Determination of self-absorption corrections for gamma analysis of environmental samples: comparing gamma-absorption curves and spiked matrix-matched samples.

Accurate determination of the massic activity of gamma-emitting radionuclides in environmental samples, particularly sediments and soils, cannot be achieved without taking into account sample self-absorption. The extent of self-absorption in the sample will depend on a number of factors including sample composition, density, sample size and gamma-ray energy. The preferred method for correcting for this effect is to use spiked or natural matrix reference materials that match each sample type to be analysed. However, for laboratories that must measure a wide variety of sample matrices this method is too costly and time-consuming. Another technique commonly used is to make direct gamma-ray transmission measurements for each sample. This method, while more practical, still requires a minimum of three measurements to be made for each sample analysed. A more convenient method is to prepare sets of gamma-absorption curves. This approach involves making a series of direct transmission measurements for samples of varying densities but similar type. Sets of matching samples, both spiked and unspiked, were prepared and density correction factors determined using the direct transmission method and the spiked sample approach. It was found that, for soil and sediment samples, these two methods typically differed by 5-9% for higher energy gamma rays and by 12-15% for the 59.54 keV 241Am peak. Gamma-absorption curves were also derived and, for the admittedly limited dataset, 95% confidence intervals of +/-7% for the curve generated using the spiked samples method were obtained.