Experimental and theoretical studies of quasifission

Abstract

The quest to synthesise superheavy elements is at the frontier of nuclear physics research. These elements can only be formed by the fusion of two heavy nuclei. The repulsive electrostatic energy between such nuclei is extremely large and more often than not, the system re-separates prematurely into two heavy fragments, intermediate in mass compared to the original nuclei. This non-equilibrium process is called quasifission. Only occasionally does fusion occur resulting in the formation of a compound nucleus. Finding the variables determining the competition between quasifission and fusion is a problem currently challenging experimentalists and theoreticians. The dynamic evolution of the dicnulear system is governed by several degrees of freedom, fluctuations and quantum properties. A self consistent and reliable calculation of the competition between quasifission and fusion is beyond current theoretical capabilities. Prediction of the most favorable reactions to form superheavy elements, thus currently relies on empirical systematics. To aid in the development of a complete, self-consistent, realistic and tractable model, it is important to determine which degrees of freedom are critical in quasifission dynamics and what is the dynamical nature of quasifission. This thesis addresses this problem by studying reactions forming heavy and superheavy elements using experimental and theoretical methods. In total eight reactions with targets of U-238 and Th-232 were studied experimentally. Six reactions were studied in pairs forming the same compound nucleus while the two heaviest reactions were between projectiles of Ca-40 and targets of U-238 and Th-232. For the heaviest reaction (Ca-40 + U-238) a detailed theoretical study was also conducted. The experimental part of this thesis presents a detailed analysis of the binary fission events from these reactions. The large angular coverage of the CUBE fission spectrometer was used to obtain wide-ranging mass-angle distributions for each reaction, at energies spanning the Coulomb barrier. The results point to the role of shell effects around Pb-208 in the mass-asymmetric quasifission exit channel, the presence of mass-symmetric quasifission and the evolution of the balance between quasifission and fusion with increasing Zp*Zt. The theoretical part of this thesis examined the Ca-40 + U-238 reaction within the Time-Dependent Hartree-Fock (TDHF) model, using the TDHF3D code. This is the first time that the TDHF approach has been used to extensively study quasifission. The results revealed that the orientation of the heavy deformed prolate nucleus plays a major role in the reaction outcome, in agreement with experiment. It was found that aligned collisions lead to quasifission and short contact times of 5-10 zs, whilst anti-aligned collisions lead to longer contact times (> 23 zs). TDHF accurately predicted the presence of quasifission and the average mass splits in this reaction. The influence of shell effects around Pb-208 in the calculated quasifission characteristics was confirmed by an analysis of the neutron and proton numbers of the outgoing fragments. These findings are a promising step towards the formulation of a consistent theoretical picture of nuclear reaction dynamics of heavy systems. Show more