A Numerical Model of Atomic Relaxation and its Applications

Abstract

Atomic relaxation is important in many diverse disciplines, from atomic and nuclear physics to medical physics. A Monte Carlo model of the atomic relaxation is presented in this thesis. Atomic transition probabilities are taken from the Evaluated Atomic Data Li- brary (EADL). Transition energies are calculated using the relativistic self-consistent-field Dirac-Fock method for any specified distribution of atomic vacancies. This CPU-intensive computational approach is adopted to account for the effect of spectator vacancies resulting from non-radiative transitions that occur during the atomic relaxation. The shake theory from Carlson and Nestor is adopted to account for simultaneous ionisation of two electrons in the near closed-shell atomic systems. The model has been extensively benchmarked against the best calculations and available experimental data. The model is applicable to the atomic relaxation resulting from inner-shell ionisation for a wide range of atomic numbers, with particular emphasis on radioactive atoms. In addition, the model can be used to study the femtosecond evolution of the atomic relaxation. A novel technique, which combines the atomic-relaxation modelling and a Bayesian statistical approach, has been developed to analyse the perturbation in the charge-state distribution of fission fragments due to the decays of nanosecond isomers. This study is, to the author’s knowledge, the first theoretical attempt to study the effect of isomeric decays on the charge distributions of fission fragments. This technique has been demonstrated to be applicable for the extraction of isomeric ratios in fission fragments. Furthermore, this study has laid the groundwork to hunt for unknown nanosecond isomers that are low-lying excited states of fission fragments. Finally, the impact of adopting a rigorous approach to assess transition energies in the atomic-relaxation modelling on dosimetric evaluation of Auger-electron emitting medical radioisotopes, is also investigated. This study showed that the sub-micrometre dimension is where a realistic Auger-energy spectrum is essential for radiobiological studies of Auger- emitting radioactive or irradiated atoms. Therefore, the Monte Carlo model developed for this thesis can provide significantly improved energy spectra to existing radiobiological models for better research outcomes.