Description of cross sections for photonuclear reactions in the energy range between 7 and 140 MeV
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CLEI Theory
Description of Cross Sections for Photonuclear Reactions in the Energy Range between 7 and 140 MeV B. S. Ishkhanov and V. N. Orlin* Institute of Nuclear Physics, Moscow State University, Moscow, 119992 Russia Received March 27, 2008; in final form, July 3, 2008
Abstract—A combination of the exciton and evaporation models is used to describe photonuclear reactions induced in light, medium-mass, and heavy nuclei by photons of energy in the range 7 ≤ Eγ ≤ 140 MeV. Two mechanisms of the photoexcitation of nuclei are considered. These are the formation of a giant dipole resonance at energies in the range Eγ 30 MeV and quasideuteron photoabsorption, which is dominant at energies in the region Eγ 40 MeV. The density of particle–hole states, which appears in the exciton model, is calculated on the basis of the Fermi gas model. The emission of two preequilibrium particles is taken into account in describing the quasideuteron reaction channel. The effect of isospin conservation on giant-dipole-resonance decay accompanied by photonucleon emission is examined. The model in question is used to describe cross sections for photon-induced reactions on 26 Mg, 54 Fe, 112,118,119,124 Sn, and 181 Ta nuclei. PACS numbers: 25.20.-x DOI: 10.1134/S1063778809030041
1. INTRODUCTION In the energy range between 7 and 140 MeV—that is, up to the pion-production threshold—photonuclear reactions proceed predominantly via two photoabsorption mechanisms. These are the formation of a giant dipole resonance and quasideuteron photoabsorption [1]. The first prevails at energies in the range Eγ 30 MeV, while the second is dominant at > 40 MeV. energies in the region Eγ ∼ The character of photonucleon emission depends greatly on the nature of the primary excitation of a target nucleus. For example, preequilibrium nucleon emission plays a significant (dominant at high energies) role in the decay of a quasideuteron excitation (see, for instance, [2–4]), because, in quasideuteron photoabsorption, the entire hard-photon energy is transferred to a single neutron–proton pair. On the other hand, the giant-dipole-resonance component of photonuclear reactions on medium-mass and heavy nuclei is determined primarily by the evaporation of nucleons from a compound-nucleus state that arises upon the thermalization of the energy of collective dipole vibrations. The contribution of preequilibrium processes to this component plays a significant role only for light nuclei and manifests itself primarily in the region of the giant-resonance maximum, where the formation of the coherent one-particle–one-hole *
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(1p1h) doorway state occurs. It should also be borne in mind that the decay properties of the giant dipole resonance in light and medium-mass nuclei (A 100) are strongly affected by isospin effects, since the decay of the T> giant-dipole-resonance component is dominated by proton emission because of totalisospin conservation. With allowance for isospin splitting, the cross section for giant-dipole-resonance formation can be calc
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