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$$^{255}$$Noの$$alpha$$-$$gamma$$核分光

$$alpha$$-$$gamma$$ spectroscopy of $$^{255}$$No

浅井 雅人 ; 塚田 和明 ; 羽場 宏光*; 豊嶋 厚史; 石井 哲朗 ; 永目 諭一郎 ; 西中 一朗; 市川 隆敏*; 小島 康明*; 末木 啓介*

Asai, Masato; Tsukada, Kazuaki; Haba, Hiromitsu*; Toyoshima, Atsushi; Ishii, Tetsuro; Nagame, Yuichiro; Nishinaka, Ichiro; Ichikawa, Takatoshi*; Kojima, Yasuaki*; Sueki, Keisuke*

超重核の殻構造を実験的に明らかにするため、$$^{255}$$Noの$$alpha$$崩壊に伴う$$gamma$$線を$$alpha$$-$$gamma$$同時計数法を用いて測定し、中性子一粒子軌道のエネルギー間隔や順序を調べた。7本の$$gamma$$線が観測され、$$^{255}$$Noの娘核$$^{251}$$Fmの励起準位のエネルギーを決定することができた。また$$^{251}$$Fmの励起準位及び$$^{255}$$Noの基底状態のスピン・パリティ,中性子軌道配位を同定し、中性子数N=151原子核における中性子一粒子軌道のエネルギー間隔と順序を明らかにした。その結果、Fm及びNo原子核におけるN=152変形閉殻のエネルギーギャップが、それより軽いCf, Cm, Pu原子核に比べて大きいことがわかり、超重核の殻構造を明らかにするうえで重要な情報が得られた。

$$gamma$$ rays following the $$alpha$$ decay of $$^{255}$$No have been measured by means of $$alpha$$-$$gamma$$ coincidence spectroscopy to establish neutron single-particle states in odd-mass superheavy nuclei. Seven $$gamma$$ transitions were observed, and excitation energies of the daughter nucleus $$^{251}$$Fm were determined. Spin-parities and neutron single-particle configurations of the excited states in $$^{251}$$Fm as well as the ground state of $$^{255}$$No were identified, and energy spacings and order of neutron single-particle states in N=151 isotones were established. It was found that the N=152 deformed neutron shell gap is larger in Fm and No nuclei than in lighter nuclei of Cf, Cm and Pu, which provides important information to clarify the shell structure of superheavy nuclei.

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