 Neutron scattering lengths and cross sections by NIST

The NIST Center for Neutron Research table lists bound coherent and incoherent scattering lengths, cross sections and isotopic abundances, citing the 1992 Neutron News compilation. Natural-element coherent lengths include K $3.67$ fm, Cl $9.5770$ fm, Al $3.449$ fm and Si $4.1491$ fm. Nuclear contrast is not fixed by the number of atomic electrons.