Anisotropy of disorder accumulation and recovery in 6H-SiC irradiated with Au ions at 140 K

Single crystal 〈0 0 0 1〉-oriented 6H-SiC was irradiated with Au²⁺ ions to fluences of 0.032, 0.058 and 0.105 ions/nm² at 140 K and was subsequently annealed at various temperatures up to 500 K. The relative disorder on both the Si and C sublattices has been determined simultaneously using in situ D⁺ ion channeling along the 〈0 0 0 1〉 and 〈〉 axes. A higher level of disorder on both the Si and C sublattices is observed along the 〈〉. There is a preferential C disordering and more C interstitials are aligned with 〈0 0 0 1〉. Room-temperature recovery along 〈〉 occurs, which is associated with the 〈0 0 0 1〉-aligned interstitials that annihilate due to close-pair recombination. Disorder recovery between 400 and 500 K is primarily attributed to annihilation of interstitials that are misaligned with 〈0 0 0 1〉 and to epitaxial crystallization. Effects of stacking order in SiC on disorder accumulation are insignificant; however, noticeable differences of low-temperature recovery in Au²⁺-irradiated 6H-SiC and 4H-SiC are observed.     

Reflection properties of small hydrocarbons impinging on tungsten and carbon surfaces

Sticking coefficients of deuterium from are quantified on fusion relevant plasma sprayed tungsten and carbon fibre composite in the incident energy range from about 0-100 eV. The samples that were cut from ASDEX-Upgrade tiles are exposed to a beam of of specific incident energy, E_inc, in the tandem mass spectrometer BESTOF in Innsbruck. Nuclear reaction analysis is performed ex-situ at IPP Garching for the quantification of deuterium content. The deuterium content difference measured on a spot before and after ion-beam exposure of the sample is assigned to the above mentioned species of hydrocarbon molecules sticking on the surface, allowing the calculation of the sticking probability of a specific deuterated molecular ion. The sticking coefficient, S, is found to depend on the incident energy and shows a maximum of about S ∼ 0.4 around E_inc = 30 eV on CFC and about S ∼ 0.1 near E_inc = 20 eV in case of PSW.     

First principle studies on the electronic structures and absorption spectra in KMgF3 crystal with fluorine vacancy

The experiments indicate that the perfect KMgF₃ crystal has no absorption in the visible range, however the electron irradiation induces a complex absorption spectrum. The absorption spectra can be decomposed by five Gaussian bands peaking at 2.5 eV (488 nm), 3.4 eV (359 nm), 4.2 eV (295 nm), 4.6 eV (270 nm) and 5.2 eV (239 nm), respectively. The purpose of this paper is to seek the origins of the absorption bands. The electronic structures and absorption spectra either for the perfect KMgF₃ or for KMgF₃: with electrical neutrality have been studied by using density functional theory code CASTEP with the lattice structure optimized. The calculation results predicate that KMgF₃: also exhibits five absorption bands caused by the existence of the fluorine ion vacancy and the five absorption bands well coincide with the experimental results. It is believable that the five absorption bands are related to in KMgF₃ crystal produced by the electron irradiation.     

Analysis of oxygen potential of (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x based on point defect chemistry

Stoichiometries in (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x were analyzed with the experimental data of oxygen potential based on point defect chemistry. The relationship between the deviation x of stoichiometric composition and the oxygen partial pressure P_O2 was evaluated using a Kröger-Vink diagram. The concentrations of the point defects in uranium and plutonium mixed oxide (MOX) were estimated from the measurement data of oxygen potentials as functions of temperature and P_O2. The analysis results showed that x was proportional to near the stoichiometric region of both (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x, which suggested that intrinsic ionization was the dominant defect. A model to calculate oxygen potential was derived and it represented the experimental data accurately. Further, the model estimated the thermodynamic data, and , of stoichiometric (U_0.7Pu_0.3)O_2.00 and (U_0.8Pu_0.2)O_2.00 as −552.5 kJ·mol⁻¹ and −149.7 J·mol⁻¹, and −674.0 kJ · mol^-1 and −219.4 J · mol⁻¹, respectively.     

Threshold stress intensity factor for delayed hydride cracking of a recrystallized N18 alloy plate along the rolling direction

The objective of this study is to obtain the threshold stress intensity factor, K_IH, for an initiation of delayed hydride cracking in a recrystallized N18 (Zr-Sn-Nb-Fe-Cr) alloy plate which was manufactured in China, gaseously charged with 60 ppm of hydrogen by weight. By using both the load increasing method and load drop method, the K_IH’s along the rolling direction were investigated over a temperature range of 150-255 °C. The results showed that K_IH along the rolling direction was found to be higher in the load increasing method than that in the load drop method. In the load increasing method, K_IH’s of the N18 alloy plate appeared to be in the range of and K_IH in the load drop method appeared to be in the range of . This means that the N18 alloy plate has high tolerance for DHC initiation along the rolling direction. The texture of a N18 alloy plate was investigated using an X-ray diffraction and the K_IH was discussed based on texture and analytically as a function of the tilting angle of hydride habit planes to the cracking plane.     

