129I determination by direct gamma-X spectrometry and its application to concentration variations in two seaweed species

The quantification of radionuclides by direct gamma-X spectrometry with energy below 100 keV requires knowledge the elementary composition of the sample or the development of a device for determining the mass attenuation coefficients. This is especially true for 129 I which is characterised by a 29.8 keV X-ray and 39.6 keV gamma ray. Experimental equipment has been developed in order to obtain this mass attenuation coefficient as a function of energy. 129 I concentrations were measured in samples of seaweed (Fucus serratus and Laminaria digitata) collected monthly over a period of one year nearby La Hague reprocessing plant in France. This paper describes the measurement methodology used to determine 129 I concentrations and variations in the two seaweeds over a one-year period. Mean mass attenuation coefficients for 129 I energies were established in order to determine the self-attenuation corrective factor for both seaweed species, regardless of the sampling date. Iodine-129 is a beta emitter (T1/2 = 1.57 × 10 7 years) with both natural and artificially produced radionuclides existing in the environment. This radionuclide will decay to emit photons of energies below 40 keV at high intensity. Direct gamma-X spectrometry is therefore well suited to quantifying this radionuclide, provided that self-attenuation corrections are applied. These corrections must not only take into account the density but also the elementary composition of the matrix seeing that the photoelectrical process becomes the dominating interaction phenomenon under 100 keV. We recently developed a corrective method based on transmission measurements (1-2) to determine these self-attenuation correction factors in relation to standard calibration sources. Temporal variations in the self-attenuation coefficient are related to variations in the elementary composition of the matrix, including variations in iodine in the case of seaweed. Our study focused on brown seaweed, more specifically Fucus serratus and Laminaria digitata, which are used as bio-indicators in environmental studies to monitor various pollutants (3-9). The specificity of such seaweed resides in their sedentary nature on the one hand and the fact that they concentrate elements that are only soluble in their surrounding environment on the other hand.