Mean excitation energies extracted from stopping power measurements of protons in polymers by using the modified Bethe–Bloch formula

Recent stopping power measurements in thin polymeric films have been performed for protons of 0.4–3.5 MeV energies using the indirect transmission technique [H. Ammi, S. Mammeri, M. Chekirine, B. Bouzid, M. Allab, Nucl. Instr. and Meth. B 198 (2002) 5]. Experimental stopping data have been analyzed with the modified Bethe–Bloch formula and the mean excitation energies I have been then extracted from the data. Resulting values for each thin film are 76 ± 1 eV in Mylar, 70.8 ± 1 eV in Makrofol, 82.2 ± 1.2 eV in LR-115 and 55.4 ± 1 eV in Polypropylene. The I-extracted values are compared to those I_B calculated by using the Bragg’s rule.     

Influence of protecting gel film on oxidation of zirconium alloys

Thin oxide layers of Zr1Nb and Zry-4S on tubes, used for fuel cladding in light water reactors, which had been protected by thin gel films, were compared with oxide layers on unprotected specimens of the same kind, grown together in two batches in water at 360 °C for 21 and 42 days, respectively. The analysis of the I–V characteristics at constant temperatures up to 180 °C showed a strong decreasing resistivity with oxidation time of the gel-protected layers of the Zry-4S specimens, with indication of an energy dependent trap distribution, although the gel and the oxide alone showed evidence of single, but different, energy traps. The changes are believed to be due to diffusion of different trap centers in both directions. The Zr1Nb specimens retained their single energy trap behaviour also in the combination with the protecting gel film and resistivity dropping with increasing oxidation time.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Thermal stability of radiation-induced optical centers in non-stoichiometric spinel crystals

The successive anneals of the neutron-irradiated non-stoichiometric spinel crystals MgO · 2.2Al₂O₃ leads to incremental change of optical spectra that demonstrates two main bands whose intensity and spectral position depends on annealing temperature. While temperature increases from 450 to 750 K, one of the bands shifts from photon energy of 4.2–5.1 eV. Another one shifts in the opposite direction from 6.4–5.6 eV. This effect can be attributed to coagulation of radiation induced defects near cation vacancies and change of the energy levels and transitions in F⁺- and F-centers in neutron-irradiated crystals. The final position of these two bands 5.1 and 5.6 eV corresponds to transitions in F⁺- and F-centers in non-stoichiometric spinel, respectively. The investigation of optical centers induced at subsequent UV-illumination of neutron-irradiated crystals annealed to temperature of 750 K, and comparison with as-grown crystals, shows the existence of residual concentration mainly of the antisite defects and partially of the anionic vacancies in the neutron irradiated and annealed crystals. The existence of two temperature stages where optical centers at antisite defects are effectively destroyed may indicate the presence of spatially correlated and isolated antisite defects in irradiated spinel.     

Low energy nitrogen ion doping into GaAs using combined ion-beam and molecular-beam epitaxy method

Low energy nitrogen (N) ions were irradiated during the epitaxial growth of GaAs using combined ion beam and molecular beam epitaxy (CIBMBE) method as a function of N⁺ ion acceleration energy (E_a) and N⁺ ion beam current density (I_N). E_a was varied from 70 to 170 eV I_N from 900 pA/cm² to 75 nA/cm². GaAs growth rate was fixed to 1 μm/h. In 2 K photoluminescence (PL) spectra of the samples with I_N = 3 nA/cm² and E_a = 70–100 eV, two sharp emissions at 1.508 eV (X₁) and 1.495 eV (X₂), which have been attributed to the emissions of excitons bound to isolated N atoms, and another one at 1.443 eV (X₅) were observed. These results show that nitrogen (N) atom in GaAs becomes optically active as an isoelectronic impurity at least in as-grown condition. For N⁺ ion-irradiated samples with rather high I_N, e.g., with I_N = 75 nA/cm² and E_a = 100 eV, a broad emission together with multiple sharp ones were observed after furnace annealing at 750°C which were ascribed to emissions of excitons bound to nitrogen-nitrogen (N---N) pairs.     

