Nuclear fusion is one of the most dramatic physical processes in the universe -- from the fusion of light nuclei that power the stars, through to the fusion of heavy nuclei exploited at laboratories around the world as physicists seek to add new elements to the periodic table. Our understanding of both extremes are continually tested as experimental techniques and theoretical predictions develop and challenge each other, both working towards a fundamental understanding of this complex process. 

Precise measurements of the fusion cross section of heavy ions above and below the potential barrier have revealed limitations of the standard coupled-channel model of fusion. For reactions involving nuclei with atomic number Z>8, suppression of experimental cross sections is found relative to coupled-channels predictions.
At above-barrier energies, the magnitude of the disagreement is correlated with charge product of the colliding nuclei. This is correlated with nuclear matter overlap at the average barrier radius, suggesting this is a key quantity contributing to the disagreement. While the dissipation of the collision energy into internal excitations is necessary for fusion, this process is not explicitly treated within the coupled channels model, which models the collision as a coherent superposition of quantum states, and mocks up the irreversible dissipation process leading to fusion by applying an incoming wave boundary inside the barrier.

In this thesis I have sought to examine the behaviour of above-barrier fusion suppression using cross-bombardment reactions forming the compound nucleus 220Th. In order to isolate and probe the fusion characteristics, I have measured xn evaporation residues, which are an unambiguous signature of fusion/compound nucleus formation. These measurements were supplemented with fission measurements, which include both fusion-fission, arising from compound nucleus formation, and the faster quasifission. Whilst both may have similar characteristics, quasifission results from nonequilibrium reactions occurring on timescales shorter than compound nucleus formation. 

Both evaporation residue and fission measurements were performed at the Australian National University, using the 14UD tandem accelerator at the Heavy Ion Accelerator Facility. A new gas-filled solenoidal separator with an exceptionally high efficiency (>90%) was used for the evaporation residue measurements. As part of my thesis, I devised and refined a method for accurately characterising the efficiency of this device, which shows promise for use in future cross section measurements. The fission measurements were made with the fission spectrometer CUBE, which is well supported by continually refined analysis codes and experimental techniques. Both experimental methods have provided precise cross section measurements.

Following this experimental investigation, I have found that as the charge product of the colliding nuclei increases, compound nucleus formation is suppressed. Evolution of the nonequilibrium processes is seen in the fission characteristics. The absence of a mass-angle correlation in the fission characteristics for reactions induced by 28Si and 34S projectiles, despite significant suppression of the xn-evaporation residue yield, is new evidence that suggests nonequilibrium process are competing with compound nucleus formation on longer timescales than previously assumed.