In nuclear physics experiments involving neutron measurements, spectroscopic information such as neutron energy, width, and intensity can be measured using the time-of-flight (TOF) method. Analyzing TOF spectra is often challenging because neutrons suffer severe scattering from various materials in the surrounding experimental apparatus. Therefore, it is inevitable to evaluate scattered neutrons with the experimental setup and decompose TOF lines into direct and out-scattered components. We demonstrate the utility of the large-area neutron detector MAGRO for counting low-energy neutrons down to 300 keV in the β-delayed neutron-decay study of rare isotopes. We performed Monte Carlo simulations using GEANT4 to investigate the detector's responses to incident radiation. In the simulations, physics models for optical physics and high-precision neutron physics were invoked. The optical physics parameters were adjusted to reproduce experimental data for light attenuation and propagation time, which were measured with the 90Sr β source. The simulation codes were further validated with in-beam tests at HIMAC (Heavy Ion Medical Accelerator in Chiba) of QST (National Institutes for Quantum Science and Technology) and RCNP (Research Center for Nuclear Physics) of Osaka University for intrinsic efficiency and TOF measurements using 16C and 17N beams. The neutron efficiency was calculated for a single MAGRO detector from 200 keV to 7 MeV, and it is shown that the calculated efficiency curve follows the measured data below 2 MeV. TOF spectra were reproduced in the simulations with the details of the experimental setup for β-n-γ coincidence, providing a unique way to identify out-scattered neutron events in the measurements.

Center for Accelerator and Beam Applied Science of Kyushu University has been established to promote activities in various fields such as, medical, educational and engineering fields at Kyushu University. An accelerator facility, consist of a 10 MeV proton cyclotron, 8 MeV tandem accelerator and of 150 MeV FFA, has been constructed in the center. In this paper, status of the hardware developments and results of the beam commissioning of the FFA in Kyushu University is described.

We are developing a laser spectroscopic method to study the nuclear structure of radioactive isotopes utilizing superfluid helium (He II) as an efficient stopper for highly energetic ion beams and as an in-situ laser spectroscopic environment. Recently, we conducted an ion stopping experiment for the ^84 Rb ^37+ beams with the energy of approximately 350 A MeV at QST-HIMAC with a cryostat system used for 66 A MeV at RIKEN-RIPS experiment (Appl. Phys. Express 12, 016502 (2019)). The radioactive ^84 Rb ions were produced via a projectile fragmentation reaction using accelerated ^84 Kr ^36+ beams and a 12-mm thick Be target. As a first step, we measured the stopping range distribution of the injected ion beams in liquid N _2 to estimate the stopping range distribution in He II and the spot size of the injected ion beams using a plastic scintillator. Then, a laser-induced fluorescence (LIF) detection experiment was performed using He II. We successfully observed the LIF from ^84 Rb atoms. We estimated the longitudinal range straggling of ^84 Rb ions in superfluid helium from the obtained results. The details of experiment at QST-HIMAC and results are given in this report.

Background: The nuclear structure of low-lying excited states in I-139, a neutron-rich nucleus with Z = 53 and N = 86 above Sn-132 and in the proximity of A similar to 140, is investigated. Purpose: The decay scheme for I-139 is obtained after the first beta-decay measurement of Te-139. Transitions in I-138 are detected after the beta-delayed neutron emission of Te-139. The Pn ratio is investigated based on the gamma-ray emissions. Methods: beta-delayed. -ray spectroscopy is employed to study excited states in I-139, populated in the decay of a mass-separated beam of Te-139, produced in the in-flight fission of U-235 on a Be-9 target. Results: The new level scheme of I-139 with 26 new transitions, established for the first time in beta decay, is reported. Two new transitions are observed also in I-138. The beta-delayed neutron emission probability P-n of Te-139 is deduced to be 17.6(48)%. New and/or more restrictive spin/parity assignments of states in the daughter I-138 nucleus are reported as well. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell-model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

