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The nuclear level density $\rho (E)$ is the number of distinct nuclear energy levels per unit excitation energy, after specifying which spin and parity classes are included. In a small interval, the expected level count is approximately $\rho (E)\Delta E$. It is commonly expressed per MeV.

At high excitation, individual levels can be too numerous to resolve, so reaction models use a statistical density instead. A density counting each level once differs from a state density that includes every magnetic substate with multiplicity $2J+1$. Known low-lying levels and average neutron-resonance spacings provide experimental normalization constraints.^[ref]

A nuclear resonance is an enhanced nuclear reaction or scattering response near the energy of an accessible quasi-bound nuclear state. For an isolated resonance, a common energy dependence is ${\sigma }_{ab}(E)\propto {\Gamma }_a{\Gamma }_b/[(E-E_r{)}^2+(\Gamma /2{)}^2]$, where $E_r$ is the resonance energy, $\Gamma$ the total width and ${\Gamma }_a,{\Gamma }_b$ partial widths for entrance and exit channels.

The lifetime scale is $\tau \sim \hslash /\Gamma$: narrower states persist longer. Spin factors, wavelength factors and interference with background amplitudes complete a cross-section model. Resonance spacings and widths provide input to statistical nuclear-reaction calculations.^[ref]