Particle-hole optical model and strength functions for high-energy giant resonances
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CLEI Theory
Particle–Hole Optical Model and Strength Functions for High-Energy Giant Resonances М. H. Urin* National Research Nuclear University MEPhI, Kashirskoie sh. 31, Moscow, 115409 Russia Received December 28, 2009
Abstract—A formulation of the particle–hole optical model is proposed for describing the contribution of the fragmentation effect to the formation of strength functions for high-energy giant resonances. The model is based on the Bethe–Goldstone equation for the energy-averaged particle–hole Green’s function. In this equation, the particle–hole interaction that is induced by a virtual excitation of multiquasiparticle configurations and in which, upon averaging over energy, an imaginary part is contained is taken into account. An analogy with the single-quasiparticle optical model is discussed. DOI: 10.1134/S1063778810080119
1. INTRODUCTION The relaxation of simple nuclear excitations (single-quasiparticle and collective particle–hole) characterized by a relatively high excitation energy has been intensively investigated from a theoretical point of view for a long time (see, for example, [1–3]). Their coupling to a single-particle continuum and their coupling to multiquasiparticle configurations that is induced by pair interactions of nucleons are the main modes of relaxation. Coupling of the latter type leads to a fragmentation effect—that is, a distribution (spreading) of the strength of a simple configuration (doorway state) over complex ones—and to a shift of the configuration energy. There are two implementable (in practice) methods (microscopic and phenomenological) for taking into account the fragmentation effect. The microscopic method consists in taking directly into account the coupling of simple excitations to some configurations next in complexity: two particles– one hole (2p−1h) in the case of single-particle excitations and two particles–two holes (2p−2h) in the case of collective particle–hole excitations (giant dipole resonances). An example of the application of the microscopic approach to describing the fragmentation effect for both single-quasiparticle excitations and giant resonances can be found in [4]. Theoretical investigations of giant-resonance strength functions within the procedure where the fragmentation effect is treated on the basis of the microscopic method and where a single-particle continuum is taken precisely into account were continued until recently [5]. In this *
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procedure, one can interpret the total width of a giant resonance in terms of an additional smearing parameter, but it is hardly possible to describe correctly the fragmentation shift of the giant-resonance energy because of the need for taking into account, within such a microscopic description, a formally complete basis of 2p−2h configurations. It also seems that, in what is concerned with “multilevel” giant resonances, the disregard of thermalization of simple configurations is a serious drawback of the microscopic approach, since this thermalization may lead to an independent fragm
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