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.     

Neutron irradiation studies on low density pan fiber based carbon/carbon composites

Carbon has been extensively used in nuclear reactors and there has been growing interest to develop carbon-based materials for high-temperature nuclear and fusion reactors. Carbon-carbon composite materials as against conventional graphite material are now being looked into as the promising materials for the high temperature reactor due their ability to have high thermal conductivity and high thermal resistance. Research on the development of such materials and their irradiation stability studies are scant. In the present investigations carbon-carbon composite has been developed using polyacrylonitrile (PAN) fiber. Two samples denoted as Sample-1 and Sample-2 have been prepared by impregnation using phenolic resin at pressure of 30 bar for time duration 10 h and 20 h respectively, and they have been irradiated by neutrons. The samples were irradiated in a flux of 10¹² n/cm²/s at temperature of 40 °C. The fluence was 2.52 × 10¹⁶ n/cm². These samples have been characterized by XRD and Raman spectroscopy before and after neutron irradiation. DSC studies have also been carried out to quantify the stored energy release behavior due to irradiation. The XRD analysis of the irradiated and unirradiated samples indicates that the irradiated samples show the tendency to get ordered structure, which was inferred from the Raman spectroscopy. The stored energy with respect to the fluence level was obtained from the DSC. The stored energy from these carbon composites is very less compared to irradiated graphite under ambient conditions.     

Swift heavy ion induced modifications of fullerene C70 thin films

Modifications of the C₇₀ molecule (fullerene) under swift heavy ion irradiation are investigated. C₇₀ thin films were irradiated with 120 MeV Au ions at fluences from 1 × 10¹² to 3 × 10¹³ ions/cm². The energetic ion impacts lead to the destruction of the C₇₀ molecule. To investigate the stability of C₇₀ fullerene, the damage cross-section and radius of the damaged cylindrical zones are evaluated by fitting the evanescence of C₇₀ vibration modes recorded by Raman spectroscopy. Conductivity measurements together with Raman and optical absorption studies revealed that an irradiation fluence of 3 × 10¹³ ions/cm² results in complete amorphization of the carbon structure of the fullerene molecules.     

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.     

Effect of swift heavy ion irradiation on nanocrystalline CaS:Bi phosphors: Structural, optical and luminescence studies

Luminescence studies of CaS:Bi nanocrystalline phosphors synthesized by wet chemical co-precipitation method and irradiated with swift heavy ions (i.e. O⁷⁺-ion with 100 MeV and Ag¹⁵⁺-ion with 200 MeV) have been carried out. The samples have been irradiated at different ion fluences in the range 1 × 10¹²-1 × 10¹³ ions/cm². The average grain size of the samples before irradiation was estimated as 35 nm using line broadening of XRD (X-ray diffraction) peaks and TEM (transmission electron microscope) studies. Our results suggest a good structural stability of CaS:Bi against swift heavy ion irradiation. The blue emission band of CaS:Bi³⁺ nanophosphor at 401 nm is from the transition ³P₁ → ¹S₀ of the Bi³⁺. We have observed a decrease in lattice constant (a) and increase of optical energy band gap after ion irradiation. We presume this change due to grain fragmentation by dense electronic excitation induced by swift heavy ion. We have studied the optical and luminescent behavior of the samples by changing the ion energy and also by changing dopant concentration from 0.01 mol% to 0.10 mol%. It has been examined that ion irradiation enhanced the luminescence of the samples.     

Swift heavy ion induced phase transition in CdTe films deposited by spray pyrolysis in presence of electric field

CdTe polycrystalline thin films possessing hexagonal phase regions are obtained by spray deposition in presence of a high electric field. Thin film samples are irradiated with 100 MeV Ag ions using Pelletron accelerator to study the swift heavy ion induced effects. The ion irradiation results in the transformation of the metastable hexagonal regions in the films to stable cubic phase due to the dense electronic excitations induced by beam irradiation. The phase transformation is seen from the X-ray diffraction patterns. The band gap of the CdTe film changes marginally due to ion irradiation induced phase transformation. The value changes from 1.47 eV for the as deposited sample to 1.44 eV for the sample irradiated at the fluence 1×10¹³ ions/cm². The AFM images show a gradual change in the shape of the particles from rod shape to nearly spherical ones after irradiation.     

