Applications of positron annihilation spectroscopy in nuclear materials research:Dissertation

In this work, positron annihilation spectroscopy was used in studying lattice point defects in some of the technologically relevant nuclear materials. In addition, the behaviour of the detectors were studied to deepen understanding of their influential properties in lifetime and Doppler broadening spectroscopy. Detectors play a significant role in the performance of a measurement setup and in the quality of the obtained results, both in lifetime and Doppler broadening spectroscopy. Lifetime spectroscopy is very sensitive to the excess activity of studied samples, which limits its use in the study of nuclear materials. The only way of improving the tolerance of lifetime spectrometry is to add an extra coincidence detector to the setup. In Doppler broadening spectroscopy a detector energy resolution function directly affects the obtained results. The sensitivity of two-detector lifetime spectroscopy to false coincidence events was studied by adding a cobalt-60 source next to the studied Si reference samples. The improvement of the radiation tolerance in a three-detector setup was estimated based on simple theoretical models. To study energy resolution function in Doppler broadening spectroscopy, a sodium-22 positron source was directly attached to the studied samples enabling simultaneous recordings of Doppler broadening and discrete photoabsorption spectrum. The impact of the source-detector geometry on the obtained results was studied and the reason behind the changing results is discussed based on systematic experiments supported by Monte Carlo simulations. Microstructural defects play an important role in the properties of nuclear materials, such as corrosion and irradiation resistivity. Zircaloy-4 is currently the most used cladding alloy in nuclear power plants, but its oxidation in reactor conditions is a complex process including cyclic oxidation rates. In fusion power plants, materials close to the plasma are exposed to a very hostile environment including high temperatures and high particle irradiation fluxes. Tungsten is considered a promising candidate material for this purpose. The fundamental oxidation properties of zirconium are discussed based on microstructural defect evolution in the studied Zircaloy-4 samples that were oxidized in pressurized water reactor -type conditions. Doppler broadening spectroscopy supported by theoretical modelling was harnessed to characterize lattice point defect behaviour in the sample oxide layers. Positron lifetime spectroscopy was applied in the study of mono-vacancies in pre-annealed proton-irradiated tungsten samples. Migration barriers for interstitial atoms and mono-vacancies were directly detected with a positron lifetime spectrometer connected to the cold temperature irradiation facility