Linking of direct and compound chains in multistep nuclear reactions

We remove the sharp separation between multistep compound and multistep direct emission in the Feshbach-Kerman-Koonin (FKK) derivation of preequilibrium processes. In addition to the original multistep compound mechanism, we find a new class of multistep processes arising from linking of the direct and compound chains. There can be additional scatterings in unbound P space before the quasibound compound Q space is entered, or after it is left. We provide a theoretical justification for the presence of P→Q transitions, which are needed to account for experimentally observed preequilibrium spectra. Our formalism is applied to the analysis of the 14 MeV ⁹³Nb(n,n’) reaction, using modified distorted-wave Born approximation (DWBA) matrix elements which include an inverse S-matrix factor. Since the dominant contribution to multistep compound emission comes from the 2p1h Q stage, the linking of the multistep chains results in flux bypassing this stage, resulting in a reduced multistep compound emission and an increased emission from the compound nucleus.