Neck Rupture and Scission Neutrons in Nuclear Fission.

Just before a nucleus undergoes fission, a neck is formed between the emerging fission fragments. It is widely accepted that this neck undergoes a rather violent rupture, despite the absence of unambiguous experimental evidence. The main difficulty in addressing the neck rupture and saddle-to-scission stages of fission is that both are highly nonequilibrium processes. Here, we present the first fully microscopic characterization of the scission mechanism, along with the spectrum and the spatial distribution of scission neutrons (SNs), and some upper limit estimates for the emission of charged particles. The spectrum of SNs has a distinct angular distribution, with neutrons emitted in roughly equal numbers in the equatorial plane and along the fission axis. They carry an average energy around 3±0.5  MeV for the fission of ^{236}U, ^{240}Pu, and ^{252}Cf, and a maximum of 16-18 MeV. We estimate a conservative lower bound of 9%-14% of the total emitted neutrons are produced at scission.