= Neutron-induced fission threshold
{title2=$S_n(A+1,Z)+E_n^{\rm CM}\gtrsim B_f(A+1,Z)$}
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.
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