Zakopane 2026 Conference on Nuclear Physics

This study presents a major advancement in understanding the nuclear structure of superheavy nuclei through spectroscopy of the high-K 8- isomer in 254No. Leveraging the enhanced sensitivity of the GABRIELA detector array at JINR (Dubna), γ-ray emissions and internal conversion electrons (ICE) were observed simultaneously for the first time. The results reveal a new 4+ state at 1203 keV, identified as the spin partner of the previously known 3+ state. Together, they form a Gallagher-Moskowski doublet; their 216 keV energy difference provides the first experimental measurement of spin splitting between two-quasiproton states in the actinide region. Furthermore, analysis of the ICE spectrum uncovered an intense electric monopole (E0) strength, indicating a second 0+ state at 884 keV excitation energy. These findings are well-reproduced by a new extension of the Shell Model (DNO-SM(VAP)), offering a vital benchmark for validating beyond-mean-field theoretical models in heavy deformed nuclei. All experimental results and comparison with theoretical model will be presented.