Raman investigation of incident N-, Xe-ions induced effects in ZnO thin films

Highly c-axis orientation ZnO thin films with hundreds nanometers in thickness have been deposited on (1 0 0) Si substrate by RF magnetron sputtering. These films are implanted at room temperature by 80 keV N-ions with fluences from 5.0 × 10¹⁴ to 1.0 × 10¹⁷ ions/cm², implanted by 400 keV Xe-ions with 2.0 × 10¹⁴ to 2.0 × 10¹⁶ ions/cm², irradiated by 3.64 MeV Xe-ions with 1.0 × 10¹² to 1.0 × 10¹⁵ ions/cm², or irradiated by 308 MeV Xe-ions with 1.0 × 10¹² to 5.0 × 10¹⁴ ions/cm², respectively. Then the ZnO films are investigated using a Raman spectroscopy. The obtained Raman spectra show that a new Raman peak located at about 578 cm⁻¹ relating to simple defects or disorder phase appears in all ZnO films after ion implantation/irradiation, a new Raman peak at about 275 cm^-1 owing to N-activated zinc-like vibrations is observed in the N-implanted samples. Moreover, a new Raman peak at about 475 cm⁻¹ is only seen in the samples after 400 keV and 3.64 MeV Xe-ions bombardment. The area intensity of these peaks increases with increasing ion fluence. The effects of ion fluence, element chemical activity, atom displacements induced by nuclear collisions as well as energy deposition on the damage process of ZnO films under ion implantation/irradiation are discussed briefly.     

Investigations on the effect of 100 MeV Ni ions irradiated chloride vapour phase epitaxy (Cl-VPE) grown GaN epilayers

Gallium nitride (GaN) epilayers have been grown by chloride vapour phase epitaxy (Cl-VPE) technique and the grown GaN layers were irradiated with 100 MeV Ni ions at the fluences of 5 × 10¹² and 2 × 10¹³ ions/cm². The pristine and 100 MeV Ni ions irradiated GaN samples were characterized using X-ray diffraction (XRD), UV-visible transmittance spectrum, photoluminescence (PL) and atomic force microscopy (AFM) analysis. XRD results indicate the presence of gallium oxide phases after Ni ion irradiation, increase in the FWHM and decrease in the intensity of the GaN (0 0 0 2) peak with increasing ion fluences. The UV-visible transmittance spectrum and PL measurements show decrease in the band gap value after irradiation. AFM images show the nanocluster formation upon irradiation and the roughness value of GaN increases with increasing ion fluences.     

Light ion irradiation-induced phase transformation in the monoclinic polymorph of zirconia

Ion irradiation damage experiments were performed at ∼80 K on polycrystalline samples of monoclinic, slightly sub-stoichiometric zirconia (ZrO_1.98). Following irradiation with 150 keV Ne⁺ ions, the monoclinic phase was gradually replaced by a new phase. Transmission electron microscopy (TEM) observations in cross-sectional geometry and electron microdiffraction (μD) measurements revealed that the irradiated layer in a sample irradiated to a fluence of 5 × 10²⁰ Ne/m² is partially transformed to a higher symmetry phase of high crystallinity. This phase transformation is accompanied by reduction of the initial micron-sized, highly-twinned grain distribution, to a nano-phased grain structure. Grazing incidence X-ray diffraction (GIXRD) measurements revealed that the radiation-induced phase is a tetragonal polymorph of zirconia. This was verified by the existence of strong (1 0 1) diffraction maxima and weak (1 0 2) reflections (body-centered cell). Raman spectroscopy (RS) measurements were also performed in an attempt to corroborate GIXRD results obtained from the irradiated material. RS measurements in the confocal geometry agreed with GIXRD measurements, although RS was not as definitive as GIXRD. In addition to RS showing the existence of a band corresponding to a tetragonal structure at 262 cm⁻¹, a new mystery band appeared at 702 cm⁻¹ that increased in intensity as a function of irradiation fluence.     

