E. Paumier

The Se sensitivity of metals under swift-heavy-ion irradiation: a transient thermal process

In the framework of the thermal-spike model the present paper deals with the effect of the electronic stopping power (Se) in metals irradiated by swift heavy ions. Using the strength of the electron-phonon coupling g(z) with the number of valence electrons z as the unique free parameter, the increment of lattice temperature induced by swift-heavy-ion irradiation is calculated. Choosing z=2, the calculated threshold of defect creation by Se for Ti, Zr, Co and Fe is about 11, 27.5, 28 and 41 keV nm-1, in good agreement with experiment. Taking the same z value, the calculation shows that Al, Cu, Nb and Ag are Se insensitive. Moreover, in Fe, the differences in the damage created by U ions of different energies but exhibiting the same value of Se may be interpreted by a velocity effect. Using z=2, other calculations suggest that Be (Se>or=11 keV nm-1), Ga (Se>or=5 keV nm-1) and Ni (Se>or=49 keV nm-1) should be sensitive to Se but Mg should not. These examples put the stress on the effect of the physical parameters governing the electron-phonon coupling constant apart from z determination: the sound velocity linked to the Debye temperature and the lattice thermal conductivity. Furthermore, a simple criterion is proposed in order to predict the Se sensitivity of metals.

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.

A high-resistivity phase induced by swift heavy-ion irradiation of Bi: a probe for thermal spike damage?

Pure bismuth samples were irradiated at 20 K with swift heavy ions from 18O to 238U in the GeV range. The rate of the induced damage was deduced from in situ electrical resistance measurements. Above a threshold in the electronic stopping power Se equal to 24 keV nm-1, the damage is due to electronic slowing down. Above 30 keV nm-1, the electronic slowing down is efficient enough to induce latent tracks attributed to the appearance of a high-resistivity phase. The induced latent tracks radii can be up to 21.9 nm for Se=51 keV nm-1 which is the largest value reported so far for non-radiolytic materials. The evolution with Se of the latent tracks radii is calculated on the basis of the thermal spike model, assuming a realistic value for the electron-phonon coupling constant. A rather good agreement is obtained which supports the idea that the thermal spike could be operative in the observed radiation damage.

Nanometric size effects on irradiation of tin oxide powder

A nanometric powder of tin oxide (SnO2) has been irradiated with lead ions. The same grains have been observed by Transmission Electron Microscopy (TEM) before and after irradiation at a fluence of 5×1012 Pb cm−2. The shape of largest grains strongly changes while the smallest ones disappear. This phenomenon has been explained by using the thermal spike model. It appears that the irradiation induces an increase of the internal pressure in the grains leading to their explosion. In the smallest grains, the calculated maximal temperatures exceed the boiling point so that these grains disappear.

Phase transformation induced in pure zirconia by high energy heavy ion irradiation

Abstract Samples of monoclinic zirconia were irradiated with heavy ions having incident energies in the range of a few hundred MeV giving then rise to a slowing down essentially caused by high electronic excitations. The characterizations of the samples by X-ray diffraction and complementary Raman spectroscopy analyses revealed two main features. First, in the electronic stopping power regime, it is only when the electronic energy loss is above a threshold near 13 keV nm−1 that monoclinic zirconia undergoes a transformation to the tetragonal phase. Second, the evolution of the amount of the tetragonal phase with the ion fluence exhibits a sigmoidal shape suggesting a mechanism for phase transformation which very likely needs two ion impacts.

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.

Phase transformation of polycrystalline Y2O3 under irradiation with swift heavy ions

Abstract The effects of swift heavy ions in bulk solid targets have been extensively studied. Some effects are specifically related with the large densities of energy absorbed by the target electron gas in the projectile wake. One remarkable effect observed in the present work is the crystalline solid state phase transition of Y 2 O 3 as a result of ionizing irradiation by GeV heavy ions. Our X-ray diffraction measurements give the first evidence of the cubic to monoclinic transformation of Y 2 O 3 under high energy heavy ion irradiation.

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 .