NATURE AND STABILITY OF RADIATION-INDUCED DEFECTS IN NATURAL KAOLINITES - NEW RESULTS AND A REAPPRAISAL OF PUBLISHED WORKS

A new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented kaolinite samples originating from various environments. Three different centers were identified. Two of the centers, A- and A'-centers, are trapped holes on oxygen from Si-O bonds. They have a distinct signature and orthogonal orientation, i.e., perpendicular and parallel to the (ab) plane, respectively. The third center, the B-center, is a hole trapped on the oxygen bonding Al in adjacent octahedral positions (Al(VI)-O--Al(VI) bridge). This confirmed some previous assignments from the literature, some others are no longer considered as valid. A least squares fitting procedure is proposed to assess the RID concentration in any kaolinite. It allows a quantitative approach of the thermal stability of RID. Isochronal annealing shows that the thermal stability of the centers decreases in the order A, A', B over the temperature range 0-450-degrees-C: (1) B-center is completely annealed above 300-degrees-C; (2) A'-center can be annealed by heating at 400-degrees-C for more than two hours; (3) A-center is stable up to 450-degrees-C. The activation energy and the magnitude of the mean half-life for A-center is evaluated through isothermal annealing at 350, 375 and 400-degrees-C, with E(a) = 2.0 eV +/- 0.2, and t1/2 > 10(12) years at 300 K. The stability of A-center seems to decrease with increasing crystalline disorder. Nevertheless, it is high enough for radiation dosimetry using kaolinites from any environment on the Earth's surface.