Doctoral Dissertation

15.6.2018 M.Sc. Ilkka Pohjalainen (Facul­ty of Mat­hema­tics and Science, Physics)

M.Sc. Ilkka Pohjalainen defends his doctoral dissertation in Physics "Gas-phase chemistry, recoil source characterization and in-gas-cell resonance laser ionization of actinides at IGISOL".

Opponent Professor Dave Morrissey (Michigan State University, USA) and Custos Professor Iain D. Moore (University of Jyväskylä). The doctoral dissertation is held in English.

Abstract: The underlying theme of this thesis focuses on buffer gas purification and relevant gas-phase ion chemistry which critically affects ion beam purity at gas cell-based radioactive ion beam facilities. The achievement of attaining a sub-parts-per-billion level of impurity at the IGISOL facility has enabled subsequent gas cell developments for production of the actinide elements, plutonium and thorium, required for a programme of high-resolution optical spectroscopy. Firstly, the construction and characterization of the new IGISOL buffer gas purification system is presented. Off-line ion beam production of plutonium and thorium using in-gas-cell laser resonance ionization combined with filament dispensers has resulted in successful collinear laser spectroscopy of several long-lived plutonium isotopes and has revealed unique collisional phenomena significantly affecting resonance laser ionization in gaseous environments.

Thorium is of particular interest due to 229Th and its low-energy nuclear isomeric state. By stopping 229Th recoils from the alpha decay of 233U in a helium-filled gas cell, a 229Th ground and isomeric state ion beam can be produced. The recoil efficiency determination of two 233U sources using direct and implantation foil gamma- and alpha-ray spectroscopy as well as surface characterization by Rutherford back scattering (RBS) measurements have shown the importance of good source quality. The development of a new gas cell to house several such recoil sources is also presented, emphasizing the interplay between gas pressure, size of the gas cell, and diffusion losses during extraction. Finally, this thesis presents the first on-line experiments for the production of 229Th via proton-induced fusion-evaporation reactions using a 232Th target.

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Ilkka Pohjalainen

Department of Physics

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