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論文

Multi-reflection time-of-flight mass spectroscopy for superheavy nuclides

Schury, P.*; 伊藤 由太; 庭瀬 暁隆*; 和田 道治*

Atoms (Internet), 11(10), p.11100134_1 - 11100134_13, 2023/10

https://doi.org/10.3390/atoms11100134

The atomic masses of isotopes of elements beyond fermium, which can presently only be produced online via fusion-evaporation reactions, have until recently been determined only from $$alpha$$ decay chains reaching nuclides with known atomic masses. Especially in the case of lower-yield nuclides, for which the sufficiently detailed nuclear spectroscopy required to fully determine the nuclear structure is not possible, such indirect mass determinations may suffer systematic errors. For many superheavy nuclides, their decay chains end in spontaneous fission or in $$beta$$-decay prior to reaching nuclides of known mass. To address this dearth of accurate atomic masses, we have developed a multi-reflection time-of-flight mass spectrograph that can make use of decay-correlations to accurately and precisely determine atomic masses for the very low-yield superheavy nuclides.

論文

Discovery of new isotope $$^{241}$$U and systematic high-precision atomic mass measurements of neutron-rich Pa-Pu nuclei produced via multinucleon transfer reactions

庭瀬 暁隆*; 伊藤 由太; 他18名*

Physical Review Letters, 130(13), p.132502_1 - 132502_6, 2023/03

https://doi.org/10.1103/PhysRevLett.130.132502
 被引用回数：39 パーセンタイル：98.35(Physics, Multidisciplinary)

新同位体$$^{241}$$Uを合成し、KISS施設の$$^{238}$$U+$$^{198}$$Pt系の多核子移行反応において、19種類の中性子過剰Pa-Pu同位体の系統的原子量測定が行われた。本実験結果は、この領域の核種において、$$N=152$$シェルギャップに向かう未踏の中性子過剰アクチニド同位体へのアクセスに多核子移行反応が重要な役割を果たすことを実証した。

論文

Study of the $$N=32$$ and $$N=34$$ shell gap for Ti and V by the first high-precision multireflection time-of-flight mass measurements at BigRIPS-SLOWRI

飯村 俊*; Rosenbusch, M.*; 高峰 愛子*; 角田 佑介*; 和田 道治*; Chen, S.*; Hou, D. S.*; Xian, W.*; 石山 博恒*; Yan, S.*; et al.

Physical Review Letters, 130(1), p.012501_1 - 012501_6, 2023/01

https://doi.org/10.1103/PhysRevLett.130.012501
 被引用回数：13 パーセンタイル：88.67(Physics, Multidisciplinary)

The atomic masses of $$^{55}$$Sc, $$^{56,58}$$Ti, and $$^{56-59}$$V have been determined using the high-precision multireflection time-of-flight technique. The radioisotopes have been produced at RIKEN's Radioactive Isotope Beam Factory (RIBF) and delivered to the novel designed gas cell and multireflection system, which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For $$^{56,58}$$Ti and $$^{56-59}$$V, the mass uncertainties have been reduced down to the order of 10 keV, shedding new light on the $$N=34$$ shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species $$^{58}$$Ti and $$^{59}$$V. With the new precision achieved, we reveal the nonexistence of the $$N=34$$ empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied $$nu$$p$$_{3/2}$$ orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the $$nu$$d$$_{5/2}$$ and $$nu$$g$$_{9/2}$$ orbits and compare the results with conventional shell model calculations, which exclude the $$nu$$g$$_{9/2}$$ and the $$nu$$d$$_{5/2}$$ orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at $$N = 34$$.

論文

Ground-state $$beta$$-decay spectroscopy of $$^{187}$$Ta

向井 もも*; 平山 賀一*; 渡辺 裕*; 渡邉 寛*; 小浦 寛之; Jeong, S. C.*; 宮武 宇也*; Brunet, M.*; 石澤 倫*; Kondev, F. G.*; et al.

