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原子力発電に伴い生成される放射性核種のうち，長寿命核分裂生成物の処理方法の一つの解として，加速器を用いた核変換処理が提案されている。この処理には，広いエネルギー範囲にわたる長寿命放射性核種とその周辺核の核反応データの知見が必要である。逆運動学法を用いると，通常は標的として用いにくい放射性核種の反応断面積の測定が可能となる。生成される同位体が識別可能となるので，結果として同位体毎断面積が得られる。得られた同位体毎断面積は理論モデルと照会され，その精密化に貢献した。最終的には核変換プロセスや大強度加速器の仕様策定に利用されると期待される。

The spectroscopic structure of 19C, a prominent one-neutron halo nucleus, has been studied with a 20C secondary beam at 290 MeV/nucleon and a carbon target. Neutron-unbound states populated by the ... one-neutron knockout reaction were investigated by means of the invariant mass method. The preliminary relative energy spectrum and parallel momentum distribution of the knockout residue, 19C*, were reconstructed from the measured four momenta of the 18C fragment, neutron, and beam. Three resonances were observed in the spectrum, which correspond to the states at Ex = 0.62(9), 1.42(10), and 2.89(10) MeV. The parallel momentum distributions for the 0.62-MeV and 2.89-MeV states suggest spin-parity assignments of 5/2+ and 1/2−, respectively. The 1.42-MeV state is in line with the reported 5/22+ state. more

A spectroscopic study of 17C was performed via the one-neutron knockout reaction of 18C on a carbon target at RIKEN-RIBF. Three unbound states at excitation energies of 2.66(2), 3.16(5), and 3.97(3) ... MeV (preliminary) were observed. The energies are compared with shell-model calculations and existing measurements to deduce their spin-parities. From the comparison, the states at 2.66(2) and 3.97(3) MeV are suggested to be 1/2− and 3/2−, respectively. From its decay property, the state at 3.16(5) MeV is indicated to be 9/2+. more

The generalized two-centre cluster model (GTCM), which can handle various single-particle configurations in general two-centre systems, is applied to the light neutron-rich system, 12Be = α+α+4N. We ... discuss the change of the neutrons' configuration around two α cores as the excitation energy varies. We show that the excess neutrons form various chemical-bonding-like configurations around the two α cores in the unbound region above the α-decay threshold. The possibility of α-cluster formation in a heavier system with neutron excess, 28Ne, is also discussed. more

The generalized two-center cluster model, which can handle various single particle configurations in general two center systems, is applied to the light neutron-rich system, 12 Be = α+α+ 4 N . We ... discuss the change of the neutrons’ configuration around two α -cores as a variation of an excitation energy. We show that the excess neutrons form various chemical-bonding-like configurations around two α cores in the unbound region above the α decay threshold. The possibility of the α cluster formation in the heavier neutron-excess system, 28 Ne, is also discussed. more

