HICS: Highly charged ion collisions with surfaces

The layout of a new instrument designed to study the interaction of highly charged ions with surfaces, which consists of an ion source, a beamline including charge separation and a target chamber, is presented here. By varying the charge state and impact velocity of the projectiles separately, the dissipation of potential and kinetic energy at or below the surface can be studied independently. The target chamber offers the use of tunable metal-insulator-metal devices as detectors for internal electronic excitation, a time-of-flight system to study the impact induced particle emission and the possibility to transfer samples in situ to an ultra high vacuum (UHV) scanning probe microscope. Samples and detectors can be prepared in situ as well. As a first example data on graphene layers on SrTiO₃ which have been irradiated with Xe³⁶⁺ are presented.     

Potential energy threshold of surface erosion on GaN by impact of slow highly charged heavy ions

In the present work the erosion behavior on the surface of GaN epi-layer by the impact of various slow highly charged heavy ions (SHCIs, including Ar^q+, Xe^q+ and Pb^q+, in two incidence geometries) was investigated. Atomic force microscopy reveals a well-defined threshold of potential energy carried by the incident heavy ions accounting for the surface erosion. This threshold also depends on the projected range of the SHCIs, the longer the projected range, the higher the potential energy required for the onset of surface erosion. And the etched depth is close to a linear function of potential energy deposited, increasing with the potential energy increases. Moreover, the etching rate for 60° off normal incidence is by more than a factor of 2 larger than etching rate for normal incidence, and the etch rate by Xe^q+ is larger than by Pb^q+ under the same potential energy and incident direction. And a mechanism is discussed.     

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.     

A deceleration system at the Heidelberg EBIT providing very slow highly charged ions for surface nanostructuring

Recently, it has been demonstrated that each single-impact of a slow (typically 1-2 keV/u) highly charged ion (HCI) creates truly topographic and non-erasable nanostructures on CaF₂ surfaces. To further explore the possibility of nanostructuring various surfaces, using mainly the potential energy stored in such HCIs, projectiles with kinetic energies as low as possible are required. For this purpose a new apparatus, capable of focusing and decelerating an incoming ion beam onto a solid or gaseous target, has been installed at the Heidelberg electron beam ion trap (EBIT). An X-ray detector and a position-sensitive particle detector are utilized to analyze the beam and collision products. First experiments have already succeeded in lowering the kinetic energy of HCIs from 10 keV/q, down to ∼30 eV/q, and in focusing the decelerated beam to spot sizes of less than 1 mm², while maintaining the kinetic energy spread below ∼20 eV/q.     

Untersuchungen von natriumkorrosionseffekten an austenitischen stählen mit dem raster-elektronenmikroskop

Corrosion by sodium in a closed loop with temperature gradients is characterised by solution of components of structural materials in the high-temperature zone, transport of dissolved matter along the thermal gradient and deposition in the low-temperature zone. The scanning electron microscope reveals the morphology of the corrosion effects in the hot zone and of the deposited matter in the cool zone. The use of non-dispersive X-ray analysis in conjunction with the scanning microscope can also give information about shifts in chemical composition in both zones. Even single particles 10^?4 cm in diameter can be qualitatively analysed. The possibilities of corrosion research with the scanning electron microscope are exemplified by investigation of sample which have been corroded in the hot zone, or covered partly or wholly with deposited layers in the cool zone.     

Modification of polymer membranes by ion implantation

This paper presents an experimental study of the transport properties of ion implanted isotactic polypropylene (iPP) polymer membranes. The polymer samples have been irradiated with O⁷⁺, S⁹⁺ and F⁶⁺ ions having the energy 10 keV/q with fluence up to 10¹⁶ ions/cm². In this experiments the incident ions were produced by the ECR ion source. The transport properties (diffusive permeability), changes of the chemical structure and wettability of ion implanted membranes were investigated. The transport properties were studied by measurements of the solute concentration profiles. The changes of the chemical structure were investigated by the differential scanning calorimetry (DSC) thermograms and the atom force microscopy (AFM) images measurements. Finally, the wettability – by the contact-angles methods. The results of the AFM imaging show that ion implantation induce changes in surface topography of polypropylene samples. The degree of crystallinity of iPP foils is decreased by ion implantation. The hydrophilicity of the modified iPP is increased considerably in comparison to the unimplanted foils. Also the hysteresis in the contact-angle gives some indication about the surface roughness or composition. The behavior of the iPP membranes modified by ion implantation shows greater permeability for transported substance.     

