Contribution of uranium diffusion on creep behavior of uranium dicarbide

Compressive creep tests of uranium dicarbide (UC₂) have been conducted. The general equation best describing the creep rate over the temperature range 1200–1400°C and over the stress range 2000–15000 psi is represented by the sum of two exponential terms ge =A(σ/E)^0.9 exp(−39.6 ± 1.0/RT) + B(σ/E)^4.5 exp(−120.6 ± 1.7/RT), where pre-exponential factors are A(σ/E)^0.9 = 12.3/h at low stress region (3000 psi) and B(σ/E)^4.5 = 3.17 × 10¹³/h at high stress region (9000 psi), and the activation energy is given in kcal/mol. Each term of this experimental equation indicates that important processes occurring during the steady state creep are grain-boundary diffusion of the Coble model at low stress region and the Weertman dislocation climb model at high stress region. Both mechanisms are related to migration of uranium vacancies.     

The thermal conductivity of UO2 vapor

The thermal conductivity, λ of a saturated vapor over UO_1.96 is calculated in the temperature range 3000–6000 K. The calculation shows that the contribution to λ from the transport of reaction enthalpy dominates all other contributions. All possible reactions of the gaseous species UO₃, UO₂, UO, U, O, and O₂ are included in the calculation. We fit the total thermal conductivity to the empirical equation λ = exp(a+ b/T+cT+dT² + eT³), with λ in cal/(cm s K), T in kelvins, a = 268.90, B = − 3.1919 × 10⁵, C = −8.9673 × 10⁻², d = 1.2861 × 10⁻⁵, and E = −6.7917 × 10⁻¹⁰.     

A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

Characterization of extended defects in SiGe alloys formed by high dose Ge implantation into Si

The synthesis of SiGe/Si heterostructures by Ge⁺ ion implantation is reported. 400 keV Ge⁺ ions were implanted at doses ranging from 3 × 10¹⁶ to 10 × 10¹⁶ ions/cm² into (001) Si wafers, followed by Si⁺ amorphisation and low temperature Solid Phase Epitaxial Regrowth (SPER). TEM investigations show that strained alloys can be fabricated if the elastic strain energy (E_el) of the SiGe layer does not exceed a critical value (E′_el) of about 300 mJ/m², which is independent of the implantation energy. Our analysis also suggests that “hairpin” dislocations are formed as strain relieving defects in relaxed structures. A “strain relaxation” model is proposed to explain their formation.     

2 MeV Si ion implantation damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3 μm thick relaxed Si_1−xGe_x layers with 2 MeV Si⁺ ions has been measured as a function of Ge content (x = 0.04, 0.13, 0.24 or 0.36) and Si dose in the dose range 10¹⁰–10¹⁵ cm⁻². The accumulation of damage with increasing dose has been studied as a function of Ge content by Rutherford Backscattering Spectrometry, Optical Reflectivity Depth Profiling and Transmission Electron Microscopy and an increased damage efficiency in Si_1−xGe_x with increasing x is observed. The characteristics of implantation-induced defects have been investigated by Electron Paramagnetic Resonance. The results are discussed in the context of a model of the damage process in SiGe.     

Dissociation of fast HeH ions at glancing angle scattering from a crystal surface

Scattered H⁺ fragments resulting from glancing angle incidence of MeV HeH⁺ ions on the clean (001) surface of a SnTe crystal are distributed on an ellipse in the E-φ plane, where the angle φ is measured in the plane parallel to the surface and E is the energy of the fragment. The kinetic energies released upon dissociation derived from the diameters of the ellipses are smaller than those calculated from the explosion of fully ionized fragments.

Based on a simplified model, we have simulated the dissociative scattering of HeH⁺ ions at the surface, where the dissociation takes place via excitation of HeH⁺ ions and the charges of fragments change along the trajectory. It is concluded from the simulation that the cross section for dissociation in HeH⁺-electron collisions is of the order of 10⁻¹⁶ cm².     

Surface swelling of sapphire induced by MeV and GeV heavy ions

-Al₂O₃ single crystals were bombarded with MeV xenon ions from 10¹⁵ to 10¹⁷ ions cm⁻² and GeV uranium ions from 10¹¹ to 10¹³ ions cm⁻² to study the surface swelling of sapphire at 77 and 300 K due to atomic collision processes (Xe) and electronic energy loss processes in the 20–45 keV/nm regime (U). The induced damage was studied by channeling Rutherford backscattering. Surface swelling was measured with a profilometer. The step height induced by nuclear cascades of MeV xenon increases with the ion fluence and saturates. With GeV uranium, an electronic stopping power threshold for surface swelling was observed and the step height increased with the damage for dE/dx higher than this threshold.     

Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.     

