MeV Xe Ions Research Articles

Ion irradiation effects on WNxOy thin films

We have investigated ion irradiation effects on modifications of electronic- and atomic-structures of WNxOy films on C-plane-cut-sapphire (C-Al2O3) substrate. Rutherford backscattering spectroscopy (RBS) of 1.8 MeV He+ ions leads to the composition, x = 1.1 ± 0.1 and y = 0.4 ± 0.2. X-ray diffraction (XRD) with Cu-Kα radiation shows a strong peak at the diffraction angle (2θ) ≈37° and a weak peak at ∼78°. These peaks are assigned as hexagonal-WN and no peaks were observed other than WN and C-Al2O3 (2θ = 41.7°). It is found that the electrical resistivity of unirradiated film (25 × 10−3 Ωcm) is reduced to 3 × 10−4 and 2 × 10−3 Ωcm by 100 MeV Xe ion irradiation at 4 × 1014 cm−2 and 100 keV N at 1016 cm−2, respectively, and that the resistivity of unirradiated film increases with decreasing temperature from RT to 30 K like semiconductor and irradiated film shows very weak temperature dependence. It also appears that the XRD intensity decreases by more than a factor of 10 under these ion irradiation, and lattice expansion of ∼1% at low ion fluence turns into lattice compaction for further ion irradiation. Optical absorption monotonically decreases with increasing the wavelength from 200 to 2500 nm, implying that the bandgap does not exist in the region of 0.5–6 eV and little change in absorption is observed by the ion irradiation.

Modifications of WNxOy films by ion impact

We have investigated ion-impact-induced modifications of electronic- and crystalline-structures of WNxOy polycrystalline films on C-plane-cut-sapphire (C-Al2O3) substrate. Rutherford backscattering spectroscopy (RBS) of 1.8 MeV He+ ions leads to the composition, x = 1.1 ± 0.1 and y = 0.4 ± 0.2, respectively. X-ray diffraction (XRD) with Cu-Kα radiation shows a strong peak at the diffraction angle (2θ) ≈ 37° and a weak peak at ≈78°. These peaks are assigned as WN, referring to JCPDS data. No peaks were observed other than WN and C-Al2O3 (2θ = 41.7°). It appears that the electrical resistivity of unirradiated film (0.025 Ω cm) is reduced to 9 × 10−4 and 3 × 10−4 Ω cm by 100 keV Ne ions at 1.5 × 1017 cm−2 and 100 MeV Xe ions at 4 × 1014 cm−2, respectively, and that the XRD intensity decreases to ~1/240 and 1/170 by these ion impacts at 3 × 1015 cm−2 and 8 × 1012 cm−2, respectively. It also appears that the resistivity of unirradiated film increases with decreasing temperature from RT to 30 K like semiconductor and irradiated film shows very weak temperature dependence. Optical absorption monotonically decreases with increasing the wavelength from 200 to 2500 nm, indicating that the bandgap does not exist in the region of 0.5–6 eV and a small change was observed in the optical absorption by the ion impact.

Nano-crystallization induced by high-energy heavy ion irradiation in UO2

Advanced microstructure investigations of the high-burnup structure (HBS) in UO2 produced by high-dose 84 MeV Xe ion irradiation are reported. Spark plasma sintered micro-grained UO2 was irradiated to 1357 dpa at 350 °C. The characteristic nano-grains and micro-pores of the HBS were formed. The grain size and grain boundary misorientation distributions of the HBS were measured using transmission electron microscopy based orientation imaging microscopy. Grain polygonization due to accumulation of radiation-induced dislocations was found to be the mechanism of nano-crystallization. The morphology of Xe bubbles was quantitatively investigated. This study provides crucial references for advanced fuel performance modeling of high-burnup UO2.