Physical Review C, 105(3), p.034331_1 - 034331_6, 2022/03

https://doi.org/10.1103/PhysRevC.105.034331
 被引用回数：8 パーセンタイル：71.48(Physics, Nuclear)

Beta-decay spectroscopy of the $$^{187}$$Ta ground state was performed at the KEK Isotope Separation System. $$beta$$-delayed $$gamma$$ rays corresponding to the previously reported in-beam transitions were observed. The $$beta$$-decay half-life of the $$^{187}$$Ta ground state was determined to be 283(10) s by analyzing a time spectrum of $$beta$$-$$gamma$$ coincidence events. The $$beta$$-decay branching ratio and log($$ft$$) values were evaluated for the first time. Based on the newly evaluated log($$ft$$) values of $$>$$6.0 and a decay scheme, spin-parity values of I$$^pi$$ = 7/2$$^+$$ originating from the odd-proton orbit $$pi$$7/2[404] were assigned with high confidence, which is consistent with the systematics of neighboring odd-A nuclides.

論文

$$alpha$$-decay-correlated mass measurement of $$^{206,207g,m}$$Ra using an $$alpha$$-TOF detector equipped multireflection time-of-flight mass spectrograph system

庭瀬 暁隆*; 伊藤 由太; 他26名*

Physical Review C, 104(4), p.044617_1 - 044617_8, 2021/10

https://doi.org/10.1103/PhysRevC.104.044617
 被引用回数：6 パーセンタイル：57.44(Physics, Nuclear)

The atomic masses of the isotopes 206,207Ra have been measured via decay-correlated mass spectroscopy using a multireflection time-of-flight mass spectrograph equipped with an $$alpha$$-TOF detector. The Ra isotopes were produced as fusion-evaporation products in the 51V+159Tb reaction system and delivered by the gas-filled recoil ion separator GARIS-II at RIKEN. The $$alpha$$-TOF detector provides for high-accuracy mass measurements by correlating time-of-flight signals with subsequent $$alpha$$-decay events. The masses of 206Ra and 207g,mRa were directly measured using a multireflection time-of-flight mass spectrograph equipped with an $$alpha$$-TOF detector. The mass excesses of 206,207gRa and the excitation energy of 207mRa were determined to be ME=3540(50)keV/c$$^{2}$$, 3538(15)keV/c$$^{2}$$, and Eex=552(42) keV, respectively. The $$alpha$$-decay branching ratio of 207mRa $$b_alpha$$=0.26(20), was directly determined from decay-correlated time-of-flight signals, and the reduced $$alpha$$ width of 207mRa was calculated to be $$delta^2$$=43 keV from the branching ratio. The spin-parity of 207mRa was confirmed to be J$$^pi$$=13/2$$^+$$ from decay correlated mass measurement results.

論文

Beta decay of the axially asymmetric ground state of $$^{192}$$Re

渡邉 寛*; 渡辺 裕*; 平山 賀一*; Andreyev, A. N.; 橋本 尚志*; Kondev, F. G.*; Lane, G. J.*; Litvinov, Yu. A.*; Liu, J. J.*; 宮武 宇也*; et al.

Physics Letters B, 814, p.136088_1 - 136088_6, 2021/03

https://doi.org/10.1016/j.physletb.2021.136088
 被引用回数：8 パーセンタイル：63.10(Astronomy & Astrophysics)

The $$beta$$ decay of $$^{192}_{75}$$Re$$_{117}$$, which lies near the boundary between the regions of predicted prolate and oblate deformations, has been investigated using the KEK Isotope Separation System (KISS) in RIKEN Nishina Center. The ground state of $$^{192}$$Re has been assigned $$J^{pi}$$ = $$(0^{-})$$ based on the observed $$beta$$ feedings and deduced log$$ft$$ values towards the $$0^{+}$$ and $$2^{+}$$ states in $$^{192}$$Os, which is known as a typical $$gamma$$-soft nucleus. The shape transition from axial symmetry to axial asymmetry in the Re isotopes is discussed from the viewpoint of single-particle structure using the nuclear Skyrme-Hartree-Fock model.