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10¹⁵ to 2 × 10¹⁶ ions/cm². The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr-Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10¹⁶ ions/cm². For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10¹⁵ ions/cm². The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10¹⁶ ions/cm². The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.     

Method based on isotope ratio mass spectrometry for evaluation of carbon activation in the reactor graphite

The general idea of this work is to introduce an evaluation method to restore the irradiation parameters of graphite or other carbonaceous materials using experimental and modelling results of ¹³C generation in the irradiated material. The method is based on coupling of stable isotope ratio mass spectrometry and computer modelling of the reactor core to evaluate the realistic characteristics of the reactor core such as the neutron fluence in any position of the reactor graphite stack or other graphite constructions.The generation of carbon isotopes ¹³C and ¹⁴C in the irradiated graphite of the RBMK-1500 reactor has been estimated by modelling of the reactor core with computer codes MCNPX and CINDER90. Good agreement of simulated and measured Δ¹³C/¹²C values in graphite of the central part of the reactor core indicates that the neutron flux (1.40 × 10¹⁴ n/cm² s) is modelled accurately in the graphite sleeve of the fuel channel. The simulated activity of ¹⁴C is compared with the one measured by the β spectrometry technique. Results indicate that production of ¹⁴C from ¹⁴N in the RBMK-1500 reactor is considerable and has to be taken into account in order to make proper evaluation of ¹⁴C activity. Measured ¹⁴C specific activity values correspond to 15 ± 4 ppm impurity of ¹⁴N in graphite samples from the RBMK-1500 reactor core.     

Morphological change in FePt nanogranular thin films induced by swift heavy ion irradiation

We have investigated morphology change of FePt nanogranular films (FePt)₄₇(Al₂O₃)₅₃ under irradiation with 210 MeV Xe ions. Here, electron tomography technique was extensively employed to clarify three-dimensional (3D) structure in irradiated specimens, in addition to conventional transmission electron microscopy (TEM) techniques such as bright-field observation and scanning TEM energy dispersive X-ray spectroscopy (STEM-EDX) analysis. The ion irradiation induces the coarsening of FePt nanoparticles with elongation along the beam direction. Electron tomography 3D reconstructed images clearly demonstrated that when the fluence achieves 5.0 × 10¹⁴ ions/cm², well-coarsened FePt balls have been formed on the irradiated surface, and the particles in the film interior have been deformed into rods along the ion trajectory. The alloy particles become inhomogeneous in composition after prolonged irradiation up to 1.0 × 10¹⁵ Xe ions/cm². The particle center is enriched with Pt, while Fe is slightly redistributed to the periphery.     

Investigations on the structural and optical properties of the swift heavy ion irradiated 6HSiC

Single crystal 6HSiC wafers have been irradiated with 150 MeV Ag¹²⁺ ions with fluences ranging from 1 × 10¹¹ to 1 × 10¹³ ions/cm² at 300 K. The defect accumulation as a function of fluence was studied to determine changes in structural and optical properties. The variation in the fundamental Raman modes of the crystalline 6HSiC due to irradiation has been correlated with the disorder accumulation. The creation of defect states due to irradiation in the bandgap affects the blue-green photoluminescence emission in the irradiated samples. The UV-Visible absorption studies support the existence of defect states in the bandgap which is observed by the shift in the absorption edge towards the lower energy side with increasing fluence. Time Correlated Single Photon Counting photoluminescence decay results suggest that the existing defect states are radiative, exhibiting three lifetimes when irradiated with a fluence 5 × 10¹¹ ions/cm². The total number of lifetime components was reduced for a fluence 1 × 10¹³ ions/cm² as the defect states produced increase the non-radiative defect centres. These results suggest that the accumulation of defects due to irradiation at fluences 5 × 10¹¹ and 1 × 10¹³ ions/cm² are degenerate configurations which exhibit multiple lifetimes in photoluminescence studies. It is inferred that the optically active defect states influence the transition rate of charge carriers in this device material.     