Oxygen potential of (U0.88Pu0.12)O2±x and (U0.7Pu0.3)O2±x at high temperatures of 1673-1873 K

The oxygen potential of (U_0.88Pu_0.12)O_2±x (−0.0119 < x < 0.0408) and (U_0.7Pu_0.3)O_2±x (−0.0363 < x < 0.0288) was measured at high temperatures of 1673-1873 K using gas equilibrium method with thermo gravimeter. The measured data were analyzed by a defect chemistry model. Expressions were derived to represent the oxygen potential based on defect chemistry as functions of temperature and oxygen-to-metal ratio. The thermodynamic data, and , at stoichiometric composition were obtained. The expressions can be used for in situ determination of the oxygen-to-metal ratio by the gas-equilibration method. The calculation results were consistent with measured data. It was estimated that addition of 1 wt.% Pu content increased oxygen potential of uranium and plutonium mixed oxide by 2-5 kJ/mol.     

Photoluminescence study of swift heavy ion (SHI) induced defect centers in sapphire

Single crystals of sapphire (Al₂O₃: Fe, Ti, Cr) were irradiated at room temperature with different fluence of 100 MeV Ni ions. Photoluminescence (PL) spectra of pristine and irradiated sapphires were recorded at room temperature under 2.8 eV blue excitation. A broad emission band consists of two bands centered at 516 nm corresponding to F₂ defect center and 546 nm corresponding to defect center was observed. The intensity of these defect centers was found to vary with the fluence. defect center develops at low fluence reaching maximum at 5 × 10¹⁶ ions/m² and finally decreasing at higher fluence. The behavior is interpreted in terms of creation of defect centers, their clustering and annihilation.     

Linear free energy relationship applied to trivalent cations with lanthanum and actinium oxide and hydroxide structure

Linear free energy relationships for trivalent cations with crystalline M₂O₃ and, M(OH)₃ phases of lanthanides and actinides were developed from known thermodynamic properties of the aqueous trivalent cations, modifying the Sverjensky and Molling equation. The linear free energy relationship for trivalent cations is as , where the coefficients a_MvX, b_MvX, and β_MvX characterize a particular structural family of MvX, r_M³⁺ is the ionic radius of M³⁺ cation, is the standard Gibbs free energy of formation of MvX and is the standard non-solvation free energy of the cation. The coefficients for the oxide family are: a_MvX = 0.2705, b_MvX = −1984.75 (kJ/mol), and β_MvX = 197.24 (kJ/mol nm). The coefficients for the hydroxide family are: a_MvX = 0.1587, b_MvX = −1474.09 (kJ/mol), and β_MvX = 791.70 (kJ/mol nm).     

Absorption and photoluminescence study of Al2O3 single crystal irradiated with fast neutrons

Colour centers formation in Al₂O₃ by reactor neutrons were investigated by optical measurements (absorption and photoluminescence). The irradiation’s were performed at 40 °C, up to fast neutron (E_n > 1.2 MeV) fluence of 1.4 × 10¹⁸ n cm⁻². After irradiation the coloration of the sample increases with the neutron fluence and absorption band at about 203, 255, 300, 357 and 450 nm appear in the UV-visible spectrum. The evolution of each absorption bands as a function of fluence and annealing temperature is presented and discussed. The results indicate that at higher fluence and above 350 °C the F⁺ center starts to aggregate to F center clusters (F₂, F₂⁺ and ). These aggregates disappear completely above 650 °C whereas the F and F⁺ centers persist even after annealing at 900 °C. It is clear also from the results that the absorption band at 300 nm is due to the contribution of both F₂ center and interstitial ions.     

Role of grain/phase boundary nature on the formation of hydrides in Zr-2.5%Nb alloy

Hydride formation in a fully recrystallized Zr-2.5%Nb alloy having equiaxed grains of α and β was studied. Primarily the electron back scatter diffraction (EBSD) technique was used for the characterization of the hydrides in conjunction with optical and transmission electron microscopy. Hydrides were found to have preferentially formed along the α/β interfaces. Microtexture measurements showed that the orientation relationship (OR) between α and δ-hydride phase was (0 0 0 1)_α || (1 1 1)_δ and || [1 1 0]_δ. It was shown that the hydrides have higher preference to form along such α/β interfaces which have one of the low index planes of the β phase constituting the interface.     

