Structural Modifications Induced by the Electronic Slowing Down of Swift Heavy Ions in Matter
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STRUCTURAL MODIFICATIONS INDUCED BY THE ELECTRONIC SLOWING DOWN OF SWIFT HEAVY IONS IN MATTER* J.C. JOUSSET, E. BALANZAT, S. BOUFFARD and M. TOULEMONDE Centre Interdisciplinaire de Recherches avec les Ions lourds, (CIRIL) (CEA-CNRS) B.P. 5133, rue Claude Bloch, 14040 Caen C&lex, France * Experiments performed at GANIL National Laboratory, Caen, France
In the last five years, the use of GeV heavy ions, such as those accelerated at GANIL, has lead to a breakthrough in the knowledge of the effects induced by the huge energy deposition which occurs during the slowing down of a swift heavy ion in matter. The specific interest of GeV heavy ions comes from the fact that during - 0.9 of their large range (- 100 gtm) the electronic stopping power dominates the elastic stopping power by a factor of 2.103 whatever the ion or the target. Moreover, the electronic stopping power is very large : a few keV/A. If we consider now how this energy spreads radially around the ion path, it can be assumed, as an order of magnitude, that a few tens of eV/atom are transmitted to the target electrons during 10-15s in a cylindrical volume containing some tens of atomic units (1-4). This situation is quite unusual, compared to the case either of the displacement cascades, where the energy is transmitted by elastic events to the target atoms, or of the short powerful laser pulses irradiations where the energy transmitted to the target electrons is lower. This unusual situation constitutes the very interest of swift heavy ions irradiation experiments. As previously discussed [51, it has to be underlined that the duration of the electronic interactions is much shorter, by two or three orders of magnitude, compared to the target atomic vibration period. The main questions to be answered are: how is this high energy transmitted to the target electrons, either by direct ionization or excitation, relaxed in the atomic (or molecular) structure of the irradiated solid ? Or, more precisely, how can the high density electronic excitation created in the wake of the ion-projectile be converted in atomic motion leading to induced stable lattice defects or phase modifications ? None of the models which have been so far proposed are really able to account for the whole of the observed effects. To give only one example : the well known Coulombian explosion spike model [61 which may account for the latent tracks induced by heavy ions in some insulators is completely unable to explain any of the effects of atomic mobility induced by electronic stopping power in bulk metallic materials [7-11]. Numerous experimental results have been obtained at GANIL for many different types of target : magnetic oxides, polymers, ionic crystals, semi-conductors, organic conductors, metallic conductors and high Tc superconductors. We shall present hereafter only two typical results for i) an insulator : a magnetic oxide, the yttrium iron garnet, ii) a metallic conductor, the amorphous Fe85B 15 alloy. IRRADIATION FACILITY The experiments are performed in the IRABAT facility set u
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