RIBF Facility Upgrade Project

July 2023

RIKEN Nishina Center for Accelerator-Based Science (RNC)-1-

Contents

Page
1. RIBF as an internationally open-accessed leading facility for the low energy
nuclear physics--------------------------------------------------------- 3

2. Progress in the last 15 years and Prospects for the next 20 years------------------------------------------------------------------------------- 5
2.1 Programs FY2007 – 2022-------------------------------------------- 5
2.2 Prospects for the next 20 years -------------------------------------- 9
3. Accelerator Complex----------------------------------------------------- 15
3.1 Performances FY2007-2022---------------------------------------- 15
3.2 Upgrade plan --------------------------------------------------------- 24
4. In-flight separator BigRIPS --------------------------------------------- 34
5. Experimental devices ---------------------------------------------------- 46
6. Role of RIBF upgrade in the regional and international community----------------------------------------------------------------------------- 62
7. Cost, Schedule, and Status ---------------------------------------------- 64
8. About this document ----------------------------------------------------- 65

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1

RIBF as an internationally open-accessed leading facility for the low energy
nuclear physics

The RIKEN Nishina Center for Accelerator-Based Science promotes science and technology development
for “element transmutation” pioneered by Dr. Yoshio Nishina approximately 80 years ago, and is dedicated toanswering two questions: “How were elements created by the universe?” and “Can humans change the elements
at will?” Since the heavy-ion accelerator facility “Radioactive Isotope Beam Factory (RIBF)” constructed
officially started operation in 2007, the facility has become a global center for research on the low energy
nuclear physics while playing a global leadership role in the field.
As the name implies, the RIBF is a facility that supplies radioactive isotopes (RIs) as “beams,” and produces
RI beams like a “factory.” Its RI beam intensity is famous worldwide, and has gathered users from around theworld, and produced a wealth of outstanding research results. The intense RI beams are realized withcombination of the high-performance heavy-ion accelerator complex and the large-acceptance inflightseparator. The superconducting ring cyclotron, the energy breeder at the final acceleration stage has delivered
the intense primary beams, which are converted to radioactive isotopes via reactions at target. The reaction
products of interest are collected and separated at inflight separator and hence are delivered as secondary beams.
It would be emphasized that the accelerator system and inflight separator are optimized to obtain high intense
fission products as secondary beams. The acceleration scheme was designed to deliver an intense uranium beamat an intermediate energy of 345 MeV/u and momentum and angular acceptance of the separator was fit for
momentum and angular spread of fission products.
In addition to the intensity of RI beams, the other uniqueness of RIBF is three magnetic spectrometers
equipped, which are complementary each other in terms of resolution and solid angle and are suitable for any
styles of reaction studies. A storage ring dedicated to mass measurement is also uniquely designed to storage
RI ions produced with continuous beams delivered at the cyclotron-based accelerator complex.
Since 2007, experimental programs with these spectrometers and the ring have been organized underinternational collaboration. Based on in-kind contributions of users in the world such as detectors or newly
developed system, large-size collaborations have been formed to lead to so called “campaign-style”
experiments to achieve efficient operation in terms of machine time, data production and manpower.
RIBF has contributed to the expansion of the nuclear chart by producing more than 150 new isotopes andhas provided a multitude of opportunities to discover new phenomena originating from high-isospin asymmetry.
Very selected research highlights in studying the nuclear structure are the determination of the neutron driplinein the F and Ne isotopes, appearance of a new magic number of N=34, discovery of the deformed halo nuclei
29
F, 31Ne and 37Mg, the double magicity in 78Ni. Recently, the tetra-neutron state was observed with the two
independent experiments, which has stimulated the community to investigate T=3/2 three-nucleon forces. The
world-first heavy ion-collision with RI beams is giving the information on Equation-of-State of asymmetricnuclear matter. RIBF produced the world’s first neutron-rich nuclei involved in the r-process and succeeded ininvestigating their properties and demonstrated the impacts of experiment data. It is not an exaggeration to saythat the RIBF pioneered the r-process research based on experimental results for the first time in history.
It would be emphasized that based on the intense RI beams available at RIBF, a new nuclear engineering
program was launched to solve the problem of the high-level radioactive waste from nuclear power plants. In
this program long-lived fission products in the waste were delivered as RI beams for reaction studies.
All the highlighted results above have been obtained due to the nice facility performances of RIBF. The
success of RIBF has brought the great growth of the low energy physics community and has advanced
construction plans of new inflight facilities abroad. FRIB (US) was completed in 2022 and started operation.

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RAON (Korea), HIAF (China), and FAIR (Germany) are projects with their budget plan and construction
currently underway. The designs of these worthy rival facilities also focus on the production of RI beams via
fission reaction with a uranium beam.
Considering the uniqueness of RIBF and competitiveness with other facilities, we are proposing an RIBF
upgrade project. The target region newly accompanied with the upgrade is proton-rich and heavy mass region
at Z>50, where dynamical phenomena in nucleus such as alpha-clustering and fission will be studied withreactions. Such nuclear physics research aims to bring safe and nuclear-waste free energy source in the society
50-100 years later. The region will be accessed via projectile fragmentation not via fission with a more intense
uranium beam. Not only the accelerator complex but also the inflight separator and detector systems are being
upgraded to access the heavy region.

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2

Progresses and prospects of physics

In this chapter, we will present the achievements of physics obtained at the RIBF since 2007 to present and thefuture perspective with the upgraded RIBF.

2.1

Programs FY2007 – 2022

2.1.1 New knowledge from newly observed isotopes
Since the RIBF started to provide beams, more than 150 new isotopes are produced using a variety of heavyion primary beams. Among them, discoveries of 31F, 34Ne [AHN19], 39Na [AHN22] result in extension of theneutron dripline from previous Z = 8 (N = 16) to Z = 9 (N = 22), 10 (N = 24), and 11 (N = 28), which provides
us with invaluable information on nuclear stability in the region between N=20 and 28 magic numbers.
Discovery of the doubly magic 60Ca [TAR18] also impacts theoretical mass models and implies existence ofanother doubly magic 70Ca isotope.
On the neutron deficient side, it is found that 72Rb with odd-number protons (Z = 37) and odd-number
neutrons (N = 35) exists, while 73Rb with even-number neutrons is unbound [SUZ17]. This contradicts, at least
at a glance, a common sense of traditional nuclear physics which tells that a nucleus with even-number
neutrons is more stable than one with odd-number neutrons, and hints existence of an unknown mechanism tostabilize nuclei near the proton dripline.

2.1.2 Shell evolution and universality of level inversion
One of the subjects focused during the last 15 years is shell evolution in the region far from the stability line,
in particular appearance and disappearance of magic numbers. RIBF has discovered that 1) the island of
inversion, known to be localized around Z = 12 and N = 20, has an extended structure covering N = 28 to east,and Z ~ 8/N = 20 to south [29F: DOR17, 28F: REV20]. It was also found the mechanism similar to the islandof inversion is responsible for nuclear structure around Z ~ 28, N = 40. 2) A new magic number of N = 34

-5-appears in the neutron-rich calcium isotopes [STE13]. 3) The Z = 28, N = 50 magicity in 78Ni is persistent[TAN19].

2.1.3 New forms of existence around the neutron drip-line
High-intensity neutron-rich beams from RIBF have been used to reveal new forms of existence near andbeyond the neutron drip-line. Coulomb and nuclear breakup experiments at SAMURAI show that a newmechanism leading to occurrence of a neutron halo with a deformed core stabilize 31Ne [NAK14] and 37Mg
[KOB14].
The (p,pn) knockout reaction experiment clarified previously-unknown feature of dineutron in Borromean
nuclei, namely localization of the dineutron in 11Li around the surface of the 9Li core [KUB20]. This dineutron
localization seems to be universal and occurs in 14Be and 17B, too [COR23].
The most remarkable outcomes relevant to the physics are discoveries of a tetraneutron state. A clear peakslightly above the four neutron threshold was first found in the double charge exchange 4He(8He,8Be)4nexperiment at SHARAQ [KIS16]. Existence of the same structure was successfully confirmed in the recent
SAMURAI experiment using 8He(p,pα)4n reaction [DUE22].

