Developing the recoil-in-vacuum technique for magnetic moment measurements on fission fragments and radioactive beams

Abstract

This thesis reports on work to develop and refine the recoil in vacuum (RIV) technique as a method to measure the g factors for picosecond lifetime excited nuclear states. The challenge has been to calibrate the hyperfine interactions of free ions moving in vacuum, so that their influence on the angular correlations of the radiation de-exciting the nuclear state can be interpreted to extract its magnetic moments. The RIV interaction was characterised, calibrated and modelled on stable isotopes, with a view to using the resulting improved understanding to measure the magnetic moments of quantum states in isotopes away from the valley of stability. Magnetic moment measurements illuminate the internal structure of the nucleus and are an important observable to understand nuclear structure away from the valley of stability. This research at the ANU has involved work with the even, stable isotopes of germanium, selenium, ruthenium and palladium at energies of the order of 2MeV/nucleon. These four elements were chosen for study because they have atomic numbers in regions where fission fragment distributions peak. Therefore they provide useful RIV calibrations for future radioactive beam measurements in which fission fragments are re-accelerated. The stable isotope beams were Coulomb excited in inverse kinematics on carbon, aluminium, and nickel targets. The beam energies and target thicknesses were chosen to provide a range of velocity values which would result in different distributions of charge states in the recoiling ions. The work demonstrated that understandings of both the charge state distributions and atomic structure of the recoiling ions is crucial. Show more