PL and XPS study of radiation damage created by various slow highly charged heavy ions on GaN epitaxial layers

Photoluminescence (PL) spectrum, in conjunction with X-ray photoelectron spectroscopy (XPS) is used to evaluate the surface damage of GaN layer by highly-charged Xe^q+ (18 ? q ? 30), Ar^q+ (6 ? q ? 16) and Pb^q+ (q = 25,35) ions. The intensity of PL emission of GaN layer, including near band-edge peak and yellow luminescence, decreases with increasing fluence and charge state of the incident ions. Finally the PL emission is completely quenched after irradiation to high fluences at high charge state. A new peak at 450 nm appeared in PL spectra of the specimens irradiated with Xe¹⁸⁺, Ar⁶⁺ and Ar¹¹⁺, indicating that radioactive recombination within donor-acceptor pairs (DAPs) during irradiation. After irradiation, XPS spectra show N deficient or Ga rich on GaN surface and XPS spectra of Ga3d core levels indicate spectral peak evidently shifts from a Ga-N to Ga-Ga and Ga-O bond. The relative content of Ga-N bond decreases and the content of Ga-Ga bond increases with the increase of ion fluence and ion charge state. The binding energy of Ga3d_5/2 electron corresponding to Ga-Ga bond of the irradiated GaN film is found to be smaller than that of metallic Gallium (Ga⁰), which can be attributed to irradiation damage.     

Electrochemical Properties of Uranyl Ion in Ionic Liquids as Media for Pyrochemical Reprocessing

We have proposed a new reprocessing process by using ionic liquids (ILs) instead of molten salts of alkali chlorides in pyrochemical process. In the proposed process, spent nuclear fuels are dissolved in ILs by using Cl₂ as an oxidant, and UO₂ ²⁺ and PuO₂ ²⁺ ions in ILs are recovered as UO₂ and PuO₂ by electrochemical reduction. In order to examine applicability of ILs as media for reprocessing, we have studied electrochemical behavior of UO₂ ²⁺ in 1-butyl-3-methylimidazolium chloride (BMICl), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄), and 1-butyl-3-methylimidazolium nonafluorobutanesulfonate (BMINfO). Electrochemical properties of uranyl chloride dissolved into ILs were examined by cyclic voltammetry. In BMICl, an almost reversible redox couple was observed, and the formal potential and the diffusion coefficient were evaluated as _0:758V vs. Ag/AgCl and 4:8 × 10^?8 cm²s^?1, respectively. On the other hand, the electrochemical reactions of UO₂ ²⁺ in BMIBF₄ and BMINfO were irreversible. In BMINfO, some reduction peaks and one sharp oxidation peak were observed in the range of ?0:6～–0:2V and around 0.85V vs. Ag/AgCl, respectively. The reduction and oxidation peaks were assigned to multi step reduction of UO₂ ²⁺ to U(IV) via U(V) and/or direct reduction of UO₂ ²⁺ to U(IV), and the oxidative dissolution of the resulting U(IV) compounds, respectively. The electrochemical reduction of UO₂ ²⁺ in BMINfO at ?1:0V vs. Ag/AgCl produced the deposits on a carbon electrode as a cathode. Analyses of the deposits with the scanning electron microscope and the energy dispersive X-ray spectrometer indicated that the deposits are compounds containing uranium, oxygen, and chlorine. As a result, it is expected that the UO₂ ²⁺ in IL can be recovered electrolytically as uranium compounds such as UO₂ and uranium oxychlorides.     

Prompt fission neutron multiplicity and spectrum evaluations for n+238U reaction

The prompt fission neutron multiplicity and spectra for n+²³⁸U reaction are calculated using an improved Los Alamos model which includes the linear relation between the average prompt gamma ray energy and the prompt neutron multiplicity and also the average fission fragment kinetic energy dependence on the incident neutron energy. The coefficients describing the quadratic variation of the fission fragment kinetic energy versus the incident energy are obtained by extrapolation of the data and procedure used for n+²³⁵U reaction. The inverse process compound nucleus cross-section of the fissioning nucleus is calculated using the coupled channel method. In the incident energy range where only the first fission chance is involved the comparison of present spectrum evaluation with spectrum calculation using multi-modal model is made too. The calculated prompt neutron multiplicity and spectra of ²³⁸U neutron induced fission are in good agreement with the experimental data for the entire incident energy range required in evaluations, proving the validity of the used procedure.     

