Clarification of high density electronic excitation effects on the microstructural evolution in UO2

In order to understand the properties of ion tracks and the microstructural evolution under accumulation of ion tracks in UO₂, 100 MeV Zr¹⁰⁺ and 210 MeV Xe¹⁴⁺ ions irradiation examinations have been done at a tandem accelerator facility of JAEA-Tokai, and it has been observed the microstructure by means of a transmission electron microscope (TEM) and a scanning electron microscope (SEM) in CRIEPI.Comparison of the diameter of ion tracks between UO₂ and CeO₂ under irradiation with 100 MeV Zr¹⁰⁺ and 210 MeV Xe¹⁴⁺ ions at room temperature clarify that the sensitivity on high density electronic excitation of UO₂ is much less than that of CeO₂. By the cross-sectional observation of UO₂ under irradiation with 210 MeV Xe¹⁴⁺ ions at 300 °C, elliptical changes of fabricated pores that exist till ∼6 μm depth and the formation of dislocations have been observed in the ion fluence over 5 × 10¹⁴ ions/cm². The drastic changes of surface morphology and inner structure in UO₂ indicate that the overlapping of ion tracks will cause the point defects, enhance the diffusion of point defects and dislocations, and form the sub-grains at relatively low temperature.     

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.     

Studies on Ion Emission from the Plasma Focus Device by Using Ion Collector and Track Detector

Ion beam emission from a neon gas filled plasma focus device has been studied by using ion collector and solid state nuclear track detector. The neon ion beam emission is found to be highly pressure dependant and it is maximum at a pressure of 0.3 Torr. The maximum ion energy at 0.3 Torr is estimated to be 1 MeV. Preliminary results on solid state nuclear track detector indicate the formation of tracks in CR-39 due to exposure of neon ions. The average rim diameter of tracks is measured to be 4.35 μm and the number of track is of the order of 10¹⁰ track/m².     

A track detector technique for determining ion source emission profiles

Solid state nuclear track detectors have been used to directly measure for an ion source the beam emission cross sections along the ion beam trajectory. Results are demonstrated for an ion source of the surface ionization type using solid source materials to obtain ¹⁴⁷Sm or ⁶Li isotopes. Cellulose nitrate LR 115 was used as the detector to register the alpha tracks from the radioactive decay of the implanted ¹⁴⁷Sm and from the ⁶Li (n, α) reaction induced by thermal neutrons in a nuclear reactor.     

Swift heavy ion irradiation induced modification of the microstructure of NiO thin films

NiO nanoparticle films (200 nm thick) grown on Si substrates by pulsed laser deposition method were irradiated by 200 MeV Ag¹⁵⁺ ions. The films were characterized by glancing angle X-ray diffraction, atomic force microscopy and optical absorption spectroscopy. Though electronic energy loss of 200 MeV Ag ions in NiO matrix was higher than the threshold electronic energy loss for creation of columnar defects, films remained crystalline with the initial fcc structure even up to a fluence of 5 × 10¹³ ions cm⁻², where ion tracks are expected to overlap. Irradiation however modified the microstructure of the NiO films considerably. The grain size decreased with increasing ion fluence, which led to reduced surface roughness and increased optical band gap due to quantum confinement. These results correlate well with variation of the power spectral density exponent with ion fluence, which indicate that at high ion fluences, the evolution of surface morphology is governed by surface diffusion.     

Erosion of lithium coatings on TZM molybdenum and graphite during high-flux plasma bombardment

The rate at which Li films will erode under plasma bombardment in the NSTX-U divertor is currently unknown. It is important to characterize this erosion rate so that the coatings can be replenished before they are completely depleted. An empirical formula for the Li erosion rate as a function of deuterium ion flux, incident ion energy, and Li temperature was developed based on existing theoretical and experimental work. These predictions were tested on the Magnum-PSI linear plasma device capable of ion fluxes >10²⁴ m⁻² s⁻¹, ion energies of 20 eV and Li temperatures >800 °C. Li-coated graphite and TZM molybdenum samples were exposed to a series of plasma pulses during which neutral Li radiation was measured with a fast camera. The total Li erosion rate was inferred from measurements of Li-I emission. The measured erosion rates are significantly lower than the predictions of the empirical formula. Strong evidence of fast Li diffusion into graphite substrates was also observed.     

