Ch. Dufour

Transient thermal processes in heavy ion irradiation of crystalline inorganic insulators

Abstract A review of matter transformation induced in crystalline inorganic insulators by swift heavy ions is presented. The emphasis is made on new results obtained for amorphizable materials such as Gd3Ga5O12, GeS, and LiNbO3 and for non-amorphizable crystals such as SnO2, LiF and CaF2. Assuming that latent tracks result from a transient thermal process, a quantitative development of a thermal spike is proposed. The only free parameter is the electron–lattice interaction mean free path λ. With this parameter it is possible to quantitatively describe track radii, whatever the bonding character of the crystal is, in a wide range of ion velocities assuming two specific criteria: tracks may result from a rapid quenching of a cylinder of matter in which the energy deposited on the lattice has overcome either the energy necessary to reach a quasi-molten phase in the case of amorphizable materials or the cohesion energy in the case of non-amorphizable materials. The evolution of the λ parameter versus the band gap energy of the considered insulator will be presented. On the basis of this discussion some predictions are developed.

Track creation in SiO2 and BaFe12O19 by swift heavy ions: a thermal spike description

Abstract The thermal spike model is used in order to calculate the track radii variation versus electronic stopping power Se in two radiolysis resistant oxides: SiO2 quartz and BaFe12O19. The mean diffusion length λ of the energy deposited on the electrons is determined by fitting latent track radii versus Se: 4.0 ± 0.3 and 8.2 ± 1.3 nm respectively for both materials. A decrease in the band gap Eg (12 and 1 eV respectively) means an increase in λ.

An original approach for the fabrication of Si/SiO2 multilayers using reactive magnetron sputtering

Composite and multilayered Si/SiO2 structures have been grown by means of reactive magnetron sputtering of a pure silica target. This method is based on the different nature of the plasma during deposition: the plasma consists of either pure argon or an argon/hydrogen mixture for the growth of silica or Si-rich films, respectively. Using this process, Si-rich single layers and Si/SiO2 multilayer structures have been successfully elaborated and characterised by infrared absorption spectroscopy, optical transmission measurements and transmission electron microscopy (conventional and high resolution). The superlattices have shown a significant photoluminescence signal whose energy is governed by the Si sublayer thickness.

Ion-matter interaction: the three-dimensional version of the thermal spike model. Application to nanoparticle irradiation with swift heavy ions

In the framework of swift heavy ion?matter interaction, the thermal spike has proved its worth for nearly two decades. This paper deals with the necessary refinement of the computation due to the kind of materials of interest, i.e. nanomaterials such as multilayered systems or composite films constituted of nanocylinders or nanospheres embedded in an insulating matrix. The three-dimensional computation of the thermal spike model is applied for the first time in the case of ions striking layers containing spherical nanoparticles embedded in a silica matrix. The temperature profiles calculated at each point (x, y, z) of the target for a total duration up to 10?10?s and different values of ion impact parameter allow a possible explanation of the particle shape change under irradiation with swift heavy ions having an energy of several MeV?amu?1

Atomic and cluster ion bombardment in the electronic stopping power regime: A thermal spike description

Abstract A brief review of experimental results of defect creation in metallic materials supports the assumption that the electron-phonon coupling is the main physical parameter which determines their sensitivity against the irradiation in the electronic stopping power ( d E d x ) regime. Following this idea the thermal spike model is developed using a numerical solution of two coupled equations describing the energy diffusion on the electrons and on the lattice atoms respectively and their coupling. Assuming that the experimental observations may be interpreted by a rapid quench of the induced molten phase, radii of latent tracks in Ti and Zr irradiated by atomic and cluster ions will be calculated and compared to experimental results with quite a good agreement.

Velocity effect on the damage creation in metals in the electronic stopping power regime

