The $(p,p\alpha )$ reaction offers a direct means to probe preformed $\alpha$-cluster structures in nuclei under quasi-free scattering conditions. Previous studies around 100 MeV provided valuable insights into $\alpha$ clustering, but quantitative comparison with microscopic cluster wave functions remained limited due to strong distortion effects. At higher energies, the reaction mechanism becomes simpler and the distorted-wave impulse approximation (DWIA) provides a more reliable framework for quantitative analysis. In the present work, the ${}<^>{40}$Ca$(p,p\alpha )$ reaction was measured at an incident energy of 392 MeV using the high-resolution Grand Raiden and Large Acceptance Spectrometer at RCNP. Despite the small cross section in this energy region, the achieved resolution allowed clear separation of the ground and excited states of the residual ${}<^>{36}$Ar nucleus, and corresponding momentum distributions were extracted. DWIA calculations using a Woods-Saxon $\alpha + {}<^>{36}$Ar bound-state wave function yielded an experimental spectroscopic factor of $S_{\mathrm{FAC}}<^>{\mathrm{WS}} = 0.51 \pm 0.05$, consistent with the previous result at 101.5 MeV ($0.52 \pm 0.11$). This agreement demonstrates that the reaction mechanism is well described across a wide energy range. The present study establishes the feasibility of high-precision $(p,p\alpha )$ measurements at several hundred MeV and highlights their potential as a quantitative probe of $\alpha$ clustering in medium-mass nuclei, forming the basis for systematic studies in both stable and unstable systems.

The proton-induced deuteron knockout reaction $(p,pd)$ provides a unique opportunity to exploit proton-neutron correlations and the deuteron cluster structure of nuclei. Direct deuteron knockout experiments on $<^>{12}$C and $<^>{16}$O under quasi-free scattering conditions have been carried out under the ONOKORO project using a 226-MeV proton beam at the Research Center for Nuclear Physics, Osaka, Japan. Deuterons from the knockout reaction, acting as clusters, were unambiguously detected at the focal plane of the Large Acceptance Spectrometer. Outgoing protons, following the knockout of clusters, were detected at the focal plane of the Grand Raiden spectrometer to correctly reconstruct the expected events. $(p,pd)$ reactions are established successfully, and excitation energy spectra for the residual nuclei $<^>{10}$B and $<^>{14}$N are obtained. Results showed that a large difference in transition strength toward the low-lying energy level of residual nuclei, including the ground state, was found in comparison to other studies. Theoretical calculations using the Proton-Induced KnockOut reaction calculation for Exclusive processes (PIKOE) package based on the distorted-wave impulse approximation are conducted to deduce the triple differential cross section of the $(p,pd)$ reaction, and experimental spectroscopic factors of the deuteron cluster are obtained by normalizing the experimental cross section to the theoretical computation. Consistent spectroscopic factors in comparison to shell-model expectations are obtained for transitions involving the orbital angular momentum $L=0$ of the cluster. The present work demonstrates that the $(p,pd)$ reactions at 226 MeV as a spectroscopic tool have an advantage in examining spectroscopic aspects of deuteron clustering and the mechanism to form a proton-neutron pair in the spin triplet state with negligible interference of final-state interactions and refraction effects.

We present measurements of the hyperfine structure of the neutron-deficient isotope 21Na. The experiment was performed at the newly commissioned CLaSsy beamline at the RAON facility using collinear laser spectroscopy on the D1 transition in both collinear and anti-collinear geometries. An RFQ coolerbuncher was utilized together with time-gated data acquisition to perform the present measurement under bunched-beam operation. A simultaneous linked-fit analysis yields A(3P1/2) = 103.6(10)stat[22]sys MHz and A(3S 1/2) = 952.5(11)stat[40]sys MHz for 21Na. The successful observation of the 21Na resonances demonstrates the commissioning performance of the CLaSsy setup for high-resolution spectroscopy. These results provide the spectroscopic basis for future isotope-shift measurements and for comparisons with atomic-structure calculations required for charge-radius studies of unstable sodium isotopes.

