Particle size and radionuclide levels in some west Cumbrian soils

Four west Cumbrian soils of contrasting types, together with an estuarine silt sample, have been separated into different particle size fractions by a combination of sieving and settling techniques. The sub-samples thus isolated were analysed by quantitative gamma-ray spectrometry for several nuclides, principally 137Cs, 106Ru and 241Am, followed by chemical separation and alpha spectrometric determination of 238,239 & 240Pu. A simple empirical method of correction for differing sample sizes, and hence counting geometries, was developed for gamma spectrometry and was found to give good results. The correction technique is based on calibrating each detector over a range of sample depths in the container using a multi-nuclide standard, and hence calculating an efficiency correction factor. In all the samples, the radionuclides were concentrated into the finer size fractions, with the clay-sized (< 2 μm) particles showing enrichments of plutonium from 1.5 to 40 times that in the bulk samples, of americium from 2 to 40 times and of 137Cs from 3 to 35 times. The enhancement was greatest for all radionuclides in a sandy soil with a very low clay content (0.2% by weight) and it was found that, as the abundance of fine particles increased, so the concentration effect decreased. No evidence was found for a simple relationship between organic content and radionuclide activity, although the organic matter does have some effect.