A. Dunlop

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.

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.

Track separation due to dissociation of MeV C60 inside a solid

Abstract Samples of yttrium iron garnet were irradiated at 300 K with 10–40 MeV Au4 and C60 ions at normal and grazing incidences. The samples were then observed in a transmission electron microscope. It is seen that the cluster ions create large size amorphous continuous tracks and that strong sputtering occurs. Irradiations at grazing incidence allow direct observations of the track shape evolution as the projectiles slow down in the target. For the first time it was possible to visualize the dissociation of C60 ions inside a solid. The stochastic nature of this separation process results in tracks of different shapes and lengths. The tracks generated in the target keep a constant diameter during the correlated slowing-down of the C60 projectile constituents during travelled distances Lconst ≈ 100 nm, although the area on which the constituents are spread increases by almost two orders of magnitude between the target entrance and the depth Lconst.

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.

Tracks induced in CaF2 by MeV cluster irradiation

Abstract Samples of fluorite (CaF 2 ) crystals were irradiated at 300 K with cluster ions of a few tens MeV at normal incidence. After irradiation the samples were observed in a transmission electron microscope. The cluster ions create intermittent tracks which are stable under the electron beam. The tracks consist of aligned faceted anion voids, which can also be viewed as calcium inclusions. Furthermore, surface contrasts associated to sputtering are sometimes observed at the track entrance and exit.

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.

A comparison between tracks created by high energy mono-atomic and cluster ions in Y3Fe5O12

Abstract Samples of yttrium iron garnet (Y3Fe5O12 or YIG) were irradiated with Au4, Cn (2⩽n⩽20), C60 ions in the MeV energy range. Using transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS), the induced latent tracks are compared with those generated with GeV mono-atomic ions. The cluster ions create amorphous and continuous tracks as in the mono-atomic case. Irradiation at grazing incidence allows to visualise the dissociation of cluster ions inside a solid. Some of the tracks induced by high energy C60 ions contain a “bubble-like” structure inside the amorphous part of the tracks, which is in contrast to the tracks induced by smaller cluster and GeV mono-atomic ions for which no structure is seen. The diameters of the tracks induced by MeV cluster ions and by the low velocity mono-atomic ions are seen to follow the same curve as a function of the linear rate of energy deposition (dE/dx)e.

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.

Evidence for atomic mixing induced in metallic bilayers by electronic energy deposition

Abstract High levels of energy deposition by electronic excitations can induce damage creation in some bulk metallic targets as soon as the rate of energy deposition in electronic excitation is higher than a few 10 keV/nm. The present study is aimed at determining whether such excitations can also induce interdiffusion at the interface of metallic bilayers. Ni Ti bilayers were irradiated at 80 K with GeV Pb, Ta and U ions up to a few 10 13 ions/cm 2 . Damage creation and mixing were followed using various methods: electrical resistance, grazing X-ray and neutron reflectometry, X-ray diffraction, electron microscopy and electron energy loss on target cross-sections. A very strong mixing is indeed observed at the Ni Ti interface as a result of electronic energy deposition: the observed mixing efficiency is at least two orders of magnitude higher than that induced by elastic collisions with target nuclei. We finally study the influence of various parameters (irradiation temperature, symetry of the interface) on the mixing efficiency and follow the kinetics of the mixing process.