Direct Measurement of Hexacontatetrapole, $E6$ $\gamma$ Decay from ${}^{53m}\mathrm{Fe}$

The only proposed observation of a discrete, hexacontatetrapole ($E6$) transition in nature occurs from the $T_{1/2}=2.54(2)\text{−}\min$ decay of ${}^{53m}\mathrm{Fe}$. 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 ${}^{53m}\mathrm{Fe}$. 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 $fp$ 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 lower-multipole, electric transitions observed in atomic nuclei.

Figure 1

Level scheme showing the energies (in keV) of excited states and $\gamma$-ray transitions observed in the decay of ${}^{53m}\mathrm{Fe}$ [6], together with nucleon configurations that couple to form the $19/2^{-}$ isomer. The $\gamma$-ray intensities were determined in this Letter. Proton (neutron) particles are depicted by red (blue) solid spheres; proton (neutron) holes are shown as faded spheres. Coupling of the proton- and neutron-hole configurations leads to formation of the $19/2^{-}$ isomeric state at 3040 keV.

Figure 2

(a) Time spectrum from the ADC clock recorded with each $\gamma$-ray event illustrating the irradiation and out-of-beam collection period split into two parts, gates A and B. Lower panels show (b) the total $\gamma$-ray spectrum recorded (gate A plus gate B) and (c) the subtracted spectrum (gate A minus gate B) described in the text. The inset spectrum is on a linear scale and expands the region near the 3041-keV, $E6$ transition.

Figure 3

Proton effective charges calculated for $\lambda =2$, 4, and 6 with seven different interactions (red and blue lines) [20] compared to experimental values for $\lambda =2$ (open circle) [19] and $\lambda =4$, 6 (closed circles) from this work.