The Graded Screen Technique for Measuring the Diffusion Coefficient of Radon Decay Products

ABSTRACT

The main focus of this paper is the graded screen array (GSA) sampler, a specialized diffusion battery developed modified at our Laboratory to measure the size distribution of unattached ²¹⁸Po, a naturally occurring “atomic aerosol,” implicated in causing lung cancer. The GSA, as used in this study, consists of four stainless steel wire screens with increasing mesh count, each positioned on a circular mount. For sampling, the four mounted screens are stacked into a holder and air is drawn through them in the direction of increasing mesh count. A backup filter is placed behind the fourth screen. After sampling, the alpha radioactivity on each screen, and on the backup filter, is measured by standard methods. Size distributions are retrieved from these data using both the Nelder-Mead downhill simplex method and the expectation-maximization algorithm. To illustrate the usefulness of the GSA sampler, size distributions from ²¹⁸Po samples taken from the radon chamber at Clarkson University, Potsdam, NY, are discussed in detail. It was found that 96% or more of the radioactivity was associated with a size mode centered at diffusion coefficient =0.06 cm² s⁻¹. In contrast, ²¹²Pb samples taken at the Inhalation Toxicology Research Institute, Albuquerque, NM, yielded size distributions that were clearly bimodal (three of six cases) or possibly bimodal (two of six cases). All samples had one mode at about 0.044 cm² s⁻¹. The location of the other mode averaged 0.0054 cm² s⁻¹, but varied with test conditions. Additionally, data from our earlier publications have been reexamined in light of our added experience with the GSA sampler.