EXPERIMENTAL-STUDY OF ION-INDUCED NUCLEATION BY RADON DECAY

In the environment, the presence of ions from natural radioactivity ma y increase the rate of new particle formation through ion-induced nucl eation. A thermal diffusion cloud chamber (TDCC) has been built to exp erimentally study ion-induced nucleation where the ions are produced b y gaseous radioactive sources. The critical supersaturation values and nucleation rates for methanol, ethanol, 1-propanol, and 1-butanol vap ors on ions produced within the volume of the chamber by alpha decay o f Rn-222 have been measured quantitatively at various radioactivity co ncentrations and supersaturations. The presence of ion tracks and the effect of an external electric field were also investigated. The alpha tracks and ion-induced nucleation formed by Rn-222 Rn decay become vi sible at the critical supersaturation that is below the value needed f or homogeneous nucleation. At this supersaturation, the nucleation rat es increase substantially with increasing Rn-222 at low activity conce ntrations, but attain limiting values at higher concentrations. The ex perimental results indicate that the ionization by radon decay will pr omote ion-cluster formation and lower the free energy barriers. The fo rmation of visible droplets is strongly dependent on the supersaturati on. This study also confirms that the external electric field has a si gnificant effect on the observed rates of nucleation.