Signature of a possible α -cluster state in N= Z doubly-magic 56 Ni

An inelastic α-scattering experiment on the unstable N= Z, doubly-magic 56Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for α-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at kT = 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of α particles corresponds to an excitation energy around 96 MeV. The method of distributed mα-decay ensembles is in agreement with the experimental results if we assume that the α-gas state in 56Ni exists at around 113-17+15 MeV. These results suggest that there may exist an exotic state consisting of many α particles at the excitation energy of 113-17+15 MeV.