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2.1.4 Nuclear data to ground the r-process research
RIBF is the first accelerator facility that enables access to r-process nuclei. Using inflight fission of 238U,
nuclei along and close to r-process paths are produced and provided to experiments. RIBF r-process studies
started with lifetime measurements with the EURICA setup. From the EURICA campaign, 150 new half-life
data and 326 improved half-life data have been obtained [XU14, LOR15]. It is followed by decay branchingratio measurements using high-efficiency neutron detectors under the BRIKEN collaboration. BRIKEN has
succeeded in measuring one or two neutron emission branching ratio data after β-decays for 80 nuclides.
Now mass measurements of r-process nuclei are starting. Combined use of MR-TOF [IIM23], Rare RI Ring
[LI22], and SHARAQ-OEDO [MIC18] when necessary will open broad research opportunities at RIBF.

2.1.5 Properties of neutron-rich matter
Evolution of nuclear matter properties with an increasing neutron to proton ratio is another focused subject-7-

at RIBF and has been investigated in a variety of viewpoints.
SπRIT at SAMURAI is the heavy-ion collision program at 200 to 300 MeV/u aiming at determination of
the equation of state of neutron matter. The collaboration set up a large-volume time projection chamber in the
gap of the SAMURAI magnet and observed particle emission from the collision of neutron-rich Sn isotopes.
The measured π+/π− [EST21] and t/p [KAN21] ratios clearly support smaller values of L of 40 to 60 whencompared with results of the recent transport theories.
Direct reactions also shed new lights on the properties of neutron-rich matter: The first observation of
Gamow-Teller giant resonances in an unstable 132Sn nucleus [YAS18] indicates the Landau-Migdal parameterg’ doesn’t significantly change with the neutron excess and gives a new constraint on occurrence of the pion
condensation in dense neutron-rich matter. Quasi-free knockout reaction study of single particle strengths inneutron-rich and deficient oxygen isotopes shows a weak dependence of the reduction factor on the neutronproton separation energy difference [KAW18]. Other direct reaction programs such as the active-target
measurements of isoscalar monopole strengths in 132Sn, the proton elastic scattering experiments to determine
proton- and neutron-density distributions in 132Sn are ongoing. Recently, the SCRIT collaboration has
succeeded, for the first time, in performing the electron elastic scattering experiment for an unstable 137Cs[TSU23]. The neutron and proton density distributions determined by the proton and electron elastic scatterings,
respectively, will provide us with accurate data of the neutron-skin thickness in neutron-rich nuclei.

2.1.6 The first step towards transmutation of nuclear waste
Nishina Center initiated a basic science program aiming at reduction of high-level radioactive waste throughnuclear transmutation, in particular, on fission products. Taking full advantages of high-intensity RI beams
produced from in-flight fission of a uranium beam and methodologies of inverse-kinematics reaction, thecollaboration took cross section data for spallation reactions induced by proton and deuteron in a wide range
of incident energies. The data clearly show that long-lived fission products can be efficiently transmuted tostable or short-lived radioactive nuclides by the reactions.

2.1.7 References[AHN19] D.S. Ahn et al., Phys. Rev. Lett. 123, 212501 (2019)https://doi.org/10.1103/PhysRevLett.123.212501[AHN22] D.S. Ahn et al., Phys. Rev. Lett. 129, 212502 (2022)
https://doi.org/10.1103/PhysRevLett.129.212502[COR23] A. Corsi et al., Phys. Lett. B 840, 137875 (2023)
https://doi.org/10.1016/j.physletb.2023.137875[DOR17] P. Doornenbal et al., Phys. Rev. C 95, 041301 (R) (2017)https://doi.org/10.1103/PhysRevC.95.041301[DUE22] M. Duer et al., Nature 606, 678 (2022)
https://doi.org/10.1038/s41586-022-04827-6[EST21] J. Estee et al., Phys. Rev. Lett. 126, 162701 (2021)
https://doi.org/10.1103/PhysRevLett.126.162701[IIM23] S. Iimura et al., Phys. Rev. Lett. 130, 012501 (2023)
https://doi.org/10.1103/PhysRevLett.130.012501[KAN21] M. Kaneko et al., Phys. Lett. B 822, 136681 (2021)
https://doi.org/10.1016/j.physletb.2021.136681-8-

[KAW18] S. Kawase et al., Prog. Theor. Exp. Phys. 2018, 021D01 (2018)https://doi.org/10.1093/ptep/pty011[KIS16] K. Kisamori et al., Phys. Rev. Lett. 116, 052501 (2016)
https://doi.org/10.1103/PhysRevLett.116.052501[KOB14] N. Kobayashi et al., Phys. Rev. Lett. 112, 242501 (2014)
https://doi.org/10.1103/PhysRevLett.112.242501[KUB20] Y. Kubota et al., Phys. Rev. Lett. 125, 252501 (2020)
https://doi.org/10.1103/PhysRevLett.125.252501[LI22] H.F. Li et al., Phys. Rev. Lett. 128, 152701 (2022)
https://doi.org/10.1103/PhysRevLett.128.152701[LOR15] G. Lorusso et al., Phys. Rev. Lett. 114, 192501 (2015)
https://doi.org/10.1103/PhysRevLett.114.192501[MIC18] S. Michimasa et al., Phys. Rev. Lett. 121, 022506 (2018)https://doi.org/10.1103/PhysRevLett.121.022506[NAK14] T. Nakamura et al., Phys. Rev. Lett. 112, 142501 (2014)
https://doi.org/10.1103/PhysRevLett.112.142501[REV20] A. Revel et al., Phys. Rev. Lett. 124, 152502 (2020)
https://doi.org/10.1103/PhysRevLett.124.152502[STE13] D. Steppenbeck et al., Nature 502, 207 (2013)https://doi.org/10.1038/nature12522[SUZ17] H. Suzuki et al., Phys. Rev. Lett. 119, 192503 (2017)https://doi.org/10.1103/PhysRevLett.119.192503[TAN19] R. Taniuchi et al., Nature 569, 53 (2019)https://doi.org/10.1038/s41586-019-1155-x[TAR18] O.B. Tarasov et al., Phys. Rev. Lett. 121, 022501 (2018)https://doi.org/10.1103/PhysRevLett.121.022501[TSU23] K. Tsukada et al., Phys. Rev. Lett. in press.
[XU14] Z.Y. Xu et al., Phys. Rev. Lett. 113, 032505 (2014)
https://doi.org/10.1103/PhysRevLett.113.032505[YAS18] J. Yasuda et al., Phys. Rev. Lett. 121, 132501 (2018)https://doi.org/10.1103/PhysRevLett.121.132501

2.2

Prospects for the next 20 years

In this section, we will present the long-term goals of physics that will be pursued in the upgraded RIBF.
The goal of the upgraded RIBF in a long-term perspective will be the understanding of the structure anddynamics of heavy element RIs, especially in the region accessed by the projectile fragmentation reaction of auranium beam. The upgraded RIBF will offer great opportunities to investigate the north area of the nuclear
chart, which presently remains an unreached territory for the RIBF. The richness in the structure and dynamics
expected in this region, as described below, will be fascinating in terms of the physics of many-body nuclear
systems and far-reaching on other disciplines such as astrophysics, medical application, or energy resources.
This ultimate goal, defined as Pilar 3 in this report, will require the totality of the knowledge on atomic nuclei
and of the theoretical and experimental techniques that have been developed at the RIBF. Therefore, Pillar 3
will be based on other two Pillars that represent evolutional extensions of the research initiatives practiced at
the present RIBF; Pillar 1, the understanding of the nuclear force, especially the three-nucleon force (3NF),
and Pillar 2, the understanding of the physics of the continuum. Pillar 1 (3NF) and Pillar 2 (continuum) are ofutmost importance in low-energy nuclear physics today, and closely linked to various phenomena expected in-9-heavy element RIs, thus providing a foundation to challenge Pillar 3.
In the following, we will overview three Pillars with physics interests and envisioned experiments at the
upgraded RIBF.

