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.     

Mechanical properties of Eurofer 97 in Pb-16Li and irradiation effect

To be used in a fusion reactor, structural materials, and in particular steels, has to be selected and optimised in their composition to achieve a reduction in the long-term radioactive waste. A reduction in the long-term radioactive inventory could be reached substituting elements like molybdenum, niobium and nickel with other ones like tantalum and tungsten which have the same functions as alloying elements and, if irradiated, do not produce long lived radioisotopes. The martensitic steel belonging to the family of 8-9% Cr Eurofer 97 is considered the reference structural steel for fusion application. However, only few information are available about its mechanical properties in the liquid eutectic alloy Pb-16%Li. Particularly, the problem of liquid metal embrittlement (LME) has not been studied in detail and the effect of neutron irradiation on LME has not been investigated at all so far. This work presents the results obtained irradiating tensile specimens of Eurofer 97 up to 5.9 dpa in lead lithium. Tensile tests of samples have been performed out of pile in the same alloy at the same temperature at which irradiation was carried out.     

Radiation damage effects in the uranium-bearing δ-phase oxide Y6U1O12

Ion irradiation damage effects in delta (δ) Y₆U₁O₁₂ were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Experimental results revealed no amorphization transformation occurs in Kr-ion irradiated Y₆U₁O₁₂ to a maximum displacement damage dose of ∼50 displacements per atom at cryogenic temperature. Density functional theory calculations indicate that δ-Y₆U₁O₁₂ possesses a relatively low cation antisite formation energy, which may help to explain the observed resistance of δ-Y₆U₁O₁₂ to irradiation-induced amorphization of δ-Y₆U₁O₁₂.     

Confinement of motion of interstitial clusters and dislocation loops in BCC Fe-Cr alloys

This work is devoted to the study of the effect of Cr solutes on the mobility of self interstitial atom (SIA) clusters and small interstitial dislocation loops (of size up to a few nanometers) in concentrated Fe-Cr alloys. Atomistic simulations have been performed to characterize the variation of the free energy of interstitial loops in the Fe-15Cr alloy using the experimentally determined profile of Cr distribution along the path of a loop. It is shown that the presence of randomly distributed Cr in Fe leads to the creation of local trapping configurations for small SIA clusters. The strength (trapping energy) and density of these configurations depend on the Cr content. On the contrary, large SIA clusters (which can be described as 1/2〈1 1 1〉 dislocation loops) are strongly affected by the presence Cr-Cr pairs and larger Cr clusters, which act as barriers to their motion.     

Secondary electron yield of Au and Al2O3 surfaces from swift heavy ion impact in the 2.5-7.9 MeV/amu energy range

We report on the secondary electron yields of Au and oxidized aluminum (Al₂O₃) by impact of heavy ions with energies ranging from 7.92 MeV/amu (¹²C₆) to 2.54 MeV/amu (¹⁰⁷Ag₄₇). The obtained results, the first in this energy range using medium-heavy ions, extend the validity of proposed scaling laws obtained with lighter ions. Measurements have been performed using the SIRAD irradiation facility at the 15 MV Tandem of the INFN Laboratory of Legnaro (Italy), to evaluate the performance of ion electron emission microscopy at SIRAD.     

Dosimetric calibration of solid state detectors with low energy β sources

A PTW Optidos plastic scintillation and a PTW natural diamond detectors were calibrated in terms of absorbed dose to water with β fields produced by ⁹⁰Sr + ⁹⁰Y and ⁸⁵Kr reference sources. Each source was characterized at the Italian National Metrological Institute - the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti of ENEA (ENEA-INMRI) - for two different series, 1 and 2, of ISO reference β-particle radiation fields. Beam flattening filters were used for the series 1 β fields to give uniform absorbed dose rates over a large area at a source-to-reference plane distance of 30 cm. The series 2 β fields were produced at source-to-reference plane distance of 10 cm, without the beam flattening filters, in order to obtain higher absorbed dose rates.The reference absorbed dose rate values were directly determined by the Italian national standard for β-particle dosimetry (a PTW extrapolation ionization chamber) for the series 1 β fields and by a calibrated transfer standard chamber, (a Capintec thin fixed-volume parallel plate ionization chamber) for the series 2 β fields. Finally the two solid state detectors were calibrated in terms of absorbed dose to water with the series 2 β field.The expanded uncertainties of the calibration coefficients obtained for the plastic scintillation dosimeter were 10% and 12% (2SD) for the ⁹⁰Sr + ⁹⁰Y and the ⁸⁵Kr sources, respectively. The expanded uncertainties obtained for the diamond dosimeter were 10% (2SD) and 16% (2SD) for the ⁹⁰Sr + ⁹⁰Y and the ⁸⁵Kr sources, respectively.The good results obtained with the ⁹⁰Sr + ⁹⁰Y and the ⁸⁵Kr β sources encourage to implement this procedure to calibrate this type of detectors at shorter distances and with other β sources of interest in brachytherapy, for example the ¹⁰⁶Ru source.     

Measurement of specimen thickness by phase change determination in TEM

A non-destructive method for measuring the thickness of thin amorphous films composed of light elements has been developed. The method employs the statistics of the phase of the electron exit wave function. The accuracy of this method has been checked numerically by the multislice method and compared with that based on the mean inner potential.     

Bright-field TEM imaging of single molecules: Dream or near future?

We examine the suitability of spherical aberration (C(S))-corrected (CS) and uncorrected (UC) transmission electron microscopes (TEM) for conventional bright-field imaging of radiation-sensitive materials. We have chosen an individual molecule suspended in vacuum as a hypothetical example of a well-defined radiation-sensitive sample. We find that for this particular sample, CS instruments provide about 30% improvement over an UC instrument in terms of signal/noise ratio per unit electron dose at 300kV. The lowest imaging doses can be achieved in CS instruments equipped with high-brightness electron source operated at low incident electron energies. Our calculations suggest that it may be possible to image individual, iodine- or bromine-substituted organic molecules in bright-field mode, at doses lower than the accepted values for radiation damage of aromatic molecules.     

Column-by-column compositional mapping by Z-contrast imaging

A phenomenological method is developed to determine the composition of materials, with atomic column resolution, by analysis of integrated intensities of aberration-corrected Z-contrast scanning transmission electron microscopy images. The method is exemplified for InAs_xP_1−x alloys using epitaxial thin films with calibrated compositions as standards. Using this approach we have determined the composition of the two-dimensional wetting layer formed between self-assembled InAs quantum wires on InP(0 0 1) substrates.