A System for Aerodynamically Sizing Ultrafine Radioactive Particles

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Abstract
A hypersonic impactor similar to that originally developed by Fernandez de la Mora et al. (1990a) has been modified to measure the aerodynamic diameter, dae, of radioactive particles. The first application is the measurement of the size of the radiolytic particles/clusters formed in the decay of 222Rn. The particles are generated in 32-liter radon chamber under well-controlled experimental conditions. The initial system test was with the particles generated in a gas-mixture of 222Rn and high-purity nitrogen. N2, for various relative humidities, < 1%, 10%, 15%, and 25% at 22°C. The measured particle size agrees with previously measured diffusion diameters. Thus, the size-discriminating capability of the system is good if an appropriate nozzle diameter is used. A particle's d ae, unlike its diffusion coefficient, is independent of the number of charges carried by the particle. There is then the possibility of obtaining a better size distribution resolution than can be achieved with a diffusion-based measurement system for the case when the aerosol is an admixture of both neutral and charged particles. Also, the particle density information contained in the d ae will be utilized in the latter studies to estimate these particle's actual chemical composition.