The determination of radon exhalation rates.

The exhalation of radon from porous materials like mining wastes and building materials can be determined by studying the growth of radon activity in vessels containing samples of the materials. It is demonstrated that the initial part of the activity growth curve determines the total free exhalation rate of the sample used, independently of container leakage and back diffusion. It is furthermore shown that a specific area exhalation rate adequately describes the exhalation process, when the exhalation is restricted to take place in one direction only, and when the dimension in the exhaling direction is greater than approx. 2 times the diffusion length. A determination of the free surface exhalation rate as a function of the sample thickness also yields information on the diffusion length, as well as the radon production rate per unit volume of material. In case of sample dimensions smaller than about half the diffusion length, the total exhalation rate is shown to be proportional to the mass or volume of the sample.