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

TL;DR

This study investigates potential alpha-cluster states in the doubly-magic nucleus $^{56}$Ni through inelastic alpha scattering, revealing possible exotic alpha-gas states at high excitation energies that challenge traditional statistical decay models.

Contribution

The paper provides experimental evidence and theoretical modeling suggesting the existence of an alpha-gas state in $^{56}$Ni at high excitation energy, a novel insight into nuclear clustering phenomena.

Findings

High alpha-particle multiplicity observed in $^{56}$Ni

Statistical decay models cannot explain the data

Evidence supports an alpha-gas state at around 113 MeV

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 $113^{+15}_{-17}$ 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.