The measurement of activity-weighted size distributions of radon progeny: methods and laboratory intercomparison studies.

Over the past 5 y, there have been significant improvements in measurement of activity-weighted size distributions of airborne radon decay products. The modification of screen diffusion batteries to incorporate multiple screens of differing mesh number, called graded screen arrays, have permitted improved size resolution below 10 nm such that the size distributions can now be determined down to molecular sized activities (0.5 nm). In order to ascertain the utility and reliability of such systems, several intercomparison tests have been performed in a 2.4 m3 radon chamber in which particles of varying size have been produced by introducing SO2 and H2O along with the radon to the chamber. In April 1988, intercomparison studies were performed between direct measurements of the activity-weighted size distributions as measured by graded screen arrays and an indirect measurement of the distribution obtained by measuring the number size distribution with a differential mobility analyzer and multiplying by the theoretical attachment rate. Good agreement was obtained in these measurements. A second set of intercomparison studies among a number of groups with graded screen array systems was made in April 1989 with the objective of resolving spectral structure below 10 nm. Again, generally good agreement among the various groups was obtained although some differences were noted. It is thus concluded that such systems can be constructed and can be useful in making routine measurements of activity-weighted size distributions with reasonable confidence in the results obtained.

A measurement system for Rn decay product lung deposition based on respiratory models.

Using models of particle deposition in the nasal cavity and in the tracheobronchial region of the respiratory tract, it is possible to design a measurement system based on wire screen penetration theory. This system provides direct estimates for the quantities of Rn decay product activity deposited in the nasal cavity and in the tracheobronchial region. Historically, measurements of the "unattached" fraction of the airborne Rn decay product size distribution have been made in order to better estimate the dose resulting from the deposition of these 222Rn progeny in the human respiratory tract. However, previous measurement systems have been developed based on choices of screen parameters and sampling flow conditions in an attempt to separate the "unattached" fraction from the rest of the activity size distribution. By taking into consideration the deposition behavior of particles in the respiratory tract, a specific choice of wire screens and flow rates can be made that can provide better information for the estimation of deposited dose in the respiratory tract.

On improving the validity of wire screen "unattached" fraction Rn daughter measurements.

Wire screens are commonly used to estimate "unattached" Rn daughter fractions in ambient and mine atmospheres. However, it is now recognized that the "unattached" fraction is in reality an ultrafine cluster mode in the 0.5-3 nm size range and that the collection efficiency versus particle diameter characteristics of wire screens do not permit a distinct separation of the "unattached" and "attached" fractions. Wire screen penetration theory and a semi-empirically corrected diffusion coefficient equation are used to characterize "unattached" fraction measurements as a function of experimental parameters. Collection efficiency curves are estimated for previously published wire screen "unattached" fraction measurements, and improvements in wire screen methods for such measurements are discussed.