Abstract The energy deposited on target electrons by a swift heavy ion has been studied for several years. The deposited energy density in the vicinity of the ion path is large as the velocity of the incident ion is low. The influence of the ion velocity on irradiation effect induced by electronic energy loss ( S e ) has been clearly demonstrated after irradiation experiments performed on the insulator Y 3 Fe 5 O 12 and suspected in metals. In that case the damage creation is enhanced at low ion velocities. In the present work, pure bismuth samples have been irradiated at 100 K by 42.9 MeV amu −1 Xe and 4 MeV amu −1 Kr ions with similar S e values (∼ 17 KeV nm −1 ). The resistivity increment Δϱ was measured in situ as a function of the ion fluence, Φt. The analyses of Δϱ(Φt) curves show that the Kr irradiation leads to a damage efficiency which is two to four times larger than that obtained after Xe irradiation. In addition, 2, 10 and 20 MeV amu −1 Pb-ion irradiations were performed at 20 K. The comparison of deduced latent track radii for the same S e values with previous results gives that lower velocity in irradiations produce larger tracks. These phenomena evidence that a strong ion velocity effect exists in bismuth. The thermal spike model with the ion velocity dependence of the initial energy deposition is used to determine the track radii as a function of S e values. We now find an agreement between calculated and experimental track radii in relation with the ion velocity. Moreover the electron number density n e of the quasi-free gas in bismuth is found to be 1.5 times the atomic density n a ( n e = 1.5n a ) and the S e threshold for continuous latent track formation, S et , is ranged from 24 to 31 keV nm −1 for ion energies ranging from 2 to 25 MeV amu −1 .

Phase transformation of polycrystalline zirconia induced by swift heavy ion irradiation

Abstract Polycrystalline samples of monoclinic zirconia (α-ZrO 2 ) have been irradiated at room temperature with 190 MeV 36 Ar and 170 MeV 84 Kr ions in the electronic slowing down regime. Room-temperature X-ray diffraction (XRD) and micro-Raman spectroscopy measurements show consistently that a phase transition to the tetragonal form (β-ZrO 2 ) occurs for 170 MeV 84 Kr ion irradiation above an electronic stopping power value around 15 MeV μm −1 . The kinetics of the transition were monitored by on-line XRD measurements on the same sample. No such phase transformation is seen with 190 MeV 36 Ar ion irradiation for an electronic stopping power value around 6 MeV μm −1 . The plot of the tetragonal phase fraction deduced from XRD measurements vs fluence is analysed with single-impact and double-impact kinetic models. The data seem to be in favour of a double ion impact process.

TEM study of irradiation effects on tin oxide nanopowder

Abstract The effects of swift heavy ions in bulk materials have now long been studied and some models — such as the thermal spike — have been developed in order to explain the observed phenomena. Nevertheless, some questions remain debatable, among which the following ones: i) What is the spatial extension of the huge energy deposited by the swift heavy ions on the target electrons? ii) Can we evidence a pressure effect in some materials due to the lattice temperature increase? In most cases, we are able to predict whether a bulk material may be sensitive or not to the electronic energy loss S e of the incident ion. According to the thermal spike, a given material could be all the more sensitive to S e as the energy density deposited on the electrons is high. Therefore, an interesting way to increase this energy density is to confine the electrons in small target grains (i.e. submicrometric grains). This work reports the first experimental results obtained on irradiated tin oxide (SnO 2 ) nanopowders. The same grains have been observed by TEM and HREM before and after lead ion irradiation at several fluences (from 0.3 to 7.5 × 10 12 Pb cm −2 ). A modification gradually appears as the fluence increases up to a critical fluence above which the grains split into nanodomains. A possible explanation is given through the thermal properties of SnO 2 .

Electron-phonon coupling and the sensitivity of metals to irradiation with swift heavy ions

Abstract In the framework of the thermal spike model, the present paper shows a relationship between the behaviour of metals under irradiation with swift heavy ions and two main parameters: the electron-phonon coupling strength g and the energy ΔH necessary to melt the material. A correlation is made between the electronic stopping power threshold (Seth) of track formation, the order of magnitude of the track radii, and the values of g and ΔH. Three cases are discussed: bismuth that shows large track radii and a rather high value of Seth, titanium that exhibits low values of track radii and Seth, and iron which is an intermediate case (low values of track radii and high value of Seth). The correlation deduced from those cases is successfully applied to C60 fullerene crystals.

Sensitivity of metallic materials under irradiation with swift heavy ions

Abstract We report on experimental results on metallic materials (Ag,Sn) after irradiation with swift heavy ions. These results, in addition to previous ones, show that the behaviour of that kind of materials could be well described in the framework of the thermal spike model. We find a good correlation between theoretical results and experimental ones provided that the only free parameter of the model should be known: the number z of electrons involved in the thermal spike mechanism via the electron phonon coupling constant. It is shown that the values of z must lie in the range of 1 and 2 electrons per atom. A simple criterion is deduced and used to know whether a given material could be sensitive or not to the electronic slowing down of swift heavy ions.