Attachment of decay products to nuclei

Careful reviews by Raes (1985) and Raes et al. (1985) leave unanswered the question of the role of humidity, and of acid or organic vapours, in modifying the diffusivity of decay product ions. By comparison with the mobility in normal air of decay product and ordinary atmospheric small ions, the diffusivity of decay product small ions is probably 2 to 3 x 10-6 m2 s-1. For neutral atoms, or possibly oxide molecules, most measurements give D in the range 5 to 8 x 10-6 m2 s-1, except where radiolytic reaction products or reactive trace gases are present in sufficient concentration to form intermediate ions. 

Attachment is mainly by diffusion, although, if the decay products are ions, electrostatic attraction to charged nuclei of opposite sign makes a small additional effect (Bricard Pradel, 1966). The rate constant for attachment XA, is given by an equation originally applied to evaporation from small droplets (Fuchs, 1959). 

A is about 15 nm, and if r is much less than this the second term in the denominator of (1.18) is small and 

For particles of about 1 /tm radius, r(r + A)-1 is near unity. Either term in the denominator of (1.18) may then be dominant, depending on whether D(rVma) l is greater or less than unity. At room temperature and pressure, Vm for a decay product atom is 43 m s-1 and if D = 7 x 10-6 m2 s-1 and r = 1 m, the condition D(rVma) l 1 is equivalent to a 0.14. Thus if a is less than 0.1, its value determines XA, which then depends on r2 up to r = 1 iim. 

As the particle size increases further, a becomes unimportant unless it is very small and (1.18) reduces to Smoluchowski s equation 

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Attachment of decay products to nuclei greatly affects the process of deposition, because the Brownian diffusivity of nuclei is typically about four orders of magnitude less than the molecular diffusivity of unattached decay products. The lifetime of decay products in air before deposition on surfaces is shorter if the air is clean than if it is dirty. 

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