Microstructural evolutions of three-dimensional carbon–carbon composite materials irradiated by carbon ions at elevated temperatures

Carbon–carbon composites are deemed as candidate materials for applications in very high temperature reactors (VHTRs). In a VHTR, carbon–carbon composite materials would experience severe environmental impacts of both a high radiation fluence (about 3 × 10¹⁶/cm² per year) and high temperatures (∼1200 °C), and radiation damage is accordingly expected. In this study, samples prepared from a three-dimensional pitch-based carbon–carbon composite material were irradiated with a 4 MeV C²⁺ ion beam emitted by an accelerator to deliberately induce various levels of radiation damage on the samples at 1000 °C. Transmission electron microscopy analyses were then conducted to examine the evolution of microstructures in the samples, and a scanning electron microscope was used to examine the changes in surface morphology. It was found that an accumulated fluence of 7.0 × 10²¹/m² at 1000 °C resulted in cracks between the matrix and the fiber in the sample. These interfacial cracks were all parallel to the fiber orientation. The fiber/matrix bonding strength might play an important role in the mechanical property of the carbon–carbon composite material. In the meantime, cracks were also found at the fiber side of the sample irradiated under the same condition.     

Single- and multiple-electron loss cross-sections for fast heavy ions colliding with neutrals: Semi-classical calculations

Extensive calculations of single, multiple and total electron-loss cross-sections of fast heavy ions in collisions with neutral atoms are performed in the semi-classical approximation using the DEPOSIT code based on the energy deposition model and statistical distributions for ionization probabilities. The results are presented for Ar¹⁺, Ar²⁺, Kr⁷⁺, Xe³⁺, Xe¹⁸⁺, Pb²⁵⁺ and U^q+ (q = 10, 28, 39, 62) ions colliding with H, N, Ne, Ar, Kr, Xe and U atoms at energies E > 1 MeV/u and compared with available experimental data and the n-particle classical-trajectory Monte Carlo (nCTMC) calculations. The results show that the present semi-classical model can be applied for estimation of multiple and total electron-loss cross-sections within accuracies of a factor of 2.From calculated data for the total electron-loss cross-sections σ_tot, their dependencies on relative velocity v, the first ionization potential I₁ of the projectile and the target atomic number Z_A are found and a semi-empirical formula for σ_tot is suggested. The velocity range, where the semi-classical approximation can be used, is discussed.     

Proton sputtering from polycrystalline Al surface interacting with highly charged ions at grazing incidence angle

Sputtering processes of protons from a polycrystalline Al surface interacting with Ar^q+ (q = 3-14) ions at a grazing incidence angle (∼0.5°) were investigated. The intensity of protons (I_H) detected in coincidence with scattered Ar atoms was measured as a function of q. I_H saturated at q ? 10, although it increased rapidly with q at 3 ? q ? 8. The angular distribution of protons with low kinetic energy (?2 eV) began to deviate from the cosine distribution and assumed a rather flat equidistribution as q increased. To analyze the sputtering processes of protons at the grazing incidence angle, a modified model of the “above-surface potential sputtering model” was proposed by considering image acceleration of projectile ions.     

Argon/Hexamethyldisiloxane Plasma Effects on Poly Propylene Film Surface Properties

In this work a DC plasma reactor was used for deposition of plasma polymerized coating from hexamethyldisiloxane–Ar (35/65%) mixture on polypropylene films. Surface energy parameter have been calculated using Owens–Wendt approaches with the sessile drop method are used to obtain the dispersive γ^D and polar γ^P component of surface free energy. The surface morphology of samples were investigated using scanning electron microscope. Also the chemical properties and wetability of prepared samples were tested using Fourier transform infrared spectroscopy and contact angle measurement, respectively.     

Use of keV-energy ion beams in inducing neutron resonance reactions

It is possible that discrete-resonances in slow-neutron reactions can be induced in inverse-kinematics when an ion-beam of reactant-nuclei of right kinetic energy is made to pass through a column of thermal-neutrons. A scheme for effective cross-section calculation in this type of reactions has been discussed. Effective cross-sections have been calculated for the lowest 17 large s-wave resonances in ²³⁸U+n and ²³⁹U+n capture-reactions, induced by low energy ion-beams of ²³⁸U and ²³⁹U, respectively. Use of ion-beams in this manner will facilitate online-separation of transmuted-isotopes and this may be a useful feature for future nuclear energy technology.     

Isotropic pyrocarbon deposited at 1250 °C by means of thermal gradient chemical vapor deposition

Isotropic pyrocarbon was fabricated at 1250 °C via thermal gradient chemical vapor deposition. The density of the deposit was measured using Archimedes methods and the microstructure was examined by polarized light microscope and scanning electron microscope. The results showed that the density of the deposit was about 1.85 g/cm³ and it was constituted of spherical pyrocarbon grains which was about 0.6-0.8 μm in diameter and compacted densely and there are some microcracks in the deposit.     