Figure 41 Summary of three Pillars of physics in the upgraded RIBF. The nuclear chart is taken from Chapter
3.

2.2.1 Pillar 1: How does the three-body force organize nuclear many-body systems?
Understanding the nature of nuclear forces, particularly the properties of three-body forces, and how they
manifest in the properties of nuclear many-body systems is a crucial task that remains in nuclear physics.
Obviously, high-precision theoretical and experimental studies of the scattering and resonances of few-nucleon
systems are indispensable.
Furthermore, three-body forces have a significant impact on the properties of finite mass nuclei, for example,
the position of the dripline. RIBF can provide such valuable information. It is also important to promote thetheoretical studies that make use of the data from RIBF to reveal the nature of 3NF and predict the propertiesof nuclei in a broader area in the nuclear chart.
Deeper understanding of the three-body forces will shed light on the properties of the unstable nuclei as wellas the EOS of nuclear matter at high densities. For example, we have an intriguing question of how muchthree-body forces influence the magnitude of LS splitting. This question will be answered through the researchof the mass region beyond 78Ni and 132Sn.

Short to long-term future
A. Nature of (sub-)shell closures at 100Sn (Z = 50, N = 50), 140Sn (Z = 50, N = 90) and 60Ca (Z = 20, N = 40)
These RIs with (semi-)magic numbers are currently at the limit of production for spectroscopy and will be
more available in the upgraded RIBF. Most of the data obtained so far supports that the shell gaps persist
in these nuclei. Nonetheless, the shell closures have not yet been tested directly with these nuclei lacking- 10 -

the level scheme information. The direct observation of their first 2+ states by in-beam γ-ray spectroscopy
will be very important. A comprehensive data of RIs in each region will help quantify the detailed
properties of the shell closures, such as the shell gap energies.
B. Island of Inversion in the region beyond doubly-magic 78Ni (Z = 28, N = 50)
The Ni isotopes are now within reach for ab-initio methods such as Coupled Cluster and Valence Space
In-Medium Renormalization Group (VS-IMSRG). Yet, the most important features of the region of 78Ni
are seemingly picked up only by effective interactions, such as PFSDG-U. This interaction, using a 60Ca
inert core, the pf-shell for protons and the sdg-shell for neutrons, strikingly reveals a rapid change in theshell structure in form of low-lying intruder configurations in 78Ni, which may lead to deformed groundstates in 76Fe, and 74Cr.
The possible breakdown of the shell closures at Z = 28 and N = 50 would create an Island of Inversion in
the region beyond 78Ni, where the deformed ground states dominate. This would impact not only the
structural characteristics of RIs in this region, but also the nuclear ground state masses. The latter governs
the r-process path running across the region beyond 78Ni. The consequence would thus be far-reaching.
The even-even 76Fe, 74Cr or 80Ni nuclei beyond 78Ni will be available in the upgraded RIBF for in-beam γray spectroscopy with the new scintillators array. The information of the first 2+ states will be critical toestablish the onset of the fifth Island of Inversion, if it occurs.

2.2.2 Pillar 2 : How do nuclear many-body systems get organized in continuum?
The advancement of experimental techniques has enabled the exploration of continuum across highly excitedstates and driplines. These are governed by many-body correlations rather than average single-particle behavior,and exhibit rich structures and dynamics, as illustrated by the discovery of the 4n resonance. The physics inthis domain is a challenging task for theoretical research, requiring seamless discussions bridging bound andunbound states. Understanding resonances near the neutron drip line and phenomena like 2p decay is crucial
for comprehending the multi-particle tunneling effects observed in heavier systems, including α decay and rarecluster decays.
Short to long-term future
A. Next step for tetra neutrons research
Near-future SAMURAI experiments will probe nature of multi-neutron states and their stabilizing
mechanisms more deeply from several different angles: 1) Neutron-neutron correlations in multi-neutronstates will be investigated through detection of all the decay neutrons. The neutron detection capabilityof the SAMURAI setup reinforced by addition of NEBULA-Plus arrays from LPC-Caen and HIME arraysfrom TITech and TU Darmstadt will result in unprecedentedly high statistics multi-neutron data. 2) Multineutron states will be produced from different initial states using several cluster knockout reactions. 3)
Superheavy hydrogen (7 − 9H), helium (10 − 12He), and litium (13 − 15Li) isotopes , and their decays
accompanying multi-neutron emissions will be studied. In all the experiments, newly-developed telescopearrays for charged particles, Straβe-CATANA+ and TOGAXSI, will be introduced to significantly enhancethe experimental efficiencies.
B. Multi-neutrons correlations in the vicinity of 24O and 60Ca
In terms of the multi-neutron physics, it is also important to investigate the multi-neutron correlations inatomic nuclei and connect them to the resonances of 4n and possibly 6n in continuum. The region aroundthe doubly-LS-closed shell nuclei 24O and 60Ca will be ideal playgrounds. For instance, 26,28O with extra
neutrons will be interesting for the investigation of 2n and 4n correlations. The SAMURAI-NEBULA+
system will be powerful for the complete kinematical reconstruction of emitted neutrons.
C. Magnetized nuclear bulk in heavy ion collision
Heavy ion collisions, primarily aimed at the investigation of EOS, are also a tool to create a drip of bulk
nuclear matter in the laboratory. For instance, it is predicted that a nuclear bulk by heavy ion collisions
generates a very high magnetic field of about 1016 Gauss. A similar effect is studied at RHIC-STAR by- 11 -

measuring the polarization of Λ. It will be interesting to investigate this possible magnetization with RI
beam collisions at the SAMURAI-SpiRIT TPC system by measuring, for instance, the spin polarization ofneutrons. Studies on the spin degree of freedom in heavy ion collisions may also be important for theunderstanding of spin-polarized nuclear matters in magnetors.

2.2.3 Pillar 3 : How do nuclear many-body systems get organized heavy element RIs under an
extraordinarily high Coulomb field.
The proton-rich nuclei with heavier masses delivered by RIBF has the potential to open new era of modern
nuclear physics. Due to the Coulomb barrier, there will be many unbound but long-lived nuclei, which compels
us to reassess the concept of the dripline. In this domain, the correlations between nucleons will dominate oversingle-particle features, and there will be a delicate balance between the Coulomb and nuclear forces.
Consequently, we will discover various exotic decays and cluster formations.
We also note that protons and neutrons occupy the closer orbits which will emphasize the shell effect andinduce stronger proton-neutron correlations. As a result, we expect coexistence of various nuclear shape andemergence of exotic nuclear deformations. A highlight may be the emergence of the octupole deformation
which simultaneously breaks the rotational and reflection symmetries.
Heavy-mass proton-rich nuclei produced through nuclear fragmentation will enable the study of various
isomers, including high-spin isomers and shape isomers. For instance, the spin response of high-spin states
represents entirely novel physics. It would also be possible to systematically investigate the nuclear fission as
a function of proton and neutron numbers, which will reveal the physics behind the symmetric/asymmetricfission processes.

Short to long term future
A. The third peak of r-process and freezeout
The nuclear mass information in the region of N = 126 is essential to understand the third peak of theelemental abundance, which is governed by the (n,γ) equilibrium during the hot phase. Heavy element RIs
near N = 126 will be produced more efficiently by the upgraded BigRIPS and by KISS II. As a massspectrograph, MRTOF-MS and R3 can be chosen depending on the mass and lifetime ranges.
The neutron capture cross sections play an important role in the freezeout phase. A study by the (d,p)
surrogate reaction will be carried out at OEDO for key nuclei, for instance, around 132Sn to understandespecially the competition between two types of neutron capture, the direct capture process and thecompound process.
It should be noted that both data will not only tune or benchmark theoretical frameworks, but also be used
to improve the prediction powers by the machine learning toward the r-process path that is still out of thereach with the upgraded RIBF.
Long term future
The physics of heavy element RIs will be a new challenge for the upgraded RIBF. The availability of heavy
element RIs will be enhanced by the primary beam intensity upgrade (Chapter 5) and also by the upgraded
BigRIPS separator (Chapter 3). It should also be emphasized that the opportunities of reaction studies using
fast RI beams are one of the greatest advantages of the RIBF compared to previous studies in the heavy region
often based on the stable beam fusion or decay spectroscopy of actinides. Various reaction probes with different
sensitivities, developed also for Pillar 1 and 2, will greatly assist the challenges in Pillar 3.

