Oxygen defects created in CeO2 irradiated with 200 MeV Au ions

CeO₂ films were irradiated with 200 MeV Au ions in order to investigate the damages created by electronic energy deposition. In the Raman spectra of the ion-irradiated films, a broad band appears at the higher frequency side of the F_2g peak of CeO₂. The band intensity increases as ion fluence increases. Furthermore, the F_2g peak becomes asymmetric with a low-frequency tail. In order to understand the origin of these spectral changes, an unirradiated CeO₂ film was annealed in vacuum at 1000 °C. By comparing the results for the irradiation and for the annealing, it is concluded that the broad band obtained for irradiated samples contains the peak observed for the annealed sample. The F_2g peak becomes asymmetric with a low-frequency tail by the irradiation as well as the annealing. Therefore, the above-mentioned changes in the Raman spectra caused by 200 MeV Au irradiation is closely related to the creation of oxygen vacancies.     

Structural changes in helium implanted Zr0.8Y0.2O1.9 single crystals characterized by atomic force microscopy and EXAFS spectroscopy

The present work is devoted to investigate the local atomic environment (of Zr, Y and O) as well as surface modifications associated with excess helium in the cubic phase of (1 0 0)-oriented Zr_0.8Y_0.2O_1.9 single crystal substrates. Commercially available oxide crystals have been implanted at various fluences in the range 0.15-2.0 × 10¹⁶ He-atoms/cm² using a 2.74 MeV He⁺ ion beam passing through a 8.0 μm Al foil. The microstructure and surface morphology of the irradiated surface are examined using atomic force microscopy (AFM). The local atomic environments of Zr, Y and O in the implanted layer are studied using synchrotron radiation and by extended X-ray absorption fine structure (EXAFS) measured at glancing angles to probe the implanted layer. From AFM studies it was observed that the surface roughness increases as fluence increases and above a critical fluence stage, small blister-like structures originating from helium bubbles are scattered on the irradiated surface. The radial distribution functions (RDFs), derived from EXAFS data at the Zr K-edge, have been found to evolve continuously as a function of ion fluence describing the atomic scale structural modifications in YSZ by helium implantation. From the pristine data, long range order (beyond the first- and second-shell) is apparent in the RDF spectrum. It shows several nearest neighbour peaks at about 2.1, 3.6, 4.3 and 5.4 Å. In the implanted specimens, all these peaks are greatly reduced in magnitude and their average positions are changed, typical of damaged material. A simple model taking into account only the existence of lattice vacancies has been used for the interpretation of measured EXAFS spectra.     

Irradiation effects in an HfO2/MgO/HfO2 tri-layer structure induced by 10 MeV Au ions

In this report, we present radiation damage effects in a thin film, tri-layer structure, HfO₂/MgO/HfO₂. Irradiations were performed with 10 MeV Au ions in a recently developed medium energy ion irradiation facility at Los Alamos National Laboratory, which is described in this paper. Energy deposition by 10 MeV Au ions corresponds to a mixed regime, wherein electronic and nuclear stopping contribute to radiation damage. In this study, we investigated modifications of both surface and bulk properties in order to assess the structural stability of our oxide tri-layers under the severe irradiation conditions employed here. The most dramatic structural changes were observed to occur on the surfaces of the tri-layer samples. Surface features consisted of large craters and spires. The dimensions of these craters and spires exceed those of the individual ion tracks by almost three orders of magnitude. As for the bulk tri-layer structure, our conclusions are that this structure is stable in terms of: (i) resistance to amorphization; (ii) resistance to compositional mixing and (iii) resistance to pronounced nucleation and growth of extended defects. The main effect observed in the tri-layer structure was the transformation of the first HfO₂ layer from a monoclinic to either a tetragonal or cubic form of HfO₂.     

Amorphisation of ZnAl2O4 spinel under heavy ion irradiation

ZnAl₂O₄ spinels have been irradiated with several ions (Ne, S, Kr and Xe) at the IRRSUD beamline of the GANIL facility, in order to determine irradiation conditions (stopping power, fluence) for amorphisation. We observed by transmission electron microscopy (TEM) that with Xe ions at 92 MeV, individual ion tracks are still crystalline, whereas an amorphisation starts below a fluence of 5 × 10¹² cm⁻² up to a total amorphisation between 1 × 10¹³ and 1 × 10¹⁴ cm⁻². The coexistence of amorphous and crystalline domains in the same pristine grain is clearly visible in the TEM images. All the crystalline domains remain close to the same orientation as the original grain. According to TEM and X-ray Diffraction (XRD) results, the stopping power threshold for amorphisation is between 9 and 12 keV nm⁻¹.     

