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論文

核データ研究の最前線; たゆまざる真値の追及、そして新たなニーズへ応える為に,3; 核分裂データの最前線; 実験と理論

西尾 勝久; 千葉 敏*

日本原子力学会誌, 59(12), p.717 - 721, 2017/12

核分裂は原子力エネルギー利用の基礎となる現象であるが、その複雑さのため質および量ともに限られた実験データを現象論によって記述し、核データとして利用されるにとどまっていた。近年、新規の実験技術確立に加え高性能計算機を活かした理論計算に大きな進展があったので、最新成果を見ながら議論する。

論文

Study of fission using multi-nucleon transfer reactions

西尾 勝久; 廣瀬 健太郎; Vermeulen, M. J.; 牧井 宏之; Orlandi, R.; 塚田 和明; 浅井 雅人; 豊嶋 厚史; 佐藤 哲也; 永目 諭一郎; et al.

EPJ Web of Conferences (Internet), 163, p.00041_1 - 00041_6, 2017/11

 パーセンタイル:100

We are promoting a study of fission using multi-nucleon transfer (MNT) reactions, where excited states in neutron-rich actinide nuclei, which cannot be accessed by particle capture and/or fusion reactions, are populated. Also, the excited states in the fissioning nucleus are widely populated by the MNT reactions, from which effects of excitation energy on fission properties can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the $$^{18}$$O beam and several actinide target nuclei such as $$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm. Ejectile nucleus was identified by a silicon $$Delta$$E-E telescope to identify transfer channel and hence the compound nucleus. Fission fragments were detected by multi-wire proportional counters, and fission fragment mass distributions (FFMDs) were measured for each isotope. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model, and importance of multi-chance fission concept is investigated. Fission fragment angular distribution relative to the recoil direction suggested the increase of the spin of the fissioning nucleus with the number of transferred nucleons.

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

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.

口頭

Experimental study of fission using multinucleon transfer reactions

西尾 勝久; 廣瀬 健太郎; 牧井 宏之; Orlandi, R.; 塚田 和明; Vermeulen, M. J.; Andreyev, A.; 千葉 敏; Tsekhanovich, I.*; 大槻 勤*

no journal, , 

We are promoting a campaign to study multi-nucleon transfer reactions to populate excited states in neutron-rich actinide nuclei which cannot be accessed by particle capture and/or fusion reactions. Main purpose of this program is to study fission in new region of chart of nuclei. As a first step we studied reactions using the $$^{18}$$O beam ($$sim$$ 9 MeV/u) and actinide target nuclei such as $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm. The experiment was carried out at the tandem facility of Japan Atomic Energy Agency. Ejectile nuclei generated in the reaction were identified by a newly developed silicon $$Delta$$E-E detectors. Using this detector, clear separation of oxygen isotopes ($$^{16,17,18,19}$$O) was obtained as well as lighter element isotopes (Be, B, C, N). The number of produced nuclei amounted to more than fifteen in one reaction. Two fragments emitted in multi-nucleon transfer fission were detected by multi-wire proportional counters, and the fragment mass distributions were obtained for each isotopes. The measured FFMDs agreed with a calculation based on the fluctuation-dissipation model up to about excitation energy 30 MeV. Measurement of fission fragment angular distribution relative to the recoil direction suggested the increase of the spin with the number of transferred nucleons.

口頭

Study of fission using multi-nucleon transfer reactions

西尾 勝久

no journal, , 

We are promoting a study of fission using multi-nucleon transfer reactions, where excited states in neutron-rich actinide nuclei, which cannot be accessed by particle capture and/or fusion reactions, are populated. This allows us to study fission mechanism in the new region of chart of nuclei. Also, the excited states in the fissioning nucleus are widely populated by the multi-nucleon transfer reactions, from which effects of excitation energy on fission properties can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the $$^{18}$$O beam ($$sim$$ 9 MeV/u) and several actinide target nuclei such as $$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm. Ejectile nuclei were identified by a newly developed silicon $$Delta$$E-E detectors ($$Delta$$E = 75 $$mu$$m, E = 300 $$mu$$m). Clear separation of oxygen isotopes ($$^{16,17,18,19}$$O) was obtained as well as lighter element isotopes (Be, B, C, N). Number of produced nuclei reached to more than fifteen in a single measurement. Both fission fragments were detected by multi-wire proportional counters, and fission fragment mass distributions (FFMDs) were measured for each isotopes. The data allows us to obtain the systematic trends of FFMDs in detail. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model, from which the shell damping energy was determined.

口頭

Fission study using multi-nucleon transfer reaction at JAEA tandem

西尾 勝久

no journal, , 

This talk will discuss the use of multi-nucleon transfer (MNT) reactions to study fission properties of multitude exotic nuclei in the neutron-rich actinide region. Most of these nuclei cannot be accessed by the traditional method of complete-fusion reactions. The MNT transfer channels of the $$^{18}$$O+$$^{232}$$Th reaction were 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. Fission fragment mass distributions (FFMDs) are measured for each transfer channel. In particular, the FFMDs of $$^{234}$$Th and $$^{234,235,236}$$Pa were 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. By using the same method, the measurements with $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm, and $$^{249}$$Cf targets were recently performed. The obtained FFMDs are compared with a calculation based on the fluctuation-dissipation model, where effect of multi-chance fission (neutron evaporation prior to fission) was considered. It was found that multi-chance fission has significant role on the shape of FFMD, particularly at the high-excitation energies.

口頭

Fission studies using multi-nucleon transfer reactions

西尾 勝久

no journal, , 

We are promoting a program of fission studies using multi-nucleon transfer reactions. This approach allows us to investigate fission of neutron-rich nuclei, which cannot be populated by particle capture reactions. From the measurement of fission fragment mass distributions (FFMDs) for various nuclides and their evolution with initial excitation energy, role of multi-chance fission, i.e. neutron emission prior to fission, on FFMD was investigated. The obtained large data set of FFMDs was used to improve the Langevin model to describe low-energy fission, which was applied to the fissions of fermium isotopes. The results explain the origin of the onset of sharp symmetric fission appearing in neutron-rich fermium isotopes. In the presentation, we will also discuss other data obtained in our setup, such as fission barrier height and prompt neutrons in correlation with fission fragments.

口頭

Measurement of fission fragment mass distributions of actinide nuclides using multi-nucleon transfer reactions

西尾 勝久

no journal, , 

Using the JAEA tandem accelerator facility, we are promoting a program to take fission data using multi-nucleon transfer (MNT) reactions. The MNT reaction allows us to produce neutron-rich actinide nuclei, which cannot be populated by particle capture reactions. Fission fragment mass distributions (FFMDs) and fission barrier heights for various nuclei were obtained. The FFMDs and their evolution with initial excitation energies were explained only by invoking a concept of multi-chance fission, i.e. neutron-evaporation before fission.

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