Investigation of α-particle states of 8Be and 12C Nuclei in photonuclear reactions on 12C, 14N and 16O nuclei

The method of diffusion chamber in the magnetic field making use of a bremsstrahlung beam with a maximum photon energy of 150 MeV is applied to study the reactions of multiparticle photodisintegration of ¹²C, ¹⁴N and ¹⁶O nuclei with the formation of several α-particles in the final state (¹²С(γ,3α), ¹⁶O(γ,4α) і ¹⁴N(γ,np)3α). An experimental complex with a digital data bank of stereo frames and a set of graphic programs that allows to restore events and carry out physical analysis of the obtained data is created. The distribution of events in regard of the excitation energy of the subsystem of two alpha particles has been measured. For all three reactions, the resonance is found which is identified as the ground state of the ⁸Be nucleus and the partial channels for the formation of this state have been separated (¹²C(γ,α)⁸Be₀, ¹⁶O(γ,2α)⁸Be₀ and ¹⁴N(γ,np)α⁸Be₀). For the channels ¹²С(γ,α)⁸Be₀ and ¹⁶О(γ,2α)⁸Be₀, the partial cross-sections are measured. The dependences demonstrate the presence of four maxima, whose positions coincide for both reactions. The rcorrelation between the energy E_γ corresponding to the cross-section maximum and the excitation energy E₀ of ⁸Be nucleus is determined as E_γ – Q = E₀, where Q are the threshold values. The obtained results are qualitatively explained in the framework of the model which describes the absorption of a γ-quantum by a virtual α-particle pair. For the channels ¹⁶О(γ,2α)⁸Be₀ and ¹⁴N(γ,np)α⁸Be₀ the distribution of the excitation energy of three α-particles is measured. The maximum is found in the near-threshold 3α-particle region. This maximum can correspond to the Hoyle state of ¹²C nucleus. Events corresponding to the partial channel of the ¹⁴N((γ,np)¹²C*(0⁺) reaction with the subsequent two-particle decay ¹²C→α+⁸Be*(0⁺) are separated, and the energy and angular distributions of α-particles at each decay stage are analyzed. The channel of ¹²C nucleus formation in the Hoyle state in photonuclear reactions has not been identified earlier.