Determination of sulphur and copper depth distribution in patina layers using nuclear reaction techniques

A method for Cu and S profiling in patina layers was developed by applying a combination of nuclear reaction analysis (NRA) and Rutherford backscattering spectroscopy (RBS). The copper profiling was performed by using the 1327 keV γ-ray deexciting the third excited state to the ground state of ⁶³Cu produced by the reaction ⁶³Cu(p,p^′γ)⁶³Cu. For the determination of sulphur the 2230 keV γ-ray was used deexciting the first excited state to the ground state of ³²S formed through the reaction ³²S(p,p^′γ)³²S, which exhibits three sharp resonances at projectile energies 3.094, 3.195 and 3.379 MeV. The relevant cross-sections were measured in the energy range between 3.0 and 3.7 MeV in steps of 20 keV at 125° to the incident proton beam direction. The technique was tested using artificially produced and natural copper patina layers. Supporting information on the depth distribution of the constituent elements of the patina samples was obtained by p-RBS (E_p: 1.5 MeV, θ: 160°).     

Evidence for neutron irradiation-induced metallic precipitates in model alloys and pressure-vessel weld steel

We have used positron Doppler-broadening spectroscopy to examine a series of neutron-irradiated model alloys (1 × 10²³ n/m², E>0.5 MeV) and 73W-weld steel (to 1.8 × 10²³ n/m², E>1 MeV. The copper, nickel and phosphorus content of the model alloys was systematically varied. The samples were examined in the as-irradiated state and after post-irradiation isochronal anneals to temperature up to 600 °C. By following the S and W parameters, and especially by plotting the results in (S,W) space, we can infer that the damage is a combination of irradiation-induced metallic precipitates and vacancy-type defect clusters. Samples with either high Cu or with a combination of high Ni and medium Cu (and the pressure-vessel weld steel) showed evidence for both irradiation-induced metallic precipitation, and vacancy-type clusters, while samples without either high Ni or high Cu showed predominantly evidence of annihilations in vacancy-type clusters. These results are discussed in terms of embrittlement models.     

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.     

Elastic recoil detection analysis on the ANSTO heavy ion microprobe

The heavy ion microprobe at the Australian Nuclear Science and Technology Organisation is capable of focussing heavy ions with an ME/q² of up to 100 amu MeV. This makes the microprobe ideally suited for heavy ion elastic recoil detection analysis (ERDA). However, beam currents on a microprobe are usually very small, which requires a detection system with a large solid angle. We apply microbeam heavy ion ERDA using a large solid angle ΔE−E telescope with a gas ΔE detector to layered structures. We demonstrate the capability to measure oxygen and carbon with a lateral resolution of 20 μm, together with determination of the depth of the contamination in thin deposited layers.     

High energy resolution PIXE with high efficiency using the heavy ion microbeam

An X-ray crystal spectrometer using a position sensitive proportional counter combined with tandem microbeam line at Osaka National Research Institute have been developed. This system realizes high energy resolution PIXE analysis using a heavy ion microbeam (E < 6 MeV) with reasonable detection efficiency. The design of the spectrometer, such as detection geometry, detectable energy range and energy resolution, are described. This system was applied to high energy resolution PIXE analysis of Ti, SUS and Si with 2 MeV proton and 5 MeV Si³⁺ focused or collimated beams. The best energy resolution was 2 eV for the Si K line.     

Layout of a nuclear microprobe beamline using a liquid metal ion source

The Bochum solenoid lens microprobe will be installed in a new configuration which can be fed both by the 4 MV Dynamitron tandem accelerator and by a new 500 kV accelerator of extremely low energy-spread (ΔE/E 10⁻⁵). Apart from a conventional duoplasmatron ion source, a commercial gallium liquid metal ion source (LMIS) will be implemented in this accelerator. Microprobe optics will benefit from the high brightness of the LMIS ( 10⁵ Am⁻²sr⁻¹eV⁻¹), thus enabling an increased lateral resolution. Ion optical ray tracing, simulating the accelerator tube and the solenoid lens, has allowed one to specify the beam parameters required at the accelerator entrance to yield a micrometer focus at the target position. Using this information and further simulations, the accelerator injection optics can be particularly optimized for the new microbeam line.     

Cobalt cross sections for 14 MeV neutrons

14 MeV cross sections for the reactions ⁵⁹Co(n,2n) ^58m+gCo, ⁵⁹Co(n,p) ⁵⁹Fe and ⁵⁹Co(n,) ⁵⁶Mn were measured relative to the ⁵⁶Fe(n,p) ⁵⁶Mn reaction employing the activation technique. Accuracies of about 1% were achieved for the (n,) reaction and 2% for the others. The isomeric cross section ratio was measured for the ⁵⁹Co(n,2n) reactions.

