Detection of beryllium treatment of natural sapphires by NRA

Since the 1990’s, artificial treatment of natural sapphires (Al₂O₃ crystals coloured by impurities) by diffusion of beryllium at high temperature has become a growing practice. This process permits to enhance the colour of these gemstones, and thus to increase their value. Detection of such a treatment – diffusion of tens of μg/g of beryllium in Al₂O₃ crystals – is usually achieved using high sensitivity techniques like laser-ablation inductively coupled plasma mass spectrometry (LA-ICP/MS) or laser-induced breakdown spectrometry (LIBS) which are unfortunately micro-destructive (leaving 50–100-μm diameter craters on the gems). The simple and non-destructive alternative method proposed in this work is based on the nuclear reaction ⁹Be(α, nγ)¹²C with an external helium ion beam impinging on the gem directly placed in air. The 4439 keV prompt γ-ray tagging Be atoms are detected with a high efficiency bismuth germanate scintillator. Beam dose is monitored using the 2235 keV prompt γ-ray produced during irradiation by the aluminium of the sapphire matrix through the ²⁷Al(α, pγ)³⁰Si nuclear reaction. The method is tested on a series of Be-treated sapphires previously analyzed by LA-ICP/MS to determine the optimal conditions to obtain a peak to background appropriate to reach the required μg/g sensitivity. Using a 2.8-MeV external He beam and a beam dose of 200 μC, beryllium concentrations from 5 to 16 μg/g have been measured in the samples, with a detection limit of 1 μg/g.     

Determination of differential cross-sections for the natK(p, p0) and 39K(p, α0) reactions in the backscattering geometry

In the present work, new, differential cross-section values are presented for the ^natK(p, p₀) reaction in the energy range E_lab = 3000–5000 keV (with an energy step of 25 keV) and for detector angles between 140° and 170° (with an angular step of 10°). A qualitative discussion of the observed cross-section variations through the influence of strong, closely spaced resonances in the p + ³⁹K system is also presented. Information has also been extracted concerning the ³⁹K(p,α₀) reaction for E_lab = 4000–5000 keV in the same angular range. As a result, more than ～500 data points will soon be available to the scientific community through IBANDL (Ion Beam Analysis Nuclear Data Library – http://www-nds.iaea.org/ibandl/) and could thus be incorporated in widely used IBA algorithms (e.g. SIMNRA, WINDF, etc.) for potassium depth profiling at relatively high proton beam energies.     

Combination of resonance integral and Maxwellian 30 keV data – A sensitive test of the resonance region

This paper presents the approach of a combined use of resonance integrals and average Maxwellian cross sections (MACS) at kT = 30 keV to test and validate the resolved resonance range or its reconstructed cross section curve. Based on these two integral measurements a sensitive and energy dependent test can be provided. These two integral quantities cover with their neutron spectra the energy region between E_n = 0.5 eV up to several hundred keV, respectively, with different weighting. Our principal motivation is to produce a validation tool, sensitive to the lower and upper parts of the resonance region through the difference in the applied 1/E and kT = 30 keV Maxwell–Boltzmann spectra of the resonance integral and MACS data.     

CdTe detector use for PIXE characterization of TbCoFe thin films

Peltier cooled CdTe detectors have good efficiency beyond the range of energies normally covered by Si(Li) detectors, the most common detectors in PIXE applications. An important advantage of CdTe detectors is the possibility of studying K X-rays lines instead the L X-rays lines in various cases since CdTe detectors present an energy efficiency plateau reaching 70 keV or more. The ITN CdTe useful energy range starts at K-K_α (3.312 keV) and goes up to 120 keV, just above the energy of the lowest γ-ray of the ¹⁹F(p, p’γ)¹⁹F reaction. In the new ITN HRHE-PIXE line, a CdTe detector is associated to a POLARIS microcalorimeter X-ray detector built by Vericold Technologies GmbH (an Oxford Instruments Group Company). The ITN POLARIS has a resolution of 15 eV at 1.486 keV (Al-K_α) and 24 eV at 10.550 keV (Pb-L_α1). In the present work, a TbCoFe thin film deposited on a Si substrate was analysed at the HRHE-PIXE system. The good efficiency of the CdTe detector at 45 keV (Tb-K_α), and the excellent resolution of POLARIS microcalorimeter at 6.403 keV (Fe-K_α), are presented and the new possibilities open to the IBA analysis of systems with traditionally overlapping X-rays and near mass elements are discussed.     

Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO₂ layers followed by thermal annealing post-treatment.Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on ²⁸Si. Indeed, for high incident energy, nuclear levels of ³²S and ³¹P can be reached, and cross-sections for (α,α) and (α,p₀) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO₂, even for low fluences.In this work, (α,α) and (α,p₀) reactions are applied to study depth distributions of 70 keV ²⁸Si⁺ ions implanted in 200 nm SiO₂ layers with fluences of 1 × 10¹⁷ and 2 × 10¹⁷ cm^?2. Analysis is performed above E_R = 3864 keV to take advantage of resonances in both (α,α) and (α,p₀) cross-sections. We show how (α,p₀) reactions can complement results provided by resonant backscattering measurements in this complex case.     

Dose-dependent bonding environment of oxygen implanted in GaN

The bonding environment of oxygen implanted in GaN is studied using Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The implantation of 70 keV O ions in GaN results in the formation of a 200 nm – thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1 × 10¹⁵ to 1 × 10¹⁷ cm^?2. The NEXAFS spectra are simulated using the FEFF8 code assuming models that account for the formation of point defects (various configurations of O interstitial and O substitutional in N and Ga sites) as well as chemical effects such as the formation of various polymorphs of Ga oxides and oxynitrides. The implantation-induced lattice disorder is modeled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulations reveal that when the fluence is 1 × 10¹⁵ cm^?2, the O implants occupy interstitial sites preferentially in the empty channels aligned parallel to the c-axis in the plane that contains the Ga atoms and/or in the columns that consist of Ga and N atoms along the c-axis. When the fluence is equal to 1 × 10¹⁶ cm^?2 the O ions substitute for N while at 1 × 10¹⁷ cm^?2 they participate in the formation of mixed GaO_xN_y phases.     

Theoretical critical angels of ion channeling in carbon nanotubes

We have developed a mass- and charge-dependent equation to predict theoretical critical angles for ion channeling in carbon nanotubes. We focus M (ion mass) effects how to reduce Ze (ion nucleus charge) effects on Ψ_C (critical angles). As an instance, we give theoretical critical angels of He, Ne, Ar, Kr, Xe and Rn ion channeling in carbon nanotubes. We find that for (10, 10) single-wall carbon nanotubes, Ψ_C(He) ≈ Ψ_C(Ne) ≈ Ψ_C(Ar) ≈ Ψ_C(Kr) ≈ Ψ_C(Xe) ≈ Ψ_C(Rn) ≈ 23.3 (keV/E)^1/2 deg. This is because (Z/M)^1/2 ≈ 0.66 [amu]^?1/2.     

The lower hybrid current drive system for steady-state operation of the Vulcan tokamak conceptual design

The steady-state current drive system for the Vulcan tokamak concept has been designed, taking into account requirements of high field, small size, and high operational wall temperature (B₀ = 7 T, R₀ = 1.2 m, T_wall > 800 K). This lower hybrid current drive system allows steady-state operation by utilizing high field side launch, high RF source frequency (8 GHz), and dedicated current drive ports. An iterative MHD and current drive solver is used to determine the ideal launching spectra and location to assure strong single pass absorption. It is found that with nominal Vulcan operational parameters (n_e ≈ 4 × 10²⁰ m^?3, T_e ≈ 2.8 keV, I_p = 1.7 MA, P_LHCD = 19.8 MW) bootstrap currents of ～70% and lower hybrid current drive efficiencies of 1.16 × 10¹⁹ A W m^?2 could be achieved. The optimized solution yielded advanced tokamak profiles with q values on-axis above 2. A conceptual design of the system is presented, which takes into account space, power, cooling, and launched spectrum requirements. The system is found to be compatible with the vacuum vessel design and requires cooling power of <1 MW per waveguide bundle.     

Combined Rietveld refinement of Zn2SiO4:Mn2+ using X-ray and neutron powder diffraction data

The behavior of Mn²⁺ ions doped into the crystal lattice of Zn₂SiO₄ is closely related to the luminescent properties of Zn₂SiO₄:Mn²⁺ as a color-emitting phosphor. The combined Rietveld refinement using X-ray and neutron powder diffraction was used to determine the site preference and the amount of Mn²⁺ ions in Zn₂SiO₄:Mn²⁺. Of possible cation-disorder models, the best Rietveld refinement was obtained from the model that Mn²⁺ ions partially substituted for Zn²⁺ ions in two crystallographically non-equivalent Zn sites. The final converged weighted R-factor, R_wp, and the goodness-of-fit indicator, S (=R_wp/R_e) were 8.12% and 2.28, respectively. The occupancy of Mn²⁺ ions for the two non-equivalent Zn sites was 0.034(4) and 0.003(2), respectively. The refined model described the crystal structure in space group R?3 (No. 148) with Z = 18, a (=b) = 13.9612(1) Å, c = 9.3294(1) Å and γ = 120°.     

