RI Beam Research Articles

Discovery of Neutron-Rich Silicon Isotopes 45,46Si

Abstract New isotopes $^{45,46}$Si, which extend beyond the most neutron-rich Si isotopes confirmed thus far, have been discovered at the RI Beam Factory at the RIKEN Nishina Center. The isotopes were produced by the in-flight fragmentation reactions of $^{70}$Zn at 345 MeV/nucleon on a Be target. All the projectile fragments were analyzed and identified using the large-acceptance two-stage separator BigRIPS. Consequently, six $^{45}$Si events and one $^{46}$Si event were confirmed. The neutron-bound nature of odd-N$^{45}$Si may have a significant impact on the nuclear stability of neutron-rich proton $sd$-shell nuclei at approximately $N=40$.

Production Cross Sections of Residual Nuclides from 93Zr + p at 27 MeV/Nucleon

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.

Correction to: Nuclear-moment measurement using highly spin-aligned RI beams: Recent activities at RIBF

Correction to: Nuclear-moment measurement using highly spin-aligned RI beams: Recent activities at RIBF

The p+238U Reaction for the RI Beam based on the Langevin Method

The p+²³⁸U Reaction for the RI Beam based on the Langevin Method

Pre-experiment of 70 MeV H - cyclotron for producing ISOL RI beam.

The Rare Isotope Science Project (RISP) is currently constructing equipment and facilities for the Rare Isotope Accelerator complex for ON-line experiments (RAON). The first experiment being conducted involves the acceleration of a proton to 70MeV using a cyclotron, which is then delivered to a SiC target to generate a radioactive isotope beam in the ISOL system. However, before conducting this experiment, a preliminary experiment was performed to confirm the exact location and shape of the proton beam to prevent loss of the proton beam. The pre-experiment involved configuring a faraday cup, wire grid scanner, collimator, and slit inside the proton beam diagnostic module to understand the characteristics of the proton beam at the target position. The beam current ranged from 1 μA to 1.5 μA, and the beam size was confirmed to be within the 2×2cm 2 slit size.

Development of the segmented plastic scintillation detector GARi for studying $r$-process nucleosynthesis at RIBF

\(\beta\)-decay properties of very neutron-rich nuclei, such as the half-life and the \(\beta\)-delayed neutron emission probabilities, play an essential role in the astrophysical rapid neutron capture process (r-process), where elements heavier than iron may be synthesized. Investigating those crucial properties has been one of the main objectives of the experimental programs at the RIKEN RI Beam Factory (RIBF). Recently, the development of a fast-timing detector system comprising a highly segmented plastic scintillation detector GARi (Gas-cell Active detector for Radioisotope decay), a neutron time-of-flight detector array, and a LaBr\textsubscript{3} detector array is being conducted for the measurement of \(\beta\)-delayed neutron emission and other related \(\beta\)-decay properties of neutron-rich nuclei at the F11 decay station of RIBF. The GARi detector has been constructed from a fast, segmented plastic scintillator coupled with nine multi-anode photomultiplier tubes (PMTs), resulting in enhanced position sensitivity. Therefore, this detector can be employed as an implantation detector and can also serve as a trigger signal in neutron time-of-flight experiments. This work details the development of the GARi detector and the results tested with the radioactive sources.

Shell evolution and emerging paradigm changes

The shell structure conceived by Mayer and Jensen in 1949 has been shown to be quite appropriate for stable and near-stable nuclei, but substantial deviations from it have also been observed more recently for exotic nuclei with notable neutron excess. Such changes of the basic picture of the nuclear shell structure, called the shell evolution, seem to be the subject studied most extensively and most elaborately by RI beam experiments worldwide. An overview of the shell evolution is presented in this talk, from both theoretical and experimental perspectives. The shell structure is shown to be varied, for instance, from the one presented by Mayer and Jensen, by particular types of the monopole components of the effective nucleon-nucleon interaction in nuclei. Among various contributions, the importance of the tensor force is illuminated here, with an outstanding example: the emergence of new neutron magic number 34 in neutron-rich Ca isotopes. The mechanism of the shell evolution produces significant impacts also on the nuclear shapes. Type II shell evolution shifts the excitation energies of intruder deformed bands, for instance, in some Ni isotopes. In other more general cases, the monopole interaction is shown to produce unexpected crucial effects on the patters of rotational bands of heavy deformed nuclei. In fact, the shape of the ellipsoidal deformation is investigated by large-scale shell model calculations, which is nothing but the Monte Carlo Shell Model. The unique role of the monopole component of the tensor force is clarified: the interplay between this monopole interaction and the quadrupole interaction provides us with various patterns of triaxial shapes for many nuclei, such as 166Er, one of the traditional prolate deformed heavy nuclei. Thus, the prolate preponderance hypothesis by Aage Bohr is investigated for its microscopic validity. Some of the nuclear paradigms are changing now in this way, due to emerging aspects of nuclear-force effects.

