Neutron Emission from Fission Fragments during Acceleration

Fission-neutron angular correlations following fusion of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ have been measured. Conventional analysis, based on the approximation that post-fission neutrons originate only from fully accelerated fission fragments, gives unexpectedly large numbers of "prefission" neutrons. Comparison with the considerably less fissile system ${}_{}{}^{200}{}_{}{}^{}\mathrm{Pb}$ gives the first convincing evidence that this approach is inadequate. Consideration of neutron emission from the accelerating fragments gives results consistent with expectations.