Effect of keV ion irradiation on mechanical properties of hydrogenated amorphous silicon

The susceptibility of mechanical properties of hydrogenated amorphous silicon (a-Si:H) to the implantation-enhanced disorder has been studied with the aim to extend the application field of this material in the technology of micro-electromechanical systems. Effect of keV ion irradiation on the elastic modulus, E, of hardness, H, and of root-mean-squared roughness to silicon ion implantation has been determined. The mechanical properties were evaluated by nanoindentation testing. E of 119 GPa and H of 12.3 GPa were determined for the as-prepared a-Si:H film. The implantation of silicon ions leads to a decrease in E and H, evaluated for a series of the implantation fluences in the range of 1.0 × 10¹³–5.0 × 10¹⁶ cm^?2. Surface smoothing has been observed at high fluences and low ion energy of 18 keV, suggesting that ion beam may be used as a tool to reduce the roughness of the a-Si:H surface, while keeping intact the mechanical properties inside the film. The conducted experiments show that it is possible to prepare a-Si:H films with hardness and smoothness comparable to crystalline silicon.     

Measurement of thermal neutron capture cross section and resonance integral of the 138Ba(n, γ)139Ba reaction using 55Mn(n, γ)56Mn as a monitor

The thermal neutron capture cross section (σ_o) and the resonance integral cross section (I_o) of the ¹³⁸Ba(n, γ)¹³⁹Ba reaction have been measured by the activation method using the Ghana Research Reactor-1 (GHARR-1). The barium and manganese targets were irradiated within and without a cadmium capsule. The result of the thermal neutron capture cross section for the ¹³⁸Ba(n, γ)¹³⁹Ba reaction is 0.53 ± 0.01barns. The result was obtained relative to the reference value 13.2 barns of the ⁵⁵Mn(n, γ)⁵⁶Mn reaction. The resonance integral cross section for the ¹³⁸Ba(n, γ)¹³⁹Ba reaction was also measured relative to the reference value of 13.9 barns for the ⁵⁵Mn(n, γ)⁵⁶Mn reaction. The present resonance integral cross section for the ¹³⁸Ba(n, γ)¹³⁹Ba reaction is 0.380 ± 0.005 barns. The previous measurements of the σ_o and I_o of the reaction ¹³⁸Ba(n, γ)¹³⁹Ba were reviewed and the difference between the present values and the previous results were discussed. The present work was undertaken with the aim to contribute to the experimental basis of σ_o and I_o evaluations.     

Microstructural modifications induced by swift ions in the NiTi intermetallic compound

The conditions for track formation by irradiation with swift ions, in the shape memory NiTi metallic compound are studied. We observe that tracks are only induced in the monoclinic structure but not in the cubic one and only when the linear rate of energy deposition by electronic excitation ([dE/dx]_e) exceeds 46 keV/nm. We show that the tracks are amorphous in their centre and that a decrease of the monoclinic towards cubic transformation temperature is observed at the periphery of the track region. For [dE/dx]_e=32 keV/nm, no individual tracks can be observed at low fluences, only a monoclinic → cubic structure transformation is observed at high fluences. If [dE/dx] ⩽ 17 keV/nm, swift ions are unable to induce any visible structural modification by electronic excitations. The track formation in this alloy is discussed within the framework of the classical Coulomb explosion and thermal spike models.     

Effects of hydrogen implantation into GaN

Proton implantation in GaN is found to reduce the free carrier density through two mechanisms – first, by creating electron and hole traps at around E_C − 0.8 eV and E_V + 0.9 eV that lead to compensation in both n- and p-type material, and second, by leading to formation of (AH)° complexes, where A is any acceptor (Mg, Ca, Zn, Be, Cd). The former mechanism is useful in creating high resistivity regions for device isolation, whereas the latter produces unintentional acceptor passivation that is detrimental to device performance. The strong affinity of hydrogen for acceptors leads to markedly different redistribution behavior for implanted H⁺ in n- and p-GaN due to the chemical reaction to form neutral complexes in the latter. The acceptors may be reactivated by simple annealing at ⩾600°C, or by electron injection at 25–150°C that produces debonding of the (AH)° centers. Implanted hydrogen is also strongly attracted to regions of strain in heterostructure samples during annealing, leading to pile-up at epi–epi and epi–substrate interfaces. IR spectroscopy shows that implanted hydrogen also decorates V_Ga defects in undoped and n-GaN.     

