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論文

Origin of the dramatic change of fission mode in fermium isotopes investigated using Langevin equations

宮本 裕也*; 有友 嘉浩*; 田中 翔也; 廣瀬 健太郎; 西尾 勝久

Physical Review C, 99(5), p.051601_1 - 051601_7, 2019/05

The fission of even-even fermium nuclides $$^{250-260}$$Fm at low excitation energy was studied using Langevin equations of three-dimensional nuclear-shape parametrization. The mass distributions of fission fragments show a dramatic change from an asymmetric shape for the lighter fermium isotopes to sharp symmetric fission for the heavier isotopes. The time evolution of the nuclear shape on the potential surface reveals that the lighter fermium isotopes showing asymmetric fission are trapped in the second minimum for a substantial length of time before overcoming the second saddle point. This behavior changes dramatically for the compact symmetric fission found in the heavier neutron-rich fermium nuclei that disintegrate immediately after overcoming the first saddle point, without feeling the second barrier, resulting in a fission time two orders of magnitude shorter.

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

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.

論文

Study of fission using multi-nucleon transfer reactions

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

Proceedings of 6th International Conference on Fission and Properties of Neutron-rich Nuclei (ICFN-6), p.590 - 597, 2017/11

We are promoting a fission study 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 in the new region of chart of nuclei. Also, the excited states in the fissioning nucleus are widely populated in the reactions, thus the effects of excitation energy on fission can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the $$^{18}$$O beam ($$sim$$9MeV/u) and several actinide target nuclei such as $$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm. Ejectile nuclei and thus the transfer channels were identified by a newly developed silicon $$Delta$$E-E detectors. The produced nuclei in one experiment reached more than fifteen. Fission fragment mass distributions (FFMDs) were measured for each isotopes. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model (Langevin-type calculation), where effects of multi-chance fission were included. We also started to measure the prompt neutrons accompanied by fission.

論文

Experimental 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.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; 田中 翔也*; et al.

EPJ Web of Conferences (Internet), 146, p.04009_1 - 04009_6, 2017/09

 被引用回数:2 パーセンタイル:4.05

The objective of this work is to extend fission data for actinide nuclei not investigated so far in order to understand the fission mechanism, especially for neutron-rich nuclei. Multi-nucleon transfer reactions were used to populate the compound nuclei which cannot be accessed by particle capture or fusion reactions. The experiment was carried out at the tandem facility of Japan Atomic Energy Agency. In the multi-nucleon transfer reactions using $$^{18}$$O beam, a wide variety of nuclei around a target nucleus was produced. By identifying transfer channels, fission data of nuclei more than fifteen compound nuclei were generated in one experiment. Another feature of transfer reaction is that the excitation energies 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. From the excitation function of fission probabilities, fission-barrier heights of neutron-rich actinide nuclei were obtained. Experiments were carried out in the reactions of $$^{18}$$O + $$^{238}$$U, $$^{232}$$Th, $$^{248}$$Cm, $$^{237}$$Np. The obtained fission fragment mass distributions were reproduced by a model calculation based on a fluctuation dissipation model.

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.

論文

A Comprehensive approach to determination of nuclear data of unstable nuclei

千葉 敏*; 西尾 勝久; 有友 嘉浩*; 小浦 寛之; 岩本 修; 牧井 宏之; 西中 一朗*; 廣瀬 健太郎

EPJ Web of Conferences (Internet), 106, p.04004_1 - 04004_9, 2016/02

 パーセンタイル:100

A comprehensive approach to determine nuclear data of unstable nuclei will be described. It consists of a measurement of fission and capture cross sections, mass distribution of fission fragments (independent fission yields) and number of prompt fission neutrons by the method of surrogate reactions. A multi-dimensional Langevin model is being developed to estimate the independent fission yields theoretically. Furthermore, the $$beta$$ decay properties of the fission fragments, almost all are neutron-rich unstable nuclei, are investigated systematically by improving the gross theory of $$beta$$ decay, which will yield information on the decay heat and delayed-neutron data.

論文

Surrogate reactions research at JAEA/Tokyo Tech

千葉 敏; 西尾 勝久; 牧井 宏之; 有友 嘉浩*; 西中 一朗; 石井 哲朗; 塚田 和明; 浅井 雅人; 古高 和禎; 橋本 慎太郎; et al.