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Figure 41 Predicted decay modes mapped over the nuclear chart (by H. Koura of JAEA). The nuclei with thepredicted lifetimes longer than 1 ns are plotted in colors.
Below several major cases of physics are listed.
A. Proton dripline without a drip
The proton dripline, especially beyond Pb, is expected to give a very complex landscape due to multiple
decay modes (beta, alpha, proton, 2p …) competing with each other and due to the Coulomb barrier which
delays proton emissions. As a result, the dripline, defined by the masses, and the existential limit of nuclei,
governed by the lifetimes, split apart. The complex dual coast lines should reflect structural and dynamical
properties, such as alpha cluster formation, 2p correlation, or multi-dimensional tunneling through the
Coulomb potential, providing challenges to both experiments and theories. Experimentally, not only thedetermination of the dripline, but also measurements of lifetimes and decay modes are required. New
isotope searches and decay spectroscopy with the upgrade BigRIPS, and mass measurements by MRTOF
or R3 will be instrumental.
B. Rare mode of nuclear decay
Rare decay modes involving charged-particle emissions are phenomena bridging alpha decays to fission.
The first of this kind was found in 223Ra decaying by a 14C emission to 209Pb in 1984, and about 30 examples
are presently known. Another important decay mode is two or multifold alpha emissions, predicted in 1985,
but still elusive. The upgraded BigRIPS allows to directly deliver an RI of interest to the decay station.
The beam purity is a key to suppress background radiations, which will be improved by the upgraded firststage of BigRIPS.
The excitation of RI beams by reactions will also be an advantageous method. This allows to measure therare decay mode in excites states. The decay residue in-flight will be identified by the upgraded
ZeroDegree.
C. Shape and clustering
Due to the symmetry breaking or to the emphasized proton-neutron interactions, exotic shapes andclustering are expected in heavy element RIs. In-beam reaction studies by using fast RI beams will be a
powerful tool to shed light onto the structural richness of heavy element RIs. In addition to SAMURAI
and ZeroDegree adapted to a few hundred MeV/u, low energy reactions are also feasible at the energy
degrading beamline OEDO down to 10 MeV/u. The upgraded RIBF will also be equipped with newspectroscopic instruments, for instance, for γ-rays (the new scintillator array and the germanium clusters
array GT-5) or for knockout particles (TOGAXI or STRASSE).

- 13 -Examples include:-

Octupole deformation. Higher-order shapes beyond the quadrupole are expected due to the breakingof the reflection symmetry. Nuclei around N = 134 and Z = 88 are predicted to favor the octupole
deformation. In-beam γ-ray spectroscopy with inelastic scattering, knockout reactions or Coulomb
excitations will be essential to characterize the deformation.

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Shape coexistence. A variety of coexistence phenomena involving various deformation types areforeseen. A special interest goes to the magicity region near Pb and the north area with N = 126.

-

Alpha cluster formation. It is a longstanding question as to whether and how an alpha particle is formed
prior to the alpha decay. It is also interesting whether multiple alpha clusters are formed and possibly
condensed at the nuclear surface. Alpha knockout reaction studies can be carried out by using TOGAXI
for various alpha-decaying RIs. The upgraded ZeroDegree will serve to identify the knockout residue.

D. Isomerism
Isomerism is another interesting subject in the heavy element region. Extremely high spin states (e.g. 151Er
67/2- state at 10.3 MeV with a lifetime of 420 ns; 178Hf 16+, 2.45 MeV, 31 y), highly-deformed fissile states(235U Jπ unknown, 2.5 MeV, 3.5 ms), or extremely low energy states ( 229Th 3/2+, 8.2 eV, 7 μs) represent
extreme conditions that are not often met in ground states. The upgraded BigRIPS and KISS II with
MRTOF-MS will offer great opportunities to extensively survey still unknown isomeric states up to theregion of actinides from the proton dripline towards very neutron-rich side.
The isomer RI beam filter based on R3 is an innovative technology envisioned at the upgraded RIBF. This
instrument will deliver isomeric RI beams for reaction studies. The R3 storage ring will serve to tag (forthe states with lifetimes greater than 50 μs) and/or physically filter (＞50 ms) RIs in isomeric states andextract them in-flight toward the reaction station. Spectroscopy of isomeric states by reaction probes will
help shed light onto the structure or nuclear properties in extreme conditions. For instance, the extremely
large deformative nature of fission isomers, so far inferred only indirectly or theoretically, can becharacterized by Coulomb excitations or inelastic scattering to measure quadrupole transition strengths or
giant dipole strengths. High-spin isomers, if regarded as a state more magnetized than ordinally groundstates, would offer interesting playgrounds to investigate the spin response of nuclear matters. GamowTeller giant resonances can be investigated by the (p,n) reaction.
E. Fission factory
Induced fission studies by various fast RI beams will be feasible at the upgraded BigRIPS. An advantageof the RIBF is that the fission fragments are moving in-flight and can be analyzed by the upgraded
SAMURAI or ZeroDegree to obtain a complete set of information (the particle identity and momentum).
This allows not only to evaluate the symmetry or asymmetry in fragment mass distributions, but also to
analyze correlations in the multi-dimensional parameter space. Systematic data of this kind for nuclei withdifferent proton and neutron numbers, close to and away from N = 126, will be important to further our
understanding of nuclear dynamics. This will also give a feedback to related phenomena such as the fission
recycling at the r-process termination, albeit still difficult to directly access.
F. RI beam reactions toward the next-next phase beyond the perspective of 20 years
The regions beyond uranium, for instance, related to the Island of Stability or to the fission recycling, willremain inaccessible even at the upgraded RIBF. It is important to continue the efforts to investigate newreaction techniques to produce RIs. The two-step RI beam production method, where an RI beam isbombarded onto a secondary production target, is promising. The understanding of reaction dynamics of
RI beams, especially at low energies, is especially important. For instance, nuclear fusion or deep inelastic
scattering with RI beams are interesting in terms of understanding the dynamics, such as the fusion
hindrance, the cross-section enhancement with side collisions, or the spin distributions in compound states.
The energy degrading OEDO beamline can be used for reaction studies down to the beam energy of about
10 MeV/u. The future RUNBA recycling ring facility is an innovative luminosity booster for low-energy
reactions, where target thicknesses are limited. Reaction studies with low-rate RI beams will be facilitated.

- 14 -

3

Accelerator Complex

3.1 Performances FY2007-2022
This section gives an overview of the RIBF accelerators, followed by description of some technicaldevelopments since 2007 through 2022, and present status of the accelerator system.
3.1.1 Overview of RIBF
A: AVF (K70)
B: RILAC-SRILAC
C: RILAC2

D: RRC (K540)
E: fRC (K700)
F: IRC (K980)
G: SRC (K2600)

B

b c
E

a

A

C

k

D

g

d

G
f

e

Low
-ene
rgy
F
a: GARIS III
b: GARIS II
c: RIPS
d: CRIB (CNS)
e: KISS (KEK)

