ASPECTS IN BREAK-UP REACTIONS

Abstract

Break-up reactions offer many opportunities to study the structure and reactions of exotic nuclei. In this thesis we use the theoretical formalism of elastic Coulomb break-up reactions to study medium mass exotic nuclei and also applications in nuclear astrophysics. This work is divided into two parts consisting of five chapters and two appendices. The first three chapters, which form the part I, contain the introduction, a brief theoretical formalism of break-up reactions leading to the distorted wave Born approximation and applications of our theory to 37Mg. The last two chapters contain the application of our theory as an indirect method to calculate radiative capture reaction cross-sections in nuclear astrophysics and the summary and future outlook of our work. Along with two appendices this forms the part II of the thesis. In chapter 1 we give an introduction to the field of exotic nuclei elaborating on their general characteristics and also the fact the field has moved to the medium mass region of the nuclear chart, where nuclei can also be deformed. We also briefly discuss the experimental facilities where experiments with exotic nuclei are being performed. The role of radiative capture reactions in nuclear astrophysics is also discussed here. In chapter 2 we describe the basic theoretical formalism of break-up reactions i ii leading to the post and prior forms of the transition matrix. We then focus on the distorted wave Born approximation for the elastic break-up of a two-body projectile in the field of a target. Specializing to pure Coulomb break-up and more specifically to the case where we have an uncharged fragment in the final channel, we show that the transition matrix can be split into two parts - a dynamics part which can be evaluated analytically and a structure part where the ground state wavefunction of the projectile is an input. This theory is further developed in the next chapter when the break-up of a deformed projectile is studied in a semi-analytic way. In chapter 3 we study the elastic Coulomb break-up of 37Mg on a Pb target at 244 MeV/nucleon beam energy. By calculating several reaction observables like the total one-neutron removal cross-section, the neutron-core relative energy spectrum, the parallel momentum distribution of the core fragment, the valence neutron angular, and energy-angular distributions, we try to set limits on the ground state spin-parity and one neutron separation energy of 37Mg. In chapter 4 we study the 15N(n, γ)16N radiative capture cross-section and its subsequent reaction rate by an indirect method and in that process investigate the effects of spectroscopic factors of different levels of 16N to the cross-section. We calculate the Coulomb break-up of 16N on Pb at 100 MeV/nucleon and relate this to the photodisintegration cross-section of 16N(γ, n)15N. Subsequently invoking the principle of detailed balance, the 15N(n, γ)16N capture cross-section is calculated. The calculated reaction rate is also compared with various other charged particle rates and consequences of the same as a function of temperature in stellar environments are discussed. The summary and future directions of our work are presented in chapter 5. Some mathematical details of the formalism and of the approximations are also given in various appendices.