Nuclear Data Sheets, 119, p.229 - 232, 2014/05

 パーセンタイル:100(Physics, Nuclear)

We will present a status of researches in surrogate reaction method using heavy-ions to determine neutron fission and capture cross sections of unstable nuclei. We take advantage of having (1) an electrostatic tandem accelerator which can deliver highly mono-energetic beams of heavy ions, (2) rich experience in in-beam $$gamma$$ spectroscopy, (3) rich experience in measuring fission fragments induced by heavy-ions, (4) nuclear theory and evaluation experiences. We have constructed apparatus to measure fission fragments and $$gamma$$-rays in coincidence with ejectiles by which we can identify the populated compound nuclei. Primarily, we used $$^{18}$$O-induced reactions as well as $$^3$$He-induced reactions. We also investigated conditions under which such measurements lead to correct neutron cross sections. Results of the theoretical researches were published in a series of papers. In this presentation, status of results of the above researches will be summarized.

論文

In-beam fissio study at JAEA for heavy element synthesis

西尾 勝久; 池添 博; Hofmann, S.*; Ackermann, D.*; 有友 嘉浩*; Comas, V. F.*; D$"u$llmann, Ch. E.*; Heinz, S.*; Heredia, J. A.*; He${ss}$berger, F. P.*; et al.

AIP Conference Proceedings 1524, p.68 - 72, 2013/04

 パーセンタイル:100

Fission fragment mass and kinetic energies distributions were measured in the heavy-ion induced reactions using $$^{238}$$U target nucleus at the energies around the Coulomb barrier. We observed strong variation of the fragment mass distribution with incident energy, which was explained by an energy dependence of the probabilities for fusion and quasifission. A calculation based on a fluctuation-dissipation model was carried out to determine the fusion probability in the reactions of $$^{30}$$Si+$$^{238}$$U and $$^{34}$$S+$$^{238}$$U. The fusion probabilities were consistent with those determied from the evaporation residue cross sections for both reactions, showing that in-beam fission measurement and analysis can give a reasonable estimation for the cross sections to produce super-heavy nuclei. We also discss the results for in-beam fission measurement of $$^{48}$$Ca+$$^{238}$$U.

論文

Fusion probabilities in the reactions $$^{40,48}$$Ca + $$^{238}$$U at energies around the Coulomb barrier

西尾 勝久; 光岡 真一; 西中 一朗; 牧井 宏之; 若林 泰生*; 池添 博; 廣瀬 健太郎*; 大槻 勤*; 有友 嘉浩*; Hofmann, S.*

Physical Review C, 86(3), p.034608_1 - 034608_6, 2012/09

 被引用回数:43 パーセンタイル:6.26(Physics, Nuclear)

Fission cross sections and fission fragment mass distributions were measured in the reactions of $$^{40}$$Ca + $$^{238}$$U and $$^{48}$$Ca + $$^{238}$$U at energies around the Coulomb barrier. Fusion probabilities were calculated based on the fluctuation dissipation model. The measured mass distributions for both reactions showed an asymmetric shape at low incident energies, whereas the distribution changed to a flat shape at higher energies. The variation of the mass distribution is explained by a change of the ratio between fusion and qasifission with nuclear orientation. The calculation reproduced the mass distributions and their energy dependence. The trajectories for fusion-fission were used to determine the fusion probability. Fusion probabilities for both reactions are identical as function of the center-of-mass energy, but they differ when plotted as function of the excitation energy. The calculated evaporation residue cross sections for the reaction $$^{48}$$Ca + $$^{238}$$U using a statistical model showed that sub-barrier fusion reaction can be applied to form the element 112.

