Role of the Δ Resonance in the Population of a Four-Nucleon State in the Fe 56 → Fe 54 Reaction at Relativistic Energies

Abstract

The Fe54 nucleus was populated from a Fe56 beam impinging on a Be target with an energy of E/A=500 MeV. The internal decay via γ-ray emission of the 10+ metastable state was observed. As the structure of this isomeric state has to involve at least four unpaired nucleons, it cannot be populated in a simple two-neutron removal reaction from the Fe56 ground state. The isomeric state was produced in the low-momentum (-energy) tail of the parallel momentum (energy) distribution of Fe54, suggesting that it was populated via the decay of the Δ0 resonance into a proton. This process allows the population of four-nucleon states, such as the observed isomer. Therefore, it is concluded that the observation of this 10+ metastable state in Fe54 is a consequence of the quark structure of the nucleons.