Interaction between implanted fluorine atoms and point defects in preamorphized silicon

We have studied, by means of B diffusion analyses, the effect of F on the point defect density in preamorphized Si. Through molecular beam epitaxy (MBE) Si samples containing a special B multi-spike were grown. These samples were amorphized to a depth of 550 nm by implanting Si at liquid nitrogen temperature and then enriched with F at different energies (65–150 keV) and fluences (0.7–5 × 10¹⁴/cm²). After solid phase epitaxy (SPE) of the samples, we induced, by thermal annealing at 850 °C, the emission of Si self-interstitials (Is) from the end-of-range (EOR) defects. We studied the diffusion of the B spikes, demonstrating that F effectively reduces the B diffusion. This reduction is shown to be caused not by a direct B–F chemical interaction, but by a F interaction with point defects. In particular, F is able to reduce the density of Is, which are responsible for the B diffusion. Still, we showed that F does not appreciably influence the Is emission from the EOR defects, but a local interaction occurs between F atoms and Is after the release of these defects from the EOR region. This interaction results in a consistent reduction of B diffusivity in F enriched regions.     

Determination of stoichiometry in silicon carbide materials using elastic backscattering spectrometry

Elastic backscattering spectrometry (EBS) was performed on SiC materials, using ⁴He particles at energies ranging from 2 to 4 MeV, in order to establish the energy values that lead to an accurate measurement of the Si/C ratio. Analysis of the random yield of “bulk” SiC single crystals indicates that energy values of 3.25 and 3.75 MeV are the most suitable for chemical composition determination; backscattering yield of carbon is enhanced compared to the yield measured at 2 MeV, while the excitation of strong resonances above 3.75 MeV are suppressed. Random backscattering yield measurements were then carried out at an energy of 3.25 MeV on unhydrogenated SiC thin films grown on Si(1 0 0), by pulsed laser deposition, at different substrate temperatures. The Si and C atomic concentrations in the films were determined with an uncertainty of 1% and little interference from the underlying substrate. The films were found to be stoichiometric with a Si/C ratio of 1.03 ± 0.05, independent of deposition temperature, which indicates that the films were grown under congruent ablation conditions. The analysis proved to be applicable to both amorphous and crystalline SiC layers, as confirmed by the results obtained for films deposited at 400 and 950 °C, respectively.     

Structural modifications in He-implanted waveguide layers of LiNbO3

He-implanted and heat-treated optical waveguide layers in Y-cut LiNbO₃ wafers were studied by means of high-resolution X-ray diffraction. The depth-resolved profiles of the lattice parameter were derived from the measured diffraction spectra as a function of annealing temperature. Using these data, the spatial distribution of the electron density related to the post-implantation lattice swelling was determined, which shed light on the waveguide formation in damaged layers. Heat treatments at temperatures T > 200°C resulted in the deterioration of waveguide properties due to annealing of the implantation-induced damage. Lattice recovery was found to be a thermally activated process with an activation energy of E_a=0.32 ± 0.03 eV. This is accompanied by defect clustering, giving rise to diffuse scattering components in the diffraction spectra.     

Elemental dissolution study of Pu-bearing borosilicate glasses

Single-pass flow-through tests were conducted to study the effects of self-radiation damage from alpha decay on dissolution kinetics of three radiation-aged Pu-bearing (1 mass% PuO₂) borosilicate glasses over a pH interval of 9–12 at 80–88 °C. The chemical compositions of the glasses were identical except the ²³⁹Pu/²³⁸Pu isotopic ratio, which was varied to yield accumulated doses of 1.3 × 10¹⁶, 2.9 × 10¹⁷ and 2.6 × 10¹⁸ -decays/g at the time of testing. Release of Al, B, Cs, Na, Si and U to solution increased with increasing pH, whereas Ca, Pu and Sr were invariant over the pH interval. Average dissolution rates, based on B release, were identical within experimental uncertainty for all three glass compositions and increased from 0.17 ± 0.07 at pH(23 °C) 9 to 10.6 ± 2.7 (g/(m² d¹)) at pH(23 °C) 12. Release rates of Pu were 10²- to 10⁵-fold slower compared to all other elements and were not affected by isotopic composition, self-radiation damage sustained by the glass, or pH. These data demonstrate that self-radiation damage does not affect glass dissolution rates, despite exposure to internal radiation doses for >20 years.     

