First-principles investigation of energetics and site preference of He in a W grain boundary

We perform first-principles calculations based on density functional theory to investigate energetics and site preference of He in a bcc-W Σ = 5 grain boundary (GB). The segregation energy is calculated to be −1.37 eV, indicating that He prefers to segregate in the W GB. The formation energy of He in the W GB is positive and thus He is quite hard to dissolve in the W GB, similar to its behavior in the bulk. Because of its closed-shell electronic structure, He is shown to preferably occupy either interstitial or substitutional site with larger space provided by the GB, changing the GB electronic structure. Moreover, segregation of He gives rise to the W GB expansion. These structure variations can have a large effect on the mechanical properties of the W GB.     

Binary effect of He and H on the intra- and inter-granular embrittlement in Fe

Both helium and hydrogen are known to degrade the mechanical performance of reactor materials, but their binary effect has not been well understood. Using abinitio density functional theory calculations, we have investigated the He-H interactions both in the bulk and at the grain boundary in bcc Fe. We find an attraction of 0.56 eV between one substitutional He and one interstitial H in the bulk and a repulsion of 0.20 eV at the Σ3 (1 1 1) grain boundary. The attraction of He and H inside a grain means a reduction of combined embrittling effect in intra-granular fracture and the repulsion at the grain boundary means an enhancement of combined embrittling effect in inter-granular fracture. The interaction of He and H are interpreted with electronic structure analyses.     

Simulating radiation damage in δ-plutonium

Radiation events in δ-Pu (fcc) have been simulated in an attempt to understand the fundamental mechanisms that contribute to the Pu ageing process. The Pu interactions are modelled using a potential based on the modified embedded atom method (MEAM). The energetics of point defects have been investigated using static calculations together with molecular dynamics (MD) to simulate radiation events. All MD simulations were carried out with Pu initially in the face-centred-cubic (fcc) structure, although this is not the lowest energy configuration for the pure metal.The point defect study suggests that the mono-vacancy has the lowest formation energy (0.46 eV), with interstitial defects favouring the - split orientation over occupation of the native fcc octahedral site. Displacement threshold energy calculations at room temperature give a minimum value of between 5 and 6 eV, increasing to 8-14 eV along the major crystallographic directions.Low energy collision cascades, initiated with energies in the range of 0.4-1 keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone at the cascade core, surrounded by isolated interstitial defects. Higher energy cascades show similar features but with occasional channelling of energetic atoms and sub-cascade branching which significantly reduces defect production. A common trait observed across all the cascades was the relatively slow annealing period, compared to cascades in other fcc metals, with simulations at energies above 5 keV requiring many 10’s of picoseconds before the ballistic phase was completed.     

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.     

Quantum mechanical calculations of uranium phases and niobium defects in γ-uranium

Depleted uranium (U) from fuel enrichment processes has a variety of applications due to its high density. With the addition of a small concentration of niobium (Nb), U becomes stainless. Nb is fully miscible with the high-temperature γ phase of U and tends to segregate upon cooling below 1050 K. The starting point of segregation is the configuration of Nb substitutional or interstitial defects. Using quantum mechanical calculations, the authors find that the formation energy of a single vacancy is 1.08 eV, that of Nb substitution 0.59 eV, that of Nb interstitial at octahedral site 1.58 eV, and that of Nb interstitial at tetrahedral site 2.35 eV in the dilute limit of isolated defects; all with reference to a reservoir of the pure γ phase U and pure Nb. The analysis of electronic structures reveals the correlation of formation energies of Nb defects with the local perturbations of electron distribution. Higher formation energy of Nb defects correlates with larger perturbation. Based on this study, Nb atoms thermodynamically prefer to occupy substitutional sites in the γ phase U.     

First-principles study on electronic structures and absorption spectra for BaWO4 crystal containing barium vacancy

The electronic structures, dielectric function and absorption spectra for the perfect BaWO₄ (BWO) crystal and the BWO crystal containing barium vacancy () have been studied using density functional theory code CASTEP with the lattice structure optimized. The results indicate that the optical properties of the BWO crystal exhibit anisotropy and its optical symmetry coincide with lattice structure geometry of the BWO crystal. For the BWO crystal containing , there exhibit four absorption bands peaking at 0.71 eV (1751 nm), 1.85 eV (672 nm), 3.43 eV (362 nm) and 3.85 eV (322 nm), respectively. The origins of the 370 nm absorption band should be related to the .     

