APPLICATION OF SPECTRAL AVERAGING THEORY IN LARGE SHELL-MODEL SPACES - ANALYSIS OF LEVEL DENSITY DATA OF FP-SHELL NUCLEI

In the spectral averaging theory extended to large shell model spaces (i) using a significant unitary group decomposition under which a one- plus two-body nuclear hamiltonian H decomposes into an orthogonal eff ective one-body part h and an irreducible two-body part V(H --> h+V), (ii) applying the Central Limit Theorems locally and (iii) decomposing the shell model space into distant non-interacting S-subspaces (for l ight nuclei S denotes HBAR omega excitation), the state densities I-H( E) take a convolution form, I-H(E) = Sigma(S) I,(h,S) x rho(G)(V,S)[E] , where I-h is the renormalized non- interacting particle state densit y and rho G is a normalized spreading Gaussian due to V.A similar conv olution form is available for spin-cutoff densities that give spin-cut off factors. For a first systematic analysis, using the extended spect ral averaging theory, of experimental level density data in a given re gion of the periodic table, fp-shell nuclei are chosen as an example. Calculations, using surface delta interaction, are carried out by incl uding configurations up to 2HBAR omega excitations and by employing ei ght spherical orbits (ds, fp, 1g(9/2)) for the five nuclei Mn-55, Fe-5 6, Co-59, Co-60 and Ni-60 and by extending for obvious reasons to ten spherical orbits (ds, fp, 1g(9/2), 2d(5/2) 1g(7/2)) for the three A > 60 nuclei Ni-62, Cu-63 and Cu-65. In general, spectral averaging theor y is seen to provide a good representation of the observed total level densities and spin-cutoff factors.