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

The ground-state rotational band of ^{252}Fm (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 ^{256}No 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 ^{252}Fm produced in the ^{18}O+^{249}Cf multinucleon transfer reaction. The new data reveal that, compared with neighboring fermium isotopes and N=152 isotones in the region, ^{252}Fm 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 ^{252}Fm as a deformed doubly magic nucleus.