Investigation of the reaction mechanism for the four-particle photodisintegration of a carbon nucleus
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EI Theory
Investigation of the Reaction Mechanism for the Four-Particle Photodisintegration of a Carbon Nucleus S. N. Afanas’ev* , Е. S. Gorbenko, and А. F. Khodyachikh Kharkov Institute for Physics and Technology, Akademicheskaya ul. 1, Kharkov, 61108 Ukraine Received July 21, 2006; in final form, November 7, 2006
Abstract—The four-particle photodisintegration of a carbon nucleus in the reactions 12 С(γ, p)3 Н2α and 12 С(γ, n)3 Не2α is investigated by a method that employs a diffusion chamber in a magnetic field. It is shown that these reactions proceed according a sequential-type scheme: excited states of 11 B and 11 C nuclei decay to weakly excited states of 8 Be, 7 Li, and 7 Be nuclei. It is concluded that nucleons are knocked out from the s shell. In the excitation curve for the 2α system in the reaction 12 С(γ, p)3 Н2α, a resonance is found between the maxima corresponding to the ground and the first excited state of the 8 Be nucleus, and this resonance is identified as a ghost anomaly. The branching fractions of the decay modes are determined. The angular distributions of nucleons in the reaction c.m. frame are measured. The energy dependence of the asymmetry coefficient for the angular distributions is obtained. A fast increase in this coefficient is observed in the energy range 38–40 MeV. It is concluded that the asymmetry coefficient depends on the excitation energy of the final nucleus in the region of intermediate photon energies. PACS numbers: 25.20.-x DOI: 10.1134/S1063778807050055
INTRODUCTION For many years, nucleon knockout from light nuclei via (γ, N ) processes in the energy range between the giant resonance and the meson-production threshold (hereafter, the intermediate energy range) has been a subject of intensive studies, both theoretical and experimental. However, the reaction mechanism for nucleon knockout from a nucleus has yet to be clarified conclusively. For example, calculations in the nonrelativistic approximation [1–6] revealed that the direct-knockout mechanism cannot explain the equality of the cross sections for (γ, p) and (γ, n) reactions or an identical shape of the angular distributions in these reactions. It was concluded that, at intermediate energies, a dominant contribution comes from the process of photon interaction with a nucleon pair. However, this conclusion is at odds with the results of the calculations performed in the relativistic approximation by the authors of [7–9]. For the example of several nuclei, this group of authors showed that the direct-mechanism contribution to (γ, p) reactions is greater than that in the nonrelativistic approximation [1–6], ensuring agreement with experimental data. However, the role of the direct mechanism in (γ, n) reactions was not discussed in [7–9]. According to the calculations performed in [5, 6] on the basis of the nonrelativistic approximation, the *
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role of meson-exchange currents is modest if the final nucleus is in the ground state, but its contribution grows with increasing excitation energy of the fina
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