Recent progress of ISOL facility at RAON

The RAON (Rare Isotope Accelerator complex for ON-line experiments) is currently in the stage of commissioning under the Rare Isotope Science Project (RISP) launched in 2011. RAON will utilize an advanced rare isotope beams produced with a high power target by the Isotope Separation On-Line (ISOL) facility, aiming to deliver high purity and intense, neutron-rich rare isotope beams to the post-accelerator and experimental facilities. The RAON ISOL facility consists of a target/ion source module surrounded by movable shielding blocks in a bunker, remote handling facilities for the target operation, a pre-mass separator, a RFQ cooler buncher, an EBIS charge breeder, and an A/q separator. The installation and alignment of the ISOL facility were completed in June 2021. The target/ion source module allows us to bombard a thick target with a 70 MeV proton beam of RAON, producing a variety of rare isotope beams. Rare isotope beams extracted from the target/ion source can be transported to a pre-mass separator at energies of up to 60 keV, and will be cooled in a RFQ-CB. Cooled ion beams can be sent to two different experimental facilities, such as a Mass Measurement System and a Collinear Laser Spectroscopy in ISOL experimental hall. Alternatively, for post-acceleration of ion beams, the singly charged ion beam of interest can be bunched to 108 ions in RFQ-CB and then delivered to the EBIS charge breeder through the EBIS branch system. The preparation of multi charged ion beams for the post-acceleration using the superconducting LINAC of SCL3 will be carried out through the EBIS charge breeder and A/q separator to match the energy of 10 keV/u with A/q < 6 with the requirement of RAON Injector.The first commissioning experiment of the ISOL system started in March 2021 using the 133Cs and 120Sn stable beams produced from the target container combined with the surface ion source and laser Ion source. The stable beam experiments have demonstrated the overall functioning of the RAON ISOL system, and we are planning to carry out the first RI beam test using the SiC target with 70 MeV proton of cyclotron in the coming months.

The SCRIT electron scattering facility at RIKEN RI Beam Factory

The SCRIT (Self-Confining Radioactive-isotope Ion Target) electron scattering facility has been constructed at RIKEN RI Beam Factory. After the commissioning experiment, the development towards electron scattering with short-lived unstable nuclei has been carried out. Details of the ion dynamics inside the SCRIT system were studied with stable nuclei, and the achieved luminosity was 3.2 × 1027 cm−2s−1 with an electron–beam current of 250 mA and the injection of 108 Xe ions. Electron scattering with online-produced 137Cs ions has been recently commissioned. In the near future, electron scattering with short-lived unstable nuclei will be performed.

Transfer reaction measurements using SNACK at KoBRA

KoBRA can produce RI beams with energies of 5–10 MeV/u from stable ion beams (10–40 MeV/u) delivered from the superconducting linear accelerator SLC3 of RAON. In its early phase of operation, transfer reaction measurements with these RI beams can be performed for nuclear astrophysics studies. SNACK has been developed at RISP in order to measure the light ejectiles from the transfer reactions. With the beam trajectories obtained from upstream PPACs, excitation energy levels in the heavy recoils can be reconstructed by measuring light ejectile’s energies and scattering angles. The expected results of transfer reaction measurements with SNACK and KoBRA beamline detectors were calculated by using a Monte Carlo simulation.

KoBRA Wien filter for low-energy RI beam production and recoil separation

The Wien filter is one of the key components in ion optics to improve the mass separation performance. The KoBRA Wien filter will be installed at the low-energy beamline KoBRA of RAON in Korea. The specifications of the KoBRA Wien filter were determined based on the ion beams expected in the KoBRA beamline, especially, beam energies less than about 5 MeV/nucleon suitable for nuclear astrophysics experiments. The Wien filter is designed to have the maximum field intensities of 0.2 T for the magnetic field and 2.0 kV/mm for the electric field in the ±75(H)×±50(V)×2500(L)mm3 good-field region. Performance of the Wien filter was estimated by the ion optics calculations of the KoBRA beamline for 40Ar beams at 18.5 MeV/nucleon and 14O beams at 2.5 MeV/nucleon. The mass resolving powers are 42.65 and 517, respectively. Currently, the KoBRA Wien filter is being manufactured, and will perform a factory acceptance test.

OEDO-SHARAQ system: Multifaceted performances in low-energy RI production and high-resolution spectroscopy

The OEDO-SHARAQ system is the world’s first beamline characterized by the energy-degrading of RI beams. The system also has high performance on high-resolution nuclear spectroscopy with RI beams. A minor update of the system, performed in 2021, provided multifaceted improvements to the system in both effective energy-degrading and high-resolution spectroscopy with RI beams. We report here the achievements of the renewed OEDO-SHARAQ system and the experimental studies performed recently. Additionally we compactly introduce perspectives about upcoming physics programs.

