Partial width correlations and common doorway states

Abstract

This work sets up a theory for the various correlations discovered between different nuclear reaction data. In particular, an attempt is made to understand the recently reported positive correlation of neutron and partial photon widths of neutron resonances. The older data on the positive correlation of ( d, p) and ( n, γ) cross sections is also considered. A compact formalism for discussing correlations is given, and it is shown that they are closely related to the existence of “direct” cross sections. One can see that strong correlations are associated with the existence of isolated doorway states, as suggested by Chrien and Bartholomew and co-workers; further, such doorway states must be “common” to both channels. A discussion is given in terms of current models on whether or not such common doorways are expected to occur in practice. A detailed calculation of the 1 − states of 208Pb and 60Ni shows how such states can occur. In 208Pb, they are three states having large components of the 3 p −14 s configuration, which is associated with most of the neutron and photon strength in the vicinity of 6 MeV. A by-product of this study is an explanation of the “pygmy” dipole resonance. In the course of developing the theory, the relevant experimental data on correlations is presented and discussed.