Experimental and theoretical studies of quasifission

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

Probing quantum many-body dynamics in nuclear systems

Quantum many-body nuclear dynamics is treated at the mean-field level with the time-dependent Hartree-Fock (TDHF) theory. Low-lying and high-lying nuclear vibrations are studied using the linear response theory. The fusion mechanism is also described fo

Dynamical approach to fusion-fission process in superheavy mass region

In order to describe heavy-ion fusion reactions around the Coulomb barrier with an actinide target nucleus, we propose a model which combines the coupled-channels approach and a fluctuation-dissipation model for dynamical calculations. This model takes i

Determination of the angular distribution of evaporation residues following transmission through the superconducting solenoidal separator SOLITAIRE

A highly efficient superconducting solenoidal fusion product separator has been developed at the Australian National University in order to enable separation and detection of evaporation residues following heavy-ion collisions. The determination of absol

Predominant Time Scales in Fission Processes in Reactions of S, Ti and Ni with W: Zeptosecond versus Attosecond

The inhibition of fusion by quasifission is crucial in limiting the formation of superheavy elements in collisions of heavy nuclei. Time scales of ∼10⊃-18s inferred for fissionlike events from recent crystal blocking measurements were interpreted to

Influence of entrance-channel magicity and isospin on quasi-fission

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the 40Ca+ 208Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of N/Z in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger N/Z asymmetries

Quasifission and Shell Effects in Reactions Forming 266Sg

The role of shell effects in reactions forming the heavy element 266Sg was investigated using the Mass Angle Distribution technique. For the 34S + 232Th reaction the doubly magic shell closure at 208Pb was found to strongly influence asymmetric quasifiss

Mapping quasifission characteristics in heavy element formation reactions

Mass-angle distributions carry detailed information on the characteristics of quasifission, and thus of the dynamics of heavy element formation reactions. Recent experimental results are presented and discussedThe authors acknowledge outstanding support from Dr. N Lobanov and the ANU Heavy Ion Accelerator Facility staff, without whose tireless commitment to excellence this work would not have been possible, and financial support from ARC grants DP110102858, DP130101569, DP140101337, FL110100098, FT120100760 and DE140100784

Mapping quasifission characteristics and timescales in heavy element formation reactions

Background: The formation of superheavy elements by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Purpose: Through extensive mass-angle distribution measurements, we map out the systematic dependence of quasifi

Investigating energy dissipation through nucleon transfer reactions

Nucleon and cluster transfer probabilities have been measured in the collisions of 16,18O, 19F with 204,208Pb, 209Bi for charge stripping channels down to ΔZ = - 3. Strong evidence of correlated nucleon transfer has been observed in particular channels, and neighbouring systems are seen to differ significantly in their behaviour. New measurements were made using an improved ΔE-E telescope. The back-scattered projectilelike fragments were measured in the telescope at θlab = 160.6°, and in combination with monitor detectors at forward angles allowed determination of absolute transfer probabilities. The improved design allows isotopic yields to be measured with greater precision