Characteristics of Radon-222
Fate and Transport
Monitoring in the Environment
Control and Prevention
Absorption, Distribution and Organic Sites of Toxicity
Molecular Action and Genetic Effects
Radon for Skeptics
Radon for Children
Conceptually, measurement techniques can be divided into three board categories : (1) grab sampling, (2) continuous and active sampling, and (3) integrative sampling.
The choice between these categories will depend on the costs involved, the time over which an instrument can be devoted to measurements at a single location, the kind of information required, and the desired accuracy with which measurements can be related to an estimate of risk.
- Grab sampling
Grab samples consist of essentially instaneous measurements of the radon or radon progeny concentration in air over time intervals that are short (on the order of minutes) compared to the time scale of fluctuations in concentration. The air is collected in a container and brought back to the laboratory for analysis. Typical containers include plastic bags, metal cans and glass containers. The volumes of the containers are usually between 5 liters and 20 liters.
- Continuous sampling
Continuous sampling involves the automatic taking of measurements at closely spaced time intervals over a long period of time. The result is a series of measurements which can give information on the pattern with which the concentration varied throughout the measurement interval.
- Integrating Sampling
Integrating devices collect information on the total number of radition events which occur throughout some fairly long period of time, usually on the order of several days to months. The result from integrating devices is an estimate of the approximate average concentration through the environment interval.
Measurement of Radon in Air
- Alpha-article scintillation counting with ZnS (Known as Lucas Cell)
One of earliest methods for measuring concentrations of radon in air is the scintillation cell, which usually is utilized as a grab sample. This cell has been known historically as a Lucas cell. In this technique, the radon gas sample is introduced into a counting cell. The inside wall of the cell is coasted with zinc sulfide (ZnS), except one end which is covered with a transparent window for coupling to a photomultiplier tube. When an alpha particle strikes the wall of the cell, a flash of light is emitted from the ZnS coating. The light is detected by the photomultiplier tube and translated into an electrical signal. The efficiency of these cells is typically 70 to 80%. Background rates in typical Lucas cells are low, about 0.1 or 0.2 counts per minute (cpm).
- Internal ionization chamber counters
Alpha particles from the decay of radon and its daughters can also be detected in ionization chambers. In these counters, an electrical signal is produced without the intermediary of scintillation counting. Ionization counters can be used either to count electrical pulses from individual decay events or to measure currents resulting from the integrated effect of all decays. In general, ionization chambers are not as widely used as scintillation counters, since ionization chambers are more expensive to construct than Lucas cells and for radon measurements they do not appear to have a major advantage over Lucas cells.
- Two-filter methods
For measurement of both radon and radon daughter concentrations, the tow filter method can be used. In this method, air is passed through the first filter where daughter products are removed. Then the air is passed through a long decay chamber, where daughter products are allowed to grow in and are collected on a second filter. The filters can be counted separately to determine the concentration of radon (from the second filter) and daughter products (from the first filter). The above methods are used for measurement of both grab samples and continuous samples.
Measurement of Radon Daughter in Air
Unlike radon, radon daughters deposit readily on dust particles or other surfaces. By drawing air through a filter, the radon daughters can be collected with high efficiency. In all cases, counting must begin shortly after the sample is collected because the daughters all have short half-lives (the longest is 27 minutes). In order to determine the daughter concentrations, expressed as the number of working levels, it is necessary to know the individual concentrations of RaA (polonium-218), RaB (lead-214), and RaC (bismuth-214). The concentration of RaC (polonium-214) is not relevant, because its half-life is so small (< 0.0002 sec).
- Alpha particle scintillation counting: single time period
A very simple approach is to count the alpha-particle radioactivity deposited on the filter, using a scintillation counter with a ZnS phosphor. The filter containing radon daughters can be covered with a thin sheet of ZnS phosphor and then placed on the photomultiplier tube for counting, or the ZnS can be left on the photomultiplier tubes.
- Alpha particle scintillation counting: several time periods
In principle, the three radon daughter concentrations can be individually established if counts are taken during three or more successive time periods. Because the half-lived of the three daughter nuclei are different, the relative number of counts in three time period determines their individual initial concentrations.
- Alpha particle spectroscopy
As an alternative to counting for three or more time periods, it is possible to obtain the daughter concentrations by distinguishing in the detecting system between the 6.00-MeV alpha-particle group from RaA and the7.69-MeV alpha-particle group from RaC and separately counting the two groups during two time periods.
- Combined alpha-particle and beta-particle spectroscopy
The counting of beat-particles from RaB and RaC in a plastic scintillator along with alpha-particle spectroscopy with a surface-barrier detector determines the three daughter concentrations during a single time period. This method is clearly advantageous in terms of speed of measurement, but more complex in terms of the equipment required.
Radon measurement in water
The measurement of radon in water is relatively simple compared to air measurement. The rate and magnitude of variations are much lower, and there are fewer sampling problems. Nevertheless, the measurement of radon in water has its own set of sampling issues and analytical difficulties. All methods require correction for decay of radon during the decay between sampling and analysis.
- Liquid scintillation counters
Radon concentrations in water are typically much higher than the concentrations in air. When the radon concentration in water is sufficient high (> 1000 pCi/L), as is often the case with well water, direct liquid scintillation counting is a rapid and practical method. The water sample can be mixed with the counting material and counted by the conventional liquid scintillation counters used for radon in air samples. This method lends itself to large scale counting with automation.
- Gas extraction
A more sensitive method of detecting radon in water, suitable at lower concentrations, is to extract the radon as a gas and count the emitted alpha particles in a ZnS scintillation cell. Helium is bubbled through the water, striping the radon. The mixture of gases is then passed through a cold trap, for example activated charcoal at liquid nitrogen temperature, that traps radon while the helium passes through. The trap is then warmed and radon is transferred into a Lucas counting cell by stripping with a small amount of helium.
- Direct gamma counting
When the radon concentration in water is relatively high (> 500 pCi/L), it is possible to determine the radon concentration by counting gamma rays from radon daughter decay using standard gamma-ray spectroscopy techniques with a Ge(Li) detector. The original radon concentration can be distinguished from the radium-226 concentration by repeating the count after 30 days, at which time the original radon will have virtually all decayed and the only remaining radon is that in secular equilibrium with radium-226.
C. Richard Cothern. James E. Smith, Jr Environmental Radon. Plenum Press, New York (1987)
Bodansky D. Robkin MA. Stadler DR. Indoor Radon and Its Hazards. University of Washington Press, USA (1987).