Broadening effects in Auger neutralization of 130–430 eV Ar ions at Al surfaces

We present a model for electron emission from Al surfaces by Auger neutralization of 130–430 eV Ar⁺ ions, that includes the singular response of the metal conduction band to the abrupt change of the surface potential caused by electron capture by the incident ion. This effect, previously identified in X-ray studies, produces a broadening that plays a significant role in reproducing the higher energy part of the experimental electron kinetic energy distributions.     

Effects of secondary electrons emitted from surroundings on defect formation in silica glass under γ-ray irradiation

We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in silica glass under γ-ray irradiation. SiO₂ (silica) glass plates and those sandwiched in a pair of various material disks (carbon, stainless steel or lead) were irradiated by γ-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra of the glass plates after the irradiation showed three absorption bands peaked around 2 eV, 4 eV and 5.8 eV being assigned to color centers relate metal impurities (Al and Ge) and oxygen-deficient centers like E′ center, respectively. All three bands were found to grow with γ-ray irradiation dose and saturated at higher doses, and absorbance of the bands at the saturation for the sandwiched glass plates was higher than that for the bare glass plate. Moreover, the saturated absorbance was higher for the glass plate sandwiched with heavier materials. Employing Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process, enhanced energy deposition and numbers of secondary electrons and photons emitted from sandwiching material disks to a silica glass plate were calculated. The higher deposition energy correlates well to the higher saturated absorbance, indicating that the secondary electrons and photons emitted from the disks clearly enhanced the defect formation in the sandwiched silica glass plates. This suggests the existence of the dose effect above a critical does, i.e. the irradiation with higher dose will result in higher saturated absorbance.     

Ion beam mixing of Au marker in Pt and Pt marker in Au by 7 MeV Ag ions

Mixing of a thin Au layer in Pt and in reversed conditions mixing of a thin Pt layer in Au due to bombardment with 7 MeV Ag ions has been measured. The Pt-Au multilayers deposited on a Si substrate were irradiated to doses of 1–6 × 10¹⁵ ions cm⁻² at room temperature. The mixed profiles were measured using a SIMS apparatus with O₂⁺ sputter ions at energy 2.5 keV. The width of the Pt marker increased from 90 to 260 Å with increasing dose. The width of the Au marker increased from 80 to 90 Å, respectively. The corresponding mixing efficiencies are 5 ± 3 (Au marker) and 90 ± 30 Å⁵/eV (Pt marker). The experimental results are compared with simulations based on a model which describes the atomic transport from the initial collisional phase to the late thermalized stage. The calculated values for mixing efficiencies agree reasonably well with experimental values.     

The structural and dynamical properties of Al clusters adsorbed on Ni surface

The impact-induced deposition of Al₁₃ clusters with icosahedral structure on Ni(0 0 1) surface was studied by molecular dynamics (MD) simulation using Finnis–Sinclair potentials. The incident kinetic energy (E_in) ranged from 0.01 to 30 eV per atom. The structural and dynamical properties of Al clusters on Ni surfaces were found to be strongly dependent on the impact energy. At much lower energy, the Al cluster deposited on the surface as a bulk molecule. However, the original icosahedral structure was transformed to the fcc-like one due to the interaction and the structure mismatch between the Al cluster and Ni surface. With increasing the impinging energy, the cluster was deformed severely when it contacted the substrate, and then broken up due to dense collision cascade. The cluster atoms spread on the surface at last. When the impact energy was higher than 11 eV, the defects, such as Al substitutions and Ni ejections, were observed. The simulation indicated that there exists an optimum energy range, which is suitable for Al epitaxial growth in layer by layer. In addition, at higher impinging energy, the atomic exchange between Al and Ni atoms will be favourable to surface alloying.     

