A team of physicists used detailed time-dependent simulations to show that a distinct class of neutrons, called scission neutrons, appear mainly at higher kinetic energies in nuclear fission. Scission neutrons are those emitted at the instant the dividing nucleus splits. In three systems they studied — neutron-induced fission of 235U and 239Pu, and spontaneous fission of 252Cf — the simulations show scission neutrons are essentially absent below a threshold energy of about 1.5–2.0 MeV and instead dominate the higher-energy part of the prompt fission neutron spectrum.

The authors used time-dependent density functional theory (TDDFT) extended to superfluid systems, solved with an axially symmetric solver and a standard SkM* energy-density functional (EDF) with pairing. Their computational volume was an ellipsoid with semi-axes Zmax = 70 fm and Rmax = 55 fm, roughly 3.2 times larger than in earlier work. That larger domain allowed the emitted neutrons to separate far enough from the newly formed fragments to extract their directions and energies before numerical boundary effects set in. The researchers identified scission-neutron regions by looking for regions where the flow energy of neutrons is large compared with their intrinsic kinetic energy, and they extracted reliable results in an angular window roughly between 108° and 143° measured from the light fragment direction.

Across the three fissioning systems, the computed scission-neutron energy spectrum peaks near 3–3.5 MeV, falls off exponentially toward higher energies up to about 18 MeV, and shows a clear cutoff below a threshold energy Ethr: about 2 MeV for induced fission (235U and 239Pu) and about 1.5 MeV for 252Cf. The paper reports a total scission-neutron yield for 239Pu(n_th,f) of 0.19 ± 0.01 neutrons per fission, which is about 6.6% of the total average prompt neutron multiplicity quoted for that system. For 252Cf(sf) they find 0.39 ± 0.02 scission neutrons, about 10.4% of the total prompt multiplicity.