The known I^{π}=8_{1}^{+}, E_{x}=2129-keV isomer in the semimagic nucleus ^{130}Cd_{82} was populated in the projectile fission of a ^{238}U beam at the Radioactive Isotope Beam Factory at RIKEN. The high counting statistics of the accumulated data allowed us to determine the excitation energy, E_{x}=2001.2(7)  keV, and half-life, T_{1/2}=57(3)  ns, of the I^{π}=6_{1}^{+} state based on γγ coincidence information. Furthermore, the half-life of the 8_{1}^{+} state, T_{1/2}=224(4)  ns, was remeasured with high precision. The new experimental information, combined with available data for ^{134}Sn and large-scale shell model calculations, allowed us to extract proton and neutron effective charges for ^{132}Sn, a doubly magic nucleus far-off stability. A comparison to analogous information for ^{100}Sn provides first reliable information regarding the isospin dependence of the isoscalar and isovector effective charges in heavy nuclei.

In recent years, particle discrimination methods based on digital waveform analysis techniques for neutron-transmutation-doped silicon (nTD-Si) detectors have become widely used for the identification of low-energy charged particles. Although the particle discrimination capability of this method has been well demonstrated for small incident angles, the particle discrimination performance may be affected by changes in the detector response when the detector is moved closer to the charged particle source and the incident position distribution and incident angle distribution to the detector become wide. In this study, we performed a beam test for particle discrimination in light-charged (Z≤2) particles using the digital waveform analysis method with a pad-type nTD-Si detector and investigated the dependence of the performance of the particle discrimination on the incident position and incident angle. As the incident angle increased, a decrease in the maximum current was observed, which was sufficient to affect the performance of the particle discrimination. This decrease can be expressed as a function of the penetration depth of the charged particles into the detector, which varies for each nuclide.

Much research in recent years has focused on circular accelerators that accelerate and store secondary particles with a large momentum spread, such as muons, unstable nuclei, and heavy ions with different charge states. A fixed-field alternating gradient (FFAG) accelerator with large transverse and momentum acceptance has obvious advantages for such requirements. A versatile beam injection method is required to accelerate secondary particles with a large momentum spread and different charge states with an FFAG accelerator. In the present study, a method for charge exchange injection of positive heavy ions using the large momentum acceptance of an FFAG accelerator is proposed. A charge injection system, which converts a He1+ beam to a He2+ beam, is developed for a 150 MeV FFAG accelerator at the Center for Accelerator and Beam Applied Science (CABAS) of Kyushu University. As the first step to verify the injection method, an orbit shift from one charge state to the other is demonstrated. This is the first demonstration of heavy ion injection using an FFAG accelerator.

The magnetic dipole moment and the electric quadrupole moment are the nuclear moments that provide us with key information about the proton and neutron configurations in a nucleus and the shape of a nucleus, respectively. In the study of nuclear structure through the measurement of the nuclear moments, a technique to produce spin orientation of rare-isotope beams has played important roles. Recently, a scheme of the two-step projectile fragmentation was developed to produce high spin alignment in RI beams and was applied to the frontier of the study for nuclear structure of neutron-rich nuclei, such as ^75 Cu and ^99 Zr. The recent activities of the nuclear-moment measurements using highly spin-aligned beams at RIKEN RIBF are reported.

Background: The nuclear structure of low-lying excited states in 139 I, a neutron-rich nucleus with Z = 53 and N = 86 above 132 Sn and in the proximity of A similar to 140, is investigated. Purpose: The decay scheme for 139I is obtained after the first /3-decay measurement of 139 Te. Transitions in 138I are detected after the /3-delayed neutron emission of 139 Te. The Pn ratio is investigated based on the gamma-ray emissions. Methods: /3-delayed gamma-ray spectroscopy is employed to study excited states in 139 I, populated in the decay of a mass-separated beam of 139 Te, produced in the in-flight fission of 235U on a 9 Be target. Results: The new level scheme of 139I with 26 new transitions, established for the first time in /3 decay, is reported. Two new transitions are observed also in 138 I. The /3-delayed neutron emission probability Pn of 139 Te is deduced to be 17.6(48)%. New and/or more restrictive spin/parity assignments of states in the daughter 138I nucleus are reported as well. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell-model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

