An international intercomparison of soil gas radon and radon exhalation measurements.

The Environmental Measurements Laboratory hosted the Sixth International Radon Metrology Programme Intercomparison Test and Workshop (IRMP6) from 12-15 June 1995. Thirty participants representing 24 different institutions from 11 countries attended. Laboratory exercises consisted of 220Rn and 222Rn concentration measurements from a source container, and exhalation measurements from a 226Ra-spiked concrete slab and a "normal" concrete slab. Field exercises included soil gas radon measurements and radon exhalation measurements. In this report, we pooled the participants' data and used the ratio of the standard deviation (SD) to the arithmetic mean, expressed as a percentage, to assess participant agreement for each exercise. For the exhalation measurements from the 226Ra-spiked slab, this value is 37%; for soil gas 222Rn, this value is 120%, 36% and 27% for each depth range, 0.4-0.5, 0.6-0.75 and 0.9-1.0 m, respectively; for the surface exhalation measurements, this value is 34%. For the drum 222Rn measurements, the percent SD after removing a linear trend was 13%. These results indicate that sampling errors are greater than instrument errors.

Discrimination of airborne radioactivity from radon progeny.

Naturally occurring radon and thoron progeny are the most interfering nuclides in the aerosol monitoring system. The high background and fluctuation of natural radioactivity on the filter can cause an error message to the aerosol monitor. A theoretical model was applied in the simulation of radon and thoron progeny behavior in the environment and on the filter. Results show that even a small amount of airborne nuclides on the filter could be discriminated by using the beta:alpha activity ratio instead of gross beta or alpha counting. This method can increase the sensitivity and reliability of real-time aerosol monitoring.

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.

Radon and radon daughter measurements in solar buildings.

Measurements of radon and radon daughters in 11 buildings in five states, using active or passive solar heating, showed no significant excess in concentrations over the levels measured in buildings with conventional heating systems. Radon levels in two buildings using rock storage in their active solar systems exceeded the U.S. Nuclear Regulatory Commission's limit of 3 pCi/l. for continuous exposure in uncontrolled areas. In the remainder of the buildings, radon concentrations were found to be at levels considered to be normal. It appears that the slightly elevated indoor radon concentrations result from the local geological formations and from the tightening of the buildings rather than as a result of the solar heating technology.

Radon daughter plateout--I. Measurements.

Radon daughter plateout (surface deposition) was measured directly in 1.9 and 20-m3 chambers. To test the effect of different parameters on plateout, measurements were made over a wide range of particle concentrations and sizes. The results indicate that plateout is strongly dependent on particle concentration. The ratio of surface-deposited activity to total daughter activity in the chamber varied from 4% at particle concentrations greater than 10(5)/cm3 to 86% for particle concentrations less than 10(3)/cm3. Comparison for the experimental data with a theoretical model shows that the theory overestimates plateout and underestimates the airborne concentration by factors of about 3.5 and 3.3, respectively.

Radon daughter plateout--II. Prediction model.

By a linear programming procedure, we have optimized the Jacobi room model to fit our data on the partitioning of radon daughters between air and wall surfaces (plateout), obtained at high radon concentrations in a small chamber. Subsequently, the optimized model yielded estimates that compared well with plateout data obtained at lower concentrations in a room-sized chamber. (The experimental work is described separately.) The major change made in the Jacobi model was to reduce the deposition velocity of free airborne radon daughters from 1 to 0.05 cm/sec. This value was obtained by using a fast algorithm to solve the linear programming to arrive at the "best fit". Lesser changes were made in other parameters.

Assessment of a modified cascade centripeter.

The cascade centripeter of Hounam and Sherwood was modified to permit a wider selection of filters for use in stages 1 to 3. Tests with monodisperse aerosols showed that the calibration given by O'Connor adequately describes the modified device. An iterative procedure was devised which provides more detailed particle size data than is possible with conventional data analysis. In tests sampling urban aerosol, it was found that the centripeter is a practical alternative to the cascade impactor. Using filters selected for low and stable tare weight, a sample sufficient for weighing can be obtained in 4 to 6 hours. The main advantage of the centripeter is that the size-classified samples are obtained as uniform deposits on clean, dry filters.