A. Benyagoub

Athermal crystallization induced by electronic excitations in ion-irradiated silicon carbide

Silicon carbide single crystals were irradiated at room temperature with low energy I ions and high energy Pb ions. It is found that the damaged layer formed by the elastic collisions generated during low energy I ion irradiation can readily be removed by the electronic excitations induced by swift Pb ions. This effect occurs at a temperature quite below that at which the conventional ion-beam induced crystallization process is generally achieved by nuclear energy loss. This finding is interesting both from a fundamental point of view for the understanding of the interaction of swift heavy ions with solids and for a large number of technological applications.

Evidence of a phase transition induced in zirconia by high energy heavy ions

Monoclinic zirconia samples were irradiated with 300 MeV Ge and Ni ions at increasing ion fluences. Their structural evolution was monitored in situ by x-ray diffraction and ex situ by Raman spectroscopy. No amorphization of zirconia was observed in both cases. However, a transition from the monoclinic to the tetragonal phase was found in the case of Ge ion irradiation. On the contrary, no such effect was detected upon Ni ion irradiation. A comparison of these experiments indicates that the electronic energy loss released by swift heavy ion irradiation needs to be quite in excess of ∼12 keV nm−1 in order to induce a monoclinic to a tetragonal phase transition in pure (i.e., unstabilized) zirconia.

Phase transformation induced in pure zirconia by high energy heavy ion irradiation

Abstract Samples of monoclinic zirconia were irradiated with heavy ions having incident energies in the range of a few hundred MeV giving then rise to a slowing down essentially caused by high electronic excitations. The characterizations of the samples by X-ray diffraction and complementary Raman spectroscopy analyses revealed two main features. First, in the electronic stopping power regime, it is only when the electronic energy loss is above a threshold near 13 keV nm−1 that monoclinic zirconia undergoes a transformation to the tetragonal phase. Second, the evolution of the amount of the tetragonal phase with the ion fluence exhibits a sigmoidal shape suggesting a mechanism for phase transformation which very likely needs two ion impacts.

Swelling of insulators induced by swift heavy ions

Abstract In many different insulators, the irradiation with heavy ions in the MeV to GeV energy regime induces pronounced volume expansion. This swelling increases with fluence, projected range and electronic stopping power of the ions. From the relative volume change per single ion as a function of the mean energy loss, a critical swelling threshold can be deduced. In amorphizable materials such as quartz and garnets (e.g. Y3Fe5O12 and Gd3Ga5O12), swelling is directly linked to the crystalline–amorphous phase change in each ion track, whereas for non-amorphizable ionic crystals (e.g. LiF and CaF2), the correlation to known defects is not yet clear.

Swift heavy ion induced plastic deformation

Abstract Electronic energy loss, which dominates the slowing down of swift heavy ions in solids, leads to atomic rearrangements depending on the nature of the irradiated target. Amorphous materials are subject to structural changes similar to those observed in crystalline materials and undergo anisotropic plastic deformation. The former process occurs at the beginning of the irradiation while the latter one takes place above an incubation fluence depending on the nature of the target. This plastic deformation consists of a steady increase of the sample dimensions perpendicular to the ion-beam direction together with a decrease of the dimension parallel to it, at constant volume.

Heavy-ion induced damage in fluorite nanopowder

Abstract Compressed pellets of 40 nm sized CaF2 powder were exposed to Pb ions of about 4 MeV/u. The structural change of the irradiated crystals was monitored by in situ X-ray diffraction experiments. The evolution of the diffraction spectra as a function of the ion fluence gives evidence for two different phenomena: (1) the area of the diffraction peaks slowly decreases due to the loss of the crystalline phase of CaF2, and (2) the width of the peaks broadens at a much faster rate. We assume that this latter observation can be ascribed to a grain breaking process.

Radiation-induced compaction and plastic flow of vitreous silica

It is well known that under swift heavy ion irradiation vitreous silica is subject to compaction as well as to plastic flow. The aim of this contribution is to demonstrate that both phenomena can be understood on the basis of the thermal spike concept and the fact that molten silica exhibits a density anomaly in the temperature range between 2000 and about 5000 K. It is found that the predicted values for ion-beam-induced compaction and the growth-rate for plastic flow are in reasonable agreement with the experimental data obtained previously.

Ion-beam induced plastic deformation in amorphous materials investigated by marker implantation and RBS

Abstract Ion electronic energy loss induces atomic transport (plastic deformation) in amorphous alloys irradiated with high-energy heavy ions. Such a phenomenon can be investigated by measuring, with the RBS technique, the modification of a marker profile implanted in the near-surface region of the irradiated target. Ion beams are thus used at three stages of the experiment: implantation, irradiation and analysis. Irradiation of amorphous Ni 3 B with hundreds-MeV Xe or I ions leads to a shift of the maximum of the Bi marker profile towards the surface of the sample together with a decrease of the peak width and of the peak integral. The results are in qualitative and quantitative agreement with the large plastic deformation observed in amorphous materials.

SFM study of ion-induced hillocks on LiF exposed to thermal and optical annealing

Abstract Single crystals of LiF were irradiated at 10 different temperatures from room temperature to 780 K with Pb ions of 4.1 MeV/u. The irradiated surfaces were analyzed with scanning force microscopy, which revealed ion-induced hillocks with diameters of ∼20 nm and with heights of a few nm. Above 450 K, the number of hillocks strongly decreased with irradiation temperature. No hillocks were created under irradiation at 780 K. In addition, LiF samples irradiated at room temperature with Ni (2.5 MeV/u) and U ions (11.1 MeV/u) were bleached with UV-light on part of the crystal surface. In the bleached area, the characteristic F- and F2-centers disappeared, whereas the mean diameter and height of the hillocks did not show any significant change.

Effect of temperature on track formation by energetic heavy ions in lithium fluoride

Abstract Single crystals of lithium fluoride were irradiated with 790 MeV Pb ions in the temperature range between 300 and 700 K. The creation of defects along the ion trajectory was studied using optical absorption spectroscopy and small-angle X-ray scattering (SAXS). Surprisingly, the track radius is not significantly influenced by the increase of irradiation temperature, but the ion-induced change of the electron density in the track decreases substantially.