F

acil
ity

j

h

N ew

Faci
li

ty

i
f: BigRIPS
g: ZDSh: SAMURAI
i: SHARAQ (CNS)
j: Rare RI Ring
k: SCRIT

Figure 3.1.1: Schematic of the RIKEN RI Beam Factory (RIBF). The accelerators (A−G) andexperimental devices (a−k) are presented.
The Radioactive Isotope Beam Factory (RIBF) at RIKEN is a cyclotron-based accelerator facility that uses
fragmentation or fission reactions of intense heavy-ion beams to produce RI beams over the entire atomic mass
range[3.1.1]. The RIBF started beam delivery in 2007, after the commissioning of the three ring cyclotrons,
namely, fRC (fixed-frequency Ring Cyclotron), IRC (Intermediate-stage Ring Cyclotron), and SRC(Superconducting Ring Cyclotron)[3.1.3.1.2, 3]. These cyclotrons were constructed to boost the energies ofthe beams accelerated by RRC (RIKEN Ring Cyclotron) as shown in Fig. 3.1.1. The main specifications of
the four ring cyclotrons are summarized in Table
Table 3.1.I: Specifications of the RIBF ring cyclotrons.
3.1.I. Currently, there are three injectors, AVF,
RILAC (RIKEN Linear ACcelerator), and
RILAC2, that provide a wide variety of heavyion beams.
The most important feature of the RIBF
accelerator system is the ability to accelerate allions from hydrogen to uranium up to 70% of the
speed of light. To make this possible, three
acceleration modes are used in the RIBF
accelerators, as shown in Fig. 3.1.2(a). The firstmode is the fixed-energy mode, originallydesigned for accelerating very heavy ions suchas xenon and uranium. This mode uses the
RILAC2 injector with a powerful 28-GHz
superconducting electron-cyclotron-resonance ion source (SC-ECRIS), and boosts the beam energy up to 345
MeV/u with the four booster ring cyclotrons (RRC, fRC, IRC, and SRC). Two charge strippers are used for- 15 -

Figure 3.1.2: Accelerator chain of RIBF today (a) and that in 2007 when RIBF started (b). The injectors,
RILAC, RILAC2, and the AVF cyclotron are followed by four booster cyclotrons, RIKEN Ring Cyclotron(RRC), fixed-frequency Ring Cyclotron (fRC), Intermediate-stage Ring Cyclotron (IRC), andSuperconducting Ring Cyclotron (SRC)[3.1.4]. The charge strippers are indicated by labels in red text
(ST1−ST4). The superconducting linac booster, SRILAC, has been operational since 2020[3.1.5]. The
dotted line in (a) represents the return beam line dedicated to the biological applications[3.1.6].
the uranium beam. One is a helium gas stripper located at the exit of RRC (E=11 MeV/u), and the other is arotating graphite-sheet stripper located between fRC and IRC (E=50 MeV/u). Recently, due to the highperformance of ion sources, it has become possible to accelerate zinc and krypton beams in this mode using
only the second charge stripper.
The second acceleration mode is a variable energy mode that uses RILAC, RRC, IRC, and SRC to accelerate
medium-mass ions such as calcium. The beam energy from SRC can be changed in a wide range below 400
MeV/u by changing the RF frequency. The third acceleration mode uses the AVF cyclotron as an injector andtwo boosters, RRC and SRC. This mode is dedicated for light ions such as deuterons, nitrogen, and oxygen.
By changing the RF frequency, the beam energy from SRC can also be changed in the range below 440 MeV/u.
3.1.2 Technical Development during 2007-2022
Among heavy ion beams, the uranium beam is the most effective because it can produce medium-mass RI
beams far from the stability line through fission reactions. For this reason, our research and development efforts
to date have focused mainly on increasing the intensity of uranium beams. This subsection gives someexamples related to these efforts.

- 16 -

RILAC2 Injector
As shown in Fig. 3.1.2(b), we used RILAC for the uranium beam injection in the early stage of RIBF
operation. This linac, consisting of six variable-frequency cavities, was commissioned in 1981, and is capableof the total acceleration voltage of 16 MV. We first intended to provide 238U14+ from the 18-GHz electroncyclotron-resonance ion source (ECRIS), which had been operational since 1996, and strip it into 238U35+ at theexit of RILAC in the design stage around 2001. This scheme was found to be unrealistic[3.1.7], and we had
accelerated 238U35+ directly with RILAC since the beam commissioning[3.1.8]. However, RILAC was notsuited for accelerating highly charged ions. First, the beam current of 238U35+ from the ion source was too low
to meet the demand of RIBF. Second, this linac was designed to accelerate low charged ions, and the vacuum
level was not so good. Moreover, we
encountered various difficulties causedby the RF instability in the lowfrequency and low-voltage operation.
Furthermore, the total voltage requiredfor 238U35+ is only 4.7 MV, while thevoltage gain of 16 MV of RILAC was
too high.
Therefore, a new injector that coulddeliver more intense and highly
charged uranium beams was required,and a basic design study had been
conducted by 2006[3.1.9]. The new
injector, which was named “RILAC2”,
was designed to accelerate ions with amass-to-charge ratio of 7; this wastargeted towards very heavy ions such
Figure 3.1.3: Schematic drawing of RILAC2.
as 136Xe20+ and 238U35+, up to an energyof 680 keV/u in the continuous-wave(CW) mode. As shown in Fig. 3.1.3, RILAC2 consists of the 28-GHz SC-ECRIS, a low-energy beam-transport(LEBT) line including a pre-buncher, an RFQ linac, and three Drift Tube Linac (DTL) cavities (DTL1−3). A
rebuncher is located between the RFQ and DTL1, and another rebuncher is positioned in the high-energy
beam-transport (HEBT) line after
Table 3.1.II: Main parameters of the RILAC2 cavities for the DTL3. The RF cavities (excluding theacceleration of ions with M/q=7. All the cavities are operated at pre-buncher) are operated at a fixed
36.5 MHz in the CW mode. For the RFQ, the cell number and resonant frequency of 36.5 MHz,the mean radius r0 are listed as "Gap number" and "Gap length". whereas the pre-buncher is operated at
18.25 MHz. Strong quadrupole
magnets are placed in the beam line
between the DTL cavities. The RFQ
and DTL design parameters are listed inTable 3.1.II. Owing to the highperformance of the new SC-ECRIS, it
was possible to reduce the total lengthof the linac structure to less than 8 m,
and we decided to place the new
injector in the AVF cyclotron vault as shown in Fig. 3.1.3. The length of the beam transport line to RRC could
be reduced to 36 m, which is less than half that required for the RILAC injector.
We achieved the first beam from RILAC2 in December 2010, and the uranium beam was accelerated through
SRC in May 2011[3.1.10-14]. Since then, this injector has been working very stably. A side effect of RILAC2
is that more time can be spent on the experiments for synthesizing super-heavy elements using RILAC. This
contributed to the observation of the third event of the element 113 in 2012[3.1.15], later named
nihonium[3.1.16]. There is another side effect of the introduction of the new injector. Now intense ion beamsof 6−10 MeV/u are available in the low-energy facility behind RRC, where KISS and GARIS II are underoperation.

