Tracks of 30-MeV C60 clusters in yttrium iron garnet studied by scanning force microscopy

Yttrium iron garnet (Y₃Fe₅O₁₂ or YIG), an amorphizable ferrimagnetic insulator, is probably the best studied material with respect to track formation and damage morphology. This paper presents first scanning force microscopy (SFM) of surface damage induced by energetic C₆₀ clusters. YIG single crystals were irradiated at normal incidence with 30-MeV C₆₀ cluster ions (kinetic energy ∼0.04 MeV/u) provided by the tandem accelerator of the Institute of Nuclear Physics in Orsay (IPNO). The SFM topographic images show nano-protrusions on the YIG surface; where each hillock is generated by one C₆₀ cluster. The role of stopping power and deposited energy density is discussed in terms of dimensional analysis of the nanostructures. Hillocks created by C₆₀ clusters are compared with those produced by monatomic ions.     

TEM investigation of irradiation damage in single crystal CeO2

In order to understand the evolution of radiation damage in oxide nuclear fuel, 150-1000 keV Kr ions were implanted into single crystal CeO₂, as a simulation of fluorite ceramic UO₂, while in situ transmission electron microscopy (TEM) observations were carried out. Two characteristic defect structures were investigated: dislocation/dislocation loops and nano-size gas bubbles.The growth behavior of defect clusters induced by 1 MeV Kr ions up to doses of 5 × 10¹⁵ ions/cm² were followed at 600 °C and 800 °C. TEM micrographs clearly show the development of defect structures: nucleation of dislocation loops, transformation to extended dislocation lines, and the formation of tangled dislocation networks. The difference in dislocation growth rates at 600 °C and 800 °C revealed the important role which Ce-vacancies play in the loop formation process. Bubble formation, studied through 150 keV Kr implantations at room temperature and 600 °C, might be influenced by either the mobility of metal-vacancies correlated with at threshold temperature or the limitation of gas solubility as a function of temperature.     

Modification of the etching properties of x-cut Lithium Niobate by ion implantation

In this work x-cut Lithium Niobate crystals were implanted with 0.5 MeV O ions (nuclear stopping regime), 5 MeV O ions (sub-threshold electronic stopping regime) and 12.5 MeV Ti ions (ion track regime) at the fluences required for the formation of a surface fully disordered layer. The damage depth profiles were determined by RBS-channeling. Wet etching was performed at room temperature in 50% HF:H₂O solution. The data indicated an exponential dependence of the etching rate on the damage concentration. Independently of the damage regime, once random level in the RBS-channeling spectra was attained we measured the same etching rate (50-100 nm/s) and the same volume expansion (∼10%) in all samples. These results indicate that the fully disordered layers obtained by electronic damage accumulation have the same chemical properties of those obtained by conventional nuclear damage accumulation and therefore they can be defined “amorphous”. The impressive etching selectivity of ion implanted regions makes this process suitable for sub-micro machining of Lithium Niobate.     

Confinement of motion of interstitial clusters and dislocation loops in BCC Fe-Cr alloys

This work is devoted to the study of the effect of Cr solutes on the mobility of self interstitial atom (SIA) clusters and small interstitial dislocation loops (of size up to a few nanometers) in concentrated Fe-Cr alloys. Atomistic simulations have been performed to characterize the variation of the free energy of interstitial loops in the Fe-15Cr alloy using the experimentally determined profile of Cr distribution along the path of a loop. It is shown that the presence of randomly distributed Cr in Fe leads to the creation of local trapping configurations for small SIA clusters. The strength (trapping energy) and density of these configurations depend on the Cr content. On the contrary, large SIA clusters (which can be described as 1/2〈1 1 1〉 dislocation loops) are strongly affected by the presence Cr-Cr pairs and larger Cr clusters, which act as barriers to their motion.     

Mechanisms of damage formation in semiconductors

The damage accumulation in ion-implanted semiconductors is analysed using Rutherford backscattering spectrometry (RBS). When energetic ions are implanted in a material, they transfer their energy mainly into atomic collision processes (nuclear energy loss) and in electronic excitations (electronic energy loss). For a given material this primary energy deposition is determined by the mass and energy of the implanted ions and the ion fluence (number of ions per unit area). However, the damage concentration which is measured after implantation does not only depend on the primary energy deposition, but is strongly influenced by secondary effects like defect annealing and defect transformation. For the latter processes the target temperature and the ion flux (number of ions per unit area and time) play an important role. In this presentation the influence of the various parameters mentioned above on the damage accumulation is demonstrated for various materials. Simple empirical models are applied to get information about the processes occurring and to systematize the results for the various semiconductors.     

