Positron simulations of defects in tungsten containing hydrogen and helium

An understanding of the behavior of defects containing hydrogen or helium in tungsten is an important issue. Here the properties of defects in tungsten containing hydrogen or helium atoms have been investigated by model positron lifetime quantum-mechanical simulations. The electron and positron wave functions have been obtained in the local density approximation to the two-component density-functional theory. The calculated values of the positron lifetime correlate with the magnitude of the electron density. The vacancy-clusters without hydrogen or helium are active positron traps. The lattice relaxation of atoms around vacancy reduces the effective vacancy volume and decrease the positron lifetime at a vacancy. The hydrogen and helium atoms are trapped in tungsten by lattice vacancies and nano-voids. It was established that positron lifetime depends on the density of gas atoms inside the nano-void. Hydrogen and helium presence in the larger nano-voids considerably decrease the positron lifetime.     

The effect of alloying elements on the vacancy defect evolution in electron-irradiated austenitic Fe-Ni alloys studied by positron annihilation

The vacancy defect evolution under electron irradiation in austenitic Fe-34.2 wt% Ni alloys containing oversized (aluminum) and undersized (silicon) alloying elements was investigated by positron annihilation spectroscopy at temperatures between 300 and 573 K. It is found that the accumulation of vacancy defects is considerably suppressed in the silicon-doped alloy. This effect is observed at all the irradiation temperatures. The obtained results provide evidence that the silicon-doped alloy forms stable low-mobility clusters involving several Si and interstitial atoms, which are centers of the enhanced recombination of migrating vacancies. The clusters of Si-interstitial atoms also modify the annealing of vacancy defects in the Fe-Ni-Si alloy. The interaction between small vacancy agglomerates and solute Al atoms is observed in the Fe-Ni-Al alloy under irradiation at 300-423 K.     

Ab initio study of Kr in hcp Ti: Diffusion, formation and stability of small Kr-vacancy clusters

Ab initio electronic structure calculations have been performed to study the formation and migration of Kr impurities, and the stability of small Kr-vacancy clusters for clusters with up to four vacancies and four Kr atoms, in hcp Ti. Both the substitutional and the interstitial configurations of Kr are found to be stable. The octahedral configuration is however found to be more stable than the tetrahedral. Interstitial Kr atoms are shown to have attractive interactions and a low migration barrier, suggesting that, at low temperature, Kr bubble formation is possible, even in the absence of vacancies. We also find vacancy clusters to be stable. The binding energies of an interstitial Kr atom and a vacancy to a Kr-vacancy cluster are obtained from the calculated formation energies of the clusters. The stability of small-vacancy clusters is found to be dependent on Kr-vacancy ratio. The trends of the calculated binding energies are discussed in terms of providing further insights on the behaviour of Kr in implanted Ti.     

Microdefects and 3d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques

Microdefects and 3d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3d electron signal in the spectra of all tested samples. The 3d electron signal in the spectrum of Fe₅₀Al₅₀ alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3d electrons of Fe atoms and 3p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.     

Irradiation-creep during void production

It is desirable for technological reasons to develop theories which can relate irradiation-creep with swelling. Such a correlation is attempted here by developing an idea due to Hesketh and applying it to the formation of Frank loops containing interstitial atoms produced during void production. Observation suggests that the number of interstitials in loops is equal to the number of vacancies in voids and it is this equality which allows the creep strain to be linked to the swelling. The calculated ratio of creep and swelling is compared with values recently deduced from experiments with pressurized tubes and it is found that the calculated creep strain is an order of magnitude less than the deduced value. Agreement between theory and experiment can be achieved only by either abandoning the equality of the numbers of interstitials in loops and vacancies in voids or by assuming that an interstitial loop nucleus contains about 30 atoms.     

Positron annihilation characteristics of ODS and non-ODS EUROFER isochronally annealed

Yttrium oxide dispersion strengthened (ODS) and non-ODS EUROFER produced by mechanical alloying and hot isostatic pressing have been subjected to isochronal annealing up to 1523 K, and the evolution of the open-volume defects and their thermal stability have been investigated using positron lifetime and coincidence Doppler broadening (CDB) techniques. Transmission electron microscopy (TEM) observations have also been performed on the studied samples to verify the characteristics of the surviving defects after annealing at 1523 K. The CDB spectra of ODS EUROFER exhibit a characteristic signature that is attributed to positron annihilation in Ar-decorated cavities at the oxide particle/matrix interfaces. The variation of the positron annihilation parameters with the annealing temperature shows three stages: up to 623 K, between 823 and 1323 K, and above 1323 K. Three-dimensional vacancy clusters, or voids, are detected in either materials in as-HIPed condition and after annealing at T ? 623 K. In the temperature range 823-1323 K, these voids’ growth and nucleation and the growth of other new species of voids take place. Above 1323 K, some unstable cavities start to anneal out, and cavities associated to oxide particles and other small precipitates survive to annealing at 1523 K. The TEM observations and the positron annihilation results indicate that these cavities should be decorated with Ar atoms absorbed during the mechanical alloying process.     