Radiation effects on PbO-Al2O3-B2O3-SiO2 glasses by FTIR spectroscopy

The infrared absorption spectra of PbO-Al₂O₃-B₂O₃-SiO₂ glasses have been measured in the spectral range 600-4000 cm⁻¹ before and after absorbed dose of 50 Gy, 4 kGy and 50 kGy to investigate the structural change due to irradiation. The structural change due to composition has also been discussed. The experimental results clearly indicate that after irradiation, a significant change in structure of lead alumino borosilicate glass network is observed. It was shown that BO₄ groups decreases and BO₃ groups increases with the increase of Al₂O_3.     

Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation

We observed an increase in the conductivity of a thiospinel compound, CuIr₂S₄, induced by H⁺ and He⁺ irradiation with energies of 1-2 MeV. It was indicated that the metastable conductive phase was produced by electronic excitation due to the ion beam and this phase was similar to the X-ray-induced phase. Conductivity as a function of ion fluence was analyzed by a simple model where the ion-induced change occurred in a cylindrical region around an ion trajectory. The cross-sectional area of the cylinder was obtained by analyzing the conductivity as a function of ion fluence for each ion, and it was found that an impinging ion produced a nanowire in the conductive phase. In addition, the yield of the Ir dimer displacement, which was related to the increase in conductivity, was considerably high. The ion irradiation effect reported in this paper is unique with regard to the high yield and low linear energy transfer (LET) in the formation of the conductive-phase nanowire. Both these unique aspects could be ascribed to the low band-gap energy and strong electron-lattice interaction of this compound.     

Alpha-decay induced amorphization of Cm-doped Gd2TiZrO7

The sample of pyrochlore-based ceramic doped with a ²⁴⁴Cm isotope with a target composition Gd_1.935Cm_0.065 TiZrO₇ was prepared by cold pressing and sintering. The pyrochlore structure phase was predominant in the sample but minor perovskite and gadolinium zirconate (ideally Gd₂Zr₂O_7−x) were also present. The Ti/Zr pyrochlore phase was rendered amorphous at a dose of 4.6 × 10¹⁸ α-decays/g (0.60 dpa). Volume expansion of the pyrochlore lattice was found to be 2.7 vol.% at a dose of 3.85 × 10¹⁸ α-decays/g.     

Anisotropic deformation of polystyrene particles by MeV Au ion irradiation

MeV Au irradiation leads to a shape change of polystyrene (PS) and SiO₂ particles from spherical to ellipsoidal, with an aspect ratio that can be precisely controlled by the ion fluence. Sub-micrometer PS and SiO₂ particles were deposited on copper substrates and irradiated with Au ions at 230 K, using an ion energy and fluence ranging from 2 to 10 MeV and 1 × 10¹⁴ ions/cm² to 1 × 10¹⁵ ions/cm². The mechanisms of anisotropic deformation of PS and SiO₂ particles are different because of their distinct physical and chemical properties. At the start of irradiation, the volume of PS particles decrease, then the aspect ratio increases with fluence, whereas for SiO₂ particles the volume remains constant.     

10 MeV Au ion irradiation effects in an MgO-HfO2 ceramic-ceramic (CERCER) composite

Room temperature ion irradiation damage studies were performed on a ceramic composite intended to emulate a dispersion nuclear fuel. The composite is composed of 90-mole% MgO and 10-mole% HfO₂. The as-synthesized composite was found to consist of Mg₂Hf₅O₁₂ (and some residual HfO₂) particles embedded in an MgO matrix. X-ray diffraction revealed that nearly all of the initial HfO₂ reacted with some MgO to form Mg₂Hf₅O₁₂. Ion irradiations were performed using 10 MeV Au³⁺ ions at room temperature over a fluence range of 5 × 10¹⁶-5 × 10²⁰ Au/m². Irradiated samples were characterized using both grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM), the latter using both selected-area electron diffraction (SAED) and micro-diffraction (μD) on samples prepared in cross-sectional geometry. Both GIXRD and TEM electron diffraction measurements on a specimen irradiated to a fluence of 5 × 10²⁰ Au/cm², revealed that the initial rhombohedral Mg₂Hf₅O₁₂ phase was transformed into a cubic-Mg₂Hf₅O₁₂ phase. Finally, it is important to note that at the highest ion fluence used in this investigation (5 × 10²⁰ Au/m²), both the MgO matrix and the Mg₂Hf₅O₁₂ second phase remained crystalline.     

