Neutron-induced fission threshold ($S_n(A+1,Z)+E_n^{\mathrm{CM}}\gtrsim B_f(A+1,Z)$)

In a simple compound-nucleus picture, compare the neutron-capture excitation energy to the fission barrier of the capture product. If its neutron separation energy already exceeds the barrier, slow-neutron capture can lead to prompt fission without a positive kinetic-energy threshold. If it lies below, finite neutron kinetic energy is needed for appreciable over-barrier fission; sub-barrier tunnelling and detailed resonance widths soften a literal sharp-threshold picture.