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A new tool for Monte Carlo simulation of ion channeling in crystals is described. The recently developed McChasy2 code follows the algorithm used previously in the McChasy program, but is well suited for simulations in large crystalline samples. The state of works on the validation of the code, and the possibilities of the new code for supplementing molecular dynamics with a tool supporting the experimental analysis of crystalline structures are presented and discussed.

Rare earth-doped zinc oxide (ZnO:RE) systems are attractive for future optoelectronic devices such as phosphors, displays, and LEDs with emission in the visible spectral range, working even in a radiation-intense environment. The technology of these systems is currently under development, opening up new fields of application due to the low-cost production. Ion implantation is a very promising technique to incorporate rare-earth dopants into ZnO. However, the ballistic nature of this process makes the use of annealing essential. The selection of implantation parameters, as well as post-implantation annealing, turns out to be non-trivial because they determine the luminous efficiency of the ZnO:RE system. This paper presents a comprehensive study of the optimal implantation and annealing conditions, ensuring the most efficient luminescence of RE3+ ions in the ZnO matrix. Deep and shallow implantations, implantations performed at high and room temperature with various fluencies, as well as a range of post-RT implantation annealing processes are tested: rapid thermal annealing (minute duration) under different temperatures, times, and atmospheres (O2, N2, and Ar), flash lamp annealing (millisecond duration) and pulse plasma annealing (microsecond duration). It is shown that the highest luminescence efficiency of RE3+ is obtained for the shallow implantation at RT with the optimal fluence of 1.0 × 1015 RE ions/cm2 followed by a 10 min annealing in oxygen at 800 °C, and the light emission from such a ZnO:RE system is so bright that can be observed with the naked eye.

The main goal of the McChasy code, in general, is to reproduce Rutherford Backscattering Spectrometry experimental spectra recorded in channeling direction (RBS/C) by simulating He-ions travelling inside crystalline structures and to calculate the probability of the backscattering process. The 2.0 version of the code provides the possibility to simulate channeling spectra in large (ca. 108 atoms) arbitrary structures that are created based on crystallographic data or Molecular Dynamic (MD) calculations. In this work, we present the current status of the code and the results of recent investigations of extended structural defects (edge dislocations and dislocation loops) formed inside nickel (Ni) single crystals. Two ways of modelling extended defects are described: one developed using the McChasy code (Peierls-Nabarro approach) and the other one obtained by modification and thermalization of Ni structures by MD (LAMMPS code). The atomic local environment was studied qualitatively and quantitatively by the local projectile-flux density distributions around the defects.

This chapter describes the most important features of computational software called ‘McChasy’, which is a Monte Carlo (MC) simulation code developed for the evaluation of the Rutherford Backscattering Spectrometry data, in particular, recorded in the channeling mode (RBS/C). RBS/C is an experimental technique used in the analysis of defects in single crystals. Lattice distortions affect materials modified by ion beams or exposed to irradiation. Therefore, the analysis of damage in crystals is of high importance in materials science. Various types of defects can be created due to the interaction of ions with targets. However, RBS/C has different sensitivity to each of them so the analytical analysis of experimental data is hardly possible. MC simulations are a powerful tool used to overcome this limitation. The McChasy code simulates the movement of light ions in crystals. The software provides a fitting procedure of RBS/C spectra based on independent depth profiles of different defect types: interstitials, edge dislocations, substitutions, stacking faults or grain boundaries. The code works well not only with materials containing complex defects but also with heterostructures and superlattices. Recent improvements of the code include a unique approach of 3D-interaction between ions and target atoms. Application of the McChasy code in the analysis of crystal defects is described and possible ways of its further development are pointed out.

In contrast to single crystalline materials a quantitative determination of defect formation in polycrystals is challenging. Here we use low-energy (3 keV) cathodoluminescence (CL) for structural characterization of single and polycrystalline magnesium aluminate spinel (MgAl2O4) exposed to irradiation with 300 keV Ar+ ions at room temperature in a wide fluence range of 1 × 1012–2 × 1016 cm−2. The results for single crystalline materials are also compared with conventional Rutherford Backscattering and Channeling spectrometry (RBS/C). It is demonstrated that damage formation in both single and polycrystalline spinels exhibits a multi-step character where polycrystalline material is more susceptible to ion irradiation. The results also imply that the low-energy CL is more sensitive to low damage levels as compared to RBS/C and it can be efficiently used as a complementary tool showing new perspectives in the damage accumulation studies in single- and polycrystalline materials.

