Trapped hydrogen atoms radiolytically formed in natural and synthetic kaolinites: an electron paramagnetic resonance study

Abstract

Two types of trapped hydrogen atoms (H_t) were found in a natural kaolinite, Kanpaku, irradiated with an ionizing radiation at 77 K. One (H_t(o)) showed a hyperfine coupling (hfc) of 1435 MHz, which is larger than 1420 MHz for the free hydrogen atom. The line-width of the EPR signal is very wide (ca. 0.6 mT) and its half-life is as long as several minutes even at 250 K. From these characteristics we deduced a small atomic cage consisting of six oxygen atoms (and six aluminium atoms in the next nearest shell) found in the crystalline structure of kaolinite as the trapping site for this H_t. The other (H_t(i)) showed an EPR spectrum with a hfc a little smaller than that of the free hydrogen atom. The yield of this component was saturated at a low level with a dose of above 10 kGy, and it decreased further by repeated irradiation after annealing. Thus, the trapping site of H_t(i) may be a crystalline defect, whose density is usually small and consumed by radiolysis. In the case of synthetic kaolinite, both types of H_t's were also observed as the main components, which were interpreted with the same model as above. In addition, another H_t showing a sharp signal with a short lifetime was found in the non-crystalline part.