If an EECR is not a proton, but a nucleus with A nucleons, then the GZK limit applies to its nucleons, which carry only a fraction ⁠ 1 / A ⁠ of the total energy of the nucleus. There is evidence that these highest-energy cosmic rays might be iron nuclei, rather than the protons that make up most cosmic rays. For an iron nucleus, the corresponding limit would be 2.8 × 10 eV. However, nuclear physics processes lead to limits for iron nuclei similar to that of protons. Other abundant nuclei should have even lower limits.