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論文

Role of multichance fission in the description of fission-fragment mass distributions at high energies

廣瀬 健太郎; 西尾 勝久; 田中 翔也*; L$'e$guillon, R.*; 牧井 宏之; 西中 一朗*; Orlandi, R.; 塚田 和明; Smallcombe, J.*; Vermeulen, M. J.; et al.

Physical Review Letters, 119(22), p.222501_1 - 222501_6, 2017/12

 被引用回数:12 パーセンタイル:11.2(Physics, Multidisciplinary)

JAEAタンデム加速器施設で行った$$^{18}$$O+$$^{238}$$U反応における多核子移行チャンネルを用いた実験により、$$^{237-240}$$U, $$^{239-242}$$Np、および$$^{241-244}$$Puの核分裂質量分布を励起エネルギー10$$sim$$60MeVにおいて測定した。これらのうち、$$^{240}$$U, $$^{240,241,242}$$Npのデータは本実験により初めて観測された。原子核の殻効果の減衰によって対称分裂すると予想されていた高励起エネルギーにおいても、質量分布が非対称を示すことがわかった。搖動散逸定理に基づく動力学モデル計算との比較から、この振る舞いはマルチチャンス核分裂によるものであることを明らかにした。

論文

Fission fragments mass distributions of nuclei populated by the multinucleon transfer channels of the $$^{18}$$O + $$^{232}$$Th reaction

L$'e$guillon, R.; 西尾 勝久; 廣瀬 健太郎; 牧井 宏之; 西中 一朗*; Orlandi, R.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; et al.

Physics Letters B, 761, p.125 - 130, 2016/10

 被引用回数:18 パーセンタイル:7.21(Astronomy & Astrophysics)

It is shown that the multinucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multinucleon transfer channels of the $$^{18}$$O + $$^{232}$$Th reaction are used to study fission of fourteen nuclei $$^{231,232,233,234}$$Th, $$^{232,233,234,235,236}$$Pa, and $$^{234,235,236,237,238}$$U. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions are measured for each transfer channel, in selected bins of excitation energy. In particular, the mass distributions of $$^{231,234}$$Th and $$^{234,235,236}$$Pa are measured for the first time. Predominantly asymmetric fission is observed at low excitation energies for all studied cases, with a gradual increase of the symmetric mode towards higher excitation energy. The experimental distributions are found to be in general agreement with predictions of the fluctuation-dissipation model.

口頭

Fission study of actinide nuclei using multi-nucleon transfer reactions

L$'e$guillon, R.; 西尾 勝久; 廣瀬 健太郎; Orlandi, R.; 牧井 宏之; 西中 一朗; 石井 哲朗; 塚田 和明; 浅井 雅人; 千葉 敏*; et al.

no journal, , 

Nucleon-transfer reaction allows us to produce neutron-rich nuclei, which cannot be accessed by fusion-evaporation reaction. We are promoting a campaign to measure fission-fragment mass distributions for actinide nuclei populated by transfer reactions. Unique feature in this approach in this fission study is that we can populate the fissioning nuclei from its ground state up to excitation energy of several tens MeV. We thus obtain the excitation energy dependence of the mass distribution for several nuclei. The fission barrier height can be determined from the excitation function of the fission probability. These data are important to develop a fission theory. The experiment was carried out at the 20 MV JAEA tandem facility at Tokai. We report on the data obtained in the reaction $$^{18}$$O + $$^{232}$$Th. Transfer-channels and excitation energies of the fissioning nuclei were identified using silicon $$Delta$$E-E detectors located to the forward direction of the $$^{232}$$Th target. Two fission fragments were detected in coincidence using multi-wire proportional counters. Fission fragment masses were determined by kinematic consideration. We obtained the fission fragment mass distributions for 12 nuclei of thorium, protactinium and uranium. The data include those for $$^{234}$$Th and $$^{234,235}$$Pa obtained for the first time. In this run, we also report prompt neutrons in fission obtained using liquid scintillation detectors.

