On the mechanism of formation of excited states of the 8Be nucleus in the reaction 12C(γ, 3α)

Abstract

The reaction 12C(γ, 3α) was studied with the aid of a diffusion chamber placed in a magnetic field, filled with a methane-helium mixture, and irradiated with bremsstrahlung photons of endpoint energy 150 MeV. The total reaction cross section was measured in the energy range Eγ < 40 MeV. The ground state; the first, second, and unseparated third and fourth excited states; and highly excited states (in the range 19.9 < Ex < 25.2 MeV) of the 8Be nucleus manifested themselves in the distribution of events with respect to the energy of the relative motion of two alpha particles. A resonance that is characterized by the energy of E0 = 0.72 MeV and the width of Γ = 0.80 MeV and which was identified as a ghost anomaly was found between the ground state and the first excited state. Partial cross sections were measured for various channels. Excited states are formed in narrow photon-energy intervals, and their partial cross sections are of a resonance shape. It is found that the energy corresponding to the maximum of the partial cross section for the ith level, Emi, and the excitation energy of the next level, E0i+1, are correlated: Emi = E0i+1 + ɛ, where ɛ is the reaction threshold. The results are qualitatively explained on the basis of a model that assumes photon absorption by an alpha-particle pair.