Microstructural changes of graphite irradiated with swift heavy ions

Several methods, such as electron microscopy, scanning tunnelling microscopy (STM), X-ray diffraction, and decoration technique can be used to observe radiation damage in materials. In this work, we report microstructural changes and defect clusters induced by swift heavy ions (SHI) on graphite surfaces, visualized by scanning electron microscopy (SEM). Our studies have been performed on fine-grained polycrystalline graphite (grade R6650, SGL Carbon) which is proposed as material for beam catchers and rotating target at the Super-FRS of FAIR [1]). In addition, we extended our investigations to highly oriented pyrolytic graphite (HOPG, ZYB, NTMDT company) as model material consisting of layered graphite sheets. SEM images of polycrystalline graphite irradiated with 11.1 MeV/u U ions at high fluences show a decrease of the crystallite size in comparison to the pristine samples (Fig. 1). New irregular crystallites form on the surface in a non-linear manner. This “milling” effect of swift heavy ion beams has been previously inferred from the evolution of Raman spectra as a function of fluence [2]. Highmagnification SEM images of the irradiated grain surface, in the secondary electron imaging (SEI) mode, show the presence of many point-like features with bright contrast and characteristic diameters of about 10 nm. Fig. 2 illustrates these features on a surface grain irradiated at a fluence of 5×10 U-ions/cm in comparison to the smooth defect-free surface of the pristine sample. The counted density of the features is one order of magnitude smaller than the ion track density, therefore we would rather attribute them to defect clusters than to single ion tracks. To better understand the nature of these defects, highresolution images of HOPG samples irradiated with Pbions at low (5×10 ions/cm) and high (5×10 ions/cm) fluences are shown in Fig. 3. In both cases, similar brightcontrast defect structures appear. They are larger for the low fluence sample and have a higher number density and smaller size for the large fluence sample. There are several reports on the formation of prismatic dislocation loops of interstitial nature in neutron irradiated graphite [3] and vacancy prismatic loops in quenched graphite annealed at 1300 oC [4]. This type of defects was observed by TEM [4] and STM [5]. Given by the similarity, we attribute our SEM observations in SHI-irradiated graphite to interstitial prismatic dislocation loops. They are smaller for the high fluence samples due to the higher nucleation rate at supersaturated point-defect concentrations when tracks overlap. In the low-fluence sample, the nucleation rate is lower and the defects tend to aggregate into larger clusters. Also shear as shown by the large loop with bright edges in Fig. 3a or dissociation into two loops showing stacking fault contrast (as seen in the upper left corner of Fig. 3) takes place. Additional STM on oxidized irradiated samples will be performed to support these SEM observations. Prismatic dislocation loops are sessile defects and responsible for irradiation-induced hardening by preventing dislocation movement via pinning, as seen in graphite irradiated at high fluence [6].