Nuclear reactions – ⁵⁹Co(n,2n) ^58m+gCo, ⁵⁹Co(n,p) ⁵⁹Fe, ⁵⁹Co(n,) ⁵⁶Mn, E_n = 14.3, 14.7 MeV: measured activation cross sections relative to ⁵⁶Fe(n,p) ⁵⁶Mn.

⁵⁹Co(n,2n) ^58m,gCo, E_n = 14.3 MeV; measured isomeric cross section ratio. Natural targets. Ge, NaI and 4πβ detectors.     

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.     

The diffusivities of fission-created or thermally-doped tritium in UO2

The diffusion behavior of tritium in UO₂ was studied. Two methods were adopted for the introduction of tntium into UO₂: one via ternary fission of ²³⁵U and the other via thermal doping. In the former, the diffusion constants decreased with increase in sample weight. The diffusion constants obtained from the pellet with the same specification (9 mm in diameter, 5 mm high) were D″_bulk = 3.03 × 10⁻³(^+0.369_−0.003) exp[−163±43(kJ/mol)/RT](cm²/s) for fission-created tritium and D′_bulk = 0.15(^{+ 0.94}_−0.13) exp[−76±13 (kJ/mol)/RT](cm²/s) for thermally-doped tritium. The difference of the diffusion constants between two systems was discussed in terms of the effects associated with the recoil processes of energetic tritium.     

Design of a magnetic spectrograph for surface, interface and thin-layer analysis

The design of a magnetic spectrograph of the QDQ type, to be coupled to a 6 MV Van de Graaff accelerator for the analysis with Rutherford backscattering and recoil spectroscopy of surfaces and thin layers, will be shown. Due to the high energy resolution of. the instrument (ΔE/E ≈ 2 × 10⁻⁴) the depth resolution is determined mainly by energy straggling and the combined effect of angular and lateral straggling, and is close to l monolayer near the surface. The mass discrimination of the spectrograph in combination with the energy dispersion of the detector in the focal plane makes possible the background-free detection of light (¹H−¹⁹F) atoms recoiling from a heavier substrate. The design of the total setup is such that channeling and channeling plus blocking experiments can be carried out, so that the position of light (or heavy) atoms on or in monocrystalline samples can be measured. The detector in the focal plane is position-sensitive in two dimensions so that with one setting of the spectrograph an energy range of 2% and an angular distribution of 5° (with a resolution of 0.1°) can be measured simultaneously. The opening angle in the energy-dispersive direction is 0.1°. The whole spectrograph, including the scattering chamber, can be rotated over 120° with respect to the beam line. For this purpose a bellows construction is made between the beam line and the scattering chamber, permitting the rotation while maintaining a vacuum of ≈ 10⁻¹⁰ Torr in the chamber.     

Vacancy defects induced in the track region of 132 MeV C irradiated SiC

Positron annihilation lifetime spectroscopy (PALS) and electron paramagnetic resonance (EPR) have been used in this work to investigate vacancy defects induced in the track region of 132 MeV ¹²C irradiated silicon carbide. Irradiations have been performed at room temperature at a fluence of 2.5 × 10¹⁴ cm⁻² in N-low doped 6H–SiC and 3C–SiC monocrystals. Silicon monovacancies have been detected in both polytypes using EPR. Their charge state and concentration have been determined in the track and cascade region of the C⁺ ions. PALS measurements performed as a function of temperature have shown the presence of V_Si–C divacancies in the track region for both polytypes.     

Effects of 160 MeV Ni ion irradiation on polypyrrole conducting polymer electrode materials for all polymer redox supercapacitor

Electronically conducting polymers are suitable electrode materials for high performance supercapacitors, for their high specific capacitance and high dc conductivity in the charged state. Supercapacitors and batteries are energy storage and conversion systems which satisfies the requirements of high specific power and energy in a complementary way. Ion beam {energy > 1 MeV} irradiation on the polymer is a novel technique to enhance or alter the properties like conductivity, density, chain length and solubility.

Conducting polymer polypyrrole thin films doped with LiClO₄ are synthesized electrochemically on ITO coated glass substrate and are irradiated with 160 MeV Ni¹²⁺ ions at different fluence 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². Dc conductivity measurement of the irradiated films showed 50–60% increase in conductivity which is may be due to increase of carrier concentration in the polymer film as observed in UV–Vis spectroscopy and other effects like cross-linking of polymer chain, bond breaking and creation of defects sites. X-ray diffractogram study shows that the degree of crystallinity of polypyrrole increases in SHI irradiation and is proportionate to ion fluence. The capacitance of the irradiated films is lowered but the capacitance of the supercapacitors with irradiated films showed enhanced stability compared to the devices with unirradiated films while characterized for cycle life up to 10,000 cycles.