Fission fragment mass and folding angle distributions in the reaction Be9+U238

Abstract

Background: The excitation energy dependence of the fission fragment mass and folding angle distributions for reactions with loosely bound projectiles are reported to be significantly different compared to the reactions with tightly bound nuclei. Extensive study to understand the reaction mechanism of weakly bound projectiles is required as it can simulate the mechanism of fission induced with radioactive ion beams.Purpose: The effect of loosely bound nuclei on fission fragment folding angle and mass distributions in the $^{9}\mathrm{Be}+^{238}\mathrm{U}$ reaction near the Coulomb barrier energies is investigated and the contributions of incomplete fusion fission in the total fission cross section are studied.Methods: Mass and folding angle distributions of the fission fragments have been measured near the Coulomb barrier energies using two large area multiwire proportional counters. Fragment masses were determined from the time-of-flight difference method. To understand the effect of breakup of loosely bound nuclei, the results are compared with tightly bound proton-, $\ensuremath{\alpha}$-particle-, and carbon-induced reactions, as well as with the predictions from semiempirical calculations.Results: The width of folding angle distributions is found to increase with lowering excitation energy in the $^{9}\mathrm{Be}+^{238}\mathrm{U}$ reaction. This is in contrast to the reaction $^{12}\mathrm{C}+^{232}\mathrm{Th}$ involving tightly bound projectiles, populating similar compound nuclei. The peak-to-valley ratios of the fission fragment mass distributions for all events are found to sharply increase with lowering excitation energy in the $^{9}\mathrm{Be}+^{238}\mathrm{U}$ reaction. The possible contribution of incomplete fusion into the total fission cross section could be extracted. The measured mass distributions were found to be consistent with the semiempirical calculation gef and macro-microscopic two-center shell model calculations considering the admixture of incomplete fusion fission.Conclusion: For actinide targets, mass distributions of inclusive fission are influenced by the breakup/transfer channels of the weakly bound projectiles. At the above-barrier energies, the contribution of incomplete fusion is found to be $\ensuremath{\approx}25\ifmmode\pm\else\textpm\fi{}4%$ which is similar to the reported value of the complete fusion suppression factor in reactions involving nuclei with low binding energies. The increase in the peak-to-valley ratio with lowering excitation energy in the fission fragment mass distribution could be explained by the combined analysis of folding angle and mass distributions.