Temperature dependence of swift heavy ion irradiation induced hillocks in TiO2

Irradiation temperature dependant morphology of near surface latent tracks and surface hillocks in TiO2 was investigated. Irradiation was performed on single crystal [110] oriented rutile using 220 MeV Xe ions to a fluence of 5 × 1010 ions cm−2. Irradiations were performed at 80 K, 300 K, 600 K, 800 K and 1000 K. It was found that average hillock height increased with irradiation temperature although large specimen-to-specimen and hillock-to-hillock size differences were also recorded. Similarly the low density conical zone in the near-surface region of the ion track increased in both length and continuity with increasing temperature. At lower temperatures this zone was found to consist of closely spaced low density pockets that become more densely arranged and almost continuous at 1000 K. Evidence supporting the idea that surface hillocks are due to expelled matter from the track core was found in that larger hillocks are subtended by larger conical zones. These experimental observations were discussed in the framework of a simple model based on the inelastic thermal spike (i-TS) model as well as Bernoulli’s principle which could satisfactorily recreate the observed trends in hillock height and further supports the idea of expelled matter forming hillocks. Strain contrast around latent tracks is mostly absent around the conical zones suggesting some relaxation due to matter expulsion. All hillocks were found to be crystalline and epitaxial with the original crystal surface.

Effect of elastic collisions and electronic excitation on lattice structure of NiTi bulk intermetallic compound irradiated with energetic ions

Abstract NiTi bulk intermetallic compound with the B19′ structure was irradiated with 1 MeV He, 5 MeV Al, 16 MeV Au and 200 MeV Xe ions, and the change in lattice structure near the surface by the ion bombardment was investigated by using the grazing incidence X-ray diffraction (GIXD) and the extended X-ray absorption fine structure (EXAFS). The lattice structure transformation by the irradiation strongly depends on ion species and/or energies. For the 1 MeV He irradiation, the lattice structure changed from B19′ to the A2 structure, but did not show an amorphization even after the high fluence irradiation. For the 5 MeV Al irradiation, the samples are partially amorphized. For the 16 MeV Au irradiation, the lattice structure of the NiTi samples changed nearly completely from the B19′ structure to the amorphous state via the A2 structure. The value of dpa (displacement per atom) which is needed for the amorphization is, however, much smaller than the case of the Al ion irradiation. For the 200 MeV Xe ion irradiation, the lattice structure completely changed to the A2 structure even by a small ion fluence. The dependence of the lattice structure transformation on elastic collisions (dpa), the spectrum of the primary knock-on (PKA) atoms and the density of energy deposited through electronic excitation was discussed.

Microstructure investigations of U3Si2 implanted by high-energy Xe ions at 600 °C

The microstructure investigations on a high-energy Xe-implanted U3Si2 pellet were performed. The promising accident tolerant fuel (ATF) candidate, U3Si2, was irradiated by 84 MeV Xe ions at 600 °C at Argonne Tandem Linac Accelerator System (ATLAS). The characterizations of the Xe implanted sample were conducted using advanced transmission electron microscopy (TEM) techniques. An oxidation layer was observed on the sample surface after irradiation under the ∼10-5 Pa vacuum. The study on the oxidation layer not only unveils the readily oxidation behavior of U3Si2 under high-temperature irradiation conditions, but also develops an understanding of its oxidation mechanism. Intragranular Xe bubbles with bimodal size distribution were observed within the Xe deposition region of the sample induced by 84 MeV Xe ion implantation. At the irradiation temperature of 600 °C, the gaseous swelling strain contributed by intragranular bubbles was found to be insignificant, indicating an acceptable fission gas behavior of U3Si2 as a light water reactor (LWR) fuel operating at such a temperature.

Ion-beam formation of light-emitting structures based on silicon nitride layers on silicon

The paper shows the advantage of silicon nitride for use as a light-emitting element in integrated circuits. Alongwith this, the chosen methods for obtaining the studied samples enable us to determine all possible causesresponsible for radiative recombination. The effect of irradiation with Xe+ ions of 200 MeV in the dose range109 – 1014 ions/cm2 on the optical properties and the structure of silicon enriched silicon nitride (SRN) filmsdeposited on the Si substrate by chemical vapor deposition under low pressure was studied. Based on the Ramanscattering (RS) data, it was concluded that the irradiation by 200 MeV Xe ions with a dose of 1×1014 ions/cm2leads to the dissolution of the amorphous Si phase in the nitride matrix. According to the results of transmissionelectron microscopy, preliminary irradiation with swift heavy ions (SHI) enhances the phase separation processin the nitride layer with a 22% excess of Si during the subsequent annealing at 1100°C for 60 minutes. The SHIirradiation, followed by the heat treatment, leads to a further increase in the intensity of the photoluminescence(PL) in the spectral range 600–750 nm compared with that in case of annealed films without preliminaryirradiation. It is known that radiation in this spectral range is due to Si nanocrystals (Si NCs).

