Neutralisation rate and the fraction of the positive 218Po-clusters in air

The dose relevant quantities like concentration and particle size distribution of the short-lived radon decay products are strongly affected by the electric charge of the radon decay product, 218Po. The charge of 218Po has an influence on its mobility, characterised by the diffusion coefficient, that chiefly controls the formation of the radioactive aerosol by attachment and the plate-out processes on surfaces. The neutralisation rate of the positive 218Po-clusters in air was determined quantitatively in chamber experiments. The experimental results show the strong influence of the ionisation rate and the humidity concentration on the neutralisation rate of the 218Po ions in air. In air with relative humidity between 30% and 95% and ionisation rates between 20 and 2000 μR h−1 (5.1–510×10−9 Ckg−1 h−1) mainly influenced by the variation of the ionisation rate by radon, thoron, and their decay products the neutralisation rate varies between 40 and 300 h−1. In addition, the fraction of the positive 218Po-clusters for typical air conditions was determined by model calculations and measurements. In environmental air 10–60% of the unattached 218Po-clusters are positively charged. The positive fraction depends mainly on the ionisation rate. A charged fraction of the freshly formed 218Po ions of (87, 3±1.6)% was measured.