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

Two types of trapped hydrogen atoms (Ht) were found in a natural kaolinite, Kanpaku, irradiated with an ionizing radiation at 77 K. One (Ht(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 Ht. The other (Ht(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 Ht(i) may be a crystalline defect, whose density is usually small and consumed by radiolysis. In the case of synthetic kaolinite, both types of Ht's were also observed as the main components, which were interpreted with the same model as above. In addition, another Ht showing a sharp signal with a short lifetime was found in the non-crystalline part.