Grafting of PNIPAAm on PVDF submicroscopic tracks induced by the active sites remainders of the etching process

The present work is part of a systematic study that involves different polymeric substrates and monomers with the purpose to induce grafting on etched tracks. The residual active sites produced by heavy ion beams, remaining after the etching process, were used to start the grafting process. In order to produce tracks on foils of poly(vinylidene fluoride) (PVDF) they were irradiated with ²⁰⁸Pb of 25.62 MeV/n or with 115 MeV Cl ions. Then, they were etched and grafted with N-isopropylacrylamide (NIPAAm) monomers. Experimental curves of grafting yield as a function of fluence with the etching time as a parameter were measured. Also, the grafting yield as a function of the grafting and etching time was obtained. The replica method allowed the observation of the shape of the grafted tracks using transmission electron microscopy (TEM). In addition NIPAAm grafted foils were analyzed using Fourier transform infrared spectroscopy (FTIR).     

C and D reemission from mixed W-C-D layers during single-species and simultaneous irradiations of W with C and D

We present new experimental results on the reemission of C and D from tungsten during single-species and simultaneous irradiations with 6 keV C⁺ and 1 keV D⁺ ion beams. The relatively low C fraction in the combined total beam flux (∼4.5% C⁺/[C⁺ + D⁺]) was selected to prevent the formation of a carbon over-layer during C⁺ irradiation. The results show that the temperature dependence of D reemission from a mixed W-C-D surface is similar to that from pure W. In the case of a mixed W-C surface, the reemission of C was much lower than observed for pure carbon. Post-irradiation XPS analysis of the chemical bonding states of a W specimen irradiated at 973 K with 6 keV C⁺ shows that carbon in the mixed W-C surface is primarily in the form of WC.     

Individual Injection, Cooling, and Accumulation of Rare Radioactive Ions

A scheme for accumulating radioactive ions, which is geared toward a quasicontinuous low-intensity flux, is discussed. It is based on individual correction of the trajectory and momentum deflection of each ion in the transport channel and individual ion injection into the accumulator ring. The advantages of this scheme are low accumulator acceptance 5–10 ·mm·mrad, high ion accumulation rate – up to 10³ sec^–1 – with beam intensity after the fragment separator 10³–10⁴ sec^–1, and a 10⁴–10⁵-fold decrease of the pulse intensity of the primary beam on the productive target.     

Simulation of Toroidal Rotation Effect on Heat Flux Transport in the Edge Plasma of Small Size Divertor Tokamak

The effect of toroidal rotation on heat flux transport in the edge plasma of small size divertor was simulated by B2SOLP0.5.2D transport code. The main results of simulation shows that, the following: (1) the radial heat flux is strongly influenced by toroidal rotation. (2) The amplification of conduction part of radial heat flux imposes nonresilient profile of ion temperature, under which the effect of toroidal rotation on ion temperature profile is strong. (3) The ion distribution and its gradients are lower for counter-injection neutral beam than for co-injection neutral beam. (4) Reversal of toroidal rotation during using neutral beam injection result in reverses of radial electric field and E × B drift velocity. (5) The toroidal rotation strong influence on the ion temperature scale length of the ion temperature gradient (ITG). (6) Switch on and off all drifts leads to higher change in the ion density distribution in edge plasma of small size divertor tokamak when the unbalance neutral beam injection are considered (7) the comparison between radial heat flux at different momentum input shows that, the radial ion heat flux with larger ion temperature scale length in the case of co-injection neutral beam is larger than the ion heat flux with smaller ion temperature scale length in the case of counter-injection neutral beam.     

