Influence of Charge Asymmetry in Heavy Ion Collisions at Intermediate Energies

Abstract

In heavy-ion collisions, lots of effort have been made to understand the properties of stable nuclei. With the development of radioactive beam facilities, the study of isospin degree of freedom has gained a major boost. At intermediate energies, both attractive mean field and nucleon-nucleon collisions play an important role. Both these regimes together, lead the matter from a fused state to total disassembly. Therefore, one is also interested to understand the mechanism behind this. This mechanism has motivated the nuclear physics community to study the hot and dense nuclear matter under extreme conditions of density and temperature in heavy-ion collisions at intermediate energies. This energy regime offers an excellent opportunity to study various phenomena like multifragmentation, collective flow and nuclear stopping. However, the phenomena at higher incident energy are characterized by the production of sub-threshold particles and their isospin dependence. Our aim in this thesis to study the effect of isospin asymmetry (charge asymmetry) and isospin dependent nucleon nucleon cross section on fragment production, collective flow as well as on nuclear stopping. The term charge asymmetry means the collision of nuclei in which neutron number is not equal to the proton number. Here we fix the mass of the colliding nuclei but varied their proton number (isobaric nuclei). Firstly, we have focused on multifragmentation, which constitutes fragments of different charge/mass. In the present work, we study the isospin effects on the multiplicity of free nucleons (FN’s) [Af = 1], light mass fragments (LMF’s) [2 ≤ Af ≤ 4] and intermediate mass fragments (IMF’s) [5 ≤ Af ≤ Atot/6] for isospin asymmetric nuclei. We find that the multiplicity of FN’s, LMF’s and IMF’s depends more on the mass of the colliding nuclei compared to N/Z of the system. We also find that, the isospin dependence of nucleon-nucleon cross-section shows a small influence on the multiplicity of FN’s, LMF’s and IMF’s with incident energy. Moreover, we observe a uniform effect of σiso (σnp = 3σnn = 3σpp) and σnoiso (σnp = σnn = σpp) on the N/Z dependence of the multiplicity of FN’s and LMF’s. Our theoretical calculations follow the similar trend as given by the experimental findings of ALADiN collaboration. Moreover, the multiplicity of IMF’s is not influenced by the two forms of cross-section discussed here. The another phenomena linked with the interplay between the attractive mean field and repulsive nucleon-nucleon scattering is the global stopping of nuclear matter. Therefore, as a next step, we study the isospin effects on nuclear stopping for isobaric nuclei. We have discussed the isospin dependence of nucleon-nucleon cross-section on nuclear stopping. We find that, like multifragmentation even in nuclear stopping, small influence is observed due to σiso and σnoiso. Participant (spectator) matter decreases (increases) with increase in the neutron content of the system whatever be the definition of the participant matter. The variation of the slope of hparticipantinorm (normalized participant matter) with incident energy gives an indication about the global equilibrium of the system and the variation of the slope of hspectatorinorm (normalized spectator matter) with incident energy gives an indication about the local equilibrium of the system. The isospin content of the colliding nuclei has again a small influence on hRi and h1/Qzzi. Moreover, our theoretical calculations follow the similar trend as the experimental findings of the INDRA collaboration. We also investigate the reasons of failure by earlier theoretical attempts made in this direction, and we find that in addition to the choice of centrality, strength of nucleon-nucleon cross-section has a major role to play in reaction dynamics. Additional promising observable for the understanding of isospin asymmetry in the momentum distribution includes the directed, elliptical, triangular and quadrupole flow. The absolute value of flow results from the above interplay. This interplay is responsible for the transition from a fused state to total disassembly. In the present study, we conclude that the directed flow of both neutrons and protons is affected by the isospin dependence of nucleon-nucleon cross-section. Isospin dependent cross-section (σiso) is found to be more compatible with the experimental data. Although, it has been proven experimentally that, fragment charge/mass does not influence the balance energy. We have proved this theoretically for the first time using Isospin dependent Quantum Molecular Dynamics (IQMD) model. In case of elliptical flow, we have studied that, the distribution of nucleons and fragments is not symmetric around the beam axis. Moreover, the elliptical flow depends on the isospin dependence of nucleon-nucleon cross-section. Transition energy is however not affected by the change in isotopic content of the system. Our theoretical calculations of the excitation function of elliptical flow are able to reproduce the experimental findings of GSI collaboration. Higher order anisotropic flows are more sensitive to the isotopic content and the isospin dependence of the nucleon-nucleon cross-section,indicating the strong dependence of these observables on the initial configuration of the system. Isospin effects have been found to be due to the interplay between Coulomb potential, symmetry energy, nucleon-nucleon cross-section. To shed light on the relative importance of above mentioned observables, we studied the effect of isospin degree of freedom on nuclear stopping throughout the mass range between 50 and 350 for two sets of isotopic systems as well as isobaric systems. Analysis is carried out at incident energy below, at and above the energy of vanishing flow (EVF). Our findings reveal that nuclear stopping does not show any particular behavior at EVF. Moreover, system size effects dominate the isospin effects throughout the range of colliding geometry. The Coulomb effect becomes important at higher colliding geometry. The comparative study of counterbalancing of Coulomb and mean field by removing nucleon-nucleon collisions clearly indicates the dominance of nucleon-nucleon cross-section over the repulsive Coulomb potential. Finally, we conclude that, collective flow can act as a better tool compared to multifragmentation and nuclear stopping to study the reaction dynamics involving the nuclei having isospin asymmetry.