Die Anlagerung radioaktiver Atome an Aerosole (Schwebstoffe)

Abstract

The attachment of the decay products of thorium emanation to aerosol particles has been studied. The dependence of the attached activity on the particle size was determined for spherical particles with radiiR ranging from 0·04 to 0·6 microns. The particles used were homogeneous dioctylphthalate droplets and polystyrene micro-spheres. It is found that the attached activity is proportional toR 2/(1+hR). This dependence can be derived theoretically by considering the deposition to be solely governed by the diffusion process (not by electrostatic forces) and assuming a quasi-stationary density distribution for the diffusing atoms. The constanth is uniquely determined by the average gaskinetic velocity and the diffusion constant of the diffusing atoms. For the decay products of thorium and radium emanation (atomic weight ≈210)h ≈ 7 · 104 cm−1. The derived equation holds for a wide range of particle sizes: For the particles with radii larger than about 10−4 cm this means that the attachment is proportional to the radius; for particle radii below about 10−6 cm it is proportional to the surface (R2) of the particles. It is also possible to derive an expression for the time-dependence of the attachment process from the theoretical considerations. The rate at which the average concentration of the radioactive atoms decreases is proportional to exp −t/τ where τ=1+hR/πR2 N¯ v (¯ v=average gaskinetic velocity of the diffusing atoms;N=aerosol concentration).