Study on the behavior of oxygen atoms in swift heavy ion irradiated CeO2 by means of synchrotron radiation X-ray photoelectron spectroscopy

To study the effects of swift heavy ion irradiation on cerium dioxide (CeO₂), CeO₂ sintered pellets were irradiated with 200 MeV Xe ions at room temperature. For irradiated and unirradiated samples, the spectra of X-ray photoelectron spectroscopy (XPS) were measured. XPS spectra for the irradiated samples show that the valence state of Ce atoms partly changes from +4 to +3. The amount of Ce³⁺ state was quantitatively obtained as a function of ion-fluence. The relative amount of oxygen atom displacements, which are accompanied by the decrease in Ce valence state, is 3-5%. This value is too large to be explained in terms of elastic interactions between CeO₂ and 200 MeV ions. The experimental result suggests the contribution of 200 MeV Xe induced electronic excitation to the displacements of oxygen atoms.     

Experimental study of the residual activity induced by 950 MeV/u uranium ions in stainless steel and copper

As a preparatory work for constructing the FAIR facility at GSI, samples of stainless steel and copper were irradiated by 950 MeV/u ²³⁸U ions and depth-profiles of residual activity were measured by gamma-ray spectroscopy. The isotopes with dominating contribution to the residual activity were identified and their contributions were quantified. In contrast to the previous study performed at lower energies, the activities could no longer be determined from the full-assembly target measurements. Depth-profiling of residual activity of all identified isotopes had to be completed by measurements of individual target foils. The activity contributions were then obtained by integration of the depth-profiles.     

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.     

Existence states of deuterium irradiated into LiAlO2

The existence states of deuterium in LiAlO₂ were analyzed by in situ IR absorption spectroscopy during irradiation with 3 keV at room temperature. Multiple IR absorption peaks that were related to O-D stretching vibrations were observed, mainly at 2650 cm⁻¹ (O-D_α), 2600 cm⁻¹ (O-D_β), and 2500 cm⁻¹ (O-D_γ). The O-D_α was assigned to the surface O-D. The O-D_β and O-D_γ were interpreted as two distinct O-D states for three candidates: O-D of substitutional D⁺ for Li⁺; O-D of substitutional D⁺ for Al³⁺; and O-D of interstitial D⁺. O-D_β was the dominant O-D state for deuterium irradiated into LiAlO₂, and had higher stability than O-D_γ. Heating after ion irradiation led to the desorption of D₂ and an increase in the intensity of O-D_β, which implies that some of the deuterium irradiated into LiAlO₂ exists in non-O-D states, such as D⁻ captured by F centers.     

Molecular dynamics simulations to explore the effect of chemical bonding in the keV bombardment of Si with C60, Ne60 and Ne60 projectiles

Depth profiling experiments using secondary ion spectrometry (SIMS) have shown effects that are characteristic to the pairing of the projectile with a Si target. Previous molecular dynamics simulations demonstrate that this unusual behavior is due to the fact that strong covalent bonds are formed between the C atoms in the projectile and the Si atoms in the target, which result in the implantation of carbon into the solid. The focus of this paper is to understand how the formation of chemical bonds affects the net sputtered yield. The results of molecular dynamics simulations of the keV bombardment of Si with C₆₀, Ne₆₀ and ¹²Ne₆₀ at normal incidence are compared over a range of incident kinetic energies from 5 to 20 keV. The net yields with Ne₆₀ and ¹²Ne₆₀ are significantly greater than with C₆₀ at all incident kinetic energies, with ¹²Ne₆₀ having the largest values. Application of the mesoscale energy deposition footprint (MEDF) model shows that the initial deposition of energy into the substrate is similar with all three projectiles. Snapshots of the initial pathway of the projectile atoms through the substrate show a similar lateral and vertical distribution that is centered in the region of the energy footprint. Therefore, the reason for the reduced yield with C₆₀ is that the C atoms form bonds with the Si atoms, which causes them to remain in the substrate instead of being sputtered.     

