Abstract:
A new portable type cascade impactor has been developed to determine the activity size distribution of radon and thoron progeny in a natural environment more efficiently. The modified impactor consists of 4 stages with a back up filter stage for the collection of aerosol samples. The aerosol cut points in the impactor are set for 10, 2.5, 1 and 0.5 µm at a flow rate of 4 L min-1. Five CR-39 chips were used as alpha detectors for each stage. In order to separate α particles emitted from radon and thoron progeny, CR-39 detectors are covered with aluminum-vaporized Mylar films. Thickness of each film is properly adjusted to allow α particles emitted from radon and thoron progeny to reach the CR-39 detectors. In addition, a 400-mesh metal wire screen is mounted as diffusion collector at the air inlet of the impactor to remove the unattached fraction of radon and thoron decay products. Furthermore, the particle cutoff characteristics of each stage was determined by mono-disperse aerosols particle size ranging from 0.1-1.23 ∪m from the collection efficiency curve. The cutoff characteristics were found to be similar for stage 3 and 4 only. We have investigated the influence of relative humidity (30-90%), temperature (5-30℃), unattached progeny, and air sampling at different flow-rates (≈12-75%) for the sensitivity of the developed technique whereas activity median aerodynamic diameter (AMAD) is not much affected due to change in relative humidity, temperature and air flow-rate from 20-40%. The results showed that high-unattached radon progeny enhanced deposition effect at stage 1 and 2 of impactor. Validation of the technique was performed with the commercial devices and results confirmed that the developed technique can provide significant information to estimate the activity size distribution of attached radon and thoron progeny for dose assessment. The develop technique has been successfully tested in field studies expecially inside a NORM industry in Phuket to estimate doses at working environment.
