Exit doorway states in nuclear reactions

Abstract

Compound nuclear processes should in fact exhibit all the resonances of the underlying compound nucleus. In intermediate resolution experiments where the energy resolution $\ensuremath{\Delta}E$ is considerably large compared to the width and spacing of compound nuclear states but small compared to those of optical model shape resonances, one observes the well-known intermediate structure. In the case of compound elastic processes a dynamically reasonable account of intermediate structure resonances was developed by various authors, where one assumes the existence of a doorway state through which the incident (or the final) state couples to the compound nuclear states. An energy average of the resultant amplitude exhibits intermediate structure. In the case of compound inelastic reactions the incident and the final states are different and the doorway states that relate the final state to the compound nuclear states will in general be different from the doorway states which relate the incident state to the compound nuclear states. The former doorway states will be referred to as the exit doorway states and the latter as the entrance doorway states. A dynamical theory is developed acknowledging the notion of these two kinds of doorway states. The energy averaged transition amplitude shows two sets of intermediate structure resonances corresponding to these two kinds of doorway states. It is expected that one of these sets of resonances will usually dominate a reaction in an energy domain. The present formulation can possibly explain the different sets of intermediate structure resonances observed in different exit channels of a nuclear reaction, for example, those observed in various exit channels of the $^{12}\mathrm{C}$+$^{16}\mathrm{O}$ system.