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.
Highlights
The mechanism of mass division in fission of atomic nuclei has been an intriguing problem in nuclear physics for several years
With growing excitation energy of the fissioning nucleus, the asymmetric mass distribution changes to symmetric Gaussian distribution. These characteristics are explained according to the concept of independent fission channels which corresponds to specific valleys in the potential-energy surface (PES) of the fissioning nucleus [1, 5, 6]
More recent calculation of five-dimensional PES by Moller et al well predicts the presence of two deformation paths; one path with elongated scission configuration leading to symmetric mass division and the other shell influenced more compact scission configuration leading to asymmetric mass division [11]
Summary
The mechanism of mass division in fission of atomic nuclei has been an intriguing problem in nuclear physics for several years. With growing excitation energy of the fissioning nucleus, the asymmetric mass distribution changes to symmetric Gaussian distribution These characteristics are explained according to the concept of independent fission channels (modes) which corresponds to specific valleys in the potential-energy surface (PES) of the fissioning nucleus [1, 5, 6]. The origin of these valleys has been attributed to shell structures modifying the potential energy landscape of the deformed system [7, 8, 9, 10].
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