Deformation and Magicity in Heavy Actinides: First Observation of the Ground-State Rotational Band of Fm 252

The ground-state rotational band of ²⁵²Fm (Z=100, N=152) was investigated using two different experimental setups at the JAEA Tandem Accelerator Laboratory. The 2⁺→0⁺ and 4⁺→2⁺ transitions were observed in the α decay of ²⁵⁶No via α-γ coincidence spectroscopy. The energies of the (6⁺)→(4⁺), (8⁺)→(6⁺), and (10⁺)→(8⁺) transitions were identified in the prompt γ-ray spectrum of ²⁵²Fm produced in the ¹⁸O+²⁴⁹Cf multinucleon transfer reaction. The new data reveal that, compared with neighboring fermium isotopes and N=152 isotones in the region, ²⁵²Fm has the lowest first excited 2⁺-state energy, 41.88(16) keV, and the largest kinematic moment of inertia. These properties constitute new evidence for the deformed shell gaps at Z=100 and N=152, supporting the understanding of ²⁵²Fm as a deformed doubly magic nucleus.