Epitaxial ZnO films implanted with ytterbium have been studied by combination of channeling Rutherford backscattering spectrometry and Hall measurements. This approach is able to shed more light onto the difficult problem of optical activation of Yb implanted into the ZnO lattice. ZnO films grown by atomic layer deposition on a GaN/Al 2 O 3 substrate were implanted with Yb + ions to the fluences of 5 × 10 14 , 1 × 10 15 and 5 × 10 15 at./cm 2 . RBS/c spectra show that for as implanted samples the fraction of implanted Yb ions occupying substitutional lattice positions amounts to 72%, 54% and 22%, respectively. Annealing under an ambient atmosphere at 800 °C leads to a partial recovery of the crystal lattice but also substantially reduces substitutional fraction of Yb atoms. Photoluminescence studies on the as-implanted samples revealed a very weak luminescent peak in the near-infrared region, which slightly increases with a fluence. The annealing process leads to optical activation of Yb 3 + ions and significantly increases the previously observed the near-infrared inter-ionic emission. The results obtained from Hall measurements indicate that after implantation Yb ions are mainly in the 2 + state and are transformed to the optically active 3 + state as a result of annealing. It suggests that the process of optical activation via annealing is related not only to lattice recovery, but also to Yb diffusion from substitutional to interstitial positions, where Yb ions are in the optically active 3 + state.

Epitaxial thin ZnO films grown by Atomic Layer Deposition were implanted with 150 keV Pr ions to a fluence of 1 × 10 15 at/cm 2 . Implanted samples were subjected to two different kinds of annealing: rapid thermal annealing (RTA) and millisecond-range flash lamp annealing (FLA). Structural properties of implanted and annealed ZnO and the optical response were evaluated by the Channeling Rutherford Backscattering Spectrometry (RBS/c), High-resolution X-ray diffraction and Photoluminescence Spectroscopy (PL), respectively. The results shown, that both annealing techniques lead to recrystallization of the ZnO lattice, that was damaged during the ion implantation. Upon RTA performed at 800 °C a return of Zn atoms from interstitial to their regular site positions is accompanied by rejection of primarily substitutional Pr atoms to the interstitial sites. Consequently, it leads to the out-diffusion and precipitation of Pr atoms on the surface. In contrast to RTA, the diffusion of implanted Pr during a millisecond range FLA treatment is completely suppressed. Despite differences in location of Pr inside the ZnO matrix after FLA and RTA, both annealing techniques lead to the optical activation of Pr 3 + . Interestingly, our RBS/c study for as implanted layers also revealed the anomalous damage peak, called intermediate peak (IP) located between the expected surface and the bulk damage peak. The PL spectra clearly suggest, that the defect which forms the IP, can be assigned to Zn interstitials. The long-time annealing at 800 °C in oxygen atmosphere causes the complete removal of the IP.

Bombardment of ZnO with heavy ions generating dense collision cascades is of particular interest because of the formation of nontrivial damage distribution involving a defected layer located between the surface and the bulk damage regions, as seen by Rutherford backscattering spectroscopy in the channeling mode. By correlating photoluminescence and channeling data, we demonstrate that the thermal evolution of defects in wurtzite ZnO single crystals implanted with Cd ions strongly depends on the implanted dose. Specifically, the ion dose has a profound effect on the optical response in the spectral range between the near-band-edge emission and deep-level emission bands. The interplay between interstitial and vacancy type defects during annealing is discussed in relation to the evolution of the multipeak damage distribution.

International audience; The main purpose of this paper is to summarize our results obtained during research on the radiation behavior of oxides. The paper presents various methods of damage accumulation in irradiated oxides with the special emphasis on correlations between channeling, luminescence and Raman spectroscopy measurements as well as the possibility to obtain quantitative data on polycrystalline materials. The results are interpreted in terms of Multi Step Damage Accumulation model. Without pretending to give a full review of the current trends concerning these studies, we intend to present both a reflection about recent results and a few options for next investigations. This article can thus be regarded as the opening of a discussion on further directions of the research to be conducted on the topic of radiation damage formation in advanced materials for nuclear applications.

