A. Meftah

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.

Thermal spike model applied to the irradiated yttrium iron garnet: Mean diffusion length of the energy deposited on the electrons

Abstract Single crystals of yttrium iron garnet Y3Fe5O12 have been irradiated with 270 MeV 52Te, 760 MeV 86Kr and 1040 MeV 208Pb ions. The extent of the induced damage is determined using Rutherford backscattering spectrometry (RBS) in channeling geometry with a 2 MeV 4He beam. Taking into account these new results and the previous ones, the thermal spike model was applied to calculate track radii versus electronic stopping power for different beam energies using only one fitting parameter, λ the mean diffusion length of the energy deposited on the electrons by the slowing down of a swift heavy ion in the garnet. As the latent heat of fusion is unknown for yttrium garnet, the calculation was first performed by neglecting it. A λ value of (6.3±0.3) nm is extracted by fitting all the different results taking into account a velocity effect. Such a λ value is the high limit of the mean diffusion length since it decreases if the latent heat of fusion is not neglected. This value of λ is in agreement with the previous determinations made in the same conditions for resistant insulators, showing that λ should decrease when the band-gap energy increases.

High energy heavy ion irradiation effects in α-Al2O3☆

Abstract Single crystals of Al 2 O 3 have been irradiated at GANIL with 3.5 MeV/amu Pb ions, at a temperature of ≌ 8 K. The fluence range extended from 4×10 11 to 1.2×10 12 ions cm −2 . The effects of high electronic excitation induced in the samples have been characterized by Rutherford backscattering on channeling (RBS) in conjunction with optical absorption measurements. Moreover, some samples preliminary implanted with 10 16 57 Fe + ions cm −2 at 110 keV and annealed at 1673 K during one hour were studied using conversion electron Mossbauer specroscopy (CEMS) in order to obtain complementary informations. Preliminary RBS results (77 K irradiations) indicate a damage cross section of ∼ 10 −13 cm 2 , consistent with a track radius of about 1.8 nm. The defect efficiency has been also investigated as a function of the electronic stropping power (d E /D x ) c .

High energy heavy ion irradiation damage in yttrium iron garnet

Abstract We have studied the damage in ferrimagnetic yttrium iron garnet, Y 3 Fe 5 O 12 or YIG, produced by energetic heavy ion bombardment, for which the electronic stopping power is much higher than the nuclear stopping power. Epitaxial thin films of YIG on (111)-Gd 3 Ga 5 O 12 substrates were thus irradiated at room temperature with 50 MeV 32 S, 50 MeV 63 Cu, 235 MeV 84 Kr, and with 59 MeV, 185 MeV and 792 MeV 132 Xe ion beams. The film thicknesses were always smaller than the ion mean projected ranges, in order to avoid implantation effects in the layers. The resulting damage was then studied by high resolution X-ray diffraction, channeling Rutherford backscattering (RBS) spectroscopy and room temperature magnetization measurements. After these ion irradiations, the structural as well as the ferrimagnetic long-range orders are progressively destroyed. X-ray measurements on 84 Kr irradiated samples show the presence of compressive lateral macrostresses in the films due to the coexistence of crystalline and disordered phases. These stresses are partly relaxed away at high fluences, when the amount of disordered phase is high enough (around 45%). Cross section data for this damage process are deduced from the RBS and 300 K saturation magnetization measurements for six different values of the electronic energy loss, between 7 and 27 MeV μm −1 . The damage cross section increases nonlinearly as a function of the electronic stopping power, then seems to level off above 22 MeV μm −1 . However the comparison with previous works indicates that the electronic stopping power might not be the only key-parameter in this process, where the ion beam energy parameters might play some role.

