Induced modifications of kaolinite under ionizing radiation: an infrared spectroscopic study

Radiation effects on kaolinite were investigated using He+ ions of 1.5 MeV at radiation doses up to 4.3 x 10(8) Gy, which are comparable to the doses expected for clay barriers in high-level nuclear waste repositories. The concentration of paramagnetic radiation-induced defects in kaolinite reaches 2 x 10(16) spins/mg (400 at. ppm), as determined by electron paramagnetic resonance spectroscopy. The broadening of X-ray diffraction patterns and transmission infrared (IR) absorption bands is mostly related to the structural strain induced by radiation-induced point defects. The broadening of IR absorption spectra is analyzed using an autocorrelation approach and is related to a change in the distribution of vibrational frequencies due to crystal heterogeneities. We theoretically analyze how the effective dielectric properties of kaolinite samples depend on macroscopic parameters and how irradiation can modify some of them. Irradiation leads to an increase in the electronic polarizability of kaolinite particles, related to the accumulation of radiation-induced electronic point defects.