Ion radiation damage in copper

Transmission electron microscopy techniques have been used to study dislocation loop type damage as a function of depth in copper single crystals irradiated with MeV Cu, Ni and He ions at room temperature. By comparing the location of the peak in the experimental depth profiles with calculated damage energy curves, the electronic stopping powers of Cu and Ni ions in copper were deduced. The deduced electronic stopping powers have been compared with those predicted by Lindhard et al., Bricc, and the Northcliffe and Schilling tables. It was found that the deduced stopping powers agreed well with the Northcliffe and Schilling values corrected for Z₂ oscillations by the method proposed by Ward et al. In the case of 1 MeV He ions, good agreement was obtained between the observed damage peak position and that calculated using the experimental electronic stopping of Ziegler and Chu and that of White and Mueller.     

Velocity effect of swift heavy ions in graphite studied by Raman spectroscopy

Highly oriented pyrolytic graphite (HOPG) samples were irradiated with swift heavy ions (Ar, Kr, Bi, U) of fluences between 10¹¹ and 10¹³ ions/cm² in energy range MeV-GeV. The irradiated samples were analyzed by Raman spectroscopy with laser wavelength of 532.2 nm. It is shown that the ratio between the integrated intensities of the disorder-induced D and the original G Raman bands which denotes the degree of the damage induced by ion irradiation increases as a function of ion fluence as well as the electronic energy loss. This agrees with the previous reports. However, quantitative analysis of the peak intensity at a fixed fluence discloses that ion velocity is also a significant parameter in determination of damage. The conclusion is that the extent of discontinuity of ion track may change with ion velocity besides the electronic energy loss. Considering the radial distribution of the energy deposited on the matter being velocity dependent, the energy density which combines the influence of the electronic energy loss and ion velocity may be more suitable for explaining the effect induced by swift heavy ions.     

辐射损伤潜径迹的纳米尺度观测研究

综述了近年来用扫描隧道显微镜 (STM )和扫描力显微镜 (SFM)在原子水平上观测辐射损伤潜径迹的研究及进展。详述了辐射损伤潜径迹的形貌、损伤范围、损伤数密度、损伤几率等 ,对损伤潜径迹直径与能损的关系、损伤过程及各种可能的损伤机制进行了分析和讨论。     

Calculated energy loss of a swift fullerene ion beam in InP

Bombardment of semiconductors with fullerene has been used to induce the formation of tracks. It is now accepted that target electronic excitation and ionization, which gives rise to the slowing down of the projectile is essential to calculate the track diameter. In the case of cluster beams, like fullerenes, the electronic excitation induced by each of the cluster constituents is enhanced, for certain projectile energies and target depths, by the so-called vicinage effects. Here we use a simulation code to calculate the energy lost by a swift fullerene ion beam in InP, paying special attention to the vicinage effects where they are significative. The code describes classically the movement of each cluster constituent under the influence of the self-retarding force, the Coulomb repulsion among molecular fragments, the wake forces responsible for the vicinage effects and the multiple scattering with the target nuclei. The simulation code also takes into account the possibility that the molecular fragments can also capture or loss electrons from the target, changing its charge state in their travel through the solid.Our simulations show that the energy deposited by the atomic ions that constitute the C₆₀ ion is clearly higher than the energy deposited by the same atomic ions but isolated. This difference being larger as the incident energy increases. We have predicted that track diameters of can be obtained in an InP target when using C₆₀ ions with an initial energy of 300 MeV.     

Interplay of strengthening mechanisms in the interaction of a ½〈1 1 1〉 screw dislocation with Cr precipitates in bcc Fe: An atomistic study

The interaction of Cr precipitates with a ½〈1 1 1〉 screw dislocation in bcc Fe is studied using molecular static calculations to evaluate the contributions from different strengthening mechanisms. The total interaction energy of the precipitate and the dislocation, the surface energy created due to the precipitate shearing and the shear modulus misfit interaction energy were estimated separately. Based on these data the shear modulus misfit was concluded to be the main mechanism determining the precipitate resisting force for the passage of a ½〈1 1 1〉 screw dislocation.     

