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

Abstract

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

arXiv commentAccepted for publication in the European Physical Journal A

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• Clustering in hadrons and light nuclei from Lorentz boosted form factors