Phase transformations induced by heavy ion irradiation in Gd2O3: Comparison between ballistic and electronic excitation regimes

The structural transformations induced in cubic (C-type) Gd2O3 irradiated with low-energy (4 MeV Au, ballistic regime) or high-energy (∼100 MeV Pb or Xe, electronic excitation regime) ions have been studied via the combination of X-ray diffraction (XRD) and Raman spectroscopy. Results show that Gd2O3 undergoes a monoclinic (B-type) phase transformation in both ion irradiation regimes. Nevertheless, significant differences are observed: (i) with low-energy ions the B-type polymorph seems to be produced by direct impact, whereas in electronic excitation regime it is formed via a complex phase transformation buildup; (ii) the B-type structure produced by low-energy ions is different from that induced by swift heavy ions. In the latter case the relative intensities of XRD peaks differ significantly from those found for a B-type reference. This effect is not due to the formation of a preferential crystalline orientation but likely to slight deviations of internal cell parameters of the B-phase formed by high-energy ion irradiations. Thus, electronic excitation produces a “pseudo” B-type polymorph. Finally, it is worth noting that the B-type phase induced by 92 MeV Xe ion irradiation remains stable up to a temperature of 900 °C.

Defect formation in supported graphene irradiated by accelerated xenon ions

Raman spectroscopy and Monte-Carlo simulation studies for supported graphene irradiated by 160 MeV Xe ions are presented. Changes in the density and dominating types of defects with increasing fluence were observed. In order to analyze contribution of defect formation mechanisms, in which the substrate is involved, a comparative study was performed for graphene on SiO2/Si, copper and glass substrates. The major defining mechanisms were found to be atomic recoils and formation of defects induced by hot electrons. For graphene on copper, the impact of substrate recoil atoms was found to be greater comparing to graphene on silicon oxide and glass, where the recoils participated approximately equally. Moreover, a possibility of defect formation in graphene due to hot electrons generated in the substrate near the interface was noted. Finally, a linear dependence of air-induced doping on D and G peak intensity ratio that represents defect density in graphene was found. The study is useful for solving the long-standing controversy on major mechanisms of defect formation in irradiated graphene, as well as for graphene-based nanoelectronic device engineering.

Raman study of In Ga1−N (x = 0.32–0.9) films irradiated with Xe ions at room temperature and 773 K

Abstract Wurtzite InxGa1−xN (x = 0.32, 0.47, 0.7, 0.8, and 0.9) films grown on the GaN epilayers were irradiated with 5 MeV Xe ions to fluences of 3 × 1013 and 6 × 1013 cm−2 at room temperature (RT) and 773 K. Raman spectroscopy was used to study the effects on structure and electron carrier concentration in the irradiated materials. The results show that the irradiation induces lattice relaxation and reduction of the electron carrier concentration in the films, the extent of which increases and decreases, respectively, with increasing In content x in InxGa1−xN. Compared to RT irradiation, significant simultaneous defect recovery was observed during irradiation at 773 K up to a fluence of 3 × 1013 cm−2. Further irradiation to 6 × 1013 cm−2 leads to delamination of the In-rich InxGa1−xN films (x = 0.7, 0.8 and 0.9) from the GaN epilayers.

Behaviors of fine (IG-110) and ultra-fine (HPG-510) grain graphite irradiated by 7 MeV Xe26+ ions

Developing a molten salt reactor needs molten salt–impermeable nuclear graphite. Ultra-fine grain graphite is a good choice as it is better in permeability than fine grain graphite. In this paper, ultra-fine grain graphite (HPG-510) and fine grain graphite (IG-110) samples are irradiated at room temperature by 7 MeV Xe ions to doses of 1 × 1014–5 × 1015 ions/cm2. Scanning electron microscopy, transmission electron microscopy (TEM), Raman spectroscopy and nano-indentation are used to study the radiation-induced changes. After irradiation of different doses, all the HPG-510 samples show less surface fragment than the IG-110 samples. The TEM and Raman spectra, and the hardness and modulus characterized by nano-indentation, also indicate that HPG-510 is more resistant to irradiation.

