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 $^{56}$Ni 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 $^{56}$Ni exists at around $113^{+15}_{-17}$ MeV. These results suggest that there may exist an exotic state consisting of many $α$ particles at the excitation energy of $113^{+15}_{-17}$ MeV.