Examining equilibration in heavy ion fusion using precision cross section measurements

Abstract

Nuclear fusion is one of the most dramatic physical processes in the universe - from the fusion of light nuclei that powers 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 is 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 at energies both above and below the potential barrier have revealed limitations of our understanding of capture and the subsequent matter, energy and shape equilibration that defines compound nucleus formation (a process synonymous with fusion). For reactions involving nuclei with atomic number Z > 8, suppression of experimental capture cross sections is found relative to predictions, with the magnitude of the disagreement correlated with charge product of the colliding nuclei. Fusion cross sections appear to exhibit an additional suppression as charge product increases, suggesting that the process of equilibration following capture is further inhibited. 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. These measurements were supplemented with fission measurements, which include both fusion-fission, arising from compound nucleus formation, and quasifission. Whilst both may have similar characteristics, quasifission results from nonequilibrium reactions occurring on timescales shorter than compound nucleus formation and thus provide a probe of the equilibration process. 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 (>80%) was used for the evaporation residue measurements. I devised and refined a method for accurately characterising the efficiency of this device, which shows promise for use in future cross section measurements. Specific evaporation residue channels were identified and measured with an implantation-decay technique. 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 found that as the charge product of the colliding nuclei increases, compound nucleus formation is exponentially suppressed. The trend is in contrast to the near-linear suppression of capture, confirming that the suppression of fusion results predominantly from the suppression of compound nucleus formation, not capture suppression. 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 processes are competing with compound nucleus formation on longer timescales than previously assumed. Show more