Nuclear level density

One of the characteristic properties of a nucleus is the nuclear level (state) density (number of levels (states) per unit energy interval) as a function of excitation energy. A level corresponds to several degenerate states.;Theoretically, the calculation of level density starts by defining the Hamiltonian of the system and calculating the eigenvalues and eigenfunctions. However, because of the computational limitations of this shell model approach, level density has been traditionally calculated assuming non-interacting particles. This non-interacting Fermi gas model of particles moving in orbits independently of each other has led to various expressions for the density, in particular the Bethe level density formula. The expressions have various free parameters which may be determined by fitting experimental data.;In this thesis we use the spectral distribution methods of French for the calculation of level density. The basis of this method is to determine the level density not from the nuclear spectrum itself but from the level distribution defined in terms of the moments of the Hamiltonian. The one-body (mean field) and two-body (effective interaction) parts can be treated separately and the results convoluted to calculate the total level density. Thus one can study the effects of the non-interacting and interacting particles parts of the Hamiltonian.;Following the method recently suggested, we calculate the different parts of the density for several heavy nuclei where experimental information exists in the ground state and neutron resonance region. The non-interacting particle density is fitted to the Bethe expression and the parameters determined. We then convolute this with the interacting part and determine the strength of the interaction through a fit with experimental data. The total density is also fitted to the Bethe form to study the change in parameters due to the effects of the interaction.;Nuclear level density is a basic quantity of the nucleus and plays an important role in both pure and applied physics. This importance comes from the wide needs for understanding the nuclear system properties such as the description of excited nuclei, the fission dynamics and the calculation of reaction cross sections. Given the experimentally observed spectrum of a nucleus in the ground state region, the level density is simply calculated by counting the number of levels (states) in a given energy interval. For heavy nuclei, the excitation energy up to which complete information (all levels measured (energy) and identified (angular momentum)) is usually available is around 2 MeV. Further, experimental information in terms of neutron or proton resonances is available at the neutron or proton separation energy. There is no information in the intermediate energy region. Effectively, this gives us two reference points, one in the ground state region and the other at the neutron or proton separation energy, for any theoretical calculation of nuclear level density as a function of excitation energy.;One of the purposes of this thesis is to examine the basis of the traditional non-interacting particle density calculations. We have also studied the systematics such as the dependence of the ground state energy on the strength of the interaction for several nuclei. Our results for heavy nuclei supplemented by those already available in literature indicate that the spectral methods present a practical and comprehensive way to calculate the nuclear level density with the full Hamiltonian. A detailed survey across the periodic table using various effective interactions is possible and should be undertaken in future.