Recrystallization of almost fully amorphous zircon under hydrothermal conditions: An infrared spectroscopic study

Hydrothermal experiments were carried out with powder from an almost fully amorphous, natural zircon under various P–T–t conditions mainly in a 0.1 N HCl solution. Powder infrared spectroscopic measurements on the experimental products reveal that first structural changes occurred at a fluid temperature as low as 75 °C. Significant recrystallization started at 200 °C, as indicated by an increase in the absorption intensity of the zircon fundamental IR bands and the formation of sharp OH stretching bands at 3385 and 3420 cm⁻¹. Although the powder has fully reacted at 400 °C, the zircon fundamental absorption bands are not fully recovered, indicating the occurrence of significant amounts of amorphous remnants. The experimental results in neutral to acidic solutions are consistent with the idea that water (H⁺ and possibly H₂O) diffuses into the amorphous network where it ‘catalyses’ solid state recrystallization. During this process, Zr and Si were leached from the amorphous network.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

Photoluminescence of Au formed in 12CaO · 7Al2O3 single crystal by Au-implantation

Au⁺ ion implantation with fluences from 1 × 10¹⁴ to 3 × 10¹⁶ cm⁻² into 12CaO · 7Al₂O₃ (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10¹⁵ cm⁻² exhibited photoluminescence (PL) bands peaking at 3.1 and 2.3 eV at 150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au⁻ ion having the electronic configuration of 6s², judged from their similarities to those reported on Au⁻ ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (2.3 × 10²¹ cm⁻³), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au⁺ implantation if an appropriate fluence is chosen.     

Surface excitation parameter for semiconducting III–V compounds

In the rapid development of mesoscopic science, the study of surface excitations in solids and overlayer systems plays a crucial role. The surface excitation parameter which describes the total probability of surface plasmon excitations by an electron traveling in vacuum before impinging on or after escaping from a semiconducting III–V compound has been calculated for 200–2000 eV electrons crossing the compound surface. These calculations were performed using the dielectric response theory with sum-rule-constrained extended Drude dielectric functions established by the fits of these functions to optical data. Surface excitation parameters calculated for InSb, InAs, GaP, GaSb or GaAs III–V compounds were found to follow to a simple formula, i.e. P_s = aE^−b, where P_s is the surface excitation parameter and E is the electron energy. These surface excitation parameters were then applied to determine the elastic reflection coefficient for electrons elastically backscattered from III–V compounds using the Monte Carlo simulations. Good agreement was found for the electron elastic reflection coefficient between calculated results and experimental data.     

High energy oxygen ion induced modifications in lead based perovskite thin films

The lead based ferroelectric PbZr_0.53Ti_0.47O₃ (PZT), (Pb_0.90La_0.10)TiO₃ (PLT10) and (Pb_0.80La_0.20)TiO₃ (PLT20) thin films, prepared by pulsed laser ablation technique, were studied for their response to the 70 MeV oxygen ion irradiation. The dielectric analysis, capacitance–voltage (C–V) and DC leakage current measurements were performed before and after the irradiation to high-energy oxygen ions. The irradiation produced considerable changes in the dielectric, C–V, leakage characteristics and induced some amount of amorphization. The PZT films showed partial recrystallization after a thermal annealing at 400 °C for 10 min. The phase transition temperature [T_c] of PLT20 increased from 115 °C to 120 °C. The DC conductivity measurements showed a shift in the onset of non-linear conduction region. The current density decreased by two orders of magnitude after irradiation. After annealing the irradiated films at a temperature of 400 °C for 10 min, the films partially regained the dielectric and electrical properties. The results are discussed in terms of the irradiation-induced amorphization, the pinning of the ferroelectric domains by trapped charges and the thermal annealing of the defects generated during the irradiation.     

On the useful range of application of molecular dynamics simulations in the recoil interaction approximation

The validity of the binary collision (BC) approximation and of the so-called recoil interaction approximation (RIA) in ion–solid interactions at low energies is investigated by comparison with molecular dynamics (MD) simulations. The systems studied are channeling through a (1 1 0) oriented layer of Si, implantation into a (1 0 0)-Si target, and reflection from (1 0 0)-Si. It is found that the BC approximation does not introduce significant errors in the case of channeling simulations even at very low energies. Under non-channeling conditions an upper limit to the break-down energy of the BC approximation in Si is given by 30M₁^0.55 eV, taking 5% deviation in the projected range as the criterion.     

Carbon ion-implanted optical waveguides in Nd:YLiF4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF₄ laser crystals fabricated by 6.0 MeV C³⁺ ion implantation at doses of 1 × 10¹⁵ or 2.5 × 10¹⁵ ions/cm², respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.