Droplet growth and diffusion in a low-pressure cloud chamber

A low-pressure cloud chamber which can make visible individual ions in the tracks of ionizing particles is available at Harwell. The limitation on the spatial accuracy of the information obtained is the diffusion of droplets during their growth. The initial stages of droplet growth are particularly important since most of this diffusion takes place while the 'droplet' contains only a few molecules. This paper discusses the early stages of droplet growth from ions in a low-pressure cloud chamber and derives values for droplet diffusion during growth. The growth rates for the first few attachments to an ion are derived from chemical reaction data. Later growth rates are predicted using conventional droplet growth-rate equations. The effects of droplet heating are included as these are significant in a low-pressure chamber. The values derived for diffusion are compared with estimates based on comparing the expected shape of particle tracks with their observed shape in the chamber. (author)