EFFECT OF RADON DIFFUSION IN VOLUME SAMPLES ON DETECTION EFFICIENCY FOR GAMMA SPECTROMETRICAL MEASUREMENT OF Ra-226 DAUGHTERS
FIRST INTERNATIONAL WORKSHOP ON INDOOR RADON REMEDIAL ACTION
THE SCIENTIFIC BASIS AND THE PRACTICAL IMPLICATIONS
Rimini, Italy, 27 June - 2 July, 1993
ABSTRACT FORM
O.Sima – University of Bucharest
C.Dovlete, I.Osvath, F.Baciu, Gy.Ruzsa - Environmental Radioactivity Laboratory - Bucharest
Non-destructive high resolution gamma spectrometry is a basic method applied in radioactivity studies related to environmental and health physics.
Direct evaluation of Ra-226 from the 186 keV line in gamma spectra of samples not previously subjected to radiochemical processing can however raise certain analytical problems. Therefore, frequently the content of this radionuclide in environmental and building materials is estimated using gamma lines of its daughters and assuming that radioactive equilibrium exists within the sample among the members of the decay series. Experimental tests have shown that the validity of this assumption depends only upon the fraction of radon available for diffusion in the sample and upon the permeability of the sample material and container walls to radon gas. Given the range of permeabilities involved, it is usually the case in routine measurements that only the nondiffusible component of radon remains in the sample. Hence, the degree of equilibrium is equal to the fraction of nondiffusible radon. Regarding the diffusible component, the degree of container imperviousness controls not only the total amount of the radon remaining in the sample, but also its spatial distribution, and thus, the spatial distribution of its daughters. Consequently, at least in principle, it is incorrect to use a detector efficiency corresponding to a spatially homogeneous source for quantifying these radionuclides. To see the extent of this effect, we derived analytical solutions for the spatial distribution of radon and daughters, corresponding to various boundary conditions, for one of our standard sample geometries (cylindrical, diameter 70 mm, height 30 mm). We subsequently computed the HPGe detection efficiency for the resulting spatial distributions, using a realistic Monte-Carlo simulation method.
Numerical simulations indicate that radon diffusion within the sample and through container walls generates inhomogeneities in the spatial distribution of radon daughters in the sample, which, for levels usually detectable by high resolution gamma spectrometry, affect detection efficiency by less than 1%. At very low levels, the inhomogeneity and associated change in computed detection efficiency as compared to a homogeneous source, become important, but without significant practical consequences.
Similar studies are underway for other measurement geometries.
Experiments and numerical simulations will also be carried out for the much shorter-lived thoron.