Defect agglomeration in ion-implanted compound semiconductors produces lattice stress eventually causing plastic deformation at sufficiently high fluence. Consequently, a dislocations tangle is formed which can hardly be completely removed by thermal annealing. To solve this problem, a new method of sequential processing has been developed consisting of low fluence ion implantation followed by subsequent annealing. The procedure can be then repeated until the required impurity concentration has been reached without producing excessive damage. Epitaxial ZnO layers are grown using the atomic layer deposition (ALD) technique. Structural changes in ZnO epilayers due to Yb-ion implantation and subsequent annealing are analyzed by Rutherford backscattering/channeling (RBS/c) and photoluminescence (PL). Correlation between defect transformations and PL efficiency is determined. Increased Yb-atom optical activation upon sequential processing as compared to the standard single-step annealing is observed.

Epitaxial ZnO thin films grown by atomic layer deposition on GaN/Al2O3 substrates are implanted with Yb, Dy, and Pr ions to a fluence of 5e14 atcm-2 and subsequently anneals at 800 C using a rapid thermal annealing (RTA) system. Structural properties of implanted and annealed ZnO films and the optical response are evaluated by channeling Rutherford backscattering (RBS/c) and photoluminescence spectroscopy (PL), respectively. RTA leads to a partial removal of the post-implantation defects with simultaneous native defects transformation and optical activation of RE ions. It is found that two groups of defects: defects formed during implantation process and native defects, play an important role in the luminescence in the visible region. The room temperature PL spectra obtained from annealed ZnO:RE films do not show sharp PL lines from transitions within the RE 4f shell, but show near band gap emission and defect related emission, which energy emission is controlled by the RE atoms. It suggests a presence of RE-related complexes that are formed during hightemperature annealing in oxygen atmosphere. The excitonic and defect emission modified by RE ions create an optical response of the system resulting in a specific color of the emitted light.

In this paper, we present the detailed study of optical and structural properties of Yb implanted single ZnO crystals. Hydrothermally grown wurtzite (0001) ZnO crystals were implanted with 150 keV Yb ions to fluencies of 5e14 and 1e15 at/cm2. After ion implantation, two different types of annealing were performed: rapid thermal annealing (RTA) and millisecond range flash lamp annealing (FLA). Crystalline quality, damage recovery, and Yb lattice site location were evaluated by the Channeling Rutherford Backscattering Spectrometry (RBS/c). It is shown that independent of the used annealing technique, defects formed in ZnO during ion implantation can be removed. Upon RTA performed at the temperature higher than 800C, strong out-diffusion of implanted Yb atoms and precipitation on the surface takes place. Consequently, the degradation of the photoluminescence (PL) efficiency is observed. The diffusion of implanted Yb during millisecond range FLA does not occur for such experimental conditions. Moreover, FLA treatment for 20 ms leads to the formation of single crystalline ZnO layer with Yb incorporated in the substitutional lattice sites. According to RBS/c and PL data, Yb atoms substituted in the Zn sublattice are predominantly in the 2þ oxidation state. The most intensive PL has been observed after annealing at 800C for 20 min which is accompanied with the reduction of Yb substitutional fraction and formation of octahedron Yb-oxygen clusters within ZnO.

The analysis of krypton in irradiated uranium dioxide fuel has been successfully achieved by X-ray fluorescence and X-ray absorption. The present study focuses on the analytical challenge of sample and sub-sample production to perform the analysis with the restricted conditions dictated by the radioprotection regulations. It deals also with all potential interferences that could affect the quality of the measurement in fluorescence as well as in absorption mode. The impacts of all dissolved gases in the fuel matrix are accounted for the analytical result quantification. The krypton atomic environment is ruled by the presence of xenon. Other gases such as residual argon and traces of helium or hydrogen are negligible. The results are given in term of density for krypton ( 3n m 3 ) and xenon (20 nm 3 ). The presence of dissolved, interstitial and nano-phases are discussed together with other analytical techniques that could be applied to gain information on fission gas behaviour in nuclear fuels. 2014 Published by Elsevier B.V.

