Affirmation of transfer-breakup as a source for alpha-particle emission in C-12,C-13 + Th-232 fission reactions

摘要

An enhancement in near-scission alpha-particle multiplicity was observed in the C-12 + Th-232 fission reaction at a beam energy of 69 MeV in a recent experiment [Y. K. Gupta et al., Phys. Rev. C 86, 014615 (2012)]. The excess alpha-particle emission was attributed to alpha-transfer induced fission, where alpha-particle transfer followed by breakup of Be-8 into two alpha particles in coincidence with fission fragments (FFs) was observed. In order to further verify this transfer-breakup source, alpha-particle energy spectra have been measured in coincidence with FFs in C-12,(13) + Th-232 reactions at various relative angles with respect to the beam and FF directions at a beam energy of 74.5 MeV. The alpha-particle multiplicity spectra were fitted using a moving source disentangling analysis. The features of multiplicity spectra clearly demonstrate the presence of transfer-breakup events in C-12 + Th-232 fission. The contribution of alpha-particle multiplicity from transfer-breakup events in the C-13 + Th-232 reaction is seen to be negligibly smaller than that of the C-12 + Th-232 reaction. After taking into account the transfer-breakup source, the pre- and near-scission alpha-particle multiplicities follow the recently developed heavy-ion systematics for both reactions. These results conclusively affirm that transfer-breakup is a potential source of alpha-particle emission in fission reactions, induced by alpha-cluster projectiles such as C-12.