The role of ion irradiation in activating silent Raman modes via tuning in plasmonic behaviour and surface disorder of Au/ZnO/Pt NFG system

The perceptible progression of Raman modes of zinc oxide (ZnO) is studied in nanostructures film gap (Au (10 nm)/ZnO (70 nm)/Pt (50 nm)) system with 1.2 MeV Xe ion irradiation. Unattainable silent Raman modes of ZnO turn out to be strongly visible after ion irradiation. The creation of ion-beam–induced lattice disorder, defects, and impurities in a ZnO layer leads to breakdown the translational crystal symmetry that results in the origin of silent modes. The formation of hot-spots in the ZnO layer of the NFG system also supports the enhancement of the intensity of Raman modes. Overall results are attributed to combined effects of lattice disorder, defects, and impurities along with plasmonic effect and explained in the framework of elastic-thermal-spike formation.

Magnetic properties of a stainless steel irradiated with 6 MeV Xe ions

Specimens of austenitic stainless steel were irradiated with 6 MeV Xe ions at room temperature to 2, 7, 15 and 25 dpa. The vibrating sample magnetometer (VSM), grazing incidence X-ray diffraction (GIXRD) and positron annihilation lifetime spectroscopy (PLS) were carried out to analysis the magnetic properties and microstructural variations. The magnetic hysteresis loops indicated that higher irradiation damage causes more significant magnetization phenomenon. The equivalent saturated magnetization Mes and coercive force Hc were obtained from magnetic hysteresis loops. It is indicated that the Mes increases with irradiation damage. While Hc increases first to 2 dpa and then decreases continuously with irradiation damage. The different contributions of irradiation defects and ferrite precipitates on Mes and Hc can explain these phenomena.

Bubble morphology in U3Si2 implanted by high-energy Xe ions at 300 °C

The microstructure modifications of a high-energy Xe implanted U3Si2, a promising accident tolerant fuel candidate, were characterized and are reported upon. The U3Si2 pellet was irradiated at Argonne Tandem Linac Accelerator System (ATLAS) by an 84 MeV Xe ion beam at 300 °C. The irradiated specimen was then investigated using a series of transmission electron microscopy (TEM) techniques. A dense distribution of bubbles were observed near the range of the 84 MeV Xe ions. Xe gas was also found to accumulate at multiple types of sinks, such as dislocations and grain boundaries. Bubbles aggregated at those sinks are slightly larger than intragranular bubbles in lattice. At 300 °C, the gaseous swelling strain is limited as all the bubbles are below 10 nm, implying the promising fission gas behavior of U3Si2 under normal operating conditions in light water reactors (LWRs).

Light-emitting Si nanostructures formed by swift heavy ions in a-Si:H/SiO2 multilayer heterostructures

Light-emitting nanoclusters were formed in Si/SiO2 multilayer structures irradiated with 167 MeV Xe ions to the doses of 1011–3 × 1014 cm−2 and annealed in the forming-gas at 500 °C and in nitrogen at 800–1100 °C, 30 min. The thicknesses were ~4 nm or ~7–8 for the Si, and ~10 nm for the SiO2 layers. The structures were studied using photoluminescence (PL), Raman spectroscopy, and the cross-sectional high resolution transmission electron microscopy (HRTEM). As-irradiated samples showed the PL, correlating with the growth of the ion doses. HRTEM found the layers to be partly disintegrated. The thickness of the amorphous Si layer was crucial. For 4 nm thick Si layers the PL was peaking at ~490 nm, and quenched by the annealing. It was ascribed to the structural imperfections. For the thicker Si layers the PL was peaking at ~600 nm and was attributed to the Si-rich nanoclusters in silicon oxide. The annealing increases the PL intensity and shifts the band to ~790 nm, typical of Si nanocrystals. Its intensity was proportional to the dose. Raman spectra confirmed the nanocrystals formation. All the results obtained evidence the material melting in the tracks for 10−11–10−10 s providing thereby fast diffusivities of the atoms. The thicker Si layers provide more excess Si to create the nanoclusters via a molten state diffusion.