14 MeV neutron yields from D-T operation of the MIT Cockcroft-Walton accelerator

As part of our fusion-product diagnostic development program, we have begun a series of experiments with 14 MeV neutrons generated in a Cockcroft-Walton accelerator. Two different detectors have been used to measure the neutron yield: a silicon SBD and a Cu foil. The energy of the emitted neutrons has been determined by using two spectrometers: the SBD and a³He proportional counter. The reaction rate is monitored, with about ±5% accuracy, by detecting the particles from D + T n +. The neutron yields obtained from the Si detector and the Cu activation had associated uncertainties of about ±15% and agreed well with the predicted values from measurements.     

Real-time X-ray studies of the growth of Mo-seeded Si nanodots by low-energy ion bombardment

Silicon surface evolution during room temperature low-energy (300, 500 and 1000 eV) normal incidence Ar⁺ ion bombardment in the presence of Mo seed atoms has been studied with real-time grazing-incidence small-angle X-ray scattering and ex situ atomic force microscopy. When a small amount of Mo atoms was supplied to the Si surface during ion bombardment, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (“dots”) coarsened with time until they reached a constant spatial wavelength. The longer length scale corrugations associated with kinetic roughening, however, continued to grow in amplitude during bombardment. The overall roughness is dominated by different corrugations at different times in the kinetic evolution, showing a complex behavior. The evolution of the kinetic roughening can be described by the Family-Vicsek scaling hypothesis, but measured scaling exponents are not in agreement with those of existing models.     

Removal of radionuclide Sr2+ ions from aqueous solution using synthesized magnetic chitosan beads

Strontium-90 is one of the main fission products, existing in the radioactive wastes produced in nuclear power plant. In this paper, a novel magnetic chitosan beads were synthesized, characterized and applied for removal of Sr²⁺ ions from aqueous solution. The SEM analysis indicated that the magnetic beads were in regular spherical geometry with about 1 mm diameter. The XRD and EDS analysis revealed that the chitosan beads were magnetic and can be used for the magnetic separation. The influencing factors of Sr²⁺ sorption onto magnetic chitosan beads were studied, including contact time, initial pH value, initial Sr²⁺concentration and sorbent dosage. The maximum adsorption capacity (q_m) of Sr²⁺ was calculated to be 11.58 mg/g using the Langmuir isotherm. The kinetic data were analyzed by intra particle diffusion model. The FT-IR study revealed that –NH₂ was mainly involved in Sr²⁺ sorption by magnetic chitosan beads.     

Synthesis of isotopes of fermium with mass numbers 247 and 246

The isotopes Fm²⁴⁷, Fm^247m and Fm²⁴⁶ were synthesized in the extracted beam of the 310-cm heavy-ion cyclotron in the Joint Institute for Nuclear Research by irradiating Pu²³⁹ with C¹² ions. ]he procedure of collecting the recoil atoms by means of gas jets is used, with subsequent recording of their ~- decay with Si(Au) detectors. For the isotope Fm²⁴⁷, -activities are obtained with two different half-lives - one with two obviously complex groups of -particles E=7.87±0.05MeV (70%) and EE=7.93±0.05 MeV (30%), whose half- life is T_1/2=35±4 sec; the other, with E=8.18±0.03 MeV has T_1/2=9.2±2.3 sec. Data are confirmed concerning the properties of the isotope Fm²⁴⁶(E=8.25±0.03MeV and T_1/2=1.6±0.4 sec), synthesized in 1966 at The Joint Institute for Nuclear Research.Translated from Atomnaya Énergiya, Vol. 22, No. 5, pp. 342–346, May, 1967.     

Production of highly-charged recoil-ion beams at very low energy

An apparatus for production of an intense, charge-state separated, recoil-ion beam of very low energy and good collimation has been constructed. In a test experiment, in which 30 MeV C1⁵⁺ projectiles were used to produce recoil ions, the feasibility of the method was demonstrated. The mass to charge ratio of the beam is selected by means of a Wien-filter, and the beam energy can be varied between 2qeV and 1q keV with a constant charge-state resolution. Presently, the angular definition of the beam is 2.5°, but it can be impoved by at least a factor of 2. Very low energy Ar-ion beams, of intensities usable for secondary slow collision experiments, can be created for charge states ranging from one to nine. For example, beams of Ar⁴⁺ and Ar⁶⁺ of intensities 2.5 × 10⁴ and 5 × 10³ s⁻¹, respectively, can be furnished at energies of 10 q eV, while the corresponding numbers for 2 q eV beams ar approximately five times lower.