論文

Development of an "$$alpha$$-TOF" detector for correlated measurement of atomic masses and decay properties

庭瀬 暁隆*; 伊藤 由太; 他13名*

Nuclear Instruments and Methods in Physics Research A, 953, p.163198_1 - 169198_8, 2020/02

https://doi.org/10.1016/j.nima.2019.163198
 被引用回数：12 パーセンタイル：71.69(Instruments & Instrumentation)

We have developed a novel detector, referred to as an "$$alpha$$-TOF detector", for correlated measurements of atomic masses and decay properties of low-yield, short-lived radioactive isotopes using a multi-reflection time-of-flight mass spectrograph. By correlating measured time-of-flight signals with decay events, it will be possible to suppress background events and obtain accurate, high-precision mass and half-life values even in cases of very low event rates. An offline test of the -TOF detector has shown that the time-of-flight detection efficiency for 5.48 MeV -rays is more than 90% and yields a time resolution of 250.6(68) ps and an alpha-energy resolution of 141.1(9) keV. Using a two-dimensional spectrum of the correlated -ray energy and time-of-flight, the -rays from mixed sources could be resolved. Subsequent testing using -emitting nuclei extracted offline from a gas cell provide verification of the -ray based characterization.

論文

Study of quasielastic barrier distributions as a step towards the synthesis of superheavy elements with hot fusion reactions

田中 泰貴*; 森田 浩介*; 森本 幸司*; 加治 大哉*; 羽場 宏光*; Boll, R. A.*; Brewer, N. T.*; Van Cleve, S.*; Dean, D. J.*; 石澤 倫*; et al.

Physical Review Letters, 124(5), p.052502_1 - 052502_6, 2020/02

https://doi.org/10.1103/PhysRevLett.124.052502
 被引用回数：33 パーセンタイル：85.88(Physics, Multidisciplinary)

The excitation functions for quasielastic scattering of $$^{22}$$Ne+$$^{248}$$Cm, $$^{26}$$Mg+$$^{248}$$Cm, $$^{48}$$Ca+$$^{238}$$U are measured using a gas-filled recoil ion separator The quasielastic barrier distributions are extracted for these systems and are compared with coupled-channel calculations. The results indicate that the barrier distribution is affected dominantly by deformation of the actinide target nuclei, but also by vibrational or rotational excitations of the projectile nuclei, as well as neutron transfer processes before capture. From a comparison between the experimental barrier distributions and the evaporation residue cross sections for Sg (Z=106), Hs (108), Cn (112), and Lv (116), it is suggested that the hot fusion reactions take advantage of a compact collision, where the projectile approaches along the short axis of a prolately deformed nucleus. A new method is proposed to estimate the optimum incident energy to synthesize unknown superheavy nuclei using the barrier distribution.

論文

First direct mass measurements of nuclides around $$Z$$ = 100 with a multireflection time-of-flight mass spectrograph

伊藤 由太*; Schury, P.*; 和田 道治*; 新井 郁也*; 羽場 宏光*; 平山 賀一*; 石澤 倫*; 加治 大哉*; 木村 創大*; 小浦 寛之; et al.

Physical Review Letters, 120(15), p.152501_1 - 152501_6, 2018/04

https://doi.org/10.1103/PhysRevLett.120.152501
 被引用回数：66 パーセンタイル：92.72(Physics, Multidisciplinary)

