Modified version of the combined model of photonucleon reactions
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CLEI Theory
Modified Version of the Combined Model of Photonucleon Reactions B. S. Ishkhanov1), 2) and V. N. Orlin2)* Received December 11, 2014
Abstract—A refined version of the combined photonucleon-reaction model is described. This version makes it possible to take into account the effect of structural features of the doorway dipole state on photonucleon reactions in the energy range of Eγ 30 MeV. In relation to the previous version of the model, the treatment of isospin effects at the preequilibrium and evaporation reaction stages is refined; in addition, the description of the semidirect effect caused by nucleon emission from the doorway dipole state is improved. The model in question is used to study photonucleon reactions on the isotopes 35−56 Ca and 102−134 Sn in the energy range indicated above. DOI: 10.1134/S1063778815040067
1. INTRODUCTION In accordance with Bohr’s postulate [1], it is assumed in the combined photonucleon-reaction model that a nuclear reaction can approximately be broken down into two independent stages: the formation of a compound system and the decay of this system into reaction products. In addition, it is assumed in this model that, in the mass range extending from values around A ∼ 40 to A values corresponding to transuranium elements, an analysis can be restricted to considering only three competing channels of decay of the compound system. These are the neutron, proton, and photon channels. Up to the pion-production threshold, photoabsorption on a nucleus is determined by photon interaction only with one- and two-nucleon nuclear currents [2, 3]. In the first case, only one nucleon is excited upon photon absorption. This is a dominant process in the region of low energies (Eγ 40 MeV), where the interaction of electromagnetic radiation with a nucleus leads to the formation of giant resonances [the giant dipole resonance (GDR) is the main one among them], which are a coherent mixture of one-particle–one-hole (1p1h) excitations. The quasideuteron (QD) photoabsorption mechanism becomes dominant above this region. Within this mechanism, the excited nucleon exchanges a virtual pion with the neighboring nucleon. As a result, the energy and momentum of the absorbed photon are 1)
Faculty of Physics, Moscow State University, Moscow, 119991 Russia. 2) Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia. * E-mail: [email protected]
transferred to a correlated proton–neutron pair rather to a single nucleon. In describing giant resonances, use is most frequently made of various versions of the 1p1h approximation of the shell model—for example, the randomphase approximation (RPA) [4], the theory of finite Fermi systems (TFFS) [5], and the coupled-channel method [6]. Despite the difference in substantiations, all of them lead to nearly identical results. For a reasonable choice of residual forces, these calculations reproduce by and large satisfactorily the mean energies and the sums of oscillator strengths of giant resonances in medium-mass and heavy nuclei. Howe
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