口頭

Fission study of actinide nuclei using multi-nucleon transfer reactions

L$'e$guillon, R.; 西尾 勝久; 廣瀬 健太郎; Orlandi, R.; 牧井 宏之; 西中 一朗; 石井 哲朗; 塚田 和明; 浅井 雅人; 千葉 敏*; et al.

no journal, , 

We are promoting a campaign to measure fission-fragment mass distributions for various actinide nuclei populated by multi-nucleon transfer reactions. They include neutron-rich nuclei which cannot be accessed by fusion reaction. An unique feature of the transfer-reaction is that we can populate excited states from the ground state up to several tens MeV, allowing us to obtain the excitation energy dependence of the fragment mass distributions. The experiment was carried out at the 20 MV JAEA tandem facility at Tokai. We report on the data obtained in the reaction $$^{18}$$O + $$^{232}$$Th. Transfer-channels and excitation energies of the fissioning nuclei were identified using silicon dE-E detectors. Two fission fragments were detected in coincidence using multi-wire proportional counters. Fission fragment masses were determined by kinematic consideration. We obtained data for 13 nuclei from actinium to uranium.

口頭

Study for fission using multi-nucleon transfer reaction

西尾 勝久; 廣瀬 健太郎; L$'e$guillon, R.; 牧井 宏之; 西中 一朗; Orlandi, R.; Smallcombe, J.; 千葉 敏*; 荒木 祥平*; 渡辺 幸信*; et al.

no journal, , 

We report the recent development for the fission study using multi-nucleon transfer reaction at the JAEA tandem facility. Using multi-nucleon transfer reaction, neutron-rich nuclei can be produced, which cannot be accessed by fusion-reaction, and several nuclei populated by different transfer channels can be studied in one experiment. Another feature of transfer reaction is that the excitation energy of a compound nucleus is populated from the ground state up to several tens MeV, thus the excitation energy dependence of the fission properties can be obtained. We have carried out experiments in the reactions of $$^{18}$$O + $$^{238}$$U and $$^{18}$$O + $$^{232}$$Th. Transfer channel and the excitation energy of compound nucleus were determined by using silicon $$Delta$$E-E detectors. Fission fragments were detected by position-sensitive multi-wire proportional counters. We obtained the fragment mass distributions from actinium to plutonium isotopes for 24 nuclei and their excitation energy dependence. Among them, fragment mass distributions for nine nuclei were obtained for the first time.

口頭

Fission study of actinide nuclei using multi-nucleon transfer reaction

西尾 勝久; 廣瀬 健太郎; L$'e$guillon, R.; 牧井 宏之; 西中 一朗; Orlandi, R.; Smallcombe, J.; 千葉 敏*; 荒木 祥平*; 渡辺 幸信*; et al.

no journal, , 

Recent development of the fission study using multi-nucleon transfer reaction at the JAEA tandem facility will be presented. Using multi-nucleon transfer reaction, wide variety of nuclei around the target nucleus is produced. By identifying transfer channel, fission properties of several compound nuclei can be studied in one experiment. Another feature of transfer reaction is that excitation energy of a compound nucleus can be populated continuously from the ground state up to several tens MeV, allowing us to study the excitation energy dependence of fission properties. Experiments were carried out in the reactions of $$^{18}$$O + $$^{238}$$U and $$^{18}$$O + $$^{232}$$Th. Transfer channel and the excitation energy of the compound nucleus were determined by using silicon $$Delta$$E-E detectors. Fission fragments are detected by position-sensitive multi-wire proportional counters. Results for fission fragment mass distributions for more than 20 nuclei and their excitation energy dependence will be discussed.

口頭

Fission study using multi-nucleon transfer reactions

西尾 勝久; 廣瀬 健太郎; L$'e$guillon, R.; 牧井 宏之; Orlandi, R.; 西中 一朗; Smallcombe, J.; 千葉 敏*; 荒木 祥平*; 渡辺 幸信*; et al.

no journal, , 

A recent development for fission study using multi-nucleon transfer reactions at the JAEA tandem facility will be presented. Using multi-nucleon transfer reactions, a wide variety of nuclei including neutron-rich nuclei around a target nucleus is produced. By identifying a transfer channels, fission data of several compound nuclei can be studied in one experiment. Another feature of transfer reaction is that excitation energy of a compound nucleus can be populated continuously from the ground state up to several tens MeV, allowing us to study the excitation energy dependence of fission properties. The experiments were carried out in the reactions of $$^{18}$$O + $$^{238}$$U,$$^{232}$$Th,$$^{248}$$Cm. Results for fission fragment mass distributions and their excitation energy dependence are compared with a calculation based on a fluctuation dissipation model. Also, experimental data of prompt neutron multiplicity in fission and angular distribution of fission fragments will be discussed.