Can swift heavy ions create latent tracks in silicon?

In this paper we make attempts to clarify the contradictory results for latent track formation in silicon by single heavy ions in the electronic regime of their slowing down. We show by Monte Carlo calculations that, in contrast to the assumption used by the inelastic thermal spike model, the interactions of electrons excited by ions cannot cause fast heating of the lattice at the early stage of the track evolution. In addition, thermal balance equations do not confirm the reality of latent track creation. We believe that, on an ultra short time scale, the damage introduced in the lattice by the ion is rather caused by non-thermal processes, similar to those that occur in silicon irradiated by femto-second lasers (fs-lasers). The characteristics of the damaged zone, the radius of this cylindrical zone, and the effective stopping power related to the non-thermal process for different ions of energy 1-10 MeV/amu are presented. The calculations have also shown that Auger recombination does not influence significantly the density of the e-h pairs which is the most important quantitative characteristic of the non-thermal process.     

In situ and postradiation analysis of mechanical stress in Al2O3:Cr induced by swift heavy-ion irradiation

Optical spectroscopy and TEM techniques have been applied to study the radiation damage and correlated mechanical stresses in Al₂O₃ and Al₂O₃:Cr single crystals induced by (1-3) MeV/amu Kr, Xe and Bi ion irradiation. Mechanical stresses were evaluated in situ using a piezospectroscopic effect through the shift of the respective lines in ionoluminescence spectra. It was found that dose dependence of the stress level for Xe and Bi ions, when ionization energy loss exceeds the threshold of damage formation via electronic excitations, exhibits several alternate stages showing the build-up and relaxation of stresses. The beginning of relaxation stages is observed at fluences associated with beginning of individual ion track regions overlapping. The residual stress profiles through the ion irradiated layers were deduced from depth-resolved photostimulated spectra using laser confocal scanning microscopy set-up. It was determined that stresses are compressive in basal plane and tensile in perpendicular direction in all samples irradiated with high energy ions.     

Effect of ion irradiation and indentation depth on the kinetics of deformation during micro-indentation of Zr-2.5%Nb pressure tube material at 25 °C

Micro-indentation creep tests were performed at 25 °C on radial-normal samples cut from Zr-2.5Nb CANDU pressure tube material in both the as-fabricated condition and after irradiation with 8.5 MeV Zr⁺ ions. The average indentation stress, and hence the yield stress, was found to increase with decreasing indentation depth and with increasing levels of ion irradiation. The activation energy of the indentation creep rate and hence the, activation energy of the obstacles that limit the rate of dislocation glide, was independent of indentation depth but increased from ΔG₀ = 0.185 to 0.215 μb³ with increasing ion irradiation damage. The magnitude of the activation energy indicates that ion irradiation introduces a new type of obstacle into the microstructure which reduces the low temperature indentation creep rate of Zr-2.5Nb pressure tubes. This is supported by TEM images showing that Zr⁺ ion irradiation produces small, nanometer size, dislocation loops which act as obstacles to dislocation glide and thus influence both the yield stress and the activation energy of the low-temperature thermal creep of Zr-2.5Nb pressure tube material. These findings suggest that neutron irradiation will have similar effect upon yield stress and low-temperature thermal creep as the Zr⁺ ion irradiation since both create similar crystallographic defects in Zr-2.5Nb pressure tubes.     

The production of non linear damage under molecular ion implantation

Germanium atomic (Ge₁) and molecular ions (Ge₂) of equivalent energy are implanted in silicon at an elevated temperature. The ion induced damage has been characterized by RBS channeling (RBS/C) and positron annihilation spectroscopy. The RBS/C studies indicate that the molecular ion implantation has produced more defects in the near surface regions compared to the atomic ion implantation. This paper reports a first time observation of an enhanced production of vacancy related defects in silicon implanted with molecular ions.     

