= Nuclear level density
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.<Reference Input Parameter Library (RIPL-3)>[LEVEL DENSITIES and NEUTRON RESONANCES segments]{ref}
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