A neural network approach based on more input neurons to predict nuclear mass * *Supported by National Natural Science Foundation of China (12175170, 11675066), the Fundamental Research Funds for the Central Universities (lzujbky-2017-ot04), and Feitian Scholar Project of Gansu province

The study of nuclear mass is very important, and the neural network (NN) approach can be used to improve the prediction of nuclear mass for various models. Considering the number of valence nucleons of protons and neutrons separately in the input quantity of the NN model, the root-mean-square deviation of binding energy between data from AME2016 and liquid drop model calculations for 2314 nuclei was reduced from 2.385 MeV to 0.203 MeV. In addition, some defects in the Weizsäcker–Skyrme (WS)-type model were repaired, which well reproduced the two-neutron separation energy of the nucleus synthesized recently by RIKEN RI Beam Factory [Phys. Rev. Lett. 125, (2020) 122501]. The masses of some of the new nuclei appearing in the latest atomic mass evaluation (AME2020) are also well reproduced. However, the results of neural network methods for predicting the description of regions far from known atomic nuclei need to be further improved. This study shows that such a statistical model can be a tool for systematic searching of nuclei beyond existing experimental data.

Target System for RI Beam Production at RIBF

Target System for RI Beam Production at RIBF

Performance Upgrade and Present Status of RI Beam Factory Accelerator Complex

Performance Upgrade and Present Status of RI Beam Factory Accelerator Complex

Performance of the OEDO beamline

The OEDO (Optimized Energy Degrading Optics for RI beam) beamline has been developed to obtain low-energy (10 – 50 MeV/u) RI beams by slowing down the secondary beams at RIBF, RIKEN. Such beams are useful probes to investigate nuclear structure by means of nucleon transfer and Coulomb excitation reactions. The beamline is designed to produce a well-focused beam of small energy spread with the help of the angle-tuneable wedge degrader and the radio-frequency deflector. The commissioning and first physics experiments using the OEDO beamline were carried out in 2017. OEDO successfully provided the energy-degraded beams of Se, Zr, and Pd. Its performance including the optics and transmission was evaluated using those experimental data and it functions properly as designed. We expect that our novel beamline shed a light on the experimental approaches involving nuclear reactions at several tens of MeV/u to expand our knowledge of nuclei far beyond the line of stability.

Study of spin-isospin response of 11Li neutron-drip-line nucleus with PANDORA

The spin-isospin responses of the 11Li drip-line nucleus has been measured. Preliminary results of the 11Li(p, n)11Be experiment in inverse kinematics at RI Beam Factory (RIBF) of RIKEN Nishina Center are presented including the observation of 1n, 2n, t, d, 2α and 6He+α decay channels of 11Be reaction product. Details of the experimental setup based on PANDORA (Particle Analyzer Neutron Detector Of Real-time Acquisition) low-energy neutron detector and the SAMURAI large-acceptance magnetic spectrometer are described.

Charge Stripper Ring for RIKEN RI Beam Factory

The use of charge strippers is almost inevitable for the efficient acceleration of particularly heavy ions such as uranium in heavy-ion accelerator complexes. At the RIKEN RI beam factory (RIBF), the total charge stripping efficiency of two strippers, He gas and rotating graphite sheet disk strippers, used for uranium acceleration is less than 5%, which creates a serious bottleneck for potential intensity upgrades in the near future. We have proposed using charge stripper rings (CSRs) as a cost-effective way to enhance the charge stripping efficiency in a multi-stage accelerator complex involving cyclotrons such as the RIBF. In this paper, we present some calculation results on the key design issues of a CSR.

Major Accelerator Facilities for Nuclear Physics in Asia Pacific

Asian Nuclear Physics Association (ANPhA) is the central organization representing nuclear physics in Asia Pacific. ANPhA is now preparing a list of accelerators applicable for nuclear physics experiments in Asia Pacific. Among the listed facilities, world-class “major” accelerator facilities are mainly located in China (Heavy Ion Research Facility in Lanzhou (HIRFL), and its future extension in Huizhou, i.e. HIAF/CiADS, Beijing Tandem Accelerator National Laboratory (BTANL), which is now upgraded to Beijing RI beam Facility (BRIF), in India (Variable Energy Cyclotron Centre (VECC) in Kolkata), in Korea (RISP/rAoN), and in Japan (RIBF at Riken, J-PARC, RCNP, and ELPH/LEPS). World class facilities based on the tandem accelerator are locating at Mumbai and New Deli in India, at Tokai-mura in Japan, and at Canberra in Australia. Characteristics of these “majof” facilities including their facility developments will briefly be reviewed in comparing with similar facilities in Europe and North America.

Offline ion source for laser spectroscopy of RI at the SLOWRI

The ion source combining laser ablation of solid targets in helium gas and an RF ion guide system with an RF carpet was constructed for a reference measurement of isotope shifts. It is important for performing planned collinear laser spectroscopy of RI beams from the SLOWRI facility at RIKEN, especially for medium-mass nuclei of refractory elements. They are difficult to produce via the ISOL method owing to the chemical properties of the elements. The laser-ablated ions of Ni, Zr, Ag, Ta, W, and Ba were transported by the ion guide and mass spectra consistent with the natural abundance were successfully observed corresponding to 105–107 ions per laser pulse.