- 17 -

28-GHz Superconducting ECR Ion Source
Before the RILAC2 project started, the construction of a powerful ECRIS was initiated in 2007. This source
was designed to operate at 28-GHz microwave power using fully superconducting magnets with a plasma
volume as large as 1,100 cm3[3.1.17]. The coil assembly consists of a set of sextupole coils and six solenoids,
which are used to generate a confinement magnetic field. All coils use a niobium-titanium conductor and arebath-cooled in liquid helium. The maximum magnetic field on the coils is 7.4 T. A large radial magnetic field
due to this solenoid configuration is provided on the sextupole coils, and an inhomogeneous, strong expansion
force is generated on the coils. Note that this renders the design and fabrication of the sextupole coil assembly
difficult.
After the coil assembly was fabricated, excitation tests were performed at the manufacturing company
facility in June 2008. However, quenching occurred in the sextupole coils at approximately 85% of the design
current, when the solenoids and sextupole were excited simultaneously. As the cause of this quenching was
attributed to coil motion, the sextupole-coil supporting structure was modified to fix the coils in position more
firmly. Following this modification, the design currents were successfully achieved in October 2008[3.1.18].
The ion source was then moved to RIKEN in December 2008, where it was installed on the high-voltage
platform of the RILAC pre-injector. Then, initial beam tests and beam delivery were performed using an 18GHz microwave power source. However, the beam intensity extracted from SRC in 2009 was as low as 0.8
pnA. One of the reasons for this was that the transmission efficiency through RILAC was relatively poor.
Because the budget for the new injector was approved in late 2008, it was decided to move this ion source
to RILAC2, and the operation with the 28-GHz microwave power source was initiated in August 2011. During
the first few years, a sputtering method with a metallic uranium rod was used to generate uranium ions, and a
238 35+
U beam of approximately 100 eA could be extracted from the ion source during the beam time. However,the beam stability was not satisfactory.
In 2013, we started the development of a hightemperature oven (HTO) method, because this method was
expected to control the amount of vapor supplied to the ion
source plasma. Figure 3.1.4 shows the HTO in the SCECRIS. The HTO is equipped with a pure tungsten crucible
loaded with uranium oxide. The crucible is supported by apair of copper rods that act as bus bars carrying DC current
to heat the crucible directly. A temperature of 2000 ℃ is
required for the crucible to bring the vapor pressure ofuranium oxide to 0.1−1 Pa. The HTO has been used in theuranium beam time since autumn 2016. In the first beamtime, the 238U35+ beam was successfully supplied for 34
consecutive days with a current of 120 eA or more. The
Figure 3.1.4: High-temperature oven (HTO)
uranium beam intensity supplied to the RILAC2 injector areused in the 28 GHz superconducting ECR
now kept at 100−130 eA at the ion source for more thanion source (SC-ECRIS)[3.1.19].
one month[3.1.19].
First Stripping Section
As shown in Fig. 3.1.2, two charge strippers are used in the acceleration of uranium beams. The first stripper
is located behind RRC, where the beam energy is 11 MeV/u. In the early stage of the RIBF operation, carbon
foils with an thickness of 300 g/cm2 have been used for this stripping section, and they convert U35+ into U71+
at an efficiency of 18%. Intensive study has been made for making the lifetime of carbon foils longer since
2008[3.1.20-22]. In the beam time of 2011, carbon-nanotube/sputter-deposited carbon (CNT-SDC) foil, which
has been newly developed in RIKEN, was used as this stripping stage[3.1.23]. The foil, having a shape of adisk of 100 mm, was attached to a cylinder rotating with a speed of 1/20 rpm. The longer lifetime of the CNTSDC foils, typically 30 hours at the intensity of 13 eA, made the beam delivery fairly successful. The
maximum instantaneous current was 3.6 pnA at the exit of SRC. However, the beam quality was notsatisfactory due to the inhomogeneous thickness in the foil. Our goal is 5 pA at this section, and it was obviousthat an innovative stripper was necessary to achieve this goal.

- 18 -

Therefore,
we started thedevelopment of gas strippers started
in 2010. We first measured theequilibrium charge state of 136Xe and238
U beams in nitrogen gas at 11
MeV/u[3.1.24], and found that thenitrogen gas works as the stripper forthe xenon beam; sufficient amount of
41+ state, which is acceptable byfRC, has been obtained. In contrast,
the equilibrium charge state of theuranium beam was found to be 56+,
which is far below the acceptablecharge state of 69+.
The turning point was themeasurement of electron strippingand electron capture of uranium ionsin helium gas at 11 MeV/u[3.1.25].
Figure 3.1.5: Schematic drawing of the helium gas stripper and The result was remarkable; thefluorescence of helium gas by uranium beam[3.1.26].
equilibrium charge state is estimatedto be 66+, which is not so far from the acceptable charge state in fRC. Then, based on a careful design ofdifferential pumping systems, we successfully measured the charge evolution for the uranium ions throughwindow-less helium gas cells of 8 m and 0.5 m in length, respectively[3.1.26-27]. The measured mean charge
reached up to about 65+, which is consistent with that estimated from the measured cross sections for electron
loss and capture. Thus, the use of helium gas seemed promising, but the central issue remained to accumulatea sufficiently thick helium gas in the beamline without a window.
This problem was finally solved by a novel helium-gas stripper in January 2012[3.1.28-29]. It is based on a
five-stage differential pumping system, as shown schematically in Fig. 3.1.5. The target cell of the stripper
contains helium gas at approximately 7 kPa, while the gas leaked into the next stages is re-circulated to thetarget cell with the help of mechanical booster pumps. This helium-gas stripper has played an essential role inincreasing the uranium beam intensity and stability. In fact, the present intensity of uranium beams injected
into the stripper has reached 2.5 pA, and the system is working stably.
Bending Power Upgrade of fRC
The fRC was originally designed to accelerate 238U73+ ions using a 0.6-mg/cm2-thick carbon foil at 10.8
MeV/nucleon. Beam commissioning of the fRC started in 2006, and we immediately recognized that
insufficiently uniform thickness of the carbon foil produced a momentum spread outside the tolerance for thefRC. Hence, we reduced the carbon foil thickness to 0.3 mg/cm2 and modified the injection radius of the fRC.
The most probable charge state is 71+ and the operating magnetic field of the fRC is 1.69 T in this case. This
modification resulted in satisfactory transmission efficiency of the uranium beam: >90%.
The helium gas stripper described above expected that the equilibrium charge mean was 65+ at first. The
required magnetic field exceeded the capacity of the original design: the maximum rise in the cooling watertemperature was 10 degrees for the main coils of the sector magnets, and the tolerance was sufficient forincreasing the magnetic field up to the 1.85 T required for 65+ ions. Design studies performed from May toearly July 2011 identified the devices to be upgraded. The existing power supply system for exciting the fRC
sector magnets consisted of one main power supply and four current-bypassing power supplies to compensate
for small differences between the four sector magnets. The maximum current of the system is 650 A,
corresponding to 238U69+ ions. Hence, capacity should be upgraded to the 830 A expected for 238U65+. The
current setting precision of the new main power supply was designed to be 1 mA, taking into account the lowmagnetic-field operation for the xenon beam. In addition, the injection bending magnet (BM), the magnetic
inflection channel 2 and its power supply, and the extraction BM (EBM) required upgrading because theirmaximum magnetic fields were insufficient. All these devices were newly built except for the EBM, for which
replacement of its iron cores was sufficient to generate the required magnetic field. A drawback of the newiron cores is that magnetic field uniformity is worse than it was with the old cores, though the uniformity
remains within the permissible range. Finally, two new beam steering magnets were introduced in the beam
- 19 -

injection line to compensate for the much stronger stray magnetic fields expected for 238U65+ in comparison
with 238U71+. The manufacture of these devices was completed in March 2012. The installation work andmagnetic field measurements were performed during the first three months of FY2012 coinciding with tightly
scheduled beam services for users. We measured magnetic fields for various excitation currents along thecenterline of each sector magnet. It is challenging to skip the two-dimensional magnetic field mapping forlarge-scale ring cyclotrons such as the fRC, but we gained experience doing this during commissioning of thefRC. An acceleration test of 238U65+ ions was performed in July 2012 and completed in one night. We
successfully extracted a 238U65+ beam from the upgraded fRC with transmission efficiency of 80%[3.1.30].
It was found thet the newly developed helium gas stripper had stripping efficiency that was higher for 238U64+(27%) than for 238U65+ (21%) with a 0.7-mg/cm2-thick helium gas, owing to the atomic shell effect of theuranium ion [3.1.26]. The used thickness is thinnerthan that required for charge state equilibrium.
Thus 64+ ions were accelerated on 28 October by
exploiting the design margin of the new power
supply system for the fRC sector magnets. The
isochronous magnetic fields obtained for the 64+
and 65+ uranium ions are shown in Fig. 3.1.6.
Other quantities, such as transmission efficiency
and acceleration turn pattern, exhibited no
performance decline after the upgrade of the fRC.
The beam transport system from the RRC and thefRC was also upgraded to alleviate difficulty in
Figure 3.1.6: Isochronous magnetic fields of the fRC.
beam tuning caused by an insufficient number ofquadrupole magnets[3.1.30].
Second Stripping Section
The second stripper section is located behind fRC at theenergy of 50 MeV/u. In 2007, the charge state of theuranium ions was converted into 86+ with carbon foils of
17 mg/cm2 at an efficiency of 28%. Owing to the heat loadby the intense beams, these efficiency values decreased
with an increased beam dose. The typical lifetime madefrom commercially available foils was 12 hours at an
intensity of 1 eA.
Therefore, we decided to replace the carbon-foil
stripper with an alternative. In November 2012, thecarbon-foil stripper was replaced with a 19 mg/cm2
beryllium disk stripper rotated at 300−1000 rpm as shownin Fig. 3.1.7 A. However, as shown in Fig. 3.1.7 B, theFigure 3.1.7: Charge strippers for uranium
beams at 50 MeV/u[3.1.31]. A: Beryllium disk disks were found to have many tiny cracks and wereheavily distorted following irradiation of approximately
stripper (16 mg/cm2) before use. B: Beryllium
disk
stripperafter
irradiation
with 1018 uranium ions, which corresponded to a twenty-days
approximately 1018uranium ions. C: Graphite beam time.
sheet stripper (2×7 mg/cm2) before use. D:
This final bottleneck problem was successfully
Graphite disk stripper after irradiation withapproximately 1.4×1018 uranium ions. Each overcome through the introduction of a new type of
stripper has an outer diameter of approximately stripper composed of a “highly-oriented graphite sheet”,
100 mm.
which was manufactured by a Japanese company, Kaneka
Corporation, shown in Fig. 3.1.7 C. This material, which was fabricated from a polymer sheet under highpressure and temperature, has high thermal conductivity of 1500 W/m K in the planar direction. This value
is more than three times that of copper. We employed a pair of graphite sheets, each of which had 7 mg/cm2
thickness, to form the second stripper. For the uranium beam delivery performed in March−May 2015, we
achieved a final intensity of 39.5 pnA at the exit of SRC using this stripper. The carbon sheets werenot damaged
at all, as shown in Fig. 3.1.7 D, even following irradiation with 1.4×1018 uranium ions at a 200 W heat load.
The corresponding temperature on the stripper was estimated to be 600 K via a computer simulation using the
ANSYS software. This graphite-sheet stripper has been used consistently up to date[3.1.31-32].
- 20 -

