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Jahn–Teller (JT) distortion of the diamagnetic neutral divacancy (V20) in Si is studied by positron annihilation spectroscopy. Based on the anisotropy of electron momentum distribution around the divacancies obtained by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) technique, the preferred alignment of the divacancy in a specimen by compressing uniaxial stress along the [0 1 1] axis is detected. The direction of the alignment evidences the pairing JT distortion rather than the resonant bonding distortion, which is similar to the distortion in the singly negative divacancy (V2−). [Copyright &y& Elsevier]

Abstract: We present a microstructural study of a surveillance test specimen from a reactor pressure vessel steel of a Russian-type nuclear reactor after neutron irradiation, post-irradiation annealing and re-irradiation, using atom probe tomography (APT) and positron annihilation spectroscopy (PAS). The APT results showed the formation of Cu-rich solute nano-clusters (CRCs) during the initial irradiation and their subsequent coarsening during annealing. After re-irradiation, new CRCs have been observed. The irradiation-hardening almost recovered during annealing. However, by re-irradiation, hardening comparable to that by the initial irradiation was observed. The hardening due to the CRCs formed during the initial irradiation, estimated using the Russell–Brown model, was almost the same as that observed. However, the estimated hardening after the re-irradiation was about half of the measured one. The other hardening is attributed to the newly formed MDs by the re-irradiation, which was evidenced by PAS. [Copyright &y& Elsevier]