Measurements of the first excited-state energies in even-even actinide nuclei

Asai, Masato  ; Tsukada, Kazuaki  ; Toyoshima, Atsushi; Sato, Tetsuya   ; Nagame, Yuichiro ; Sato, Nozomi; Ishii, Tetsuro  ; Kasamatsu, Yoshitaka*

The first excited-state energies E(2) in the even-even actinide nuclei Pu, Cm, Cf, and Fm were determined for the first time by means of a high-resolution energy measurement and an - coincidence measurement. The -decay parents of these nuclei were produced with the JAEA Tandem Accelerator, and their decays were measured with a rotating-wheel (-) detection system coupled with a gas-jet transport technique. From the energy difference between transitions to the ground state and to the first excited state, E(2) was determined with uncertainties of less than 0.5 keV. It was found that the E(2) takes the minimum around Cm and Cf isotopes, and then, increases with the atomic number. This trend is reasonably explained as a decreasing deformation towards the Z 114 spherical closed shell. In addition, the E(2) shows the local minimum at Fm, indicating the existence of the doubly deformed shell gaps at Z = 100 and N = 152.