Discovery of Np isomer and its decay properties

Asai, Masato  ; Suekawa, Yoshihide*; Higashi, Masato*; Kamada, Hiroki*; Tobe, Teruhisa*; Andreyev, A. N.; Hirose, Kentaro  ; Ito, Yuta   ; Makii, Hiroyuki   ; Nishio, Katsuhisa   ; Niwase, Toshitaka*; Orlandi, R.  ; Sakama, Minoru*; Sato, Tetsuya   ; Shibata, Michihiro*; Suzaki, Fumi  ; Suzuki, Hayato*; Tokoi, Katsuyuki*; Tomitsuka, Tomohiro; Tsukada, Kazuaki  

A new isomer Np with a half-life of 9 min has been discovered. It was produced in the reaction of U target with Li beam at JAEA tandem accelerator, and was identified through characteristic X-ray measurement and mass separation with an on-line isotope separator (ISOL). Gamma-ray singles, gamma-gamma coincidence, and internal-conversion electron measurements were performed. It was revealed that the Np decays mostly through an isomeric E4 transition followed by an M1 transition, and that the spin-parity of the Np should be 5. The results are introduced in detail, and the nuclear structure of the Np are discussed.