First Ab initio calculation of hexacontatetrapole E6 transition in 53FE isomer

The first ab initio calculation of the rarest electromagnetic transition in atomic nuclei, the hexacontatetrapole E6 transition in ⁵³Fe, has been performed. Using the valence-space in-medium similarity renormalization group (VS-IMSRG) methods with realistic nuclear force and bare nucleon charges, the study has successfully explained both the excitation energies and electromagnetic decay rates of the unique T_1/2 = 2.54-minutes J^π = 19/2^- isomer at 3.0 MeV. This study provides unprecedented insights into nuclear structure under extreme conditions and validates ab initio approaches for describing the high-multipole electromagnetic transitions in atomic nuclei. The research demonstrates that the formation of 19/2^- isomer arises from the pure 0f_7/2 orbital configuration.

Unique Nuclear Isomer Phenomenon

The J^π=19/2^- isomer in ⁵³Fe represents an extraordinary nuclear phenomenon which can directly decay to the ground state with a single photon with angular momentum of 6ℏ emitting. This E6 transition is exceptionally rare in nature, while similar high-multipole processes have been observed in solid hydrogen and atomic rubidium, ^53mFe remains the only nuclear system where such a high-multipole single-photon emission occurs spontaneously.

In a recently published paper, the research team employed the state-of-the-art VS-IMSRG technique with two different chiral nucleon-nucleon plus three-nucleon interactions. Unlike empirical models that require effective charges adjusted for different multipolarities, this approach uses bare nucleon charges consistently across all transition types.

Experimental Validation and Theoretical Implications

The calculations successfully reproduce the experimental excitation spectrum for the first time and provide the first ab initio calculations for the E6, M5, and E4 transition probabilities. The ab initio method allows researches to calculate electromagnetic transitions without phenomenological adjustments. The results achieved convergence with large model spaces up to e_max=14 and E_3max=24, providing reliable predictions for these extremely sensitive observables.

Scientific Significance and Future Applications

"The ab initio method allows us to calculate electromagnetic transitions without effective charges." said the leader researcher of this work, Doctor Siqin Fan. " This work opens new avenues for understanding high-multipole transitions in nuclei," commented a senior nuclear theorist involved in the study. "The ability to describe such extreme transitions with bare charges represents a significant milestone in nuclear structure theory."

The complete study is accessible by DOI: 10.1007/s41365-025-01812-2