the user should return the electret to the manufacturer for recharging.|
Best practices will involve purging the chambers regularly with clean, aged air or nitrogen. Background checks are to be performed more frequently if the instrument has been exposed to high levels of radon for extended periods of time.
All EIC voltage readers will be calibrated annually and upon returning to our custody from maintenance, repair or inspection and the instrument must be checked once a week using two reference electrets (provided by the EIC manufacturer). A voltage reading of a reference electret, which differs by more than 2 volts from that specified for it by the manufacturer must be considered unacceptable and is a cause for corrective action by the Owner in concert with the manufacturer.
Best practices will involve purging the chambers regularly with clean, aged air or nitrogen. Background checks are to be performed more frequently if the instrument has been exposed to high levels of radon for extended periods of time. All EIC voltage readers will be calibrated annually and upon returning to custody from maintenance, repair or inspection and the instrument must be checked once a week using two reference electrets (provided by the EIC manufacturer). A voltage reading of a reference electret, which differs by more than 2 volts from that specified for it by the manufacturer must be considered unacceptable and is a cause for corrective action by the owner in concert with the manufacturer.
|Advantages of Electret Ion Chambers||Disadvantages of Electret Ion Chambers|
|On-site analysis with portable voltage reader.||Sensitive to background gamma radiation; a slight error may result if gamma background is not measured & corrected for.|
|Each EIC has the potential for use many times before the voltage is depleted (depending on the radon concentration to which it was exposed).||Excessive humidity can affect the accuracy of the voltmeter.|
|Excellent Accuracy and Precision.||A final voltage reading made at much colder or warmer temperatures than the initial voltage reading may result in a light error when measuring the voltage depletion.|
|Not sensitive to humidity, airflow or temperature.||Proper hygiene [user handling] required for accurate reading.|
|Notes: EICs are considered true-time integrating devices. ES – Electret Short Term SL – Electret Long Term|
Liquid Scintillation Devices (LS)
Like charcoal devices, these are passive detectors that utilize activated carbon. A typical device consists of a 20 ml liquid scintillation vial that contains 1 to 3 grams of charcoal. In some cases, the vial contains a diffusion barrier over the charcoal, which improves the uniformity of response to variations of radon concentrations over time. Some LS devices include a few grams of desiccant, which reduces interference from moisture adsorption.
A measurement is initiated by removing the radon proof closure to allow radon-laden air to diffuse into the charcoal. At the end of deployment it is resealed and returned to the laboratory where it is prepared for analysis by radon desorption techniques that transfer a major fraction of the radon adsorbed on the charcoal into a vial of liquid scintillation fluid. The vial is sealed and must reach Secular Equilibrium before analysis. The vials of fluid containing the dissolved radon and its RDPs are placed in a liquid scintillation counter and counted for a specified number of minutes.
|Light emission = scintillation|
LS – Alpha scintillations are counted after liquid scintillation fluid is added to the vial and secular equilibrium has been achieved.
|Advantages of LS Vials||Disadvantages of LS Vials|
|Somewhat economical||Require special pre-processing at the lab, which extends the analysis time. Secular equilibrium must be achieved before counting can take place.|
|Small & easy to deploy.||Sampling period limited to a few days.Results biased to last 12-24 hours.|
|Charcoal granules sensitive to temperature, humidity & airflows.|
|Unless labeled properly by the end user, retrieval rate can diminish due to occupant handling.|
|Inability to obtain graphical data relative to diurnal cycles and interference.|
|Notes: Measure Alpha radiation.|