RRC Cavity
The RIKEN Ring Cyclotron (RRC) has two acceleration cavities based on variable-frequency, halfwavelength resonator, constructed more than 30 years ago[3.1.33]. The resonant frequency of this cavity isvaried by moving two boxes vertically. The inner height of the cavities could be made as small as 2.1 m, while
keeping a wide range of resonant frequency from 20 to 45 MHz. However, the frequency for uranium
acceleration is 18.25 MHz, which is outside the designed range. The gap length between the dee electrode andthe movable box had to be made as small as 20 mm, as shown in Fig. 3.1.8. During the acceleration of theuranium beam, the narrow gap between the dee electrode and the movable box caused a bottleneck problem
limiting the cavity voltage to a maximum of 80 kV. In
addition to frequent discharges during operation, thisnarrow gap increases the capacitance in the cavity, thus
lowering the shunt impedance. Furthermore, due to the
low acceleration voltage, the uranium beam current
reached a space charge limit in the RRC[3.1.34].
Therefore, as shown in Fig. 3.1.8, the internal
components of the cavity, the stems and the deeelectrode, were replaced, leaving the external box andmovable box of the cavity unchanged. The frequency
range of the RRC cavity is shifted downward by theinsertion of a notch into the stem, which was originallystraight. Since the RRC cavities have not operated at
frequencies above 39 MHz in recent years, thefrequency range was set to 16−38 MHz after
remodeling. The shunt impedance, voltage distribution,and frequency range were optimized by changing the
Figure 3.1.8: Calculation models for the original notch size based on 3D electromagnetic calculations
cavity (upper panel) and the modified cavity(lower panel). The gap length required for a using the computer code Microwave Studio (MWS).
resonant frequency of 18.25 MHz is shown in The new cavities have been used in beam time sinceeach panel.
May 2018 and show good operational performancefrom 18.25 to 32.6 MHz. In particular, stable operation
was achieved at a voltage of 120 kV at 18.25 MHz. This is due to an improvement in shunt impedance and anincrease in the gap length between the dee and the movable box. In fact, the frequency of voltage breakdown
has decreased significantly. The bottleneck problem during uranium acceleration was solved in this way.
3.1.3 Present Status

Figure 3.1.9: Evolution of the beam
intensity at the exit of SRC since the start ofoperation of RIBF in 2007. The main R&D
items for increasing the uranium beam arealso indicated.

Figure 3.1.10: Availability of RIBF accelerator
complex delivering beams to BigRIPS experiments.

- 21 -

The evolution of the maximum beam intensity for the ion beams accelerated at RIBF is shown in Fig.
3.1.9[3.1.35]. Through our continuous efforts including the research and development described above, theuranium intensity is now 117 pnA and the beam power has reached 9.6 kW, which exceeded goal of this midterm plan of Nishina Center. As mentioned above, the first stripper shown as ST1 in Fig. 3.1.2 became
unnecessary for medium-mass ions such as zinc and krypton, owing to the high performance of the 28-GHz
SC-ECRIS. The beam power of these ion beams has now reached 20 kW, as shown in Fig. 3.1.9.
Figure 3.1.10 shows the availability of the RIBF accelerator supplying the beam to the BigRIPS target. The
figure shows that the availability has significantly improved since 2012, when the helium gas stripper was
installed. The availability of the accelerator has also exceeded 90% in these years. In addition to thedevelopments mentioned above, members of our group have made continuous efforts to improve the stabilityof the RIBF accelerator, especially the injector section built in the 1980s. Some of these efforts are describedin the reference[3.1.36].
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[3.1.14] N. Sakamoto et al., “Commissioning of a New Injector for the RIKEN RI-Beam Factory”, in Proc.
LINAC'12, Tel Aviv, Israel, Sep. 2012, paper MO3A02, pp. 125−129.
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Chicago, IL, USA, Sep. 2008, paper MOCO−B01, pp. 8−12.
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Superconducting ECR Ion Source at RIKEN”, in Proc. ECRIS'18, Catania, Italy, Sep. 2018, pp. 180−184.
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[3.1.25] H. Okuno et al., “Low-Z gas stripper as an alternative to carbon foils for the acceleration of highpower uranium beams”, Phys. Rev. ST Accel. Beams, vol. 14, 033503, 2011.
[3.1.26] H. Imao et al., “Charge Stripping Of Uranium-238 Ion Beam with Helium Gas Stripper”, in Proc.
IPAC2012, New Orleans, Louisiana, USA, paper THPPP084, Aug. 2012, pp.3930−3932.
[3.1.27] H. Imao et al., “Charge stripping of 238U by helium gas stripper”, Phys. Rev. ST Accel. Beams, vol.
15, 123501, 2012.
[3.1.28] H. Imao et al., “R&D of Helium Gas Stripper for Intense Uranium Beams”, in Proc. Cyclotrons'13,
Vancouver, Canada, Sep. 2013, paper TU3PB03, pp. 265−268.
[3.1.29] H. Imao, “Development of Gas Stripper at RIBF”, in Proc. IPAC'18, Vancouver, Canada, Apr.−May
2018, pp. 41−46.
[3.1.30] N. Fukunishi et al., “Acceleration of Intense Heavy Ion Beams in RIBF Cascaded-Cyclotrons”, in
Proc. Cyclotrons'13, Vancouver, Canada, Sep. 2013, paper MO1PB01, pp. 1−6.
[3.1.31] H. Hasebe et al., “History of Solid Disk Improvement for Rotating Charge Stripper”, in Proc. HIAT'15,
Yokohama, Japan, Sep. 2015, paper MOA1C01, pp. 17−19.
[3.1.32] H. Hasebe et al., “Development of a high-density highly oriented graphite stripper”, EPJ Web of- 23 -

Conferences, vol. 229, 01004, 2020.
[3.1.33] T. Fujisawa et al., “The Radiofrequency System of the RIKEN Ring Cyclotron”, Nucl. Instrum. Meth.
A, vol. 292, pp. 1−11, Jun. 1990.
[3.1.34] K. Yamada et al., “Upgrade and current status of high-frequency systems for RIKEN Ring Cyclotron”,
in Proc. 23rd Int. Conf. on Cyclotrons and their Applications (Cyclotrons'22), Beijing, China, MOAI2, Dec.
2022.
[3.1.35] O. Kamigaito et al., “High Beam Power Operations at RIKEN RIBF: Technical Developments,
Challenges and Resolutions”, in Proc. HIAT'22, Darmstadt, Germany, Jun.−Jul. 2022, MO4I.
[3.1.36] H. Okuno, N. Fukunishi, and O. Kamigaito, “Progress of RIBF accelerators”, Prog. Theor. Exp. Phys.,vol. 2012, pp. 03C002, 2012.

