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

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)) show ed a hyperfine coupling (hfc) of 1435 MHz, which is larger than 1420 MHz fo r 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. Fr om these characteristics we deduced a small atomic cage consisting of six o xygen atoms (and six aluminium atoms in the next nearest shell) found in th e 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 tha t 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 repeate d irradiation after annealing. Thus, the trapping site of H-t(i) may be a c rystalline defect, whose density is usually small and consumed by radiolysi s. In the case of synthetic kaolinite, both types of H-t's were also observ ed as the main components, which were interpreted with the same model as ab ove. In addition, another H-t showing a sharp signal with a short lifetime was found in the non-crystalline part.