Electron and electron-ion sequential irradiation of borosilicate glasses: Impact of the pre-existing defects

A three-oxide sodium borosilicate glass was irradiated with 2.3 MeV electrons up to 0.15 GGy and 4.6 GGy, and subsequently with 96 MeV Xe ions. The irradiated samples were characterised using Raman spectroscopy, ToF-SIMS, microhardness and surface profilometry. Electron irradiation of the pristine glasses resulted in different structural modifications at the sample surface and in the bulk of the glass, whereas, ion irradiation of either the pristine or bulk of the electron pre-irradiated glasses induced same structural, physical and mechanical property changes. Furthermore, sample surfaces showed a different behaviour than that of the bulk upon subsequent ion irradiation. These results show that the radiation sensitivity of surfaces can significantly vary depending on the type of the irradiation. Therefore, detailed studies aimed at understanding the response of the surfaces to mono and electron-ion double-beam irradiations should be undertaken to address the long-term evolution of the nuclear waste glass matrix surfaces.

Ion irradiation-induced swelling and hardening effect of Hastelloy N alloy

The volumetric swelling and hardening effect of irradiated Hastelloy N alloy were investigated in this paper. 7 MeV and 1 MeV Xe ions irradiations were performed at room temperature (RT) with irradiation dose ranging from 0.5 to 27 dpa. The volumetric swelling increases with increasing irradiation dose, and reaches up to 3.2% at 27 dpa. And the irradiation induced lattice expansion is also observed. The irradiation induced hardening initiates at low ion dose (≤1dpa) then saturates with higher ion dose. The irradiation induced volumetric swelling may be ascribed to excess atomic volume of defects. The irradiation induced hardening may be explained by the pinning effect where the defects can act as obstacles for the free movement of dislocation lines. And the evolution of the defects' size and number density could be responsible for the saturation of hardness.

Cross section TEM characterization of high-energy-Xe-irradiated U-Mo

U-Mo alloys irradiated with 84 MeV Xe ions to various doses were characterized with transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques. The TEM thin foils were prepared perpendicular to the irradiated surface to allow a direct observation of the entire region modified by ions. Therefore, depth-selective microstructural information was revealed. Varied irradiation-induced phenomena such as gas bubble formation, phase reversal, and recrystallization were observed at different ion penetration depths in U-Mo.

X-ray diffraction study of the Y2Ti2O7 pyrochlore disordering sequence under irradiation

The disordering sequence of Y2Ti2O7 pyrochlore, a nano-oxide phase that strengthens ODS steels under irradiation is studied in the experimental and modeling framework. XRD analysis has been performed considering both swift heavy ion and low energy/low mass ion irradiations. The simulation within molecular dynamics of Frenkel pair accumulation proves able to reproduce the variation of the amorphization fluence with temperature. XRD patterns calculated from the simulations reproduce well the patterns observed experimentally in the literature. Both experiments and calculations point to a first transition from pyrochlore to fluorite before an eventual amorphization. For swift heavy ion irradiations with 93 MeV Xe ions, tracks of direct impact amorphization are visible by HRTEM. Advanced refinement shows that one third of the pyrochlore impacted by an ion transforms into fluorite, while two third are directly amorphized.

Structure of ion tracks in ceria irradiated with high energy xenon ions

Structure and accumulation behavior of ion tracks in CeO2 irradiated with 200 MeV Xe ions were examined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to obtain fundamental knowledge on the microstructure evolution induced by fission fragments in nuclear fuels and transmutation targets, which is of importance for the development of advanced fuel/target materials at high burn-up conditions. Bright-field (BF) TEM images of ion tracks from an inclined direction showed Fresnel contrast along penetrating path of incident ions. The signal intensity of high-angle annular dark-field (HAADF) STEM images was decreased at the core damage region of ion tracks along the path of ions, revealing the reduction of atomic density inside the ion track. Preferential formation of smaller and larger ion tracks was observed at a high ion fluence of 1 × 1014 cm−2 compared to a low ion fluence of 1 × 1011 cm−2. Results were discussed due to the coalescences and incomplete recovery of the core damage regions during the overlap of high density electronic excitation damage, which is induced during the repetition of the formation and recovery of ion tracks within an influence region.