Investigation of energy dissipation and capture through fission measurements

Abstract

Fusion of atomic nuclei is a process in which two nuclei overcome their mutual Coulomb repulsion and merge together to form a single nucleus. Although the concept of nuclear fusion has been known for almost a century, only the more recent development of precision experimental measurements and new theoretical ideas demonstrated how essential it is to consider quantum superposition effects.

It is now recognised that fusion cross sections at energies below and close to the barrier can only be understood if the colliding nuclei are considered to be in a coherent superposition of their internal states. The coupled channels model, which considers the coupling between the internal structure and the relative motion of the two colliding nuclei, has emerged as the most successful model of fusion. However, experimental measurements of fusion cross sections at energies above the barrier consistently fall below model predictions, the deviation appearing to increase with increasing charge product. These observations have led to a closer scrutiny of the way fusion is modelled.

Currently, the colliding nuclei are considered to remain in coherent superposition right until the separation inside the barrier where fusion is simulated by imposing a boundary condition by imposing the incoming wave boundary condition. Thus the dissipation of kinetic energy that leads to fusion is not modelled explicitly. The question is whether energy dissipation starts even at larger separations, which can lead to some of the kinetic energy being lost well before separations at which fusion is simulated in the model. Such processes can potentially reduce fusion as the colliding nuclei will have less kinetic energy at the barrier. In this work, factors leading to the reduction of fusion in heavy ion collisions are explored, in particular, focus on the role energy dissipation plays in such reaction outcomes.

To probe energy dissipative processes, a series of experiments were conducted using the 14UD tandem accelerator and superconducting LINAC at the Heavy Ion Accelerator facility at the Australian National University. In these experiments, a range of projectiles bombarded fissile targets at various beam energies. Reaction outcomes were detected in coincidence using the CUBE binary fission spectrometer. A systematic study of transfer-fission, fusion-fission and quasifission was carried out through the analysis of each reaction's kinematic properties and timescales. Additionally, a new method for determining quasifission cross sections was developed and used to extract cross sections.

In this work, transfer-fission has been used as a proxy for energy dissipative processes since fission following transfer can only occur if the system formed following transfer has an excitation energy higher than the fission barrier. Transfer-fission was found to increase relative to total fission as a function of charge product. For each reaction, the proportion decreased as a function of energy relative to the capture barrier energy. It is suggested that fusion is suppressed by transfer reactions as the charge product increases, and that self-consistent models of fusion need to properly take into account energy dissipation even outside the barrier in order to accurately predict fusion probabilities for heavy ion reactions.