The low energy neutron's scattering cross section is σs, when the target nucleus could be deemed as bound. However, when the target nucleus is unbound, the scattering cross section is [A/(A+1)]2σs. The reason for the presence of factor [A/(A+1)]2 is the reduced mass of neutron when the unbound target nucleus would be recoiled.

Some literatures say that the unbound scattering length is also [A/(A+1)] that of the bound one. Does anyone know which literatue would be one of the best to show this relationship?

Thanks.