Experimental studies have evidenced the presence of some chromium oxide precipitates in chromia (Cr 2 O 3 ) doped UO 2 fuels, currently being used in many light water reactors. The main objective of this study was to analyze the atomic scale microstructure of these precipitates in a non-destructive way employing modern synchrotron-based X-ray radiation. To investigate UO 2 microstructures and chemical compositions of the precipitates an industrial grade fresh Cr 2 O 3 -doped UO 2 pellet (produced with an initial amount of 0.16 wt.% Cr 2 O 3 powder) was examined using scanning electron microscopy (SEM) and electron probe micro analysis (EPMA). Average grain diameter of the UO 2 matrix was measured by SEM image analysis method. Precipitates were detected by EPMA. For all precipitates of micrometer scale analyzed by EPMA the composition was determined to be very close to Cr 2 O 3 . The structural property and the next neighbor Cr atomic environment in these precipitates were studied by a combination of more sensitive tools such as micro-focused X-ray diffraction (μ-XRD) and absorption spectroscopy (μ-XAS). According to the XRD data, the chromia precipitates contain structural disorder and have a somewhat distorted lattice structure as compared to that of standard and crystalline α-Cr 2 O 3 hexagonal crystal unit cell. The μ-XAS results provide insight into the local atomic environment of chromium including the oxidation state assignment.

Monte Carlo (MC) simulations are a powerful tool for the analysis of ion-solid interactions. The MC code McChasy (Monte Carlo CHAnneling SYmulation) allows the evaluation of Rutherford Backscattering Spectrometry spectra in the Channeling mode to quantify implantation damage. The code works on common personal computers and takes into account randomly displaced atoms as well as certain types of extended defects. In this paper, we report recent improvements of the McChasy code, including a unique approach to the calculation of impact parameters between ions and target atoms in three dimensions (along with computing thermal vibrations also in three dimensions). Furthermore, the use of a rotation matrix to provide different orientations of dislocation lines and an updated model of edge dislocations were also implemented in the code. Dislocation parameters are obtained directly from high-resolution Transmission Electron Microscopy micrographs. Two case studies are presented to highlight the importance of these improvements: Ni-implanted Al was analyzed as an example of a crystal mainly containing dislocations; Er-implanted ZnO was studied, revealing the strength of MC analysis for materials containing a mixture of different defect types, namely, randomly displaced atoms and dislocations.Monte Carlo (MC) simulations are a powerful tool for the analysis of ion-solid interactions. The MC code McChasy (Monte Carlo CHAnneling SYmulation) allows the evaluation of Rutherford Backscattering Spectrometry spectra in the Channeling mode to quantify implantation damage. The code works on common personal computers and takes into account randomly displaced atoms as well as certain types of extended defects. In this paper, we report recent improvements of the McChasy code, including a unique approach to the calculation of impact parameters between ions and target atoms in three dimensions (along with computing thermal vibrations also in three dimensions). Furthermore, the use of a rotation matrix to provide different orientations of dislocation lines and an updated model of edge dislocations were also implemented in the code. Dislocation parameters are obtained directly from high-resolution Transmission Electron Microscopy micrographs. Two case studies are presented to highlight the importance of these...

Plutonium uranium mixed oxide (MOX) fuels are currently used in nuclear reactors. The actinides in these fuels need to be analyzed after irradiation for assessing their behaviour with regard to their environment and the coolant. In this work the study of the atomic structure and next-neighbour environment of Am in the (Pu,U)O₂ lattice in an irradiated (60 MW d kg⁻¹) MOX sample was performed employing micro-X-ray fluorescence (µ-XRF) and micro-X-ray absorption fine structure (µ-XAFS) spectroscopy. The chemical bonds, valences and stoichiometry of Am (~0.66 wt%) are determined from the experimental data gained for the irradiated fuel material examined in its peripheral zone (rim) of the fuel. In the irradiated sample Am builds up as Am³⁺ species within an [AmO₈]¹³⁻ coordination environment (e.g. >90%) and no (

Specimens of a 4.7 wt % plutonium MOX fuel, irradiated for a period of 6 years up to 60 MW d kg −1 were investigated using synchrotron based micro X-ray diffraction technique (μXRD). The μXRD measurements at the Swiss Light Source SLS with a beam spot size of about 2 μm × 2 μm were carried out on the center and periphery region on the cross section of the irradiated fuel, as well as on un-irradiated reference MOX material. The profiles of characteristic (111), (200), (220), (311) and (222) diffraction lines of the irradiated samples were analyzed in detail and compared with the data of the non-irradiated MOX material, revealing the lattice distortion and dilatation in the irradiated fuel matrix. Broadening of reflexes can be observed due to the increasing stress finally yielding strain up to fuel grain fracture. This phenomenon is quantified thanks to the analysis of the reflexes arcs fine structure. The sub-grain number per grain volume is analysed as a function of fuel location and history.