冷たい核融合反応および熱い融合反応によって生成した変形閉殻中性子数152の近傍に位置する原子核$$^{246}$$Es, $$^{251}$$Fm、および超フェルミウム原子核$$^{249-252}$$Md, $$^{254}$$Noの質量の直接測定を、多反射時間飛行質量分析装置(MR-TOF)を用いて実施した。$$^{246}$$Esおよび$$^{249,250,252}$$Mdの質量測定は世界で初めての成果である。さらに$$^{249,250}$$Mdの質量を$$alpha$$崩壊連鎖のアンカーポイントとして用いて$$^{261}$$Bhおよび$$^{266}$$Mtまでの重い原子核の質量を決定した。これらの新測定された質量を理論質量計算と比較し、巨視的・微視的模型の予測値と良い一致が見られることを示した。近接する3つの質量値から求められる経験的殻ギャップエネルギー$$delta_{2n}$$を今回の質量値から求め、MdおよびLrに対する変形閉殻中性子数$$N=152$$の存在を裏付ける結果を得た。

口頭

Experimental studies of fission mechanism

廣瀬 健太郎; 西尾 勝久; 牧井 宏之; Orlandi, R.; 塚田 和明; 浅井 雅人; 佐藤 哲也; 伊藤 由太; 洲嵜 ふみ; 田中 翔也*; et al.

no journal, , 

At the JAEA Tokai tandem accelerator facility, we have been studying fission mechanism using multi-nucleon transfer reactions (MNT) through fission observables such as fission-fragment mass distributions, fission probabilities, prompt-fission neutrons, and so on. The MNT reaction is also expected one of viable reactions to produce super-heavy nuclei with more neutrons. The angular momentum transferred in the MNT reaction is an important property since it influences a survival probability of a compound nucleus. In the present study, the angular momentum transferred in MNT reactions are experimentally deduced using fission. In MNT reactions, the axis of the angular-momentum transfer can be defined as perpendicular to the reaction plane. The fission-fragment angular distribution measured with respect to this axis, thus, strongly reflects the angular momentum. We performed measurements of the fission-fragment angular distribution, and deduced the angular momenta transferred to the produced compound nuclei. In this workshop, details of the experiment and results will be presented as well as other experimental activities.

口頭

太陽電池の重イオン検出器としての応答

浅井 雅人; 石橋 優一*; 庭瀬 暁隆*; 牧井 宏之; 伊藤 由太; 佐藤 哲也; 塚田 和明; 坂口 聡志*; 森田 浩介*; 渡辺 裕*; et al.

no journal, , 

発電用の太陽電池はダイオード特性を持つため、放射線検出器、特に高エネルギー重イオン検出器として利用されている。本研究では太陽電池を核分裂片測定に利用することを想定し、様々な種類・エネルギーの重イオンを太陽電池に入射し、その検出器としての応答(エネルギー分解能,波高欠損)を測定した。

口頭

$$^{234}$$Np新核異性体の発見とその崩壊特性

浅井 雅人; 末川 慶英*; 東 聖人*; 鎌田 裕生*; 戸部 晃久*; Andreyev, A. N.; 廣瀬 健太郎; 伊藤 由太; 牧井 宏之; 西尾 勝久; et al.

no journal, , 

半減期約9分の新核異性体$$^{234m}$$Npを発見した。$$^{234m}$$Npは、原子力機構タンデム加速器を用いて$$^{6}$$Liビームを$$^{233}$$U標的に照射して合成し、オンライン同位体分離装置(ISOL)を用いて質量分離し、特性X線を測定することで同定した。$$gamma$$線シングルス測定, $$gamma$$-$$gamma$$同時計数測定及び内部転換電子測定を実施した。$$^{234m}$$Npはほとんどが多重極度E4の核異性体転移によって崩壊し、その後M1遷移することから、$$^{234m}$$Npのスピン・パリティを5$$^{+}$$と決定した。講演では実験結果の詳細を紹介し、$$^{234m}$$Npの核構造について議論する。

口頭

$$^{234}$$Np核異性体の崩壊と核構造

浅井 雅人; 末川 慶英*; 東 聖人*; 鎌田 裕生*; 戸部 晃久*; Andreyev, A. N.; 廣瀬 健太郎; 伊藤 由太; 牧井 宏之; 西尾 勝久; et al.

