A mobility spectrometer for measurement of initial properties of Po

Reports in the literature on the measurement of the diffusive properties of ²¹⁸Po in the so-called unattached state yield conflicting results. It is now accepted that the ²¹⁸Po ion does not remain a single atom but interacts with trace vapours in the atmosphere, forming larger radioactive particles in the 0.5–3 nm size range. The mechanisms through which these interactions occur are unexplained. Recent application of the theory of ion-induced nucleation to the ²¹⁸Po ion showed that this theory can explain the experimental results reported in the literature qualitatively. The initial phases of ion-induced nucleation involve the formation of pure water clusters around which molecules of other condensable trace vapours may nucleate. This happens within microseconds after the formation of the ion. Methods available to study this phenomenon include diffusion screens, which do not have very high resolution, and electric mobility spectrometers, which can only provide information on charged particles, as is the case for the initial stages of the ²¹⁸Po ion's life before neutralization. Ion-induced nucleation can furthermore only occur for charged particles. This paper describes the development of a mobility spectrometer with adequate resolution to study the clustering processes that may occur. The first results of measurements of pure water clustering around ²¹⁸Po ions using the spectrometer are presented. It is found that the radius of the ion increases with relative humidity. The results are compared with predictions of two representations of the clustering theory. The theory qualitatively explains the results obtained, but there are considerable differences between the experimental measurements and the theoretical predictions, as well as between the two representations of the theory.