Collective Degrees of Freedom in Nuclear Level Densities

Abstract

A finite temperature extension of the interacting boson model is developed and applied to the calculation of the collective partition function of transitional even-even Sm isotopes. The effective boson number decreases with increasing temperature, leading to a natural damping of collectivity. Positive and negative parity contributions to the boson partition function are computed and compared to rotational and vibrational enhancement factors commonly used in the generalized superftuid model.

Level densities of transitional Sm nuclei

“…Even in that case, however, a realistic microscopic calculation of N B (T ) is needed, since it was recently noticed [38] that the simple BCS calculations at finite temperature done in Refs. [18,37] do not properly deal with quadrupole and octupole bosons, inducing, as a consequence, the transition from a deformed ground state to a spherical equilibrium shape at much lower temperature than the one predicted by selfconsistent mean-field calculations. In addition, the temperature dependence of the IBM Hamiltonian parameters, in particular the d-and f-boson energies, might be non-negligible over a broad temperature range, so that a consistent microscopic model of both boson number and Hamiltonian appears to be mandatory.…”

“…In particular, the behavior of the partition function (A6) on the assumption that only the effective boson number decreases with increasing temperature was first studied at the limiting symmetries of IBM-1 for positive parity only [37], and, more recently, for both parities in the same transitional Sm chain [18] considered in the present work.…”

“…The problem was already faced in Refs. [18,37] under the simplifying assumption that only the effective boson number N B changes with temperature and goes to zero when T goes to infinity. Even in that case, however, a realistic microscopic calculation of N B (T ) is needed, since it was recently noticed [38] that the simple BCS calculations at finite temperature done in Refs.…”

Combinatorial level densities from a relativistic structure model

“…As is known, the extension to deformed nuclei will require, in addition, a rotational enhancement, as well as an appropriate treatment of the coupling of rotations and vibrations. The latter coupling is frequently neglected in level density calculations, thus leading to a significant overestimate of the collective enhancement around its maximum, as shown in a recent evaluation [30] of this effect in the framework of the interacting boson model.…”