no journal, , 

最近我々が発見した新核異性体$$^{234m}$$Npの核構造を明らかにするための実験を実施した。$$^{234m}$$Npは、原子力機構タンデム加速器を用いて$$^{6}$$Liビームを$$^{233}$$U標的に照射して合成し、オンライン同位体分離装置(ISOL)を用いて質量分離した。LX線の測定から半減期を精度良く決定し、LX-$$gamma$$同時計数測定により核異性体転移の後に放出される$$gamma$$線の観測に成功した。また内部転換電子測定も実施し、$$gamma$$遷移の多重極度を決定した。これらの実験結果を基に、$$^{234m}$$Npの励起エネルギー,スピン・パリティ,核構造を明らかにした。

口頭

アクチノイド核を標的としたアイソマー核分光

杉山 晃一*; 郷 慎太郎*; 富松 太郎*; 甲斐 民人*; 長江 大輔*; 石橋 優一*; 松永 壮太郎*; 永田 優斗*; 西畑 洸希*; 鷲山 広平*; et al.

no journal, , 

中性子過剰重アクチノイド核の励起準位構造を明らかにするため、アイソマースコープ法を用いたアクチノイド核のインビーム$$gamma$$線分光実験を実施した。中性子過剰重アクチノイド核は、原子力機構タンデム加速器で加速したO-18($$^{18}$$O)ビームをCm-248($$^{248}$$Cm)標的に照射して多核子移行反応で合成した。後方に散乱したビーム様粒子をSi E-$$Delta$$E検出器で検出して生成核を識別し、前方方向に散乱された標的様粒子の核異性体を60mm程度下流に設置したアルミ板で捕集した。アルミ板の周囲に4台のGe検出器及び4台のLaBr検出器を設置して、捕集された核異性体からの$$gamma$$線を測定した。標的と検出器の間にタングステン遮蔽を設置して標的から放出される多量の$$gamma$$線を遮蔽し、アクチノイド核異性体からの$$gamma$$線を感度良く検出することに成功した。

口頭

多核子移行反応における移行角運動量の測定

廣瀬 健太郎; 西尾 勝久; 牧井 宏之; Orlandi, R.; 洲嵜 ふみ; 塚田 和明; 浅井 雅人; 伊藤 由太; 渡辺 裕*; 平山 賀一*; et al.

no journal, , 

原子力機構東海タンデム加速器施設にて$$^{237}$$Np標的と$$^{18}$$Oビームを照射した。多核子移行反応後の散乱粒子をシリコン$$Delta$$E-Eテレスコープで識別することで合成核を同定し、合成核の核分裂片を多芯線式比例検出器で同時検出した。導出した$$^{236}$$Npから$$^{245}$$Amにわたる合計19核種の核分裂角度分布から、移行角運動量を実験的に定量した。

口頭

Development of a measurement system for the surrogate reaction method

牧井 宏之; Orlandi, R.; 石井 哲朗; 廣瀬 健太郎; 塚田 和明; 西尾 勝久; 浅井 雅人; 庭瀬 暁隆*

no journal, , 

We are developing a new measurement system for the simultaneous determination of ($$n,gamma$$), ($$n,f$$), ($$n,n'gamma$$), ($$n,2n$$) reaction cross sections using the surrogate reaction method. The system consists of LaBr$$_{3}$$(Ce) scintillators for high-energy $$gamma$$ rays, Ge detectors for low-energy discrete $$gamma$$ rays, Si detectors for scattered particles, and solar cell charged-particle detectors for fission fragment measurements. To investigate the performance of the measurement system, $$^{3}$$He beams were irradiated to a $$^{243}$$Am target at the JAEA-Tokai tandem accelerator facility to observe the scattered particles from the target, and $$gamma$$-rays and fission fragments emitted from the compound nuclei produced by the $$^{3}$$He-induced reaction. In this poster presentation, the details of the measurement system and the results of the experiment will be discussed.

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