PRE-EQUILIBRIUM STUDIES: A CASE OF 12C+175Lu SYSTEM

Abstract

The role of pre-equilibrium (PE) emission within the heavy-ion fusion process has not been fully characterized. When two heavy nuclei fuse together, they form a composite nucleus far from the statistical equilibrium, and a large fraction of its energy is considered to be in the form of an orderly translational motion of the nucleons of the projectile and the target nuclei. This orderly motion transforms slowly into chaotic thermal motion through a series of two-body interactions. The thermalization process completes when the composite nucleus reaches a state of thermal equilibrium, referred to as the compound nucleus. As soon as the state of thermal equilibrium is attained, the accumulation of sufficient energy on a single nucleon or a cluster of nucleons may occur in a random sequence of events and hence may require much longer emission times, favoring the emission of low-energy particles. The time scale at which PE emissions occur is very short, 10-21 s, while further evaporations from the equilibrated nucleus take a longer time, 10−16 s. The rate of emission of the PE nucleons depends on the sensitivity of the mean-field interaction between the projectile and the target nucleus. This determines the initial energy distribution among the nucleons in the projectile and the target nuclei, which starts a cascade of nucleon-nucleon interactions as soon as the two nuclei touch each other. With a view to study pre-equilibrium emission process, the excitation functions for the neutron emission channel occurring in the fusion of 12C with 175Lu, has been measured at incident energies from near the Coulomb barrier to 7 MeV/nucleon. The off-line -ray spectrometry–based activation technique has been used for the measurements of excitation functions. Further, the measured excitation functions are compared with theoretical predictions based on pure statistical model code PACE4 and Geometry Dependent Hybrid (GDH)-based code ALICE 91. The strength of pre-equilibrium emission is also determined from comparison of the experimental excitation functions and the PACE 4 calculations.