Ion implantation in 4H-SiC

Silicon carbide offers unique applications as a wide bandgap semiconductor. This paper reviews various aspects of ion implantation in 4H-SiC studied with a view to optimise ion implantation in silicon carbide. Al, P and Si ions with keV energies were used. Channelling effects were studied in both a-axis and c-axis crystals as a function of tilts along major orthogonal planes and off the major orthogonal planes. Major axes such as [0 0 0 1] and the and minor axis like the showed long channelling tails and optimum tilts for minimising channelling are recommended. TEM analyses of the samples showed the formation of (0 0 0 1) prismatic loops and the loops as well,in both a and c-cut crystals. We also note the presence of voids only in P implanted samples implanted with amorphising doses. The competing process between damage accumulation and dynamic annealing was studied by determining the critical temperature for the transition between crystalline and amorphous SiC and an activation energy of 1.3 eV is extracted.     

Measurement of radiation-enhanced diffusion of La in single crystal thin film CeO2

The diffusion of La, a trivalent cation dopant, actinide surrogate, and high-yield fission product, in CeO₂, a UO₂ nuclear fuel surrogate, during 1.8 MeV Kr⁺ ion bombardment over a temperature range from 673 K to 1206 K has been measured with secondary ion mass spectroscopy. The diffusivity under these irradiation conditions has been analyzed with a model based on a combination of sink-limited and recombination-limited kinetics. This analysis yielded a cation vacancy migration energy of  ∼ 0.4 eV below ∼800 K, were recombination-limited kinetics dominated the behavior. The thermal diffusivity of La in the same system was measured over a range of 873-1073 K and was characterized by an activation enthalpy of . The measurement of both the migration enthalpy and total activation enthalpy separately allows the vacancy formation enthalpy on the cation sublattice to be determined;  ∼ 1 eV. The mixing parameter under energetic heavy-ion bombardment at room temperature was measured as well and found to be ∼4 × 10⁻⁵ nm⁵/eV.     

Stability and migration property of helium and self defects in vanadium and V-4Cr-4Ti alloy by first-principles

The stability and migration behavior of helium and self defects in vanadium and V-4Cr-4Ti alloy are studied by first-principles calculations. The tetrahedral site is found as the most stable configuration for interstitial He, followed by the octahedral and substitutional sites. Among the self defects, the monovacancy has lower formation energy (1.71 eV for V and 2.14 eV for V-4Cr-4Ti alloy) than the self interstitial ones. The migration energies for He hopping between the tetrahedral sites are 0.06 and 0.09 eV for vanadium and V-4Cr-4Ti alloy, respectively. Our calculations reveal strong repulsion between two interstitial He atoms and strong attraction between He and vacancy, suggesting that vacancy acts as a trapping site for He impurity and a seed for further bubble formation.     

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.     

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.     

Monitoring the embrittlement of reactor pressure vessel steels by using the Seebeck coefficient

The degree of embrittlement of the reactor pressure vessel (RPV) limits the lifetime of nuclear power plants. Therefore, neutron irradiation-induced embrittlement of RPV steels demands accurate monitoring. Current federal legislation requires a surveillance program in which specimens are placed inside the RPV for several years before their fracture toughness is determined by destructive Charpy impact testing. Measuring the changes in the thermoelectric properties of the material due to irradiation, is an alternative and non-destructive method for the diagnostics of material embrittlement. In this paper, the measurement of the Seebeck coefficient () of several Charpy specimens, made from two different grades of 22 NiMoCr 37 low-alloy steels, irradiated by neutrons with energies greater than 1 MeV, and fluencies ranging from 0 up to 4.5 × 10¹⁹ neutrons per cm², are presented. Within this range, it was observed that increased by ≈500 nV/°C and a linear dependency was noted between and the temperature shift ΔT_41 J of the Charpy energy vs. temperature curve, which is a measure for the embrittlement. We conclude that the change of the Seebeck coefficient has the potential for non-destructive monitoring of the neutron embrittlement of RPV steels if very precise measurements of the Seebeck coefficient are possible.     