A simple way to analyze infrared reflectivity spectra of p-type Hg0.78Cd0.22Te implanted in various conditions

Different ion-implanted p-type Hg_0.78Cd_0.22Te samples were analyzed by infrared reflectivity in the 2–20 μm wavelength range. We show how to derive some characteristic values of the free carriers induced by ion implantation from simple models of the implanted samples. For low energy implantations (Al (320 keV)) an excess of electrons with concentration n⁺ ≈ 5 × 10¹⁷ cm⁻³ for doses 10¹² and 10¹⁴ ions cm⁻² is observed between the surface and the projected range R_p of the ions, in agreement with the well-known change of type of the free carriers induced by the ion implantation in this kind of samples. High energy α particle (0.8 and 2 MeV, 10¹⁴ ions cm⁻²) implantations lead to a pronounced inhomogeneous concentration of free electrons with n⁺ ≈ 9.2 × 10¹⁶ cm⁻³ between the surface and R_p where a negligible amount of defects due to the nuclear energy loss is formed, and n⁺ ≈ 1.6 × 10¹⁷ cm⁻³ between R_p and R_p + ΔR_p, ΔR_p being the longitudinal straggling, where the defect production rate through the nuclear energy loss mechanism is maximum.     

Measurement of thermal neutron cross section and resonance integral for 165Ho(n, γ)166gHo reaction by the activation method

The thermal neutron cross section and the resonance integral of the reaction ¹⁶⁵Ho(n, γ)^166gHo were measured by the activation method using ⁵⁵Mn(n,γ)⁵⁶Mn monitor reaction. The sufficiently diluted MnO₂ and Ho₂O₃ samples with and without a cylindrical Cd case were irradiated in an isotropic neutron field of the ²⁴¹Am–Be neutron sources. The γ-ray spectra from the irradiated samples were measured with a calibrated n-type high purity Ge detector. Thus, the thermal neutron cross section for ¹⁶⁵Ho(n,γ)^166gHo reaction has been determined to be 59.2 ± 2.5 b relative to the reference thermal neutron cross section value of 13.3 ± 0.1 b for the ⁵⁵Mn(n,γ)⁵⁶Mn reaction, and it generally agrees with the recent measurements within about 1 to 12%. The resonance integral has also been measured relative to the reference value of 14.0 ± 0.3 b for the ⁵⁵Mn(n,γ)⁵⁶Mn reaction using an epithermal neutron spectrum of the ²⁴¹Am–Be neutron source. The resonance integral for ¹⁶⁵Ho(n, γ)^166gHo reaction obtained was 667 ± 46 b at a cut-off energy of 0.55 eV for 1 mm Cd thickness. The existing experimental and evaluated data for the resonance integral are distributed from 618 to 752 b. The present resonance integral value agrees with most of the previously reported values obtained by ¹⁹⁷Au standard monitor within the limits of error.     

Photoluminescence induced by Si implantation into Si3N4 matrix

Up to the present, photoluminescence (PL) was obtained from near stoichiometric or amorphous Si nitride films (SiN_x) after annealing at high temperatures. As a consequence, the existence of PL bands has been reported in the 400–900 nm range. In the present contribution, we report the first PL results obtained by Si implantation into a stoichiometric 380 nm Si₃N₄ film. The Si excess is obtained by a 170 keV Si implantation at different temperatures with a fluence of Φ = 10¹⁷ Si/cm². Further, we have annealed the samples in a temperature range between 350 and 900 °C in order to form the Si precipitates. PL measurements were done using an Ar laser as an excitation source, and a broad PL band basically centered at 910 nm was obtained. We show that the best annealing condition is obtained at T_a = 475 °C for the samples implanted at 200 °C, with a PL yield 20% higher than the obtained at room temperature implantation. Finally, we have varied the implantation fluence and, consequently, the Si nanocrystals size. However, no variation was observed nor in the position neither in the intensity of the PL band. We concluded that the PL emission is due to radiative states at the matrix and the Si nanocrystals interface, as previously suggested in the literature.     

