Experimental calculations of droplet diffusion in a low-pressure cloud chamber

A low-pressure cloud chamber was used for several years to display the tracks created by the passage of ionizing particles through vapors of interest. The spatial distributions of the ions that were formed were of special interest, but the accuracy with which these distributions could be determined was reduced by the presence of diffusion. This meant that the droplets, when photographed, had moved significantly away from the point of creation of the parent ion. In the present investigation photographs obtained by previous workers have been analyzed in an attempt to quantify the extent to which the droplets had diffused. The results suggest that the diffusion, when converted to standard density (1000 kg/m3), was independent of the pressure inside the cloud chamber and the mixture used. It could be represented by a one-dimensional root-mean-square diffusion distance whose value was calculated to be 2.42 +/- 0.04 nm. Values for the diffusion of thermalized electrons (< approximately 4 eV) before capture to form negative ions were also calculated. They appeared to lie in the range 3.5-5.0 nm, and were again independent of the pressure and nature of the mixture. The magnitude of the diffusion was large enough to mask any measurable prediffusion structure for a distance in the region of 10 nm radially around the track path of the alpha-particle and proton tracks analyzed.