Radiation induced defects in BaBPO5:Ce and their role in thermally stimulated luminescence reactions: EPR and TSL investigations

Defect centers induced by gamma irradiation in Ce doped BaBPO₅ were investigated using EPR spectroscopy. From EPR studies, three phosphorous centered radicals were characterized on the basis of observed ³¹P hyperfine splitting and g values as , and radicals. In addition to this, two types of boron oxygen hole centers (BOHC) and O⁻ were also formed at room temperature. An intense broad signal in sample annealed in argon (g_⊥ = 1.9258 and g_‖ = 1.8839) was assigned to Ce³⁺ ions associated with the electron trapped at anion vacancy or nearby lattice defect. TSL studies showed two glow peaks, a relatively weaker one at 425 K and an intense one at 575 K. Spectral studies of the TSL glow peaks have shown that Ce³⁺ ion acts as emission center. From the temperature dependence of the EPR spectra of gamma irradiated samples, the glow peaks at 425 K and 575 K were attributed to thermal destruction of /O⁻ and BOHC, respectively, by trapping of electrons from elsewhere. The energy released in electron hole recombination process is used for the excitation of Ce³⁺ ions resulting in these glow peaks at 425 K and 575 K. The spectral studies of the TSL glow peaks have shown emission at 330 nm indicating Ce³⁺ acts as the luminescent centre. The trap depth and the frequency factor for the 425 K and 575 K peaks were determined using different heating rates method.     

H2 inhibition of radiation induced dissolution of spent nuclear fuel

In order to elucidate the effect of noble metal clusters in spent nuclear fuel on the kinetics of radiation induced spent fuel dissolution we have used Pd particle doped UO₂ pellets. The catalytic effect of Pd particles on the kinetics of radiation induced dissolution of UO₂ during γ-irradiation in containing solutions purged with N₂ and H₂ was studied in this work. Four pellets with Pd concentrations of 0%, 0.1%, 1% and 3% were produced to mimic spent nuclear fuel. The pellets were placed in 10 mM aqueous solutions and γ-irradiated, and the dissolution of was measured spectrophotometrically as a function of time. Under N₂ atmosphere, 3% Pd prevent the dissolution of uranium by reduction with the radiolytically produced H₂, while the other pellets show a rate of dissolution of around 1.6 × 10⁻⁹ mol m⁻² s⁻¹. Under H₂ atmosphere already 0.1% Pd effectively prevents the dissolution of uranium, while the rate of dissolution for the pellet without Pd is 1.4 × 10⁻⁹ mol m⁻² s⁻¹. It is also shown in experiments without radiation in aqueous solutions containing H₂O₂ and O₂ that ?-particles catalyze the oxidation of the UO₂ matrix by these molecular oxidants, and that the kinetics of the catalyzed reactions is close to diffusion controlled.     

Phase relations and linear thermal expansion of cubic solid solutions in the Th1−xMxO2−x/2 (M = Eu, Gd, Dy) systems

Cell parameters and linear thermal expansion studies of the Th-M oxide systems with general compositions Th_1−xM_xO_2−x/2 (M = Eu³⁺, Gd³⁺ and Dy³⁺, 0.0 ? x ? 1.0) are reported. The XRD patterns of each product were refined to specify the solid solubility limits of MO_1.5 in the ThO₂ lattice. The upper solid solubility limits of EuO_1.5, GdO_1.5 and DyO_1.5 in the ThO₂ lattice under conditions of slow cooling from 1673 K are represented as Th_0.50Eu_0.50O_1.75, Th_0.60Gd_0.40O_1.80 and Th_0.85Dy_0.15O_1.925, respectively. The linear thermal expansion (293-1123 K) of MO_1.5 and their single-phase solid solutions with thoria were investigated by dilatometery. The average linear thermal expansion coefficients () of the compounds decrease on going from EuO_1.5 to DyO_1.5. The values of for EuO_1.5, GdO_1.5 and DyO_1.5 containing solid solutions showed a downward trend as a function of the dopant concentration. The linear thermal expansion (293-1473 K) of the solid solutions investigated by high-temperature XRD also showed a similar trend.     

Cold atoms photoassociation with intense laser pulses

We study the vibrational dynamics produced when two cold atoms are photoassociated in a diatomic molecule by an intense laser pulse (with the duration of hundreds ps), inducing a resonance condition at small interatomic distances. The example analysed is the population transfer from the continuum to the 1_g(6s + 6p_3/2) excited electronic potential of the Cs₂ molecule, at a temperature T ≈ 0.11 mK. We use a non-perturbative treatment by following the wavepackets dynamics on the ground and excited surfaces. We show that cold molecules can be efficiently produced in both ground and excited electronic potentials.     

Computer simulation of the interaction of carbon atoms with self-interstitial clusters in α-iron

Static and dynamic properties of clusters of self-interstitial atoms and their complexes with carbon (C) atoms in α-iron are studied by molecular dynamics method using a pairwise interatomic potential for iron-carbon interaction and a many-body potential for iron. The effect of C atoms on the configuration, stability and migration of , and 〈1 0 0〉 interstitial clusters is investigated. In the framework of the simple model of interstitial solute used here, C atoms enhance the relative stability of 〈1 0 0〉 over clusters, but not enough to explain their common occurrence under irradiation. Clusters of seven interstitials or smaller are able to co-migrate with C atoms with a reduced mobility compared with pure iron. Bigger clusters have dislocation structure and are immobilised: C migrates along the core of their periphery as in the core of a straight edge dislocation. C dissociates from all clusters at high enough temperature.