Symmetric analog positron lifetime spectrometer utilizing charge-to-digital converters

A symmetric positron annihilation lifetime (PAL) spectrometer using a charge-to-digital converters was designed. It is based on a coincidence circuit for the trigger and recording of the timing and the charge of the pulses of two scintillation detectors. The off-line data processing allows a detailed study of the spectrometer performance in function of the width of the digital charge discriminator windows for acceptance of the pulses as starts and stops. Some technical problems when working with BaF₂ scintillators were solved. The parasitic effect of the pileup and backscattered γ-rays on the PAL spectrum is discussed.     

Positron affinity for precipitates in reactor pressure vessel steels

The sensitivity of positron annihilation spectroscopy to irradiation-induced precipitates in reactor pressure vessel steels is discussed in the light of recent positron affinity and lifetime calculations. Carbide and nitride precipitates are found to trap positrons only if they contain metal vacancies. Copper precipitates are also attractive to positrons but they are probably detected through annihilation at the precipitate-matrix interface. These findings are related to available experimental data.     

Radiation damage in a-SiO2 exposed to intense positron pulses

In addition to its numerous technological applications amorphous silica (a-SiO₂) is also well suited to the creation and study of exotic atoms such as positronium (Ps) and muonium. In particular, a dense Ps gas may be created by implanting an intense positron pulse into a porous a-SiO₂ sample. However, such positron pulses can constitute a significant dose of radiation, which may damage the sample. We have observed a reduction in the amount of Ps formed in a thin film of porous a-SiO₂ following irradiation by intense positron pulses, indicating the creation of paramagnetic centers. The data show that the primary effect of the irradiation is the inhibition of Ps formation, with no significant change in the subsequent Ps lifetime, from which we deduce that damage centers are created primarily in the bulk material and not on the internal surfaces of the pores, where they would be accessible to the long-lived Ps. We find that the damage is reversible, and that the system may be returned to its original state by heating to 700 K. The implications of these results for experiments with dense Ps in porous materials are discussed.     

Positron annihilation study and computational modeling of defect production in neutron-irradiated reactor pressure vessel steels

Positron annihilation spectroscopy (PAS) and a computer simulation were used to investigate a defect production in reactor pressure vessel (RPV) steels irradiated by neutrons. The RPV steels were irradiated at 250 °C in a high-flux advanced neutron application reactor. The PAS results showed that mainly single vacancies were created to a great extent as a result of a neutron irradiation. Formation of vacancies in the irradiated materials was also confirmed by a coincidence Doppler broadening measurement. For estimating the concentration of the point defects in the RPV steels, we applied computer simulation methods, including molecular dynamics (MD) simulation and point defect kinetics model calculation. MD simulations of displacement cascades in pure Fe were performed with a 4.7 keV primary knock-on atom to obtain the parameters related to displacement cascades. Then, we employed the point defect kinetics model to calculate the concentration of the point defects. By combining the positron trapping rate from the PAS measurement and the calculated vacancy concentrations, the trapping coefficient for the vacancies in the RPV steels was determined, which was about 0.97 × 10¹⁵ s⁻¹. The application of two techniques, PAS and computer simulation, provided complementary information on radiation-induced defect production.     

Pressure of stable He-vacancy complex in bcc iron: Molecular dynamics simulations

Molecular dynamic simulation was employed to study the stable state of He-vacancy (He-V) complex in bcc iron. The pressure of He-V complex was calculated using the concept of atomic-level stress. In the case of no initial vacancies introduced in the simulation box, self-interstitial atoms (SIAs) are emitted by the small He cluster. As the number of the He cluster is above a critical value, interstitial-type dislocation loops (I-loop) will be generated. After the interstitial-type defects (SIA or I-loop) were created, it is found that the ratio of He atoms to athermal vacancies keeps nearly constant in the He-V complex.     

First-principles study of hydrogen in perfect tungsten crystal

Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies for different interstitial sites were compared to determine the optimal trapping site for the hydrogen atom inside the tungsten lattice. The diffusion barriers for hydrogen atom between nearby trapping sites and the interaction between two interstitial hydrogen atoms were also calculated. The implication of our theoretical results on the hydrogen diffusion and accumulation behavior was discussed.     

Study of defects in implanted silica glass by depth profiling Positron Annihilation Spectroscopy

Positron Annihilation Spectroscopy (PAS) performed with continuous and pulsed positron beams allows to characterize the size of the intrinsic nano-voids in silica glass, their in depth modification after ion implantation and their decoration by implanted ions. Three complementary PAS techniques, lifetime spectroscopy (LS), Doppler broadening spectroscopy (DBS) and coincidence Doppler broadening spectroscopy (CDBS) will be illustrated by presenting, as a case study, measurements obtained on virgin and gold implanted silica glass.     

