Time-differential perturbed angular correlations of 140Ce and 19F implanted in highly oriented pyrolytic graphite

Graphite-intercalated compounds have been investigated from various aspects, and it has been found that they show unique properties depending on the doped elements. Zabel et al. succeeded, for instance, in observing dynamic motion of intercalate alkali-metal atoms (Li, K, Rb, and Cs) in graphite, estimating the diffusion constants and activation energies for each of the atoms, whereas doped borons are reported substituting for the carbon atoms rather than being intercalated in the layers. Because doped graphite is expected to exhibit unique properties on account of its two dimensional structure, we have further investigated the behavior of other elements, Ce and F, implanted in highly oriented pyrolytic graphite (HOPG) by means of the time-differential perturbed angular correlation (TDPAC) method. Implantation of a fission product, 140Cs, into an HOPG sheet was performed at the isotope separator on-line at Kyoto University Reactor. The dopant disintegrates into a TDPAC probe 140Ce in the decay process. TDPAC measurements were performed on the 329-487 keV cascade γ rays with the intermediate state of 4' having a half life of 3.5 ns. In the present work, the directional anisotropy of the angular correlations of the cascade γ rays was observed at π/2- and π-radian directions with a conventional four-detector system. For the coincidence detection, BaF2 scintillation counters were adopted. (orig.)