Direct Measurement of Hexacontatetrapole, E6? Decay from 53mFe

The only proposed observation of a discrete, hexacontatetrapole (E6) transition in nature occurs from the T1=2 = 2.54(2)-min decay of 53mFe. 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 53mFe. 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 fp 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 lowermultipole, electric transitions observed in atomic nuclei.