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.     

Study on C-W interactions by molecular dynamics simulations

By means of molecular dynamics simulations using bond-order potential (BOP), we have investigated the interactions between carbon (C) atoms and bcc tungsten (W). At finite temperature (T = 300 K) with incident energy of C atoms ranging from 0.5 to 100 eV at normal incidence, the projected range distribution as a function of incident energy and the average depth have been depicted. The properties of vacancy, vacancy migration, interstitial and substitutional C atoms in W have been determined. The most stable configuration for an interstitial C atom in W is in octahedral position and the lattice distortion around the C atom in octahedral interstitial configuration occurs along 〈1 0 0〉 and 〈1 1 0〉 directions. The mutual interaction between a vacancy and near interstitial C atom is also studied.     

Development of secondary ion mass spectroscopy using medium energy C60 ion impact

In this paper, we report time-of-flight (TOF) secondary ion mass spectroscopy using primary C₆₀ ions with an energy range from several tens of keV to several hundreds of keV. Application of the spectroscopy to the analysis of a poly(amino acid) film revealed that characteristic peaks, necessary for identification of the amino acid in proteins, show higher intensities for medium energy C₆₀ (120 keV and 540 keV ) impacts than those for low energy C₆₀ (30 keV ) impacts. This finding demonstrates that medium energy C₆₀ ion impacts are useful for highly sensitive characterization of amino acids.     

The emission process of secondary ions from solids bombarded with large gas cluster ions

We investigated the effects of size and energy of large incident Ar cluster ions on the secondary ion emission of Si. The secondary ions were measured using a double deflection method and a time-of-flight (TOF) technique. The size of the incident Ar cluster ions was between a few hundreds and several tens of thousands of atoms, and the energy up to 60 keV. Under the incidence of keV energy atomic Ar ions, mainly atomic Si ions were detected, whereas Si cluster ions were rarely observed. On the other hand, under the incidence of large Ar cluster ions, the dominant secondary ions were  (2 ? n ? 11). It has become clear that the yield ratio of secondary Si cluster ions was determined by the velocity of the incident cluster ions, and this strong dependence of the yield ratio on incident velocity should be related to the mechanisms of secondary ion emission under large Ar cluster ion bombardment.     

Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems.X- and Y-cut LiNbO₃ crystals implanted with 8 MeV Au³⁺ ions with a fluence of 1 × 10¹⁷ ions/cm² result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold.Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of and aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed.The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10¹⁷ ions/cm² during subsequent high temperature annealing is studied by Raman and Brillouin light scattering.Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10¹⁵ to 250 × 10¹⁵ ions/cm⁻² induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.     

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.     

Saturation in deuterium retention of CFC graphite targets exposed to beryllium-seeded plasmas on PISCES-B

ITER strike-plates are foreseen to be of carbon-fiber-composite (CFC). In this study the CFC bulk deuterium retention in ITER-relevant conditions is investigated. DMS 701 (Dunlop) CFC targets were exposed to plasma in PISCES-B divertor plasma simulator. Samples were exposed to both pure deuterium plasma and beryllium-seeded plasma at high fluences (up to ) and high surface temperature (1070 K). The deuterium contents of the exposed samples have been measured using both thermal-desorption-spectrometry (TDS) during baking at 1400 K and ion beam nuclear reaction analysis (NRA). The total deuterium inventory has been obtained from TDS while NRA measured the deuterium depth distribution. In the analysed fluence range at target temperature of 1070 K, no fluence dependence was observed. The measured released deuterium is . In the case of target exposure with beryllium-seeded plasma no change in the released amount of deuterium was found. The deuterium concentration inside the samples is almost constant until the probed depth of ?m, except in the first 1 μm surface layer, where it is 5 times higher than in the bulk. No C erosion/redeposition was observed in the Be-seeded plasma cases. The measured retention, applied to 50 m² of ITER CFC surface, would imply a tritium saturated value of 0.3 gT, much lower than the ITER safety limit of 350 g.