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탑티어 프로젝트: 한국 RAON과 일본 RIBF가 추진하는 극한 희귀동위원소 과학 국제공동연구 플랫폼

The RIBF Facility - A TOPTIER Playground

작성자 : Hiroyoshi Sakurai ㅣ 등록일 : 2025-09-12 ㅣ 조회수 : 6 ㅣ DOI : 10.3938/PhiT.34.024

저자약력

Dr. Hiroyoshi Sakurai received his Ph.D. degree in science from University of Tokyo in 1993. He is presently holding the position of director while contributing his expertise as an advisor to multiple international research institutions worldwide. His current research interests on development of new experimental programs with fast radioactive isotope beams. (sakurai@ribf.riken.jp)

Co-PI of TOPTIER

The Radioactive Isotope Beam Factory, RIBF, in RIKEN, Japan, is one of the premier heavy-ion accelerator facilities in the world. RIBF is serving as a playground for experimental programs in TOPTIER, in harmony and complementary with the RAON facility, in Korea. The RIBF started operation in 2007, and since then, has delivered intense radioactive isotope(RI) beams and has significantly contributed to development of low energy nuclear physics, especially for physics of exotic nuclei. Excellent achievements have been made in shell evolution, nucleon-nucleon correlation in the vicinity of the drip lines, the r-process path nucleosynthesis and equation-of-state in asymmetric nuclear matter. TOPTIER plays a significant and leading role in further promoting experimental programs in low energy nuclear physics by forming large size international collaborations.

Fig. 1. Facility layout of RIBF.
Fig. 1. Facility layout of RIBF.

The RIBF facility consists of two parts, old facility and new facility constructed in 1986 and 2006, respectively, as shown in Fig. 1.1) The old facility has a 2nd generation inflight separator RIPS which is coupled with RIKEN Ring Cyclotron, RRC.2) The RIPS was designed to deliver intense RI beams for reaction studies and experimental methods for reactions in inverse-kinematics were developed such as in-beam gamma spectroscopy, invariant-mass spectroscopy and missing mass spectroscopy. Based on experiences at the RIPS, the new facility was proposed and constructed as one of the 3rd generation in-flight facilities, where fission reactions with intense uranium beams are utilized for RI beam production.3)

Fig. 2. Super-conducting Ring Cyclotron, SRC at RIBF (Courtesy of RIKEN).Fig. 2. Super-conducting Ring Cyclotron, SRC at RIBF. (Courtesy of RIKEN)

At RIBF, uranium beams are accelerated with a cascade acceleration scheme at the RIBF accelerator complex, and the last accelerator in the cascade is the Super-conducting Ring Cyclotron, SRC. SRC accelerates uranium ions up to 345 MeV/u.4) The uranium nuclei in the beams are converted to fission fragments at targets, and fission fragments of interest are collected and separated at the inflight separator BigRIPS and then are delivered as RI beams to experimental sites.5) The combination of SRC and BigRIPS has produced more than 170 new isotopes. A typical energy of the RI beams is 200 ‒ 250 MeV/u. Pictures of SRC and BigRIPS are shown in Fig. 2 and Fig. 3, respectively.

Fig. 3. In-flight separator, BigRIPS at RIBF. (Courtesy of RIKEN)Fig. 3. In-flight separator, BigRIPS at RIBF. (Courtesy of RIKEN)

Compared with other heavy ion facilities, the uniqueness of RIBF is that three magnetic spectrometers, ZeroDegree,6) SAMURAI7) and SHARAQ/OEDO8) are equipped for reaction studies with fast RI beams. These spectrometers are complementary to each other and any style of reaction experiments can be conducted at the spectrometers. ZeroDegree is a beam-transport-line spectrometer for low-energy transfer reactions. SAMURAI is a versatile spectrometer which has a large solid and momentum acceptance for exclusive measurements. SHARAQ has nice momentum resolution achieved via a dispersion matching mode and OEDO is a device to efficiently decrease energy of RI beams down to ~20 MeV/u. Not only reaction studies, but also decay-spectroscopy and mass-spectroscopy are conducted at the downstream of ZeroDegree. In addition to the spectrometers, a storage ring, Rare-RI Ring,9) was installed to measure mass of short-lived nuclei via unique scheme of particle-by-particle injection and isochronous optical mode.

International collaborations are highly encouraged at RIBF. Large size detector arrays have been constructed under the large collaborations to enhance detection efficiencies for gamma-rays, neutrons and charged particles emitted at reactions or decay, and to maximize research outputs in a limited machine time.

