Search for α-cluster state in N = Z medium heavy nuclei

Inelastic α-scattering experiments on unstable N=Z double-magic 56Ni nuclei were performed at GANIL in inverse kinetics at an incident energy of 50 A.MeV. A high multiplicity of α-particle emission was observed within the limited phase space of the experimental setup. Such high multiplicity cannot be explained by statistical decay models. On the other hand, the simplified ideal classical gas model reproduces the experimental momentum distribution well with kT=0.4 MeV. However, since the energies of all decay particles were not measured in this experiment, nor were the kinetic energies of the recoiled α-particles, the excitation energies cannot be determined. Therefore, we assume that the α-gas state of 56Ni exists around 113 MeV and that the measured multiplicity is an ensemble of α-decays from various excitation energies, which reproduces the experimental results well. These results suggest that an exotic state consisting of a large number of α-particles may exist at the excitation energy of 113+15-17MeV. We propose a method to measure the energy and angular distribution of recoiled α-particles, which are estimated to have kinetic energies of a few MeV, in order to resolve the uncertainty of the excitation energy in this experiment.