NUCLEAR-DEBRIS FORMATION 17 for H, and 0.138 cm?/sec for CO,. The application of the equation re- quires a knowledge of the molecular species of the diffusing substance. For cases of interest in fallout formation, this is usually unknown. The case where diffusion through the gas is rate determining has been studied theoretically by Fuchs!’ and theoretically and experi- mentally by Lassen, Rau, and Weicksel.'® These authors applied Fick’s second law to the case of steady-state condensation on a spherical particle of radius R on the assumption that the law is still valid ata distance of one mean free path, A, from the surface. They further as- sumed that the flux density on the surface is that expected for the concentration at the distance \’. They thus obtained the expression for the total flux at the surface ty 27m "hR?+ R+2 where n,, is the concentration of diffusing atoms at a large distance from the sphere and h is (kT /21m)*/D. At0°C and 1 atm pressure, Ais of the order of 0.1u. The equation predicts that, for values of R > d (i.e., ~1 ), attachment is proportional to the first power of the diame- ter; whereas, for particles much smaller (0.01 yu), it is proportional to the second power. The effect was produced experimentally by Lassen and coworkers but has not been observed in fallout particles. This does not necessarily mean that transport of matter through the gas phase is never rate controlling. It may mean simply that turbulence, flow, de- pletion, and charge effects nullify the applicability ofan approach based on field-free steady-state diffusion through the gas phase. Collision with the Surface The number of type i molecules striking a unit area of surface per unit time is given by kinetic theory as The mean velocity v; is equal to (8kT/mm; y*, therefore Ji ERPnj ( kT % 27m, For O, and N, at room temperature, v, is approximately equal to the speed of sound in air, 5 x 10* cm/sec. Upon collision these molecules can either cling to the surface (condense) or rebound. The fraction that condenses is usually signified by a and called the condensation coefficient. It is frequently called the accommodation coefficient, but this term is best reserved to describe the fraction of possible heat exchange actually experienced by the molecule in bouncing off the surface. The condensation coefficient is