Interplay of fission modes in mass distribution of light actinide nuclei 225,227Pa

TL;DR

This study measures fission fragment mass distributions of 225,227Pa nuclei, revealing asymmetric fission components at low energies due to shell effects, and compares these with neighboring actinides to understand fission modes.

Contribution

It provides new experimental data on 225,227Pa fission, demonstrating the coexistence of symmetric and asymmetric fission modes influenced by shell effects at low excitation energies.

Findings

Mass variance increases at below-barrier energies.

Presence of asymmetric fission components in 225,227Pa.

Shell effects influence fission modes at low excitation energies.

Abstract

Fission-fragment mass distributions were measured for 225,227Pa nuclei formed in fusion reactions of 19F + 206, 208Pb around fusion barrier energies. Mass-angle correlations do not indicate any quasi-fission like events in this bombarding energy range. Mass distributions were fitted by Gaussian distribution and mass variance extracted. At below-barrier energies, the mass variance was found to increase with decrease in energy for both nuclei. Results from present work were compared with existing data for induced fission of 224, 226Th and 228U around barrier energies. Enhancement in mass variance of 225, 227Pa nuclei at below-barrier energies shows evidence for presence of asymmetric fission events mixed with symmetric fission events. This is in agreement with the results of mass distributions of nearby nuclei 224, 226Th and 228U where two-mode fission process was observed. Two-mode feature of fission arises due to the shell effects changing the landscape of the potential energy surfaces at low excitation energies. The excitation-energy dependence of the mass variance gives strong evidence for survival of microscopic shell effects in fission of light actinide nuclei 225, 227Pa with initial excitation energy ~30 - 50 MeV.