口頭

Fission study by multi-nucleon transfer reaction at JAEA

廣瀬 健太郎; 西尾 勝久; L$'e$guillon, R.; 牧井 宏之; 西中 一朗; Orlandi, R.; Smallcombe, J.; 石井 哲朗; 塚田 和明; 浅井 雅人; et al.

no journal, , 

We have developed a measurement system for the multi-nucleon transfer reaction and been studying fission of short-lived minor actinides and neutron-rich nuclides. The experiment was performed at the tandem accelerator facility of Japan Atomic Energy Agency where targets of $$^{238}$$U, $$^{232}$$Th, $$^{248}$$Cm were irradiated with $$^{18}$$O beams. In this experiment, more than ten nuclides were excited up to few tens of MeV at a time. Projectile-like scattered particles after the multi-nucleon transfer reaction were detected and identified using a $$Delta$$E-E silicon telescope. The mass distribution was deduced from the time difference between two fission fragments from the compound nucleus which were detected using two pairs of multi-wire proportional counters. Neutrons accompanied by fission were also measured using liquid scintillators placed around the reaction chamber. In the nucleon transfer reaction, the axis of the angular-momentum transfer can be determined from the direction of the scattered particle. This is one of the most different point from the neutron/ion-induced fusion-fission reaction where only the plane on which the axis lies can be determined. The fission angular distributions with respect to this axis as well as the mass yield, the prompt neutron multiplicity and so on will be presented.

口頭

原子力機構における融合核分裂・準核分裂および多核子移行反応の実験研究

廣瀬 健太郎; 西尾 勝久; 西中 一朗; 牧井 宏之; 池添 博*; Orlandi, R.; L$'e$guillon, R.; 塚田 和明; 浅井 雅人; 永目 諭一郎; et al.

no journal, , 

原子核に重イオンを衝突させたとき、入射核が標的核から多数の核子を受け取り、複合核を形成せずに核分裂(準核分裂)する場合や、複合核を形成して核分裂(融合核分裂)する場合がある。これらを弁別する手法を確立するために、原子力機構タンデム加速器施設を使用し、準核分裂および融合核分裂のメカニズムを調べてきた。$$^{238}$$Uに種々の原子核を衝突させ、合成した複合核の核分裂質量分布を測定した。取得した質量分布を、動的計算と比較した結果、よい一致を得た。この理論では原子核のポテンシャル上を系がどのように時間発展していくかを追跡できるため、融合核分裂が全核分裂にしめる割合(融合確率)を計算できる。得られた融合確率により、重元素合成断面積の実験結果をよく再現できた。すなわち重イオン入射反応において系が複合核を形成するか否かを知るための手法を確立した。さらに我々は、多核子移行反応を用いた核分裂の研究を行っている。これまでに、$$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm標的に$$^{18}$$Oビームを照射し、広い核種・励起エネルギー領域で複合核を合成し、その核分裂における質量分布などを測定した。一度の測定で約20核種を合成できることが多核子移行反応の利点であり、その半数程度の核種の核分裂質量分布は本研究により初めて得られた。これまでに一部の核種では質量分布の測定結果を先の理論計算により再現できることを確認した。今後さらなる比較を行い、理論計算の改良・適用領域の拡大が期待できる。

口頭

Study for fission using multi-nucleon transfer reaction

西尾 勝久; 廣瀬 健太郎; L$'e$guillon, R.; Orlandi, R.; 西中 一朗; Smallcombe, J.*; 千葉 敏; 荒木 祥平*; 渡辺 幸信*; 立澤 遼太郎*; et al.

no journal, , 

A recent development of fission study using multi-nucleon transfer reaction at the JAEA tandem facility will be presented. Using multi-nucleon transfer reactions, a wide variety of nuclei around a target nucleus is produced. By identifying transfer channels, fission data of several compound nuclei can be studied in one experiment. Another feature of transfer reaction is that excitation energy of a compound nucleus can be populated widely from the ground state up to several tens MeV, allowing us to study the excitation energy dependence of fission properties. The experiments were carried out in the reactions of $$^{18}$$O + $$^{238}$$U, $$^{232}$$Th, $$^{248}$$Cm. Results for fission fragment mass distributions and their excitation energy dependence are compared with a model calculation based on a fluctuation dissipation model. Also, experimental data of prompt neutron multiplicity in fission and angular distribution of fission fragments will be discussed.

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