Cryogenic targets are highly advantageous for nuclear physics experiments in inverse kinematics due to their high purity and high density. For use in future experiments as a reaction target, a cryogenic gas cell target system, CryoSTAR (Cryogenic Stable TARget), has been developed at the Center for Exotic Nuclear Studies (CENS) of Institute for Basic Science. During the laboratory performance tests, the system achieved a lowest temperature around 10 K, and Havar windows with a thickness of 5 μm and a diameter of 20 mm were able to withstand pressures up to 800 torr. Also, the areal densities of the target cell were determined from the energy loss of α particles emitted from a 241Am source, and the values obtained at 40 K were found to be about 7 times higher than those at 297 K. As the next step, beam experiments employing the CryoSTAR system will be conducted to further validate its performance and to provide physics results.

We have developed a new experimental technique to determine the gamma emission probability from unbound states populated by the (d, p) reaction at 20 MeV/nucleon under the inverse kinematics. This was achieved using the decelerating and focusing device OEDO at RIBF. Due to the inverse kinematics, the outgoing reaction residues can be identified by a magnetic spectrometer. By combining the missing spectroscopy with the identification of the residual ions, the γ emission channel was determined event-by-event as a function of the excitation energy, without detecting γ ray from the unbound states. As the first application, the method was used to study the neutron capture reaction on 79 Se via the (d, p) reaction as a surrogate reaction for the neutron energies up to 5 MeV. Additionally, the technique was employed to investigate the compound reaction channel of (n, γ) cross sections on 130 Sn.

In this paper, we outline the research program led by the Center for Exotic Nuclear Studies (CENS) at the Institute for Basic Science aiming to investigate the nuclear structure of proton-rich nuclei and the rich diversity of exotic features they present to further extend our knowledge of the nuclear force. In particular, our interest is focused on improving our understanding of the isospin symmetry, probing the nuclear structure of N∼Z nuclei to seek new information on isospin-non-conserving interactions. The research activities reported in this letter include recent results and details of the experimental approach, discussing recoil-β-tagged experiments using fusion-evaporation reactions as a tool to perform complete spectroscopic studies of proton-rich nuclei in the f 7/2 and fpg shells. In addition, current efforts to design and develop nuclear instrumentation are also highlighted.

The neutron-rich unbound fluorine isotope ^{30}F_{21} has been observed for the first time by measuring its neutron decay at the SAMURAI spectrometer (RIBF, RIKEN) in the quasifree proton knockout reaction of ^{31}Ne nuclei at 235 MeV/nucleon. The mass and thus one-neutron-separation energy of ^{30}F has been determined to be S_{n}=-472±58(stat)±33(sys) keV from the measurement of its invariant-mass spectrum. The absence of a sharp drop in S_{n}(^{30}F) shows that the "magic" N=20 shell gap is not restored close to ^{28}O, which is in agreement with our shell-model calculations that predict a near degeneracy between the neutron d and fp orbitals, with the 1p_{3/2} and 1p_{1/2} orbitals becoming more bound than the 0f_{7/2} one. This degeneracy and reordering of orbitals has two potential consequences: ^{28}O behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both ^{29,31}F appear to be good two-neutron halo-nucleus candidates.

Nuclear transmutation is emerging as a promising approach for reprocessing high-level waste, specifically treating long-lived nuclides like Zr-93 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 Zr-93 were measured at 27 MeV/nucleon in inverse kinematics. At the RI Beam Factory (RIBF), the Optimized Energy Degrading Optics beamline was used to deduce production cross-sections for isotopes Nb91-93, Zr-91,Zr-92, and Y-88,Y-89. 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.

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 Zr-93 + 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, Zr-93 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 Zr-93 + d may feasibly transmute the waste into short-lived/stable nuclei.