Heavy ion beam micromachining on LiNbO3

In this work 3D micromachining of x-cut lithium niobate crystals was performed using the high energy heavy ion microbeam (HIM) at the Tandar Laboratory, Buenos Aires. The samples were machined using ³⁵Cl beams at 70 MeV bombarding energy combined with wet etching with hydrofluoric acid solutions at room temperature. As the ion beam penetrates the sample, it induces lattice damage increasing dramatically the local etching rate of the material. This technique was applied to the fabrication of 3D waveguides with long control electrodes. The resulting structures indicate that well defined contours with nearly vertical sidewalls can be made. The results also show that with fluences of only 5 × 10¹² ions/cm², this technique is suitable for the fabrication of different shapes of LiNbO₃ control-waveguides that can be used in different optical devices and matched with the existing optical fibers.     

Photoluminescence and XEL in Y2O3:Eu phosphor

Eu-activated Y₂O₃ phosphors were prepared by combustion synthesis and also by precipitation techniques. Photoluminescence and X-ray excited luminescence of prepared Y₂O₃:Eu phosphor, under two different techniques were compared and reported in this paper. Y₂O₃:Eu³⁺ phosphor were prepared by precipitation technique followed by annealing at 900 °C. It gives cubic nature of the particle that may be more favourable for high lumen output. X-ray excited luminescence of Y₂O₃:Eu³⁺ phosphors also reported in this paper.     

Effects of 0.28-2.80 MeV proton irradiation on GaInP/GaAs/Ge triple-junction solar cells for space use

GaInP/GaAs/Ge triple-junction solar cells were irradiated with 0.28, 0.62 and 2.80 MeV protons with fluences ranging from 1 × 10¹⁰ cm⁻² to 1 × 10¹³ cm⁻². Their performance degradation is analyzed using current-voltage characteristics and spectral response measurements. The degradation rates of the short circuit current, open circuit voltage, and maximum power output increase with fluence, but decrease with increasing proton energy. It was also observed that the spectral response of the GaAs middle cell degrades more significantly than that of the GaInP top cell.     

Swift heavy ion induced structural modifications in zircon and scheelite phases of ThGeO4

Structural modifications in the zircon and scheelite phases of ThGeO₄ induced by swift heavy ions (93 MeV Ni⁷⁺) at different fluences as well as pressure quenching effects are reported. X-ray diffraction and Raman measurements at room temperature on the irradiated zircon phase of ThGeO₄ indicate the occurrence of stresses that lead to a reduction of the cell volume up to 2% followed by its transformation to a mixture of nano-crystalline and amorphous scheelite phases. Irradiation of the zircon phase at liquid nitrogen temperature induces amorphization at a lower fluence (7.5 × 10¹⁶ ions/m²), as compared to that at room temperature (6 × 10¹⁷ ions/m²). Scheelite type ThGeO₄ irradiated at room temperature undergoes complete amorphization at a lower fluence of 7.5 × 10¹⁶ ions/m² without any volume reduction. The track radii deduced from X-ray diffraction measurements on room temperature irradiated zircon, scheelite and low temperature irradiated zircon phases of ThGeO₄ are, 3.9, 3.5 and 4.5 nm, respectively. X-ray structural investigations on the zircon phase of ThGeO₄ recovered after pressurization to about 3.5 and 9 GPa at ambient temperature show the coexistence of zircon and disordered scheelite phases with a larger fraction of scheelite phase occurring at 9 GPa. On the other hand, the scheelite phase quenched from 9 GPa shows crystalline scheelite phase pattern.     

Variation of optical band gap with radiation dose in PbO-B2O3 glasses

The effect of dose variation of γ-irradiation on optical band gap of PbO-B₂O₃ glasses have been studied in the wavelength region from 200 to 1200 nm. Absorption of glasses in near ultraviolet/visible have been used to calculate the optical mobility gap and width of tail before and after irradiation. The decrease in transmission due to irradiation indicates the formation of colour centers and structural changes in glass matrix. The optical spectrum has been measured before irradiation and in 50 Gy-50 kGy absorbed dose range.     

d-wave superconductive gap and related observables of PuCoGa5

The real-axis formulation of the Eliashberg theory has been applied to PuCoGa₅, assuming d-wave symmetry and phonon-mediated pairing. Here, we present the calculated temperature dependence of the superconductive gap Δ(T) for a freshly prepared sample, and the variation of Δ(T = 2 K) with increasing impurity scattering rate. We also present the calculated energy dependence of the quasiparticle density of state, together with the corresponding normalized tunnelling conductance at T = 4 K. These quantities could be compared with future tunnelling experiments that would also lead to a direct determination of the spectral density function. Finally, we show that the normal phase resistivity can be well reproduced up to room temperature assuming electron-phonon scattering within a two-band model.