Microstructural changes in a set of commercial grade UO 2 fuel samples have been investigated using synchrotron based micro-focused X-ray fluorescence (μ-XRF) and X-ray diffraction (μ-XRD) techniques. The results are associated with conventional UO 2 materials and relatively larger grain chromia-doped UO 2 fuels, irradiated in a commercial light water reactor plant (average burn-up: 40 MW d kg −1 ). The lattice parameters of UO 2 in fresh and irradiated specimens have been measured and compared with theoretical predictions. In the pristine state, the doped fuel has a somewhat smaller lattice parameter than the standard UO 2 as a result of chromia doping. Increase in micro-strain and lattice parameter in irradiated materials is highlighted. All irradiated samples behave in a similar manner with UO 2 lattice expansion occurring upon irradiation, where any Cr induced effect seems insignificant and accumulated lattice defects prevail. Elastic strain energy densities in the irradiated fuels are also evaluated based on the UO 2 crystal lattice strain and non-uniform strain. The μ-XRD patterns further allow the evaluation of the crystalline domain size and sub-grain formation at different locations of the irradiated UO 2 pellets.

Synchrotron-based microprobe x-ray absorption spectroscopy (XAS) has been used to study the local atomic structure of chromium in chromia-doped uranium dioxide (UO2) grains. The specimens investigated were a commercial grade chromia-doped UO2 fresh fuel pellet, and materials from a spent fuel pellet of the same batch, irradiated with an average burnup of ~40 MW d kg(-1). Uranium L3-edge and chromium K-edge XAS have been measured, and the structural environments of central uranium and chromium atoms have been elucidated. The Fourier transform of uranium L3-edge extended x-ray absorption fine structure shows two well-defined peaks of U-O and U-U bonds at average distances of 2.36 and 3.83 Å. Their coordination numbers are determined as 8 and 11, respectively. The chromium Fourier transform extended x-ray absorption fine structure of the pristine UO2 matrix shows similar structural features with the corresponding spectrum of the irradiated spent fuel, indicative of analogous chromium environments in the two samples studied. From the chromium XAS experimental data, detectable next neighbor atoms are oxygen and uranium of the cation-substituted UO2 lattice, and two distinct subshells of chromium and oxygen neighbors, possibly because of undissolved chromia particles present in the doped fuels. Curve-fitting analyses using theoretical amplitude and phase-shift functions of the closest Cr-O shell and calculations with ab initio computer code FEFF and atomic clusters generated from the chromium-dissolved UO2 structure have been carried out. There is a prominent reduction in the length of the adjacent Cr-O bond of about 0.3 Å in chromia-doped UO2 compared with the ideal U-O bond length in standard UO2 that would be expected because of the change in effective Coulomb interactions resulting from replacing U(4+) with Cr(3+) and their ionic size differences. The contraction of shortest Cr-U bond is ~0.1 Å relative to the U-U bond length in bulk UO2. The difference in the local chromium environment between fresh and irradiated UO2 is discussed based on the comparison of quantitative structural information obtained from the two chromia-doped fuel samples analyzed.

Single-phase concentrated solid-solution alloys (SP-CSAs) have shown unique chemical complexity at the levels of electrons and atoms, and their defect evolution is expected to be different from conventional dilute alloys. Single crystals of Ni, NiFe and NiFeCoCr are chosen as model systems to understand the chemical complexity on defect formation and damage accumulation in SP-CSAs under ion irradiation. The high-quality crystals were irradiated at 16 and 300 K to different ion fluences, to form irradiated region with little to heavy damages. The ion-induced damage was determined using Rutherford backscattering spectrometry technique along a channeling direction (RBS/C) and the level of lattice damage in irradiated Ni and SP-CSAs was quantified from Monte Carlo (MC) simulations. The results are interpreted using the Multi Step Damage Accumulation model to reveal material damage accumulation kinetics. Key findings of the study are that in case of room temperature irradiations the damage level measured for complex alloys at the highest irradiation fluence of 2 × 1015 cm−2 (∼3 dpa) is significantly higher than that obtained for pure nickel samples and suggest two-step damage accumulation process with a defect transformation taking place at a fluence of about 1.5 × 1015 cm−2. Moreover, structural and damage kinetic differences clearly imply that, with increasing degree of chemical complexity and high solid-solution strengthening effects from Ni to NiFe and to NiFeCoCr, the enhanced lattice stiffness resists to randomization of atomic configurations and inhibits the growth of extended defects.