To search for low-energy resonant structures in isospin T=3/2 three-body systems, we have performed the experiments ^{3}H(t,^{3}He)3n and ^{3}He(^{3}He,t)3p at intermediate energies. For the 3n experiment, we have newly developed a thick Ti-^{3}H target that has the largest tritium thickness among targets of this type ever made. The 3n experiment for the first time covered the momentum-transfer region as low as 15  MeV/c, which provides ideal conditions for producing fragile systems. However, in the excitation-energy spectra we obtained, we did not observe any distinct peak structures. This is in sharp contrast to tetraneutron spectra. The distributions of the 3n and 3p spectra are found to be similar, except for the displacement in energy due to Coulomb repulsion. Comparisons with theoretical calculations suggest that three-body correlations exist in the 3n and 3p systems, although not enough to produce a resonant peak.

To search for low-energy resonant structures in isospin T = 3/2 three-body systems, we have performed the experiments 3H(t, 3He)3n and 3He(3He, t)3p at intermediate energies. For the 3n experiment, we have newly developed a thick Ti-3H target that has the largest tritium thickness among targets of this type ever made. The 3n experiment for the first time covered the momentum-transfer region as low as 15 MeV/c, which provides ideal conditions for producing fragile systems. However, in the excitation-energy spectra we obtained, we did not observe any distinct peak structures. This is in sharp contrast to tetraneutron spectra. The distributions of the 3n and 3p spectra are found to be similar, except for the displacement in energy due to Coulomb repulsion. Comparisons with theoretical calculations suggest that three-body correlations exist in the 3n and 3p systems, although not enough to produce a resonant peak.

Nuclei in the vicinity of 78Ni are important benchmarks for nuclear structure, which can reveal changes in the shell structure far from stability. Spectroscopy of the odd-odd isotope 78Cu was performed for the first time in an experiment with the EURICA setup at the Radioactive Isotope Beam Factory at RIKEN Nishina Center. Excited states in the neutron-rich isotope were populated following the beta decay of 78Ni produced by in-flight fission and

Excited states in Cu-78 were observed for the first time following the ss decay of Ni-78 created by in-flight fission of U-238. Based on the coincidence relationships between the observed gamma-ray transitions, it was possible to construct a level scheme comprising eight excited states with tentative spin assignments for 5 of them. In addition to the gamma-decaying states, an isomeric state with a lifetime of 3.8(4) ms was found to decay by internal conversion.

The $\ensuremath{\beta}$ decay of $^{142}\mathrm{Te}_{90}$ to $^{142}\mathrm{I}_{89}$ was investigated for the first time. The parent nucleus was produced by the in-flight fission of a $^{238}\mathrm{U}$ beam with an energy of 345 MeV per nucleon, impinging on a $^{9}\mathrm{Be}$ target at the Radioactive Isotope Beam Factory of RIKEN. Excited states in $^{142}\mathrm{I}$ were established by $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray spectroscopy. The observed $({1}^{+})$ states in $^{142}\mathrm{I}$ could be interpreted to be predominantly the $\ensuremath{\nu}0{h}_{9/2}\ensuremath{\bigotimes}\ensuremath{\pi}0{h}_{11/2}$ configuration formed by a Gamow-Teller transition between a neutron in the $0{h}_{9/2}$ orbital and a proton in the $0{h}_{11/2}$ orbital. Additional features of the $({1}^{+})$ states are discussed by comparing with neighboring heavier isotones, such as $^{144}\mathrm{Cs}$ and $^{146}\mathrm{La}$. In the context of deformed shell-model calculations, the $({1}_{1}^{+})$ state is closely related to the $\ensuremath{\nu}[5,3,2]3/2\ensuremath{\bigotimes}\ensuremath{\pi}[5,5,0]1/2$ configuration, which may be related to the weak Gamow-Teller transition strength.

