M. Toulemonde

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.

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.

Swift heavy ions in insulating and conducting oxides: tracks and physical properties

Damage induced in several oxide materials by swift heavy ions is presented. The discussion is based on results obtained on the following materials [Y3Fe5O12, AFe12O19 (A = Ba, Sr), BFe2O4 (B = Ni, Mg, Zn), ZrSi2O4, SiO2 quartz, Al2O3, high Tc superconductors (YBa2Cu3O7 − δ and Bi2Sr2CaCu2O8)] which have been irradiated by ions with atomic number ranging between 6 (12C) and 92 (238U) and energies between 0.05 GeV and 6 GeV. The damage cross section A has been deduced using several physical characterisations like Mossbauer spectrometry, saturation magnetisation measurements, channeling Rutherford backscattering, infrared absorption and electrical resistance measurements. Depending on the material and on the value of the electronic stopping power (dE/dx) the damage cross section varies between 10−17 and 10−12 cm2. Using medium and high resolution transmission electron microscopy and chemical etching of the latent track, an electronic stopping power evolution of the damage morphology has been observed leading to the definition of an effective radius Re of the latent track which can be linked to the damage (amorphous) cross section A by the relation Re = √A/π. Moreover there is a direct correlation between these values and the damage morphology: for Re > 3 nm the latent tracks are long and cylindrical, conversely for Re < 3 nm the damage is inhomogeneous along the latent track. The effect of the irradiation temperature, of the crystallographic orientation, of the initial electrical resistivity and of the oxygen stoichiometry will be presented. In opposition to what has been usually believed it will be shown that alumina (Al2O3) is indeed sensitive to the electronic stopping power. Moreover the velocity of the incident ion has a direct influence on the damage production: the lower the velocity, the higher the damage.

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.

Induced damage by high energy heavy ion irradiation at the GANIL accelerator in semiconductor materials

Abstract The advantages of using a high energy (several GeV) heavy ion accelerator for irradiation are first recalled: the ranges of ions in materials are significant; an a priori relative evaluation of the damage creation rates from elastic collisions is possible; last, the ratio of the electronic stopping power to the nuclear stopping power is very large. The experimental methods used are in situ resistance and Hall mobility measurements. The irradiated samples are also analyzed in the laboratory by means of different methods (DLTS, photoluminescence, electron microscopy). The resistance has the same behaviour in silicon and gallium arsenide. It increases continuously during the irradiation. On the other hand, in n-type germanium, the resistance first increases, passes through a maximum, and decreases afterwards. A type-conversion takes place in the material. Moreover, the comparison of damage creation rates from one irradiation to another, in germanium and gallium arsenide, seems to show that the electron excitation produces a relative decrease of the damage creation rate. This effect is not visible in silicon.

Damage structure in the ionic crystal LiF irradiated with swift heavy ions

Abstract In many insulators, swift heavy ions in the MeV to GeV energy regime create latent tracks characterized by irreversible structural and chemical changes. Based on a large data set, the present report will give a detailed description of the damage structure and defect morphology of ion tracks in lithium fluoride. The results were obtained by different complementary techniques including optical absorption spectroscopy, small-angle X-ray scattering (SAXS), chemical etching, scanning force microscopy, and surface profilometry. In a large cylindrical halo of several tens of nanometers around the ion trajectory, single defects such as F- and F2-centers are evidenced by optical absorption spectroscopy, similar to the damage known from conventional irradiations. Above a critical electronic stopping power of the ions of around 10 keV/nm, new effects occur, namely the formation of more complex defects in a very small core region with a radius of 1–2 nm. The damage in this zone is responsible for a characteristic anisotropic X-ray scattering and for chemical etching. Several observations indicate that this core consists of a quasi-cylindrical discontinuous array of complex defect aggregates (presumably small Li colloids, molecular fluorine and vacancy clusters). Profilometer measurements reveal substantial ion-induced volume expansion. This swelling can be assigned to a track radius of about 5–10 nm, an intermediate zone between the track core and the halo, and appears at a much lower threshold of around 4 keV/nm. Track data (radii and threshold) linked to the core and to swelling can be described within the frame of the thermal spike model assuming two different criteria, namely quenching of a vapor and a melt phase, respectively created along the ion path.

Surface modifications of LiNbO3 single crystals induced by swift heavy ions

Single crystals of LiNbO3 (Y-cut orientation) have been irradiated at GANIL using different ions (112Sn, 155Gd and 238U) accelerated in the GeV range. All the irradiations were performed at room temperature, with fluences extending from 1010 to 1012 ions cm−2. Rutherford backscattering spectrometry in channeling geometry (RBS-C) was employed to investigate the lattice disorder resulting from the high electronic stopping power (dE/dx)e (between 18 and 40 keV nm−1). Surface swelling of the irradiated samples was evidenced using a profilometer, in conjunction with direct observations in the nanometer scale by means of atomic force microscopy (AFM). According to RBS-C analysis, the damage cross section Ad varies from 4 × 10−13 to 1.4 × 10−12cm2 and depends on both (dE/dx)e and the velocity of the incident ions. A correlation was pointed out between the height of the out of plane step and the (dE/dx)e-induced damage. AFM observations, performed on samples irradiated at the lowest fluences, indicate a significant broadening of the latent track radius at the surface.

Thermal spike model in the electronic stopping power regime

Abstract Two models have been proposed in order to explain the appearance of latent tracks induced in matter by the slowing down process of ions in the electronic stopping power regime. The first one was the thermal spike proposed by Desauer and reconsidered for metals by Seitz and Koehler. The second one was the ionic spike proposed by Fleischer et al in order to explain that metals are insensitive to the electronic excitation produced by fission fragment irradiations. In both models the key is the high mobility of the electrons in metals. The ionic spike model was considered as ineffective because of the too quick screening by the return electrons which inhibits a Coulomb impulse. In the thermal spike model the electronic energy was considered as spread out in a too large volume to induce a significant increase of the lattice temperature. Since that time a systematic use of heavy ion accelerators has enlarged the number of materials (metals, semiconductors and insulators) which present a defect creation...

Irradiation damage in magnetic insulators

Abstract Using high energy heavy ion irradiation, the damage induced in magnetic insulators (Y3Fe5O12, BaFe12O19, SrFe12O19, NiFeO4, MgFe2O4, ZnFe2O4, Fe3O4) in the electronic stopping power (dE/dx) regime is studied. The amorphization cross section Ap is extracted from the paramagnetic fraction observed on Mossbauer spectra. Electronic stopping power threshold for damage creation appears. The damage efficiency ϵ = A/(dE/dx) is calculated and can be fitted by a general amorphization law ϵ = ϵmax(1− exp(− k(dE/dx)4)). The damage morphology has been correlated to the damage efficiency. Spherical extended defects appear for low values of ϵ at low values of dE/dx. When increasing dE/dx and consequently ϵ, the spherical defects overlap to give discontinuous cylindrical defects. Then for the higher values of dE/dx, the defects are continuous cylinders of amorphous phase. The change of the physical properties induced by the irradiation has been studied. Depending on the shape of the defects, the evolution of the electrical conductivity and the change in the orientation of the hyperfine magnetic field are different. Specific crystallographic sites in BaFe12O19 are more sensitive to the irradiation than others. Local order in the new amorphous phase is determined using X-ray absorption at Fe K-edge and Mossbauer spectroscopies. The creation of magnetization is observed in irradiated ZnFe2O4 which initially shows only a paramagnetic behavior at room temperature.