In nuclear physics protons and neutrons are usually treated as elementary particles. However, they are composite systems with their lowest energy excitation, the Delta resonance, at about 300 MeV. Therefore in reactions at such energies and above (particle/nucleon) the internal structure of the nucleons can play a role. Facilities as ISOLDE, GSI, RIKEN operate at such energies.
The 54Fe nucleus was populated at GSI from a 56Fe beam impinging on a thin 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 56Fe ground state. The isomeric state was produced in the low-momentum (-energy) tail of the parallel momentum (energy) distribution of 54Fe, 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 54Fe is a consequence of the quark structure of the nucleons.