Spectroscopy of an unbound nucleus $$^{17}$$ C was performed using the SAMURAI spectrometer at RIBF of RIKEN. Six resonances were observed for the $$^{16}$$ C+n system with relative energies of 0.52, 0.77, 1.36, 1.91, 2.22 and 3.20 MeV. The excitation energies ( $$E_x$$ ) of the observed resonances were deduced, by taking into account the states of the $$^{16}$$ C fragments identified by coincident $$\gamma $$ rays, as $$E_x$$ =(3.02), 1.51, (3.86), 2.65, (4.72) and 3.94 MeV. The orbital angular momenta of the two observed states in $$^{17}$$ C at $$E_x$$ =2.65 and 3.94 MeV were determined as 1 by comparing parallel momentum distributions with theoretical predictions.

The Optimized Energy Degrading Optics (OEDO) beamline is an upgrade of the High-resolution beamline at the RI Beam Factory at RIKEN. Its purpose is to measure nuclear reactions induced by radioactive ions at 10-50 MeV/u. The main components of the OEDO beamline are two superconducting quadrupole triplet magnets and one radio-frequency deflector, which were recently installed. This setup produces slowed-down, well-focused radioactive-isotope beams by using a degree of freedom in the longitudinal phase space of the beam. This paper describes the OEDO beamline in detail, discusses the unique concept of its ion optics, and demonstrates its feasibility by conducting measurements with medium-mass radioactive-isotope beams.

An angle-tunable wedge degrader system has been developed to be used as a monoenergetic degrader for a low-energy radioactive isotope (RI) beamline working with the slowing-down method. The system consists of a pair of aluminum degraders with quadratic cross sections, whose overlap works effectively as a wedge-type degrader; the wedge angle can be tuned by changing the relative position of the two aluminum degraders. This paper presents the operation principle, estimation of performance and application range by simulation, and the experimental validation.

Very neutron-rich $Z\ensuremath{\sim}60$ isotopes produced by in-flight fission of a 345 MeV/nucleon $^{238}\mathrm{U}$ beam at the RI Beam Factory, RIKEN Nishina Center, have been studied by delayed $\ensuremath{\gamma}$-ray spectroscopy. New isomers were discovered in the neutron-rich $N=100$ isotones $^{162}\mathrm{Sm}, ^{163}\mathrm{Eu}$, and $^{164}\mathrm{Gd}$. Half-lives, $\ensuremath{\gamma}$-ray energies, and relative intensities of these isomers were obtained. Level schemes were proposed for these nuclei and the first ${2}^{+}$ and ${4}^{+}$ states were assigned for the even-even nuclei. The first ${2}^{+}$ and ${4}^{+}$ state energies decrease as the proton numbers get smaller. The energies and the half-lives of the new isomers are very similar to those of ${4}^{\ensuremath{-}}$ isomers known in less neutron-rich $N=100$ isotones $^{168}\mathrm{Er}$ and $^{170}\mathrm{Yb}$. A deformed Hartree-Fock with angular momentum projection model suggests ${K}^{\ensuremath{\pi}}={4}^{\ensuremath{-}}$ two-quasiparticle states with $\ensuremath{\nu}7/2[633]\ensuremath{\bigotimes}\ensuremath{\nu}1/2[521]$ configurations with similar excitation energy. The results suggest that neutron-rich $N=100$ nuclei are well deformed and the deformation gets larger as $Z$ decreases to 62. The onset of $K$ isomers with the same configuration at almost the same energy in $N=100$ isotones indicates that the neutron single-particle structures of neutron-rich isotones down to $Z=62$ do not change significantly from those of the $Z=70$ stable nuclei. Systematics of the excitation energies of new isomers can be explained without the predicted $N=100$ shell gap.

A search for new isotopes in the neutron-rich rare-earth region has been carried out using a 345 MeV/nucleon 238U beam at the RIKEN Nishina Center RI Beam Factory. Fragments produced were analyzed and identified using the BigRIPS in-flight separator. We observed a total of 29 new neutron-rich isotopes: 153Ba, 154,155,156La, 156,157,158Ce, 156,157,158,159,160,161Pr, 162,163Nd, 164,165Pm, 166,167Sm, 169Eu, 171Gd, 173,174Tb, 175,176Dy, 177,178Ho, and 179,180Er.

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.

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+.

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.