Investigation of initial stage of chemical etching of ion tracks in polycarbonate

Chemical track etching and the growth of nanochannels in ion-irradiated polycarbonate foils were investigated by loss of weight measurements and IR-spectroscopy. The data provided by both methods are in good agreement and allow us to shed light on the early stage of pore formation including times where the breakthrough of the pores has not yet occurred. Clear evidence is shown that the pore growth as a function of etching time depends on the irradiation fluence. For fixed etching parameters, foils containing 7 × 10⁹ tracks/cm² exhibit much smaller pores than samples with 2 × 10⁸ tracks/cm². This effect is independent of the etching temperature and appears for irradiations with Pb ions as well as for Ca-ion tracks sensitized by UV exposure. Model calculations for different etching times and fluences show that the data for low track densities can be fitted quite well by describing the radial etching rate by the track etch rate changing into the bulk etch rate with a Gaussian-shaped transition.     

Kr ion irradiation study of the depleted-uranium alloys

Fuel development for the reduced enrichment research and test reactor (RERTR) program is tasked with the development of new low enrichment uranium nuclear fuels that can be employed to replace existing high enrichment uranium fuels currently used in some research reactors throughout the world. For dispersion type fuels, radiation stability of the fuel-cladding interaction product has a strong impact on fuel performance. Three depleted-uranium alloys are cast for the radiation stability studies of the fuel-cladding interaction product using Kr ion irradiation to investigate radiation damage from fission products. SEM analysis indicates the presence of the phases of interest: U(Al, Si)₃, (U, Mo)(Al, Si)₃, UMo₂Al₂₀, U₆Mo₄Al₄₃ and UAl₄. Irradiations of TEM disc samples were conducted with 500 keV Kr ions at 200 °C to ion doses up to 2.5 × 10¹⁹ ions/m² (∼10 dpa) with an Kr ion flux of 10¹⁶ ions/m²/s (∼4.0 × 10⁻³ dpa/s). Microstructural evolution of the phases relevant to fuel-cladding interaction products was investigated using transmission electron microscopy.     

Tungsten behavior under proton flux and high temperature

An experimental study of the physico-chemical behavior of tungsten under severe conditions is presented. High temperatures (1300 ? T ? 2500 K) generated by concentrated solar energy, high vacuum (∼10⁻⁶ hPa) and proton flux (1 keV, ∼10¹⁷ ions m⁻² s⁻¹) have been applied on polycrystalline W samples to simulate expected and also unexpected high heat loads that can occur on the ITER divertor (nominal and accidental conditions). During experiment, in situ measurements are performed and the material degradation, the mass loss kinetics, the characterization of the different species coming from the materials under coupled proton flux and high temperatures and the optical properties (reflectivity) are followed. Material characterization using SEM and XRD was investigated before and after treatment to understand the observed behavior. Bidirectional reflectivity measurements were carried out on the tested samples to explain the surface modifications, between the reference sample, the heated sample and the heated and ion irradiated one that can act on the thermo-radiative properties of tungsten.     

Versatile plasma ion source with an internal evaporator

A novel construction of an ion source with an evaporator placed inside a plasma chamber is presented. The crucible is heated to high temperatures directly by arc discharge, which makes the ion source suitable for substances with high melting points. The compact ion source enables production of intense ion beams for wide spectrum of solid elements with typical separated beam currents of ∼100-150 μA for Al⁺, Mn⁺, As⁺ (which corresponds to emission current densities of 15-25 mA/cm²) for the extraction voltage of 25 kV. The ion source works for approximately 50-70 h at 100% duty cycle, which enables high ion dose implantation. The typical power consumption of the ion source is 350-400 W. The paper presents detailed experimental data (e.g. dependences of ion currents and anode voltages on discharge and filament currents and magnetic flux densities) for Cr, Fe, Al, As, Mn and In. The discussion is supported by results of Monte Carlo method based numerical simulation of ionisation in the ion source.     

Effect of the laser focus position on characteristics of X-ray and ion emission from gold plasmas generated by a sub-nanosecond laser

X-ray and ion emission from gold plasma produced by a sub-nanosecond Nd:glass laser has been studies as a function of distance of the target from the best focus position. Thermal ion (kinetic energy <19 keV) signals and soft X-ray flux (photon energy >0.7 keV) measurements decrease as the target is moved closer to the best focus position in spite of an increase in laser intensity. We observe simultaneously a strong correlation between the onset of this drop in the flux of soft X-ray and the growth of harder X-ray (photon energy 3–5 keV), alongside a growth in fast ion (energy >67 keV) numbers. This is indicative of the onset of non-linear processes at the higher irradiances (～10¹⁴ W/cm²) associated with the best focus position. Our results show that when using laser plasmas as X-ray or ion sources, X-ray and ion emission in a desired spectral range can be optimized by adjusting the focusing on the target.     

