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.^[ref]