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$$alpha$$-$$gamma$$ and high-resolution $$alpha$$ fine-structure spectroscopy for the heaviest nuclei

超重核の$$alpha$$-$$gamma$$核分光と高分解能$$alpha$$線微細構造分光

浅井 雅人 ; 羽場 宏光*; 塚田 和明 ; 佐藤 望; 笠松 良崇*; 加治 大哉*; 森本 幸司*; 森田 浩介*; 佐藤 哲也  ; 豊嶋 厚史; 石井 康雄; 高橋 竜太*; 永目 諭一郎 ; 石井 哲朗 ; 西中 一朗; 小島 康明*; 市川 隆敏*

Asai, Masato; Haba, Hiromitsu*; Tsukada, Kazuaki; Sato, Nozomi; Kasamatsu, Yoshitaka*; Kaji, D.*; Morimoto, Koji*; Morita, Kosuke*; Sato, Tetsuya; Toyoshima, Atsushi; Ishii, Yasuo; Takahashi, Ryuta*; Nagame, Yuichiro; Ishii, Tetsuro; Nishinaka, Ichiro; Kojima, Yasuaki*; Ichikawa, Takatoshi*

超重核の核構造を明らかにするため、104番元素Rf及び103番元素Lr同位体の$$alpha$$-$$gamma$$核分光実験及び高分解能$$alpha$$線微細構造分光実験を行った。$$alpha$$-$$gamma$$同時計数測定により$$^{259}$$Rfの$$alpha$$崩壊に伴う$$gamma$$線の観測に初めて成功し、$$^{259}$$Rfの基底状態の中性子軌道配位を同定した。その結果、$$^{259}$$Rfの基底状態の配位は同じ中性子数155の軽い核$$^{255}$$Fmや$$^{253}$$Cfとは異なることを明らかにした。また、高分解能$$alpha$$線微細構造測定法を用いて、$$^{255}$$Lrの基底状態と核異性体準位並びに$$^{257}$$Lrと$$^{259}$$Lrの基底状態の陽子軌道配位を実験的に同定した。巨視的微視的模型を用いた理論計算により、中性子数155を持つ原子核の基底状態の配位の変化は、陽子数が増えることによる原子核の16重極変形度の急激な減少と強い関連があることを明らかにした。

Alpha-gamma and high-resolution $$alpha$$ fine-structure spectroscopy of Rf and Lr isotopes was performed to clarify the nuclear structure of superheavy nuclei. $$gamma$$ rays following the $$alpha$$ decay of $$^{259}$$Rf were observed for the first time through the $$alpha$$-$$gamma$$ coincidence measurement, and the neutron configuration of the ground state of $$^{259}$$Rf was assigned. It was found that the ground-state configuration of $$^{259}$$Rf is different from those of the lighter N = 155 isotones $$^{255}$$Fm and $$^{253}$$Cf. For the Lr isotopes, the proton configurations of the ground and isomeric states in $$^{255}$$Lr and the ground states of $$^{257}$$Lr and $$^{259}$$Lr were definitely identified through the high-resolution $$alpha$$ fine structure spectroscopy. Theoretical calculations based on the macroscopic-microscopic model revealed that the change of the ground-state configuration in the N = 155 isotones is strongly associated with the rapidly decreasing hexadecapole deformation with increasing atomic number.

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