D. Lesueur

Effects induced by high electronic excitations in pure metals: A detailed study in iron☆

Although high electronic excitations were neglected for a long time in radiation effect studies of metallic targets, it is now well established that they can play a dominant role in the damaging processes of some metals. Iron is especially interesting in so far as, according to the rate of energy deposition in electronic excitations (dE/dx)e, various behaviours are observed. Below (dE/dx)e ≈ 40 keV/nm, due to electronic excitations, the amount of damage introduced in the sample is smaller than that expected from the sole elastic collisions. On the contrary, at very high (dE/dx)e levels, a strong enhancement of the damage occurs. After describing the experimental results obtained during low temperature irradiations with a few 10 MeV/nucleon heavy (oxygen to uranium) ions, a phenomenological model which accounts for this intricate behaviour will be presented. Finally, a microscopic mechanism will be proposed to explain how the energy given to the electronic system can play a role in damage processes involving atomic motion. Molecular dynamics simulations validate this approach.

Latent Tracks Do Exist in Metallic Materials

Until now experiments have failed to show that latent tracks can be formed in metallic crystals. For the first time, discontinuous tracks have been observed in crystalline Ni-Zr alloys irradiated by GeV heavy ions. It is shown that their formation results from the very high value of the energy deposited in electronic excitations.

High-Energy Heavy-Ion Irradiations of Fe85B15 Amorphous Alloy: Evidence for Electronic Energy Loss Effect

Amorphous metallic Fe85B15 alloy has been irradiated at low temperature with Ar, Kr and Xe ions of initial energies of 1.8, 2.7 and 3.0 GeV, respectively. Electrical resistance was measured in situ on samples piled up along the beam direction. It is shown that above a given electronic stopping power threshold the electronic losses play a crucial role in radiation-induced damage.

Radiation Damage Induced by Electronic-Energy Loss in Amorphous Metallic Alloys

The electrical resistance of amorphous metallic Fe85B15 ribbons irradiated with 3 GeV Xe ions at different tilting angles with respect to the incident ion beam has been measured in situ at 77 K. The results show that irradiation induces large sample growth due to electronic-energy loss effects. The existence of a new mechanism leading to damage creation in metallic alloys by electronic excitation alone is also demonstrated.

High electronic excitations and damage production in GeV-ion-irradiated metals☆

Abstract The low-temperature damage induced by high-energy heavy ions in metallic targets has been followed by in-situ electrical-resistivity measurements. In Cu3Au alloys, the observed damage production can be accounted for by elastic-collision processes, whereas in Ni3Fe and iron, inelastic collisions must be put forward above an electronic stopping power threshold. In the latter case, collective effects in the wake of the incident ion could explain both (1) the increased damage efficiency and (2) the inhomogeneous damage distribution that leads to a very small saturation resistivity increase.

Phase transformation induced by swift heavy ion irradiation of pure metals

Abstract It is now unambiguously established that high electronic energy deposition (HEED), obtained by swift heavy ion irradiation, plays an important role in the damage processes of pure metallic targets: (i) annealing of the defects created by elastic collisions in Fe, Nb, Ni and Pt, and (ii) creation of additional defects in Co, Fe, Ti and Zr. For Ti, we have recently evidenced by transmission electron microscopy observations that the damage creation by HEED is very important and leads to a phase transformation. Titanium evolves from the equilibrium hcp alpha-phase to the high pressure omega-phase. We studied the influence of three parameters on this phase transformation: ion fluence, electronic stopping power and irradiation temperature. The study of Ti and the results concerning other metals (Fe, Zr, etc.) and the semi-metal Bi allow us to propose criteria to predict in which metals HEED could induce damage: those which undergo a phase transformation under high pressure. As a matter of fact, beryllium is strongly damaged when submitted to HEED and seems to behave very similarly to titanium. The fact that such phase changes from a crystalline form to another form were only observed in those metals in which high pressure phases exist in the pressure-temperature diagram, strongly supports the Coulomb explosion model in which the generation of (i) a shock wave and (ii) collective atomic movements are invoked to account for the observed damage creation.

Phonon Soft Modes and Damage Production by High Electronic Excitations in Pure Metals

It is now well known that during high-energy heavy-ion irradiation, the very high-energy deposition in electronic excitation induces a spectacular damage creation in some types of metallic targets. A selected number of pure metals has been irradiated by GeV ions in order to test some possible criteria which might be pertinent to explain such effects: electron-phonon interaction, electrical conductivity, existence of various allotropic phases,.... The present results show that the latter criterion or more precisely that the existence of a displacive transformation associated with a soft mode in the phonon spectrum seems to favour efficient energy transfers between highly excited electrons and target atoms. For titanium targets, electron microscopy observations show striations which are parallel to the incident ion beam direction.

Damage production in iron during high-energy ion irradiation: Experimental and theoretical determinations☆

Abstract High-purity polycrystalline iron targets have been irradiated at cryogenic temperatures with very energetic argon and xenon ions. In situ electrical resistivity increase measurements on piles of thin foils allow us to determine the experimental damage production rates and depth distributions of the ion-induced damage. We present theo retical calculations of the number of displaced atoms in the target, of the defect profile along the projectile path and of the range of the incident ions. We compare these results to the experimental ones in order to determine the damage production efficiency corresponding to such irradiations and to look for a possible influence of the high energy transfers to the target electrons on the defect production mechanisms.

Influence of very high electronic energy losses on defect configurations in self-ion irradiated iron

Abstract We present here an experiment in which a 11 μm thick iron ribbon is first bombarded at 5 K with 10–170 MeV iron ions in order to create a homogeneous population of defects located in displacement cascades. We measure in situ the length and electrical resistivity of the target: they both increase during this phase of the experiment. Without any warming-up, the sample is then bombarded with 500 MeV iron ions (range 30 μm in iron). The slowing-down of these ions in the target occurs mainly via inelastic collisions. The electrical resistivity and length of the sample decrease during this last irradiation: we interpret this instability of pre-existing cascade damage as a consequence of some transfer of the energy lost in electronic processes to lattice atoms. We thus show that electron energy loss processes must be taken into account to study damage creation in some metals.

Electron microscopy observations of titanium irradiated with GeV heavy ions

Abstract GeV heavy ions induce the creation of damage in some metallic targets via electronic excitation. We report here on room temperature electron microscopy observations of titanium irradiated at 15 or 90 K by xenon, tantalum and lead ions. For sufficiently high electronic energy losses ( ⪆ 2.5 keV A −1 ), black dots aligned along the incident ion beam direction are observed. The ratio of the number of such alignments to that of impinging ions depends strongly on the irradiation conditions, namely the irradiation temperature and fluence. A tentative explanation of these observations is proposed. It involves the mechanism of point defect clustering resulting either from thermally activated migration or from athermal processes occuring in the wake of the incident ions.