Projected-length distributions of fission-fragment tracks from U and Th thin film sources in muscovite

Thin films of natural uranium and thorium deposited on muscovite were used as sources of neutron-induced fission fragments. Fragment energy loss in thin-source geometry is negligible. In this way, the observed fragment range results from the interaction of the fragment with the detector material. This characteristic enables the investigation of asymmetric fission and etching, through measurements of projected track length distributions in muscovite micas coupled with thin films. The means and standard deviations of the etchable length distributions of the heavy and light fission-fragment tracks were estimated by fitting a theoretical equation to the experimental data.The light fission-fragment accounts for ∼54% and the heavy fission-fragment for ∼46% of the etchable length of a full fission track. This average partition is the same for tracks from thermal-neutron-induced fission of ²³⁵U and fast-neutron-induced fission of ²³²Th. The mean etchable length of uranium fission tracks is ∼2.5% longer than that of thorium fission tracks. This difference is at the resolution limit of these measurements but correlates with the difference in the mean combined initial kinetic energies of the fission fragments. The mean etchable length of uranium fission tracks in muscovite is ∼5% shorter than their calculated latent track length, supporting earlier estimates of a length deficit of this magnitude. The length deficit and the standard deviation of the etchable length distribution of the light fission-fragment tracks are twice the equivalent values for the heavy fission-fragment tracks. This is interpreted in terms of a v_t-profile (track etch rate) that depends on the mass of the track-forming particle.     

Creep behavior of ceramic nuclear fuels under neutron irradiation

A theoretical estimation of the irradiation-induced creep rate of UO₂ resulted in a creep rate range between about 6 × 10⁻⁶/h and 8 × 10⁻⁵/h for a fission rate of 1 × 10¹⁴ f/cm³·s and a stress of 2 kgf/mm². It is essentially due to the “thermal rods” along the fission fragment tracks. Therefore, creep rates should only weakly depend on temperature (below 1000–1200°C) and must be markedly lower for carbide and nitride fuel.     

Mechanism of elongation of gold or silver nanoparticles in silica by irradiation with swift heavy ions

It has been reported that elongated Au nanoparticles oriented parallel to one another can be synthesized in SiO₂ by ion irradiation. Our aim was to elucidate the mechanism of this elongation. We prepared Au and Ag nanoparticles with a diameter of 20 nm in an SiO₂ matrix. It was found that Au nanoparticles showed greater elongated with a higher flux of ion beam and with thicker SiO₂ films. In contrast, Ag nanoparticles split into two or more shorter nanorods aligned end to end in the direction parallel to the ion beam. These experimental results are discussed in the framework of a thermal spike model of Au and Ag nanorods embedded in SiO₂. The lattice temperature exceeds the melting temperatures of SiO₂, Au and Ag for 100 ns after one 110 MeV Br¹⁰⁺ ion has passed through the middle of an Au or Ag nanorod.     

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.     

Ion-induced electrostatic charging of ice

We studied electrostatic charging on amorphous ice films induced by the impact of 100 keV Ar⁺ ions at 45° incidence. We derived the positive surface electrostatic potential from the kinetic energy of sputtered molecular ions. Measurements were performed as a function of film thickness, ion flux and accumulated fluence. The main results are (a) films charge up to a saturation value, following an exponential time dependence. (b) The time constant for charging is approximately proportional to the reciprocal of the ion flux. (c) The maximum surface voltage depends on film thickness and ion flux. (d) Charging does not occur for films thinner than the maximum range of projectile. (e) Dielectric breakdown is observed for surface potentials above ∼100 V. We explain the measurements with a model in which charges can drift into the substrate or be trapped temporarily near the ionization range of the projectiles. A charge can be released from the trap by the electric field produced by a nearby charge injected by subsequent projectiles.