12 FREILING, CROCKER, AND ADAMS Mechanism With the help of Fig. 6, the mechanism can be described in the simple case from the viewpoint of a condensable molecule in the vapor phase. To be condensed, the molecule mustfirst diffuse through the gas to the surface of a particle. Upon striking the surface the molecule can either rebound into the vapor or cling to the surface. After LINGERING AND DIFFUSION iN SURFACE PHASE Fig. 6 DIFFUSION | INTO INTERIOR Mechanism for kinetic approach to radionuclide incorporation by fallout particles. clinging to the surface awhile, the molecule may reevaporate or dif- fuse into the bulk of the particle. If it reevaporates, it can easily be deflected back to the surface by the first collision with another vapor molecule. Inside the particle the molecule may diffuse back to the sur- face. In the actual case these processes will be occurring while the temperature is dropping. Simultaneous condensation of macroscopic quantities of evaporated carrier can occur at early stages. Particles can collide and stick. Various chemical reactions and changes of species can occur at different steps of the way. Radioactive transformations will proceed concurrently. Evaporation of volatile daughters of condensed nuclides may be important, and radiation effects may be considerable. A rigorous treatment would obviously be highly complex. However, we can visualize certain simplifications under various conditions, Equilibrium Approximation One such simplification is the thermodynamic approach already described.*> This will apply when the times for condensation and diffusion are short in comparison to the rates of radioactive decay and temperature decrease. This approximation will be