Microscopic approach to heavy-ion reactions

Abstract

Heavy-ion reactions are affected by the nuclear structure of the reactants and other dynamical processes during the collision. Theoretical studies of such reactions lead to a clearer understanding of prohibitive mechanisms to heavy-ion fusion. One application of this is superheavy element formation, where quasifission is the major competing process to the formation of a compound nucleus in this region of the nuclear chart.

Microscopic approaches that use mean field approximations, such as the time-dependent Hartree-Fock (TDHF) theory, have been increasingly used to study low energy heavy-ion reactions. In addition to reactions, nuclear structure properties can also be studied with microscopic theory. Such approaches do not require empirical knowledge of the nucleus and can be used as a predictive tool for studying structure or reactions involving exotic nuclei. In this thesis, TDHF is used to study nuclear vibrations and heavy-ion reactions at near barrier energies.

The TDHF approach is applied first to a systematic study of low-lying octupole and quadrupole vibrational modes of {40-54}Ca isotopes. Then, fusion reactions are calcuated for Ca+Sn systems, using the barrier energy as the main observable. Fusion reactions are first calculated with no couplings (static HF calculations) before including all dynamics that the mean field approximation offers (TDHF calculations). The addition of dynamics often results in a lower fusion barrier compared to the static barrier, with the exception of systems involving the neutron rich {52,54}Ca projectiles. The difference in fusion barriers between the two sets of calculations are explored by considering couplings to vibrational states and transfer channels. To separate the effects of individual couplings, the coupled-channels approach is used with the HF calculations of the vibrational states as inputs. Such couplings account for most of the lowering of the static barrier. For the most neutron rich systems, coupling to vibrational states does not account for the dynamical barrier increase. Transfer channels, in particular proton pickup, are explored within TDHF and may play a role in this increased barrier energy.

Following from fusion reactions, the factors and dynamics that may hinder fusion in heavier systems are studied in the reactions 48Ca, 50Ti, 52Cr, 54Fe, 56,64Ni, 58Zn+208Pb at energies ranging from 0% to 30% above the static HF barrier. Although most of these systems fuse at energies within this range, some systems reseparate for which emission of nucleons was apparent at scission point. The contact times were in most cases less than 10 zs and the attempt at $N/Z$ equilibration of these systems happen on fast time scales (1-2 zs). Larger amounts of mass transfer occur in systems with longer neck times and fewer magic numbers such as 34S+232Th and Cr+Pt systems that were also calculated for comparison. Further studies are required to better understand the role of shell effects as well as nucleon emission in quasifission reactions, and subsequently reactions leading to superheavy element formation. Show more