Fine structure in the intermediate resonance region of the neutron induced fission cross section

Abstract

Recently, the fine structure of the 0.7 MeV resonance in the 230 Th neutron-induced cross section was investigated within the hybrid model. A very good agreement with experimental data is obtained. It is suggested that fine structure of the cross section quantify the changes of the intrinsic states of the nucleus during the disintegration process. Due to internal excitations produced in the region of the outer barrier, where the density of single-particle states is enhanced, new transitions levels are produced. These transitions levels cause β-resonances very close in energy. A way to introduce phenomenologically this phenomenon in cross section calculations is presented. An early experimental data analysis of the 0.7 MeV resonance of the 230 Th neutron induced fission cross section evidences a high moment of inertia of the rotational band associated to this peak (1). Intuitively, a such behavior indicates the possible existence of an intermediate state at a deformation energy considerably larger than that of the second minimum. A third minimum hypothesis was confirmed in refs. (2-4). The fine structure of the 0.7 MeV resonance investigated in terms of the rotational model with two Ω = 1/2 band heads of opposite parities revealed a moment of inertia parameter 2 /2J of about 1.9 keV. This value is considered compatible with the third minimum. 2 The hybrid model A new model for intermediate resonance structures in the neutron-induced fission cross section was published recently (5). The hybrid model employs phenomenological heights of the double humped barrier while the nuclear excitations are given by theoretical dynamical calculations. The defor- mation energy surface is determined in the framework of the microscopic-macroscopic model in a configuration space spanned by the most important degrees of freedom encoun- tered in fission: elongation, necking and mass-asymmetry. The macroscopic energy is obtained with the Yukawa-plus- exponential model while the shell correction with the Strutin- sky prescriptions based on the super-asymmetric two centers shell model (6). A minimal action trajectory is determined in this configuration space in order to determine the shape of the barrier. Dynamical calculations are performed using the Landau-Zener effect in order to determine the occupation probability of each orbital in the path from the fundamental state up to scission. The occupation probabilities and the associated single-particle levels are afterwards translated in excitations of the barrier. These excitations are added to a phenomenological barrier. A great number of double barriers are obtained, each of them represents a transition level and it