Setaro, Pia Antonella (2025) Toward the Island of Stability: Study of Fission Process in 263Bh and 269Bh super-heavy nuclei. [Tesi di dottorato]

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Abstract

The aim of this work is to investigate the fission process in the transitional region of the nuclear chart, with the goal of progressively approaching the predicted island of stability through both an improved theoretical description and the systematic analysis of newly available experimental data. Two Bohrium isotopes, 263Bh and 269Bh, located between the heavy actinides and the superheavy nuclei near the island of stability, were studied through their most relevant observables: the mass and total kinetic energy distributions of the primary binary fragments, reconstructed from measured flight times and positions using a two-arm time-of-flight spectrometer. To interpret these data and to achieve a more comprehensive understanding of the fission process, the original Pashkevich routine implemented in the mymash code was extended to build a consistent theoretical framework suitable for describing the fission decay of superheavy compound nuclei formed in heavy-ion–induced reactions, i.e. systems produced at moderate excitation energies and angular momentum. The resulting model, referred to as SAF (Stationary Approach to Fission), incorporates both the damping of shell effects with increasing excitation energy and the rotational contribution associated with angular momentum. The SAF model was first validated through the re-analysis of the medium-fissility system 132Ce, produced in the reaction 32S +100Mo at high excitation energy and angular momentum. This nucleus provides an ideal benchmark, as its fission channel is not hindered by quasi-fission and requires a proper treatment of both shell damping and rotational energy. The good agreement obtained for 132Ce supports extending the approach to the transitional region. The subsequent analysis focuses on the evolution of fission modes in 263Bh and 269Bh, with the aim of determining whether the transition between elongated asymmetric and compact symmetric modes persists in these isotopes and whether it is consistent with the experimental observations. A major difficulty in this mass region arises from the overlap between fusion–fission and quasi-fission processes. One of the objectives of this work is therefore to exploit the SAF calculations to constrain the fusion–fission component in the experimental distributions, thereby enabling a more reliable interpretation of the data.

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