Diffusion cloud chamber operation and the background gas effect

Key design and operational aspects for thermal diffusion cloud chamber (TDCC) applications are discussed in the context of a two-dimensional solution to the mass and energy balances describing diffusion through a stagnant background gas. The important issue of buoyancy-driven convective disturbances and their impact upon nucleation measurements made using a diffusion cloud chamber are discussed. A new derivation of the relation that predicts the upper limit of total pressure allowed for stable (the absence of buoyancy-driven convective disturbances) operation of the diffusion cloud chamber is presented. For the first time, this limit of stable operation can be predicted prior to making experimental measurements. Nucleation data obtained in our laboratory are examined in the context of this predicted limit of stable operation. New nucleation data are presented for 1-pentanol using helium as a background gas. Only data corresponding to stable operation in the cloud chamber is used in the analysis. The effect of background gas on nucleation we have reported previously is confirmed for 1-pentanol, as well as for all the other alcohols that have been investigated in our laboratory.