Sputtering of vitreous SiO2 and Y3Fe5O12 in the electronic stopping power region: A thermal spike description

Abstract The thermal spike model was applied to calculate track radii versus electronic stopping power for different beam energies using only one fitting parameter, λ the mean diffusion length of the energy deposited on the electrons by the slowing down of a swift heavy ion in vitreous silica a-SiO2. A λ value of 2.5±0.2 nm is extracted by fitting all the different results of damage creation taking into account a velocity effect. Using the same value of λ the energy necessary to vaporize the vitreous silica is reached for a value of electronic stopping power dE/dx, which corresponds to the appearance of a huge sputtering of silicon in SiO2. So in the thermal spike model one can explain two phenomena with the same value of the free parameter λ. The present paper aims at extending this model to the yttrium iron garnet Y3Fe5O12, abbreviated as YIG. By fitting the latent track radii one can extract the unique following values: λ=5.0±0.3 nm and a latent heat of fusion LH=300±100 J g−1. Then the energy value Lv of 2.2±0.4 eV/atom necessary to sputter this material is extracted. The evolution of sputtering yield versus the electronic stopping power is in good agreement with the experimental results.

High-energy irradiation of magnetic insulators by lead ions: appearance of a plateau in the damage efficiency

Abstract Lead irradiation with an incident energy of 5.6 GeV has been used to study the induced damage in magnetic insulators (Y3Fe5O12, BaFe12O19, NiFe2O4) for high values of the electronic stopping power (dE/dx). Using Mossbauer spectrometry, the damage efficiency ϵ = A/(dE/dx), where A is the damage cross section, is extracted and compared to previous results. Saturation of the damage efficiency is confirmed and in the range of dE/dx studied in our experiments the damage yield is proportional to the deposited energy. The damage efficiency in Y3Fe5O12 and BaFe12O19 is fitted by a saturation law using the relation ϵ = ϵmax(1 − exp( − k(dE/dx)n)). A unique value of n = 4 is extracted while ϵmaxand k depend on the irradiated materials. ϵmax leads to a value indicating that the maximum damage efficiency arises when 5 eV is deposited on each atom. Using n = 4, this saturation law was applied with success to the spinel NiFe2O4 and to the amorphous metallic alloy a-Fe85B15.

Out-of-plane swelling of gadolinium gallium garnet induced by swift heavy ions

Abstract Single crystals of gadolinium gallium garnet (Gd3Ga5O12) have been irradiated with various ions (Cr 10.6 MeV/u, Cu 0.8 MeV/u, Kr 9 MeV/u, and Pb 4 MeV/u) in the electronic stopping power regime. The irradiated areas of the crystals exhibited a pronounced volume expansion. Using a profilometer, the out-of-plane swelling was measured by scanning over the border line between an irradiated and virgin area of the sample surface. The step height varied between 25 and 160 nm depending on the fluence, the electronic stopping power and the total range of the ions. In the high fluence regime, the swelling effect approaches saturation. In order to compare the results obtained for different ion species, the initial swelling per ion was normalised by the length of the damage track. Such an analysis makes evident that swelling occurs only above a critical energy loss of 7±2 keV/nm. The results of Gd3Ga5O12 will be compared with data obtained earlier in SiO2 and LiNbO3.

Electronic stopping power threshold of sputtering in yttrium iron garnet

To study the sputtering induced by the electronic stopping power dE/dx, single crystals of yttrium iron garnet Y3Fe5O12 (or YIG) have been irradiated at room temperature with 195 MeV 86Kr, 400 MeV 181Ta and 150 MeV 238U ions beam. The number of sputtered iron and yttrium atoms collected upon aluminium foil placed in front of the target was measured using Rutherford backscattering spectrometry (RBS) with a 2 MeV 4He Beam. The experimental results show that the electronic stopping power threshold of sputtering Ts in this garnet is 16 ± 3keV. This value is four times higher than the electronic stopping power threshold damage creation Te in the same range of velocity ions. The sputtering yield increases linearly within the experimental errors with dE/dx. Moreover, the sputtering is non-stoichiometric.

Electronic stopping power threshold of damage creation in yttrium iron garnet

Abstract Electronic stopping power threshold of damage creation in yttrium iron garnet (YIG) has been determined by extrapolation using the known model for nuclear and electronic damage creation calibrated by 2 MeV alpha and 510 MeV argon irradiations respectively. This value is equal 4.5