A System for Aerodynamically Sizing Ultrafine Radioactive Particles

A hypersonic impact()r similar t() that ()riginally developed by Fernandez de la M()ra et al. (1990a) has been modified to measure the aer()dynamic diameter, dacʹ ()f radi()active particles. The first application is the measurement of the size ()f the radiolytic particles / clusters formed in the decay of 222 Rn. The particles are generated in 32-liter radon chamber under well-controlled experimental conditi()ns. The initial system test was with the particles generated in a gas-mixture of 222 Rn and high-purity nitrogen. N2 , for various relative humidities, < to/c, tO%, 15%, and 25% at 2rc. 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 particleʹs d Heʹ unlike its diffusion coefficient, is independent of the number of charges carried by the particle. There is then the possibility of ()btaining 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.e will be utilized in the latter studies to estimate these particleʹs actual chemical composition.