Effects of edge dislocations on interstitial helium and helium cluster behavior in α-Fe

The properties of interstitial He in the vicinity of an edge dislocation were studied using molecular dynamics (MD) simulation. The distribution of the binding energy of a single interstitial He to the dislocation with and without a jog is calculated. The results show that the distribution of the binding energy is governed by the elastic interaction between the interstitial He and the dislocation. The interstitial He is strongly attracted to the dislocation in the tensile region of the dislocation. The jog acts as a stronger sink to absorb interstitial He. The binding energy to the jog is even larger than that of the dislocation. A small He cluster (composed of three interstitial He atoms) was trapped by the dislocation core in the form of a chain along the dislocation line. The dislocation changes the migration behavior of the He cluster, and provides a pipe for the small cluster to exhibit one-dimensional motion. The diffusion of the He cluster in the dislocation is faster than in the defect-free iron, where the He cluster migrates three-dimensionally (3D). If the dislocation is decorated by a jog, the small cluster sinks deep into the jog. The jog prevents the He cluster from escaping.     

Search for short-time phase effects in the electronic damage evolution - A case study with silicon

This work focusses on the production and decay properties of inner-shell vacancies and valence-band excitations induced by swift highly charged ions interacting with amorphous and crystalline Si. High resolution electron spectra have been taken for fast heavy ions at 1.78-5 MeV/u as well as for electrons of similar velocity incident on atomically clean Si targets of well defined phase. Various Auger-electron structures are analyzed concerning their width, their intensity and exact peak position. All measured peaks show a small shift towards lower energy when the charge of the projectile is increased. This finding is an indication for a nuclear-track potential inside the ion track. A detailed analysis of the Auger-electron spectra for amorphous Si and crystalline Si(1 1 1) 7 × 7 points to a small but significant phase effect in the short-time dynamics of ion tracks.     

Nucleation and growth of defect clusters in CeO2 irradiated with electrons

Nucleation and growth process of defect clusters in cerium dioxide (CeO₂) with fluorite-type crystal structure has been investigated in situ under electron irradiation by using high voltage transmission electron microscopy. Planar defect clusters were formed with electron irradiation ranging from 200 to 1000 keV at temperatures below 450 K. The defect clusters were determined to be faulted-interstitial type dislocation loops lying on {1 1 1} planes. The growth rate of dislocation loops was found to increase with decreasing electron energy. An analysis of the fluence dependence of the growth process of dislocation loops suggests an increase in the vacancy mobility with decreasing electron energy. The rate of the electronic excitation is discussed in terms of the radiation-induced diffusion of oxygen-ion vacancies.     

固体气泡损伤探测器探测高能重离子的研究

用高能重离子Ar和C进行的实验表明：（1）高能重离子可以在固体气泡损伤探测器中产生径迹,重离子径迹呈直线形,由一连串微小气泡组成;（2）固体气泡损伤探测器探测重离子具有阈特性,阈的实质近似为临界能量损失率（dE／dX）C,这一阈特性与蚀刻径迹探测器类似。固体气泡损伤探测器的阈值为（dE／dX）c=2220MeV／g·cm2,可用于重离子物理、宇宙射线和宇宙暗物质探测以及癌症治疗模拟等领域。     

Study on evolution of gases from fluoropolymer films bombarded with heavy ions

Ion beam bombardment provides a unique way of material modification by inducing a high degree of localized electronic excitation. The ion track, or affected volume along the ion path through the material is related to the total damage and possible structural changes. Here we study the evolution of gases emitted by poly(tetrafluorethylene-co-perfluoro-(propyl vinyl ether)) (PFA) fluoropolymer bombarded with MeV gold ions. The gas was monitored by a residual gas analyzer (RGA), as a function of the ion fluence. Micro-Raman, atomic force microscopy and optical absorption were used to analyze the chemical structure changes and sputtering yield.     

Effective energy deposition and latent track formation of swift heavy ions in solids

In the present paper, latent track formation in yttrium iron garnet (YIG) produced by high energy Ar ions is briefly reported at first. Then, in the framework of thermal spike model, a phenomenological parameter describing the effective energy transfer from excited electrons to lattice atoms, effective energy deposition Qeff, is deduced. Qeff is a function of ion velocity, electronic energy loss (Se) and mean free path λ of excited electrons in the matter, and is a time moderate term in…