Signature of a possible $$\alpha $$α -cluster state in $$N=Z$$N=Z doubly-magic $$^{56}$$56 Ni

An inelastic $$\alpha $$ -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 $$\alpha $$ -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 $$\alpha $$ particles corresponds to an excitation energy around 96 MeV. The method of distributed $$m\alpha $$ -decay ensembles is in agreement with the experimental results if we assume that the $$\alpha $$ -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 $$\alpha $$ particles at the excitation energy of $$113^{+15}_{-17}$$  MeV.