論文

Dynamical approach to heavy-ion induced fission using actinide target nuclei at energies around the Coulomb barrier

有友 嘉浩; 萩野 浩一*; 西尾 勝久; 千葉 敏

Physical Review C, 85(4), p.044614_1 - 044614_15, 2012/04

 被引用回数:42 パーセンタイル:6.62(Physics, Nuclear)

入射エネルギーがクーロンバリア近傍のエネルギー領域における、アクチノイド標的核を用いた重イオン融合反応を記述するために、チャネル結合法と散逸揺動理論に基づく動力学的手法を用いて構築した新しい模型を提案した。アクチノイド標的核は変形しているために、標的核と入射核はさまざまな配位で接触するが、それぞれの配位における接触確率をチャネル結合法で計算し、接触点から球形領域に向かう融合過程と準核分裂過程の競合過程をランジュバン方程式を用いた動力学模型を導入して記述した。それぞれの過程は変形パラメータ空間における軌道を解析することで区別が可能である。この模型を用いて原子力研究開発機構で測定された$$^{34,36}$$S+$$^{238}$$U及び$$^{30}$$Si+$$^{238}$$Uのさまざまな入射エネルギーに対する質量分布の計算を行い、実験データと比較した。またそれぞれの質量分布の特徴がどの様に現れるかを軌道解析によって明らかにした。さらにそれぞれの反応における準核分裂過程の時間スケールや分裂片の変形度の違いなどについて詳細に調べ、反応機構を明らかにした。

論文

Spin-dependent observables in surrogate reactions

千葉 敏; 岩本 修; 有友 嘉浩

Physical Review C, 84(5), p.054602_1 - 054602_5, 2011/11

 被引用回数:7 パーセンタイル:46.07(Physics, Nuclear)

代理反応の有効性を検証するため、代理反応で生成されたさまざまなスピン状態から放出される観測量についての議論を行う。放出される$$gamma$$線のエネルギースペクトルと多重度,蒸発中性子のエネルギースペクトル,核分裂生成物の質量数分布はいずれも崩壊する状態のスピンに対する依存性を有することがわかった。これにより、これらの量を観測することで、分岐比に大きな影響を与える複合核状態のスピン分布に対する情報が得られることがわかった。

論文

Investigation of fission properties and evaporation residue measurement in the reactions using $$^{238}$$U target nucleus

西尾 勝久; 池添 博; Hofmann, S.*; Ackermann, D.*; Antalic, S.*; 有友 嘉浩; Comas, V. F.*; D$"u$llmann, Ch. E.*; Gorshkov, A.*; Graeger, R.*; et al.

EPJ Web of Conferences (Internet), 17, p.09005_1 - 09005_4, 2011/10

 被引用回数:1 パーセンタイル:46.63

Fission fragment mass distributions after full momentum transfer of the projectile were measured in the reactions using a $$^{238}$$U target nucleus bombarded by $$^{30}$$Si, $$^{31}$$P, $$^{34,36}$$S, $$^{40}$$Ar, $$^{40,48}$$Ca projectiles. We observed a strong variation of the distribution as function of bombarding energy. The results were interpreted by the effects of nuclear deformation of the target nucleus on the competition between fusion and quasifission. The symmetric fission includes fusion-fission, wheres as the asymmetric fission represents quasifission. The fusion probabilities were determined with the help of fluctuation-dissipation model. The estimated evaporation residue cross sections ($$^{263,264}$$Sg and $$^{267,268}$$Hs) produced in $$^{30}$$Si + $$^{238}$$U and $$^{34}$$S+$$^{238}$$U reproduced the experimental data.

論文

Dynamical model of surrogate reactions

有友 嘉浩; 千葉 敏; 西尾 勝久

Physical Review C, 84(2), p.024602_1 - 024602_10, 2011/08

 被引用回数:6 パーセンタイル:50.64(Physics, Nuclear)

核子移行反応,生成核の熱化及び崩壊という複数の過程を含む代理反応の全プロセスを記述できる動的模型を開発した。系の集団座表の時間発展を、時間依存のポテンシャルエネルギー上での多次元ランジュバン方程式の方法で分析する。本モデルにより代理反応上重要な複合核のスピン分布を計算することが可能になった。さらに、代理反応における種々の観測量の計算も可能である。それによりモデルパラメータの決定を行い、不確定さの少ない計算を行うことができる。本模型を$$^{18}$$O+$$^{238}$$U$$rightarrow^{16}$$O+$$^{240*}$$U反応に適用し、千葉・岩本が提案した代理比反応の成立条件の検証を行った。

論文

Nuclear orientation in the reaction $$^{34}$$S+$$^{238}$$U and synthesis of the new isotope $$^{268}$$Hs

西尾 勝久; Hofmann, S.*; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; 有友 嘉浩; Comas, V. F.*; D$"u$llmann, Ch. E.*; Gorshkov, A.*; Graeger, R.*; et al.

