Study of fission dynamics through neutron multiplicity measurements and dynamical model calculations

Neeraj Kumar has a postdoc position in Ganil since last December and works on the study of different fission modes in the VAMOS facility. At 14h30, he will present us “Study of fission dynamics through neutron multiplicity measurements and dynamical model calculations”

Abstract: Study of the nuclear fission process is an immense approach to understanding the fundamental properties of nuclear matter. The theoretical and experimental efforts carried out over the last decades have established the dynamic aspects of the formation and decay of Compound Nucleus (CN). Among all the important observables, pre-scission neutron multiplicity (n-pre) and fission fragment mass distribution measurements have been emerged as one of the most effective probes to address various features of nuclear fusion-fission processes. In the present seminar, the details of a systematic study of fusion-fission dynamics of excited CN will be presented, which was carried out through both theoretical and experimental approaches. In the theoretical part, the Langevin dynamical model for fission is used to extract the effect of dynamical deformation of CN on n-pre by following the propagation of fissioning trajectories up to scission. The deformation dependence of particle emission widths is found to be relevant for highly fissile systems where the dynamics is primarily governed by the saddle to scission motion. In the experimental part of the work, the pre- and post-scission neutron multiplicities are measured for the fission of ²⁰⁸Rn, populated with the reactions ³⁰Si+¹⁷⁸Hf and ⁴⁸Ti+¹⁶⁰Gd in the excitation energy range of 54.5-80.8 MeV. These measurements have been performed in the National Array of Neutron Detectors (NAND) facility at Inter University Accelerator Centre (IUAC), New Delhi, India. In this study, n-pre are found to be significantly different for the two entrance channels and exhibit an enhancement with respect to the existing data for the ¹⁶O+¹⁹⁴Pt→²¹⁰Rn reaction. The systems involved are also analyzed within the framework of Langevin dynamical model calculations. The neutron emission from the formation phase of the CN is shown to be crucial in determining the n-pre for the present systems. It was also found that, in the ⁴⁸Ti+¹⁶⁰Gd reaction, rapid quasi fission dynamics prevents neutron emission from the thermalized target-projectile composite. These investigations help us to understand the relative importance of fusion and quasi-fission processes and their timescales depending on the entrance channel mass asymmetry.