Elastic scattering of 7Li + 27Al at several angles in the 7–11 MeV energy range

Elastic cross sections for the ⁷Li + ²⁷Al system were measured at laboratory energies between 7 and 11 MeV in steps of 0.25 MeV, and angles between 135° and 170° in steps of 5°. Excitation functions for the elastic scattering were measured using an array of eight Si surface-barrier detectors whereas a solid-state telescope was used to estimate and subtract background from other reactions. Contamination from α particles arising from the ⁷Li breakup process at E_lab ? 10 MeV makes the use of these energies inadvisable for RBS applications. The present results are compared with previous data obtained at 165° (E_lab ? 6 MeV), 140° and 170° (E_lab ? 8 MeV). The experimental data were analyzed in terms of the Optical Model. Two different energy-independent potentials were found. These optical potentials allow an interpolation with physical meaning to other energies and scattering angles. The experimental cross sections will be uploaded to the IBANDL database.     

A calorimetric study of high temperature phase stability in Fe–U alloys

A differential scanning calorimetry study of high temperature phase equilibria and phase transformation kinetics in Fe_100?xU_x binary alloys, with x varying from 0 to 95 mass% U is undertaken. Accurate measurement of transformation temperatures pertaining to: (i) α-Fe → γ-Fe → δ-Fe polymorphic phase change, (ii) UFe₂ + γ-Fe → L and U₆Fe + UFe₂ → L transformations and (iii) melting has been made as a function of uranium concentration. The measured transformation temperatures are used to construct the binary Fe?U phase diagram, which showed general agreement with the latest assessment. The L → UFe₂ + γ-Fe eutectic reaction is found to occur at 1357 ± 5 K, with the eutectic composition of 47 mass%. The heat of transformation associated with this invariant reaction is estimated to be 19,969 ± 1736 J mol of atoms^?1. The L → U₆Fe + UFe₂ reaction occurs at 89 mass%, and at 1001 ± 5 K, with a heat of transformation 20,250 ± 2113 J mol of atoms^?1. The heat of melting of stoichiometric UFe₂ is estimated to be 20,983 ± 2098 J mol of atoms^?1, which is higher than the currently assessed value by 30%. A non-faceted microstructural morphology is found to accompany the eutectic solidification process of all the alloy compositions.     

Experimental studies on single-phase flow and heat transfer in a narrow rectangular channel

The state-of-the-art studies on single-phase flow and hear transfer in narrow rectangular channels shows some difference in terms of the agreement with the conventional theory. To further make clear this issue, the experimental studies on single-phase flow and heat transfer in a narrow rectangular channel with deioned water as test fluid was carried out. The narrow rectangular channel had the following dimensions: depth (e) = 2 mm, aspect ratio (e/b) = 0.05, length to diameter ratio (L/D_h) = 300, and mean wall relative roughness (?/D_h) = 8.3 × 10^?4. The experiments were performed with the channel oriented uprightly. The parameters that were varied during the experiments included the mass flow rate, inlet temperature and heat flux.Based on the measured temperatures, mass flow rates, pressure drops and heat fluxes, the isothermal and non-isothermal friction factors and the local and mean Nusselt numbers have been calculated. The correlations for the isothermal friction factors and the mean Nusselt numbers have been developed, and have a satisfactory agreement with the conventional theory. Based on the property ratio method, the correlations for non-isothermal friction factors have been proposed, but the new exponent (m) for modifying variable-property effect need to be developed.     

Study of Er+ ion-implanted lithium niobate structure after an annealing procedure by RBS and RBS/channelling

Erbium-doped lithium niobate (Er:LiNbO₃) is a prospective photonics component, operating at λ = 1.5 μm, which could be used as an optical amplifier or waveguide laser. We have focused on the structure of Er:LiNbO₃ layers created by 330 keV erbium ion implantation (fluences 1.0 × 10¹⁵, 2.5 × 10¹⁵ and 1.0 × 10¹⁶ cm^?2 1) in the X, Z and two various Y crystallographic cuts of LiNbO₃. Five hours annealing at 350 °C was applied to recrystallize the as-implanted layer and to avoid clustering of Er. Depth distribution of implanted Er has been measured by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He⁺ ions. Defects distribution and structural changes have been described using the RBS/channelling method. Data obtained made it possible to reveal the relations between the crystallographic orientation of the implanted crystal and the behaviour during the restoration process. The deepest modified layer has been observed in the perpendicular Y cut, which also exhibits the lowest reconstruction after annealing. The shallowest depth of modification and good recovery after annealing were observed in the Z cut of LiNbO₃. Since Er-depth profiles changed significantly in the perpendicular Y cut, we suppose that the crystal structure recovery inhibits Er mobility in the crystalline structure.