Irradiation-induced swelling and creep of metals

Conclusions The assumption of the existence in the microstructure of cold-worked material of dislocations having a preference for interstitial atoms (edge), and dislocations not having any preference (screw or mixed) can explain the observed decrease in dislocation density during the early stages of irradiation. This assumption also enables us to understand why the formation of a high dislocation density in a metal has a relatively small effect on the concentration and dimensions of interstitial dislocation loops formed in irradiation. Within the framework of the existing theory it is difficult to justify the assumption that screw dislocations are neutral sinks for point defects, since these dislocations have an edge component, and it would seem they should have a preference for interstitial atoms as pure edge dislocations do. However, the efficiency of dislocations as sinks may possibly not be determined by the drift of vacancies and interstitial atoms in elastic dislocation fields, but by a structural core of dislocations where trapping of point defects occurs. Unfortunately, the problem of the effect of a core of dislocations on the trapping of vacancies and interstitial atoms has not been adequately studied. Within the framework of the Heald and Speight mechanism of irradiation creep, relations were derived and analyzed which characterize the rates of irradiation swelling and irradiation creep in the presence of different types of dislocations, including those having a preference for interstitial atoms. It was established that taking account of neutral dislocation sinks does not change the theoretical conclusion that there is no simple linear relation between swelling and irradiation creep of a metal. The direct proportionality between swelling and creep deformation observed in certain cases can be explained by assuming that the strength of the sinks for dislocations predominates over the sink for point defects.Translated from Atomnaya Énergiya, Vol. 50, No. 1, pp. 17–21, January, 1981.     

Molecular model for annihilation rates in positron complexes

The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e⁺He (electronic triplet) and e⁺Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e⁺Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.     

Calculations of the trapping and migration of vacancies and nickel self-interstitials in the presence of rare gases and dislocations

Recent models of swelling, void growth, and solute segregation under irradiation all require knowledge of the trapping and migration of vacancies and self-interstitials in the presence of lattice defects. The present calculations include trapping of both vacancies and nickel self-interstitials to substitutional and interstitial rare gas atoms. The results show a systematic dependence on rare gas atom size. It is found for example, that a vacancy is bound to a small fixed rare gas interstitial (He) by ～0.5 eV and to a large fixed interstitial (Xe) by ≥3 eV. In addition, a fixed substitutional rare gas or rare gas interstitial is found to be a strong trap for a self-interstitial. It is found that a single vacancy can significantly affect the migration energy of another vacancy. For example, a 0.4 eV decrease in migration energy is found at a distance of three half-lattice constants. However, this interaction is of limited range; at distances greater than five half-lattice constants vacancy migration is unaffected. The migration of vacancies near the core of a partial dislocation was also investigated. This partial is found to provide a 1 eV (compared to 1.4 eV in the bulk) path for the pipe diffusion of vacancies. In addition, the activation energy for vacancy migration along the slip plane is reduced by as much as 0.2 eV.     

Calculations for very low energy scattering of positrons by molecular hydrogen

We give a progress report on ongoing calculations of phase shifts for very low energy elastic scattering of positrons by molecular hydrogen, using the generalised Kohn variational method. Further, provisional calculations of Z_eff for molecular hydrogen at low energies are presented and discussed. The preliminary nature of the work is emphasised throughout.     

Microstructure evolution of Ge implanted silicon oxide thin films upon annealing treatments

The structural evolution of silicon oxide films with Ge⁺ implantation was traced with a positron beam equipped with positron annihilation Doppler broadening and lifetime spectrometers. Results indicate that the film structure change as a function of the annealing temperature could be divided into four stages: (I) T < 300 °C; (II) 300 °C ? T ? 500 °C; (III) 600 °C ? T ? 800 °C; (IV) T ? 900 °C. In comparison with stage I, the increased positron annihilation Doppler broadening S values during stage II is ascribed to the annealing out of point defects and coalescence of intrinsic open volumes in silicon oxides. The obtained long positron lifetime and high S values without much fluctuation in stage III suggest a rather stable film structure. Further annealing above 900 °C brings about dramatic change of the film structure with Ge precipitation. Positron annihilation spectroscopy is thereby a sensitive probe for the diagnosis of microstructure variation of silicon oxide thin films with nano-precipitation.     

Positron annihilation study of neutron irradiated model alloys and of a reactor pressure vessel steel

The hardening and embrittlement of reactor pressure vessel steels are of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage, and positron annihilation spectroscopy has been shown to be a suitable method for analysing most of these defects. In this paper, this technique (both positron annihilation lifetime spectroscopy and coincidence Doppler broadening) has been used to investigate neutron irradiated model alloys, with increasing chemical complexity and a reactor pressure vessel steel. It is found that the clustering of copper takes place at the very early stages of irradiation using coincidence Doppler broadening, when this element is present in the alloy. On the other hand, considerations based on positron annihilation spectroscopy analyses suggest that the main objects causing hardening are most probably self-interstitial clusters decorated with manganese in Cu-free alloys. In low-Cu reactor pressure vessel steels and in (Fe, Mn, Ni, Cu) alloys, the main effect is still due to Cu-rich precipitates at low doses, but the role of manganese-related features becomes pre-dominant at high doses.     

Experimental bremsstrahlung yields for MeV proton bombardment of beryllium and carbon

Experimental bremsstrahlung yields for 2, 3 and 4 MeV protons on thin beryllium and carbon targets have been measured. The yields have been corrected for detector efficiency, self-absorption in the target and fitted to 9th order polynomials over the X-ray energy range 1-10 keV for easy comparison with theoretical calculations.