Study of the diffraction scattering 12C + 12C with the excitation of the 12C exotic state 02+ (the Hoyle state)

New results from a series of experiments, dedicated to the study of the 12C exotic state (the so-called Hoyle state) are presented. In spite of the many investigations carried out formerly, the structure of this state, lying above the threshold for breakup into three alpha particles, is still unknown. The different models assume an abnormally large size of the nucleus in this excited state. However, until recently methods for measuring the radii of unbound states were not suggested. The best way to solve the problem seems to be the measurement of the angular distributions of elastic and inelastic scattering of 12C on different target nuclei and the determination of the radii of the ground and excited states with the help of diffraction scattering. The diffraction method of determination of the radii is based on the fact that the cross sections for direct reactions at high enough energies behave at small scattering angles as Frauenhofer diffraction on a black ball. Accordingly an experiment was performed aimed at measuring the elastic and inelastic angular distributions of 12C with an energy of (121.5 ± 0.5) MeV on a 12C target. The elastic scattering was measured in the angular range from 18 to 50 deg in the c.m. system with uncertainty in the angle measurement equal to Δθ = ±0.6 deg. The inelastic cross section was measured for the 12C excited states 2+ (4.44 MeV) and 0+ (7.65 MeV). Estimates were made for the diffraction radii for the ground and excited states. An increase was observed in the radius of the state at 7.65 MeV compared to those of the ground and first excited states