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

TL;DR

This study provides the first direct measurement and confirmation of the rare $E6$ gamma decay transition in $^{53m}$Fe, resolving previous uncertainties and revising decay branching ratios through experimental and computational analysis.

Contribution

The paper presents the first firm quantification of the weak $E6$ and $M5$ decay branches in $^{53m}$Fe using sum-coincidence methods and shell model calculations, confirming the existence of the $E6$ transition.

Findings

Confirmed the existence of the $E6$ transition in $^{53m}$Fe.

Revised the $M5$ branching ratio and transition rate.

Found that the effective proton charge for high-multipole transitions is quenched to about two-thirds of the $E2$ value.

Abstract

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.