原子核は，自然界の物質質量の大部分を担っており，物質の多様性を決める端緒となる階層にある．構成要素は主に陽子と中性子であり，それらが核力（強い相互作用）により結合し，互いに束縛している量子多体系である．中性子だけで構成される物質は，観測による中性子星の存在は示されているものの，それ以外で「中性子だけで構成される原子核が存在するか」については，1960年代から放射化法などの手法を用いて探索の報告がなされているが，その発見には至っていない．4つの中性子もしくはそれ以上の中性子が束縛あるいは強く相関した状態を持つかどうかは，原子核物理学が解くべき課題であり，他の階層での現象についても影響を及ぼす可能性がある問題である．中性子過剰核をビームとして用いることにより，中性子過剰な系の実験が精力的に行われるようになり，原子核反応を通じて直接4中性子系の観測に挑戦できる状況となってきた．理化学研究所重イオン加速器施設RIBFは，8Heビームを核子あたり150–200 MeVで，大強度ビーム（毎秒105–106）として供給できる世界唯一の施設である．加えて，高分解能磁気スペクトロメータSHARAQや広帯域磁気スペクトロメータSAMURAIと特徴ある核分光装置が建設され，このエネルギー領域で支配的な直接反応過程を利用した不安定核の核分光にその本領を発揮している．これらの装置群を駆使して，4中性子状態の探索のために実施された2つの実験，SHARAQスペクトロメータによる二重荷電交換反応（8He, 8Be）を用いた実験と，SAMURAIスペクトロメータによるαノックアウト反応（ p, pα）を用いた実験が実施された．（8He, 8Be）反応は，通常の二重荷電交換反応に加えて，プローブの質量が重いものが軽くなる（束縛エネルギーの小さいものから大きなものに変化する）という「発熱型」であるという特徴がある．これは，RIビームを使う利点であり，4Heより束縛エネルギーが小さい（つまり質量のより重い）“4n”を作る際に「反跳をほぼ受けない」という運動学条件を満たすことが可能となる．一方αノックアウト反応実験では，8Heを形成するコア4He（α粒子）を最大運動量移行である180度（近くの）散乱で叩きだすことにより，余剰となっている4中性子を「ほぼ撹乱することなく」観測することが可能となる．発熱型二重荷電交換反応とαノックアウト反応の全く手法の異なる2実験で得られた結果は，驚くべきほど類似した4中性子系の質量スペクトルを示した．この質量スペクトルは，4中性子の質量の和であるしきい値よりほんの少しだけ高いエネルギーに，非常に幅の狭い状態および4中性子が無相関か弱い相関をもって位相空間を占める成分が60 MeVにわたる広い連続状態として観測された．しきい値近傍の状態は，αノックアウト反応による高統計の結果から，強い相互作用に対しては束縛しない，“非束縛状態”であることが確定した．状態の幅は，4×10－22秒程度の寿命に対応しており，反応時間の10倍程度の間強く相関した4中性子状態が生成されていることを示している．

Abstract The drip-line 24O nucleus combines several properties which are challenging our understanding of the nuclear structure: it has no bound excited state, and is expected as a neutron-skin ... nucleus with low-lying resonances. It presents a new shell gap effect at N = 16. To investigate the neutron excitations and the characteristics of this gap, the proton elastic and inelastic scattering by 24O were measured, using the light charged particle spectroscopy technique; the excitation energies of the unbound states were deduced via missing mass method. The experiment was carried out at the RIBF facility in the BigRIPS line, combining the unique intensities of the 24O beam with the state-of-the-art Si-telescope array MUST2. The analysis and the preliminary results are discussed. more

Abstract Nuclear transmutation emerges as a promising approach for reprocessing high-level waste, specifically treating long-lived nuclides like 93Zr from spent fuel. It is essential to accumulate ... reaction data for these nuclei to advance this prominent treatment and to build a comprehensive understanding of reaction mechanisms. In this study, the residual production cross sections resulting from proton-induced reactions on 93Zr were measured at 27 MeV/nucleon in inverse kinematics. At the RI Beam Factory (RIBF) the OEDO beamline was used to deduce production cross sections for isotopes, 91 − 93Nb, 91, 92Zr, and 88, 89Y. Comparing the results from this study and prior research with calculated excitation functions, a moderate agreement is found with theoretical predictions derived from TALYS and CCONE. The measured cross sections offer valuable insights for future considerations in nuclear-waste treatment facilities. This is particularly relevant for facilities exploring innovative methods, such as accelerator-driven systems. more

Abstract The deuteron is a loosely bound system that can easily break up into its constituent proton and neutron whilst in the presence of Coulomb and nuclear fields. Previous experimental studies ... have shown that this breakup process has a significant impact on residual-nucleus production from deuteron bombardment in the high-energy range of 50–210 MeV/nucleon. However, there remains a lack of cross-section data at energies below 50 MeV/nucleon. The current study determined 93Zr + d reaction cross sections under inverse kinematics at approximately 28 MeV/nucleon using the BigRIPS separator, OEDO beamline, and SHARAQ spectrometer. Cross sections from this research were compared with previous measurements and theoretical calculations. The experimental results show a large enhancement of the production cross sections of residual nuclei, especially those produced from a small number of particle emissions, compared to the proton-induced reaction data at similar bombarding energy. The DEURACS calculation, which quantitatively takes deuteron-breakup effects into account, reproduces the data well. As a long-lived fission product, 93Zr remains a challenge for nuclear-waste disposal and treatment. This study’s low-energy data may assist future consideration of nuclear-waste treatment facilities, where 93Zr + d may feasibly transmute the waste into short-lived/stable nuclei. more