Direct measurement of hexacontatetrapole, \(\textbf{E6}\) {\gamma} decay from \(^{\textbf{53m}}\)Fe

The only proposed observation of a discrete, hexacontatetrapole (\(E6\)) transition in nature occurs from the T\(_{1/2}\) = 2.54(2)-minute decay of \(^{53m}\)Fe. However, there are conflicting claims concerning its \(\gamma\)-decay branching ratio, and a rigorous interrogation of \(\gamma\)-ray sum contributions is lacking. Experiments performed at the Australian Heavy Ion Accelerator Facility were used to study the decay of \(^{53m}\)Fe. For the first time, sum-coincidence contributions to the weak \(E6\) and \(M5\) decay branches have been firmly quantified using complementary experimental and computational methods. Agreement across the different approaches confirms the existence of the real \(E6\) transition; the \(M5\) branching ratio and transition rate have also been revised. Shell model calculations performed in the full \(pf\) model space suggest that the effective proton charge for high-multipole, \(E4\) and \(E6\), transitions is quenched to approximately two-thirds of the collective \(E2\) value. Correlations between nucleons may offer an explanation of this unexpected phenomenon, which is in stark contrast to the collective nature of lower-multipole, electric transitions observed in atomic nuclei.