Physical Review C, 82(2), p.024611_1 - 024611_9, 2010/08

 被引用回数:54 パーセンタイル:4.71(Physics, Nuclear)

Synthesis of isotopes of the element hassium was studied using the reaction $$^{34}$$S+$$^{238}$$U$$rightarrow$$$$^{272}$$Hs*. At a kinetic energy of 163.0 MeV in the center of mass system we observed one $$alpha$$-decay chain starting at the isotope $$^{267}$$Hs. The cross-section was 1.8$$^{+4.2}_{-1.5}$$pb. At 152.0 MeV one decay of the new isotope $$^{268}$$Hs was observed. It decays with a half-life of 0.38$$^{+1.8}_{-0.17}$$s by 9479$$pm$$16 keV $$alpha$$-particle emission. Spontaneous fission of the daughter nucleus $$^{264}$$Sg was confirmed. The measured cross-section was 0.54$$^{+1.3}_{-0.45}$$pb. In-beam measurements of fission fragment mass distributions were performed to obtain information on the fusion probability at various orientation of the deformed target nucleus. The distributions changed from symmetry to asymmetry when the beam energy was changed from above-barrier to sub-barrier values, indicating orientation effects on fusion and/or quasifission. It was found that the distribution of symmetric mass fragments originates not only from fusion-fission, but has a strong component from quasifission. This result significantly influences the interpretation of data obtained from measurements of symmetric mass distributions with respect to extracting evaporation residue cross-sections.

論文

Fusion-fission and quasi-fission processes in reactions using actinide target nuclei

有友 嘉浩

International Journal of Modern Physics E, 19(5&6), p.813 - 824, 2010/06

 パーセンタイル:100(Physics, Nuclear)

重・超重元素領域の原子核の融合分裂反応を理論的に解析した報告である。最近、日本原子力研究開発機構で$$^{36}$$S+$$^{238}$$U及び$$^{30}$$Si+$$^{238}$$Uに対して、さまざまな入射エネルギーにおける分裂片の質量分布や捕獲断面積が測定されたが、おもにこの実験データを用いて解析を行った。このような実験データは重・超重元素領域の原子核反応のメカニズムを解明するための強力な道具の一つである。融合分裂過程の全過程を統一的に扱える「統一模型」を用いて、散逸揺動理論に基づくランジュバン方程式により軌道を解析することで反応のメカニズムを明らかにした。軌道解析により、低エネルギー入射反応で見られる$$^{36}$$S+$$^{238}$$U及び$$^{30}$$Si+$$^{238}$$Uの質量分布の構造の違いの原因を明らかにした。また今後代理反応を扱う際に必要とされる理論計算の基礎となり、今後の発展にとって重要である。

論文

Analysis of dynamical process with mass distribution of fission fragment in heavy ion reactions

有友 嘉浩

EPJ Web of Conferences (Internet), 2, p.07004_1 - 07004_4, 2010/03

 パーセンタイル:100

重・超重元素領域の原子核の全反応過程を統一的に扱える模型として近年フレロフ原子核合同研究所の理論グループが開発した「統一模型」がある。その模型の適用例の一つとしてquasi-fission過程について解析した報告である。この領域の融合分裂反応のメカニズムは複雑で明らかにはされていないが、実験で測定されたデータをもとに、モデル計算と比較することでメカニズムの解明を試みた。ここでは分裂片の質量分布に着目し、日本原子力研究開発機構で測定された$$^{36}$$S+$$^{238}$$U及び$$^{30}$$Si+$$^{238}$$Uの実験データの解析をおもに行った。低エネルギー入射反応では両者の分裂片の質量分布の形は大きく違う。この相違のメカニズムを軌道解析により明らかにした。また、質量対称分裂片の多くが複合核を経由しない、いわゆるquasi-fission過程からきていることを明らかにした。このような情報は融合断面積を評価するうえで重要である。また今後代理反応を扱う際に必要とされる理論計算の基礎となり、今後の発展にとって重要である。

