New results on the decay spectroscopy of ²⁵⁴No with GABRIELA@SHELS

The structure of the No-254(102)152 nucleus has been studied for more than 20 years: the last publications on its decay spectroscopy are from LBNL [1], GSI [2], JYFL [3] and ANL [4]. Four decay schemes featuring two isomers have been published and are interpreted differently in terms of excitation energy and decay scheme of the 2(nd) isomer and configuration assignments of both K-isomers. These discrepancies have triggered new experiments including this one, performed with the GABRIELA [5, 6] array, at the focal plane of the SHELS [7] separator at the FLNR, Dubna. The first part of this proceeding will present the experimental setup and the analysis techniques used to reveal the electromagnetic decay of the known isomers in No-254. The second part will focus on the new results obtained with more than 1 million No-254 nuclei implanted in the focal plane detector. In particular, the internal conversion electron spectrum observed in the decay of the 8(-) K-isomer has revealed the presence of a strong transition, most likely E0, suggesting low-lying shape coexistence in this nucleus as predicted in [8, 9]. The gamma-ray spectrum obtained from the decay of the short-lived 170 mu s isomer has revealed new gamma-ray lines putting in doubt the previous interpretations about this isomer decay.