Tracks in Metals by MeV Fullerenes

Abstract

It is shown that new specific effects take place during irradiation of metals with high energy fullerene beams. The observed quasicontinuous damage is confined inside -20 nm diam cylinders around the projectile paths and is compared to the damage resulting from GeV heavy ion irradiation.The large extension of the highly damaged zones after cluster irradiations might be due to the strong localization of the deposited energy during the slowing-down process. PACS numbers: 61.80.Lj, 61.80.Jh It is now well accepted that electronic excitation and ionization arising from the slowing down of swift heavy ions can lead to structural modifications in bulk metallic targets as it has been known for a long time in insulators [1] and in thin [2] or discontinuous [3,4] metallic Alms. These modifications resulting from the high rates of energy deposition, which can reach values as high as 10 keV/nm, were unexpected in metallic materials where numerous and very mobile charge carriers allow a fast spreading of the deposited energy and an efficient screening of the space charge created in the projectile wake [5]. The main difference between insulators and conductors lies in the threshold above which the linear rate of energy deposition can lead to structural modifications.So it has been shown that irradiation of bulk metallic targets with swift heavy ions can induce effects as varied as the so-called growth phenomenon of amorphous alloys [6,7], the amorphization of metal-metalloid alloys [8] or metal-metal alloys [9], a phase transformation in pure titanium [10], and defect creation in pure iron and zirconium [11]. The question of the transformation of the energy deposited by the projectile during the slowing-down process into energy stored in the target as lattice defects is still to be answered. Two mechanisms have been proposed:(i) The thermal spike model [12] in which the kinetic energy of the ejected electrons is transmitted to the lattice by electron-phonon interaction in a way efficient enough to increase the local lattice temperature above the melting point. This temperature increase is then followed by a rapid quenching.(ii) The Coulomb explosion model [13] in which the electrostatic energy of the space charge created just after the ion passage is converted into coherent radial atomic movements leading to a cylindrical shock wave.Both mechanisms are sensitive to the rate of energy loss (dE/dx) as well as to the deposited energy density. The volume in which the energy is deposited scales with the range of the 6 electronsemitted during the ionization processwhich depends only on the projectile velocity v. In monatomic ion irradiation it is not possible to vary significantly one of these parameters, say, v, keeping the second one unchanged, say, dE/dx, whereas it is easier using cluster ions. The aim of this paper is to compare damage induced in metallic targets (Ti, Zr) by MeV cluster ions with that previously observed [10,11] after irradiation with GeV heavy ions. The linear energy loss

Materials Modifications with Cluster Beams: Bulk and Surface Modification

“…In the lower part, the samples were tilted by Pe. 30° in the microscope in order to visualize the evolution of the damaged regions as the projectiles enter deeper mto the targets (from [45]). Dislocation lines going through the sample thickness from one surface to the other one are visible (from [46]).…”

“…The first observation of energetic heavy cluster damage was performed in titanium and zirconium targets [45] . Figure 3 presents comparative electron micrographs of titanium irradiated at 300 K with 850 MeV Pb ions and with 18 MeV Cho ions.…”

“…The specificity of the fullerenes at a few 10 MeV energies is that their velocities are one order of magnitude smaller than those of GeV heavy ions: Q = v/c ≈ 0.01 for 20 MeV C60, 0.1 for 1 GeV U, so that the maximum radial range of the ejected 6-electrons is respectively of the order of a few interatomic distances and some 1000 nm. This leads in the case of cluster irradiations to huge deposited energy densities of the order of 10 eV/ Ǻ3 [45]. The relaxation of such a localized high excitation leads to very spectacular structural modifications.…”

Nano tracks in fullerene film by dense electronic excitations

“…After a certain distance, strong correlation of trajectories is lost and hence the density of electronic energy deposition reduces drastically. Recently, the formation of latent tracks in semiconductors (Si, Ge, GaAs) by cluster beams has been reported at room temperature [19,20], whereas tracks are not created by mono-atomic beams in these materials. In the present study we investigate dense electronic energy deposition effects in fullerene films, which have semiconducting properties with optical band gap ~2 eV.…”

“…The energy per unit volume is more relevant parameter to understand the track formation and to compare the diameters of tracks obtained with cluster beams and mono-atomic ion beams rather than the linear rate of electronic energy loss [19]. It is noticed that to calculate the energy deposition per unit volume, the energy or range of the secondary electrons is required.…”