Measurement of fragment-mass distributions from fission of At-214 following the( 9)Be + Tl-205 reaction

Background: The recent surprising experimental discovery of asymmetric fission in the nucleus Hg-180 at low excitation energy has renewed interest concerning our understanding of the role of nuclear shell effects in the dynamics of the fission process. New theoretical models have been developed that claim to successfully explain the observed asymmetric fission of Hg-180 and also predict the nature of mass distributions of several other preactinide nuclei for which experimental data are scarce. Purpose: We investigated fragment-mass distributions in the fission of the preactinide nucleus At-214 at different excitation energies near the Coulomb barrier. The present results were compared with the predictions from a recent state-of-the-art theoretical calculation, as well as with our previous measurement for the nearby nucleus Po-210. Methods: A pulsed heavy-ion beam was used in the experiment. Fission fragments were detected with two large-area position-sensitive multiwire proportional counters placed at the folding angle. Mass distributions were extracted from time-of-flight differences and position information (theta, phi) for the fission fragments. Results: An asymmetry in fragment-mass distribution in the fission of At-214 was observed at an excitation energy of approximate to 31 MeV, where a shell effect is found to influence the potential energy surface. At higher excitation energies, mass distributions are predominantly symmetric indicating the absence of shell effects. Conclusion: This finding is consistent with the prediction of a recent macroscopic-microscopic dynamical model. However, for the preactinides, static models with nascent fragment shell structure are likely to produce similar results.