RADIATION-INDUCED PARAMAGNETIC DEFECTS IN NATURAL KAOLINITES - ALPHA-DOSIMETRY WITH ION-BEAM IRRADIATION

Three types of paramagnetic radiation-induced defects (RID), namely A, A' (Si-O--centers) and B (Al-O--Al center), had been identified in na tural kaolinites by means of electron paramagnetic resonance spectrosc opy. The A-center, stable at the scale of geological periods, was thou ght to be of particular relevance to quantify past transits of radionu clides in the geosphere. Alpha radiation being likely the main source of RID in kaolinite, the objective of this paper is to define the role of a-particles on the creation of RID and to test the use of A-center s for an a-dosimetry. Three kaolinites with different crystalline orde r and containing other clays as impurities were irradiated with He+ io n beams. The radiation dose range (0-750 MGy) was consistent with natu ral radioactivity in environments from the Earth's surface. Irradiatio n drastically enhanced the original signals due to RID. An important i ncrease of concentration of the unstable B-center, partly due to unrea listic dose rate provided by accelerator ion beam, was observed from t he lower doses. The most stable defects remained of the A-type all alo ng experimental irradiations. The contribution of ancillary phyllosili cates to EPR spectra was negligible. The concentration of the three ty pes of RID was related to radiation doses up to 30 MGy. Dosimetry grow th curves for the A-centers exhibited variable efficiencies and satura tion levels that were related to the structural order and the chemical purity of the kaolinites: the more ordered and purer the kaolinite, t he higher the efficiency and the lower the saturation plateau. Moreove r, these results are of geochemical significance: dosimetry based on A -centers could be directly used to quantify past migrations of radioel ements in the geosphere, by determination of the naturally-cumulated d oses (paleodose) of kaolinites.