Typically used by mitigation contractors or researchers, WL measurements are primarily conducted for diagnostic purposes. These measurements are brief samplings of radon decay products [RDPs] for the purpose of determining radon entry pathways or the best location for radon mitigation strategies. See section 4.4.3 for details on WL measurement devices.
Some RDP grab samples use an air pump to collect RDPs on a filter. The air pump must be calibrated to obtain accurate airflow over the sampling time interval (normally around 5 minutes). In this method, a sample is taken by a scintillation counting system for about 10 minutes, after which total alpha activity is measured between 40 and 90 minutes. The counting system is quite similar to that used for radon grab samples. The sample collection filter is placed on a tray against a zinc sulfide phosphor disc. The tray is then placed in the counting chamber against the photomultiplier tube counting surface. The photomultiplier tube counts light pulses occurring from the interaction of the alpha decays and the zinc sulfide phosphor.
A calibration-based conversion factor allows the counts collected over the analysis time interval to be converted directly to WL. Depending on the type of detector used, this calibration requires the use of either a standard alpha-emitting source or an RDP sample traceable to a National Institute of Standards and Technology (NIST) standard.
The Radon Grab Sample technique might also use a Lucas Cell also known as an alpha scintillation cell or flask ranging in size from about 0.1 to 2.0 liters in volume with a zinc sulfide phosphor coating on the interior and a counting window on its bottom surface to prevent RDPs and other background radiation from entering the cell a filter is attached to the cell’s air inlet when filling with a sample of air.
Scintillation cells can be either a single valve type or a double valve flow through type. In preparing them for use, single valve cells are evacuated with a pump and purged with nitrogen or low radioactivity air several times to reduce background radiation within the cell. Double valve cells are flushed with nitrogen or low radioactivity air typically outdoor air. The turbulence from high flow, extended duration flushing appears to enhance cell recovery by removing attached RDPs from cell walls.
To collect a sample with a single valve cell, a vacuum is created in a low background cell then the valve is simply opened to admit air from the sample location. For the double valve flow-through cell, both valves are opened and an air volume equal to at least 10 cell volumes is pumped through the cell. Both valves are then closed to hold the sample. For greatest accuracy the radon sample is allowed to reach secular equilibrium with its RDPs. This requires about a four-hour wait after collection. The cell’s clear window is then placed in contact with a photomultiplier tube, in order to count light pulses from the interaction of the alpha particles (emitted from radon and its RDPs) as they strike the zinc sulfide coating. Appropriate corrections are made for the time elapsed after the sample was taken and for the actual counting duration. Some state radon programs, prohibit the use of grab samples for follow-up measurements or for pre- or post-mitigation testing. Always check your state regulations to be in compliance. They are most useful for diagnostic and investigatory measurements.