L. Lassen

Die Anlagerung radioaktiver Atome an Aerosole (Schwebstoffe)

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 toR2/(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/πR2N¯ v (¯ v=average gaskinetic velocity of the diffusing atoms;N=aerosol concentration).

Die Anlagerung von Zerfallsprodukten der natürlichen Emanationen an elektrisch geladene Aerosole (Schwebstoffe)

AbstractIn a previous paper an equation was derived for the attachment of the decay products of radon and thoron to aerosol particles. Considering the deposition to be solely governed bythermal diffusion (not by electrostatic forces) and assuming a quasi-stationary density distribution for the diffusing atoms, it was shown that the attached activity is proportional to

Coulomb and nuclear excitation in the sequential break-up of6Li

R^2 /(1 + h R)

Three-particle correlations from208Pb+6Li

whereR is the particle radius andh=7·104 cm−1 at NTP.As most aerosols in nature are electrically charged it is shown in the following paper how the diffusion process is modified by the influence of electrostatic forces between diffusing ions and charged aerosol particles. For the frequently occuring case of asymmetrically bipolar charged aerosol andion diffusion it is furthermore shown that the attached activity is proportional to

Sequential emission of neutrons in the reaction16O+64Ni at 7.5–12 MeV per nucleon

\frac{{R^2 F(R,x)}}{{(1 + hR)}}

Intensity interferometry with neutrons from the compound nucleus82Kr

wherex is dependent on the electrostatic charge and the radius of the aerosol particles. Applying this equation to the deposition of natural radioactivity on atmospheric aerosols and assuming that particle size-distribution can be roughly approximated by a lawN (R)=const·R−3 in the range 10−6 cm≦R≦10−3 cm calculation shows that 90 to 95% of the total natural radioactivity should be attached to particles smaller thanR=5·10−5 cm and as much as 50 to 75% to particles smaller thanR=5·106 cm. It is concluded that the distribution of natural radioactivity on the different particle sizes in atmospheric and artificial aerosols is fundamentally dependent upon the diffusion process, including ionic diffusion.