Study of deformed structure in ²⁵⁴Es by Coulomb excitation

Exploring the new elements toward the high end of the nuclear chart is one of the most interesting topics in nuclear physics. The key ingredient to stabilize nucleus in this region is a nuclear shell structure and Z=114, 120, N=184 are predicted to be new magic numbers. However, the study of such nuclei and their shell structure is limited by the very low cross sections. To investigate the shell structure, we are focusing on the nuclei in the A~250 region including ²⁵⁴Es. By studying the excited states, spin and parity, and deformation, we will be able to access the single-particle orbitals relevant to new shell structure at Z=114, 120, N=184 in the super-heavy mass region.

To study nuclear deformation in the A~250 region, we have performed Coulomb excitation experiments to determine the deformation of low-lying states of ²⁵⁴Es. The experiment was performed at the JAEA-Tokai Tandem accelerator using a 250-MeV ⁵⁸Ni beam irradiating a ²⁵⁴Es target. Particle-gamma coincidence measurements were conducted using segmented CD-silicon detectors and an array of Ge and LaBr₃ detectors. From the gamma-ray spectrum analysis, rotational band structures in ²⁵⁴Es were observed.

In the presentation, recent experimental results will be discussed.