論文

Evidence for quasifission in the sub-barrier reaction of $$^{30}$$Si+$$^{238}$$U

西尾 勝久; 池添 博; 西中 一朗; 光岡 真一; 廣瀬 健太郎*; 大槻 勤*; 渡辺 裕*; 有友 嘉浩*; Hofmann, S.*

Physical Review C, 82(4), p.044604_1 - 044604_5, 2010/00

 被引用回数:35 パーセンタイル:8.74(Physics, Nuclear)

Fragment mass distributions for fission after full momentum transfer were measured for the reaction $$^{30}$$Si,+,$$^{238}$$U at bombarding energies around the Coulomb barrier. At energies above the Bass barrier, the mass distributions were Gaussian with mass symmetry. An asymmetric fission channel with mass $$A_{rm L}/A_{rm H}$$,$$approx$$,90$$/$$178 emerged at the sub-barrier energies, where competition between fusion and quasifission was suggested from the measured evaporation residue (ER) cross-section produced in fusion $$^{30}$$Si,+,$$^{238}$$U. We thus conclude the asymmetric channel is attributed to quasifission. The observed mass asymmetry is significantly smaller than those observed in actinide targets bombarded with heavier projectile, which suggests that the system $$^{30}$$Si,+,$$^{238}$$U approaches the shape of compound nucleus before disintegrating as quasifission.

論文

Analysis of dynamical processes using the mass distribution of fission fragments in heavy-ion reactions

有友 嘉浩

Physical Review C, 80(6), p.064604_1 - 064604_12, 2009/12

 被引用回数:29 パーセンタイル:12.25(Physics, Nuclear)

最近、日本原子力研究開発機構で測定された$$^{36}$$S+$$^{238}$$U及び$$^{30}$$Si+$$^{238}$$Uの実験データの理論解析の報告である。実験は、バスバリア近傍のいろいろな入射エネルギーに対して分裂片の質量分布や捕獲断面積を測定したものであるが、このような実験データは重・超重元素領域の原子核反応のメカニズムを解明するための強力な道具の一つである。このようなデータを散逸揺動理論に基づくランジュバン方程式を用いて解析を行った。特に分裂片の質量分布の入射エネルギー依存性の系統的な解析は、今まで行われたことがなく、この論文が最初の報告である。統一模型を用いて計算された軌道を詳細に解析することで、分裂片の質量分布に見られる巨視的な特徴と細かい構造がどのようなメカニズムで起こるかを明らかにした。また、低エネルギー入射反応で見られる$$^{36}$$S+$$^{238}$$U及び$$^{30}$$Si+$$^{238}$$Uの質量分布の構造の違いの原因を明らかにした。捕獲断面積,融合断面積についても議論を行った。また今後代理反応を扱う際に必要とされる理論計算の基礎となり、今後の発展にとって重要である。

論文

Fission fragment mass distribution for nuclei in the r-process region

龍田 早由*; 橋詰 和明*; 和田 隆宏*; 太田 雅久*; 住吉 光介*; 大槻 かおり*; 梶野 敏貴*; 小浦 寛之; 千葉 敏; 有友 嘉浩*

AIP Conference Proceedings 891, p.423 - 426, 2007/03

重・超重核領域における約2000核種に対して核分裂片の質量分布の理論的研究を行った。星の元素合成の中で主要な過程の一つである速中性子捕獲過程(r-process)において重・超重核領域の核分裂の影響は重要とは考えられてきたが、今まで定量的議論はほとんどなされておらず、その理論的進展が期待されている。今回われわれはr過程にかかわる核種における核分裂片の質量分布の理論計算を、液滴模型+殻補正エネルギーによる原子核の2中心模型を用いて行った。原子核の形状を表す変形パラメータは3変数とし、2中心模型によって得られた、各形状に対するエネルギーを表したポテンシャルエネルギー面をLangevin方程式を用いて解析し、核分裂の鞍点及び分断点付近の核分裂片の対称度・非対称度を広い核種領域の各原子核に対して得た。既知実験データが示す系統性に対する再現性もよく、また未知核に対しては対称に核分裂を起こす核種の領域がかなり局所的であるという結果が得られた。

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