Excellent achievements have been made under international collaborations. Research highlights at RIBF are for example, location of the neutron drip-line at Fluorine and Neon (Physical Review Letters 2019),10) discovery of new magic number N=34 in the Calcium isotopes (Nature 2013),11) double magic natures of Ni-78 (Nature 2019),12) observation of tetra neutron system (Nature 2022)13) and observation of O-28 (Nature 2023),14) probing the symmetry energy with the spectral pion ratio (Physical Review Letters 2021),15) and the r-process path study (Physical Review Letters 2015, 2017, 2022).16) In addition to pure nuclear physics, a program for radioactive waste was conducted in 2014‒2018, where long lived fission products in the waste were delivered as RI beams and reaction studies were intensively carried out.17)

Concerning the RI production, the RIBF facility has three other sites: SCRIT,18) CRIB19) and KISS.20) SCRIT is a system to realize electron-RI scattering for charge-distribution measurement. SCRIT consists of three components: ISOL, electron storage ring and electron spectrometer. At the ISOL, the RIs are produced via photo-fission reactions with an electron beam. CRIB is an inflight separator coupled with the AVF cyclotron, and nuclear astrophysics programs are running with low energy and light RI beams. KISS is a device to produce RIs via multi-nucleon transfer reactions to access a ‘blank spot’ region. Laser ionization technique is employed in RI separation. Recently the KISS collaboration proposed a new device KISS-1.521) to access a heavy mass region, and the new device is under construction.

The nuclear physics programs at RIBF are co-operated by Center for Nuclear Study (CNS), University of Tokyo, Wako Nuclear Science Center (WNSC), KEK and RIKEN Nishina Center (RNC). CNS operates SHARAQ/OEDO and CRIB, and WNSC operates KISS device (and KISS1.5 in the near future). The other spectrometers/devices are operated by RNC. Program Advisory Committee for nuclear physics programs at RIBF is co-organized with the institutes.

In TOPTIER, RNC, CNS and WNSC on the Japanese side work coherently as well to organize experimental programs at all devices of RIBF. New detector arrays will be installed at RIBF under the Japan-Korea or international collaborations. One of them, the IDATEN array for decay spectroscopy was initiated by the Korea-UK collaboration,22) and the commissioning experiment was successfully organized at RIBF. In the coming years, physics run will be carried out. Last December, two Korean-initiative proposals were approved at the PAC meeting. Construction of a Germanium detector array for decay spectroscopy is being proposed and neutron detector arrays in Korea will be shipped to strengthen neutron detection at SAMURAI. In parallel, several detector systems are under discussion or under development.

TOPTIER successfully jump-started in 2024 and has accelerated the Japan-Korea collaborations. In addition, TOPTIER is becoming an engine in promoting low-energy nuclear physics programs in the world.

Acknowledgments

This work was supported by National Research Foundation(NRF) grant(TOPTIER, RS-2024-00436392) by the Korea government of Ministry of Science and ICT (MSIT).

각주
1)https://www.nishina.riken.jp/facility/index_e.html.
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14)Y. Kondo et al., First observation of 28O, Nature 620, 965 (2023).
15)J. Estee et al., Probing the Symmetry Energy with the Spectral Pion Ratio, Phys. Rev. Lett. 126, 162701 (2021).
16)G. Lorusso et al., β-Decay Half-Lives of 110 Neutron-Rich Nuclei across the N=82 Shell Gap: Implications for the Mechanism and Universality of the Astrophysical r Process, Phys. Rev. Lett. 114, 192501 (2015); J. Wu et al., 94 β-Decay Half-Lives of Neutron-Rich Cs55 to Ho67: Experimental Feedback and Evaluation of the r-Process Rare-Earth Peak Formation, Phys. Rev. Lett. 118, 072701 (2017); V. H. Phong et al., Efficient Many-Body Non-Markovian Dynamics of Organic Polaritons, Phys. Rev. Lett. 129, 172701 (2022).
17)H. Wang et al., Spallation reaction study for the long-lived fission product 107Pd, Prog. Theor. Exp. Phys. 2017, 021D01 (2017); N. Imai et al., Neutron capture reaction cross-section of 79Se through the 79Se(d,p) reaction in inverse kinematics, Phys. Lett. B 850, 138470 (2024).
18)M. Wakasugi et al., Electron scattering based on a novel internal target technique: SCRIT, Eur. Phys. J. A 42, 453 (2009); Tsukada et al., First Observation of Electron Scattering from Online-Produced Radioactive Target, Phys. Rev. Lett. 131, 092502 (2023).
19)S. Kubono et al., New low-energy RIB separator CRIB for nuclear astrophysics, Eur. Phys. J. A 13, 217 (2002).
20)Y. Hirayama et al., Development of a doughnut-shaped gas cell for the KEK Isotope Separation System (KISS), Nucl. Instrum. Methods B 353, 4 (2015); Y. Hrayama et al., Nucl. Instrum. Methods B 412,11 (2017).
21)Y. X. Watanabe et al., Spectroscopy of neutron-rich nuclei produced in multinucleon transfer reactions at KISS, Nucl. Phys. A 1061, 123140 (2025)
22)B. Moon et al., in preparation.
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