Effect of swift heavy ion irradiation on hydrothermally synthesized hydroxyapatite ceramics

The effect of swift heavy ion irradiation on hydroxyapatite (HAp) ceramic - a bone mineral was investigated. The irradiation experiment was conducted using oxygen ions at energy of 100 MeV with three different fluences of 10¹², 10¹³, 10¹⁴ ions/cm². The irradiated samples were characterized by glancing angle X-ray diffraction (GXRD), atomic force microscopy (AFM), dynamic light scattering (DLS), photoluminescence spectroscopy (PL), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). GXRD confirmed incomplete amorphisation of HAp with increase in fluence. There was considerable reduction in particle size on irradiation leading to nanosized HAp (upto 53 nm). PL studies showed emission in the visible wavelength region. The irradiated samples exhibited better bioactivity than the pristine HAp.     

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.     

Study of 200 MeV Ag ion induced amorphisation in nickel ferrite thin films

Thin films of nickel ferrite of thickness ∼100 and 150 nm were deposited by pulsed laser deposition. The films were irradiated with a 200 MeV Ag¹⁵⁺ beam of three fluences 1 × 10¹², 2 × 10¹² and 4 × 10¹² ions/cm². X-ray diffraction showed a decrease in the intensity of peaks indicating progressive amorphisation with increased irradiation fluence. Fourier transform infra-red and Raman spectra of pristine and irradiated films were also recorded which showed a degradation of the crystallinity of the samples after irradiation. The damage cross section of the infra-red bands was determined. It was found that the two bands at 557 and 614 cm⁻¹ did not show similar behaviour with fluence.     

Effect of swift heavy ion irradiation on structure, optical, and gas sensing properties of SnO2 thin films

Swift heavy ion irradiation has been successfully used to modify the structural, optical, and gas sensing properties of SnO₂ thin films. The SnO₂ thin films prepared by sol-gel process were irradiated with 75 MeV Ni⁺ beam at fluences ranging from 1 × 10¹¹ ion/cm² to 3 × 10¹³ ion/cm². Structural characterization with glancing angle X-ray diffraction shows an enhancement of crystallinity and systematic change of stress in the SnO₂ lattice up to a threshold value of 1 × 10¹³ ions/cm², but decrease in crystallinity at highest fluence of 3 × 10¹³ ions/cm². Microstructure investigation of the irradiated films by transmission electron microscopy supports the XRD observations. Optical properties studied by absorption and PL spectroscopies reveal a red shift of the band gap from 3.75 eV to 3.1 eV, and a broad yellow luminescence, respectively, with increase in ion fluence. Gas response of the irradiated SnO₂ films shows increase of resistance on exposure to ammonia (NH₃), indicating p-type conductivity resulting from ion irradiation.     

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.     

Ion beam modification of strained InGaAs/InP characterized by HRXRD, PL and AFM

Highly tensile strained InGaAs/InP multi quantum wells have been grown by the LP-MOVPE technique. Such samples were subjected to irradiation with 100 MeV Au⁸⁺ ions. These were studied as a function of fluence, then the irradiated samples were annealed by rapid thermal annealing at 700 °C for 60 s in nitrogen atmosphere. We used high resolution X-ray diffraction (HRXRD), photoluminescence (PL) and atomic force microscopy (AFM) characterization techniques to study the interfacial induced changes, band gap modifications and surface morphology. Multi quantum wells were then studied before and after irradiation.     

Evidence of nanostructure formation in Ge oxide by crystallization induced by swift heavy ion irradiation

Ge oxide films were irradiated with 150 MeV Ag ions at fluences varying between 10¹² and 10¹⁴ ions/cm². The irradiation-induced changes were monitored by FT-IR spectroscopy, atomic force microscopy, X-ray diffraction and photoluminescence spectroscopy. The FT-IR spectra indicate stoichiometric changes and an increase in Ge content on irradiation. X-ray diffraction shows a crystallization of the irradiated films and presence of both Ge and GeO₂ phases. The Ge nanocrystal size, as calculated from Scherrer’s formula, was around 30 nm. The morphological changes, observed in atomic force microscopy, also indicate formation of nanostructures upon ion irradiation and a uniform growth is observed for a fluence of 1 × 10¹⁴ ions/cm².