Temperature-Dependent Behavior Of Impurity Atoms Implanted In Highly Oriented Pyrolytic Graphite - An Application Of A New Online Tdpac Method

The behavior of impurity atoms in graphite is one of the most intriguing subjects in current materials science. It is well known, for graphite intercalation compounds, that the type and concentration of intercalated impurities control their physical and chemical properties. It is thus of great importance to investigate the behavior of intercalants in the two dimensional layers for further understanding of their interacting nature with the graphite matrix. In order to investigate such interactions, it is essential to introduce impurities of interest into graphite; and, for that purpose, the ion-implantation method would be the simplest method. Nuclear techniques with radioactive probes are highly suited to such studies on the behavior of dilute impurities because of their high sensitivity. In our recent study, we developed a new detection apparatus for radioactive impurities: an online time-differential perturbed angular correlation (TDPAC) measurement system. The TDPAC method is a spectroscopy using unstable nuclei as the probe; by observing the time-variant angular correlation of rays successively emitted in the disintegration process of the excited nucleus of the probe, we can obtain direct information on the local field in the vicinity of the probe nucleus through electromagnetic interactions between the probe nucleus and extranuclear field. The newly developed system enables the application of shortlived unstable nuclei to the spectroscopy as the parents of probes; they are implanted in samples of interest at high energy immediately after their production by acceleratorbased nuclear reactions. In our previous paper, we demonstrated the applicability of the present online system and reported on the residence site of F( O) implanted in highly oriented pyrolytic graphite (HOPG). In comparison with the previous data obtained at 18K, we here discuss the displacement of the F( O) nuclei at room temperature on the basis of the damping trend of the TDPAC spectra. The online TDPAC experiment was carried out at the RIKEN Nishina Center. Ionized Ne was accelerated by a two-stage acceleration with the AVF cyclotron and the ring cyclotron up to 110MeV/u at a beam intensity of 150 pnA. A variety of radioactive nuclides were produced at a Be production target by projectile-fragmentation reactions; the secondary beam of interest, O (t1=2 1⁄4 26:9 s), was separated out of these heavy ions at a purity of 98% or higher by the RIKEN projectile-fragment separator (RIPS). A wellfocused pulsed beam of O was implanted deep in an HOPG sheet at a beam intensity of 10 s . The online TDPAC measurements were performed at room temperature on the 1357–197 keV cascade rays from F nuclei, whose 197 keV intermediate state with nuclear spin of I 1⁄4 5=2 has a half-life of 89.3 ns. In the present work, the time evolution of the angular correlations of the cascade rays was observed at =2 and directions. In order to obtain better counting statistics, we used sixteen BaF2 detectors. They were arranged in four independent detector planes for the investigation of the sample-to-detector configuration dependence of perturbation patterns for a single-crystalline sample. All the events were taken in a list mode using a CAMAC system so that we could work on off-line data processing searching for the optimum conditions on energy gates for the detected rays of interest. The experimental details are described elsewhere. The TDPAC spectra of F( O) implanted in the HOPG sheet at room temperature are shown in Fig. 1 together with the previous data obtained at 18K. In the present experiment, we derive the time evolution of the angular correlation as a function of the time interval between the cascade -ray emissions, RðtÞ, from the following simple arithmetic operation: -0.2