Experimental and Theoretical Studies of Quasifission

The quest to synthesise super heavy 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 j the
syste1n re-separates pren1aturely into two heavy fragments; intermediate in mass con1pared
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 experin1entalists and theoreticians~ The dynamic evolution of the dicnulear system is governed by several degrees of freedom; fluctuations and
quantun1 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 elementsi thus currently relies on empirical systen1atics. To aid in the de, elopment of a complete self-consistent; realistic and tractable
1nodel 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 23 U and 232Th were studied experi1nentally. Six reactions were studied in pairs forming
he same compound nucleus while the two heaviest reactions were betv\ een projectiles of
4°Ca and targets of 23 U and 232Th. For. he heaviest reaction (4°Ca + 238U) a detailed theoretical stud was also conducted.
The experimental part of this thesis presents a detailed analysis of the binary fission
e ents from these reactions. The large angular co erage of the CUBE fission spectrometer was used to obtain wide-ranging ma s-angle distributions for each reaction i at energies spanning the Coulon1b barrier. The results point to the role of shell effects around 20 Pb in
the n1ass-asyn11netric quasifission exit channel, the pr sence of n1ass-sy1nmetric quasifission
and t he evolution of the balance between quasifission and fusion with increasing ZpZy .
The theoretical part of this thesis examined the 4
°Ca + 23 U reaction within the Ti1ne
Dependent Hartree Fock (TDHF) 1nodel, using the TDHF3D code. This is the first time
that the TDHF approach has b een used to extensively study quasifission. The results
revealed that the orientation of the heavy deformed prolate nucleus plays a 1najor role in
the reaction outcon1e, in agreement with experi1nent. It was found that aligned collisions
lead to quasifission and short contact tin1es 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 n1ass splits in this reaction. The influence of shell effects around 208Pb
in the calculated quasifission characteristics was confirmed by an analysis of the neutron
and proton numbers of the outgoing fr agments.
These findings are a pron1ising step towards t he forn1ulation of a consistent theoretical
picture of nuclear reaction dynan1ics of heavy syste1ns.