The beta decay of 142Te90 to 142I89 was investigated for the first time. The parent nucleus was produced by the in-flight fission of a 238U beam with an energy of 345 MeV per nucleon, impinging on a 9Be target at the Radioactive Isotope Beam Factory of RIKEN. Excited states in 142I were established by beta-delayed gamma-ray spectroscopy. The observed (1+) states in 142I could be interpreted to be predominantly the nu 0h9/2 (R) pi 0h11/2 configuration formed by a Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Additional features of the (1+) states are discussed by comparing with neighboring heavier isotones, such as 144Cs and 146La. In the context of deformed shell-model calculations, the (1+1 ) state is closely related to the nu[5, 3, 2]3/2 (R) pi[5, 5, 0]1/2 configuration, which may be related to the weak Gamow-Teller transition strength.

The ground-state magnetic dipole moment of the neutron-rich 21O isotope has been measured via beta-ray-detected nuclear magnetic resonance (beta-NMR) spectroscopy by using a spin-polarized secondary beam of 21O produced from the 22Ne primary beam. From the present measurement, the g factor |gexp(21Og.s.)| = 0.6036(14) has been determined. Based on the comparison of this value with Schmidt values, we unambiguously confirm the nu d5/2 configuration with spin and parity assignments I pi = 5/2+ for the 21O ground state, suggested by previously reported studies. Consequently, the magnetic moment has been determined as mu exp(21Og.s.) = (-)1.5090(35)mu N. The obtained experimental magnetic moment is in good agreement with the predictions of the shell-model calculations using the USD, YSOX, and SDPF-M interactions as well as random phase approximation (RPA) calculations. This observation indicates that the 21O nucleus in its ground state does not manifest any anomalous structure and is not influenced by the proximity of the drip line.

The beta-decay scheme of Te-138 and the level structure of I-138 is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN Cluster Array campaigns. Secondary radioactive ions, including Te-138 and Sb-138, were produced by the in-flight fission of a U-238 beam with the energy of 345 MeV per nucleon. From the beta decay of Te-138, the level scheme of I-138 was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity 1(+) state. This 1(+) state can be interpreted as being associated with the pi 0h(11/2) circle times nu 0h(9/2) partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h(9/2) orbital and a proton in the 0h(11/2) orbital. Details of the structure of I-138 are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.

Structure of the neutron-rich $N=86$ isotope ${}^{140}\mathrm{Xe}$, located northeast of a doubly-magic nucleus ${}^{132}Sn$, is investigated by $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy. Two $\ensuremath{\beta}$-decay isomers in $^{140}\mathrm{I}$ are newly found in the study of two different $\ensuremath{\beta}$ decays of $^{140}\mathrm{I}$ which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) $\ensuremath{\beta}$ decay of $^{140}\mathrm{Te}$ at an active stopper. Half-lives of the $\ensuremath{\beta}$ decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the $\ensuremath{\beta}$ decay of the high-spin isomer and of the mixed $\ensuremath{\beta}$ decays of the ground state and the low-spin isomer in $^{140}\mathrm{I}$ to $^{140}\mathrm{Xe}$ are constructed using the information on $\ensuremath{\gamma}$-ray coincidence relation and $\ensuremath{\gamma}$-ray intensity. Nuclear structures of the low-lying states in $^{140}\mathrm{Xe}$ and $^{140}\mathrm{I}$ are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)$\ensuremath{\gamma}$-band members of ${2}^{+}$ and ${4}^{+}$ states and the octupole collective ${1}^{\ensuremath{-}}$ state are proposed in $^{140}\mathrm{Xe}$. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.