Lrの質量対称自発核分裂の測定

Measurement of mass-symmetric spontaneous fission of Lr

浅井 雅人  ; 鎌田 裕生*; 重河 優大*; 塚田 和明  ; 佐藤 哲也   ; 豊嶋 厚史; 水飼 秋菜; 永目 諭一郎 ; 西尾 勝久   ; 廣瀬 健太郎  ; Andreyev, A. N.; 長 明彦  ; 西中 一朗*; 柴田 理尋*; 富塚 知博; 後藤 真一*; 阪間 稔*

Asai, Masato; Kamada, Hiroki*; Shigekawa, Yudai*; Tsukada, Kazuaki; Sato, Tetsuya; Toyoshima, Atsushi; Mitsukai, Akina; Nagame, Yuichiro; Nishio, Katsuhisa; Hirose, Kentaro; Andreyev, A. N.; Osa, Akihiko; Nishinaka, Ichiro*; Shibata, Michihiro*; Tomitsuka, Tomohiro; Goto, Shinichi*; Sakama, Minoru*

中性子過剰Fm領域核で特異的に観測される質量対称自発核分裂の分裂メカニズムを明らかにするため、この領域の原子核の自発核分裂片の運動エネルギー及び質量分布の精密測定を行った。実験は原子力機構タンデム加速器に付設されたオンライン同位体分離装置ISOLを用い、同位体分離したLrを薄膜に捕集し、2台のSi検出器で核分裂片を同時計数測定した。Lrの自発核分裂は分布幅の狭い質量対称分裂を示すが、全運動エネルギーTKEの値は低く、Fm同位体で観測される高TKE対称核分裂とは明らかに異なるメカニズムを持つことを明らかにした。

To clarify the fission mechanism of mass-symmetric spontaneous fissions observed around the neutron-rich Fm region, we have measured kinetic energies and mass distribution of spontaneous-fission fragments in this nuclear region precisely. The experiment was performed at the JAEA tandem accelerator facility using an on-line isotope separator (ISOL). The mass-separated Lr nuclei were implanted into thin foil, and two fragments of their spontaneous fission were coincidently measured with two Si detectors. The spontaneous fission of Lr was found to show a narrow symmetric mass distribution. On the other hand, their total kinetic energy TKE was found to be low, which is apparently different from the high-TKE symmetric fissions observed in the Fm isotopes. This indicates that the fission mechanisms of these two symmetric fissions are different.