3.2 Upgrade plan
The RIBF accelerator upgrade plan aims to dramatically increase the output intensity of primary heavyelement ion beams, such as uranium, which are particularly important in unstable nuclear physics research inthe midst of global research competition. The targeted RI-beam production ability by the upgrade plan will
exceed the targeted ability of the latest facility, FRIB [3.2.1] in the U.S., which started user operation in 2022,
as well as the next generation facilities with construction costs over $1 billion such as FAIR [3.2.2] in Germany,
RAON [3.2.3] in Korea, and HIAF [3.2.4] in China, which are scheduled to start operation within the next
decade. The upgrade plan will allow RIBF to continue to maintain the leadership in this field of science in theworld. We believe that this is in line with the national strategy in the field of science that our center should
promote.
The upgrade plan is feasible and realistic enough based on the accelerator technology accumulated at the
RIBF. It is designed to be upgraded in stages with a reasonable total construction period of ~8 years andrelatively low budget of approximately 17.3 billion JPY, which is less than half of the construction cost of the
RIBF. In particular, we believe that the intensity of the uranium beam can be increased by a factor of 20 byintroducing a revolutionary ring, the charge stripper ring (CSR) [3.2.5-6]. An important feature of CSR is thatit recycles the beam, which allows a 20-fold increase in intensity to be achieved with only a 6 MW increase inelectrical power (the current operating electrical power is approximately 17 MW for uranium acceleration).
Although CSR is a very unique and unprecedended accelerator component, the technology required for therealization is based on the cyclotron and stripper technology that has matured at RIBF. For more than a decade,the basic design and the development of elemental technologies of CSRs have been proceeded by theaccelerator group at the RIBF. Such studies of CSRs were presented in some papers [3.2.7-9] and conferences[3.2.5-6] and well recognized by domestic and international accelerator societies. Recently, a detailed technicaldesign report (TDR) has been prepared for CSR1 and a technical advisory committee (TAC) meeting was heldon June 15-16, 2023. We have had strong support from TAC in moving forward with the project with CSR1.
There are no intrinsic problems with the feasibility of CSR, which is well suited for our facility. From anaccelerator physics perspective, CSR is an attractive device with a wide range of possible applications, and therealization will have an immeasurable impact on the accelerator industry.
The recycling beam technology to be established in this project with CSRs has great scalability for the future.
Further future upgrading with various CSR-like rings for a high-intensity injector, efficient energy upgradeand efficient secondary beam productions are conceivable at the RIBF. The present upgrade plan with CSRs
will also be important cornerstone of such applications possible in the future.
General description of upgrade plan with CSR
As mentioned above, heavy-ion accelerator facilities are being operated or constructed around the world tofollow the RIBF. Among them, the FRIB project in the U.S., which aims to accelerate a uranium beam to 200
MeV/u using a long superconducting linear accelerator, started operation in 2022. The acceleration scheme
involves a single charge conversion process. Because it is a linear accelerator, it can accelerate ions in multiple
charge states simultaneously [3.2.10], and the final target beam intensity is set at 8400 pnA (400 kW).
- 24 -

To compete with the FRIB project, the intensity of the uranium beam at RIBF must be significantly increased,
but the current acceleration scheme is limited to the intensity of around 200 pnA (16 kW). The main bottleneck
for the intensity upgrade is two charge conversion processes. The present acceleration scheme for the 238U
beam at the RIBF is shown in Fig. 3.2.1(a). A He gas stripper was the first stripper to convert the charge statesfrom U35+ to U64+ at an energy of 11 MeV/u. During the second stage, a rotating graphite sheet disk stripper isused to convert U64+ into U86+ at an energy of 51 MeV/u. Although the two types of strippers are advanced
strippers that can withstand the highest-intensity uranium beams, the first and second charge conversion
efficiencies are 20% and 25%, respectively, thus the overall charge conversion efficiency is only 5%. In otherwords, in the current acceleration scheme for uranium beams, 95% of the beam is discarded.
11 MeV/u
28GHz
SC-ECRIS

RRC

SRC

51 MeV/ufRC

IRC
to BigRIPS

RILAC2He stripper
35+⇒64+ (~20%)

Rot. C stripper
64+⇒86+ (~25%)

K2200 fRC
28GHz
SC-ECRIS

SRC

RRC

IRC
to BigRIPS

RILAC2
Rot. C stripper
35+⇒86+ (~25%)

SRC
28GHz
SC-ECRIS

RRC

fRC

IRC
to BigRIPS

RILAC2

CSR1
35+⇒64+ (<~80%)CSR264+⇒86+ (<~80%)

Figure 3.2.1. Schematic view of the present acceleration scheme of 238U beam at the RIBF (a). Two strippers,
a He stripper at 11 MeV/u and a rotating graphite sheet stripper at 51 MeV/u, are used in the acceleration withfour ring cyclotrons (RRC, fRC, IRC, and SRC). The charge stripping efficiencies are approximately 20% and25%, respectively. Schematic views of possible new acceleration schemes to enhance the total charge stripping
efficiency with (b) a normal conducting K2200 cyclotron and (c) CSRs are also shown.
A simple way to increase the total charge stripping efficiency of the RIBF is to remove the He stripper andrebuild the fRC to accelerate a U35+ beam without the stripper (Fig. 3.2.1(b)). A new normal conducting ring
cyclotron (K = 2200 MeV) will increase the intensity by a factor of 5, although such a huge cyclotron would
be extremely expensive and large, almost the same size as the SRC.
We proposed using a charge stripper ring (CSR) to recycle the beams discarded during the charge conversion
process as shown in Fig. 3.2.1(c). The estimated iron weight and main coil power consumption for a normal
conducting K2200 cyclotron are approximately 7000 t and 2.2 MW [3.2.11], respectively. In contrast, those
for CSR1 as a replacement for the first He stripper are approximately 300 t and 0.3 MW (Fig. 3.2.2). The CSR1
is superior in terms of manufacturing cost, running cost and small building modification. The technology of
CSR can be applied to the second stripper (CSR2) as well. As shown in the Fig. 3.2.3, CSR1 and CSR2 will
be located in the existing Nishina Building without building expansion.
The first charge conversion efficiency will be increased from 20% to 77%, and the second charge conversion
efficiency will be increased from 25% to 63%, thereby increasing the overall charge conversion efficiency- 25 -

from 5% to 50% (10 times of the current level) with CSRs. This upgrading would allow a final beam intensityof 2000 pnA (160 kW). Although this target is lower in beam intensity than FRIB, it does not lose its advantage
in the ability to produce RI beams, since the energy per nucleon is more than 1.5 times higher. We note that
there is a room in CSR for applying other ions in the mass region between Xe and U, although CSR has been
designed primary for uranium.

Figure 3.2.2. Comparison of the size of the K2200 cyclotron and the CSR1: The K2200 cyclotron weighs morethan 7000 tons and is expected to consume more than 4 MW of electricity during operation. On the other hand,
the CSR1 weighs about 300 tons and is estimated to consume only 1.5 MW of electricity during operation.

Figure 3.2.3. Proposed construction sites for CSR1 and CSR2 at Nishina Building.
- 26 -

Effective upgrading of existing accelerators is also essential to enable the acceleration of intensity-enhanced
beams by CSR. The major items for upgrading existing accelerators include: ion source enhancement, SRCRF system enhancement, SRC extraction system enhancement, fRC-RF system enhancement, IRC injection
system modification and upgrade of infrastructures (Fig. 3.2.4). The following sections provide an overview
of each task.

Figure 3.2.4. CSRs and upgrade of existing accelerators at the RIBF
Design work of CSR1
The CSR is a new concept ring with a built-in charged stripper, unprecedented worldwide. Here the CSR
scheme is explained as shown in Fig. 3.2.5, using the first ring CSR1 as an example and comparing it with thepresent scheme with the He stripper. In the present charge stripping scheme for the first stripper at the RIBF,
the 238U35+ beam (10.8 MeV/u, 18.25 MHz) passes through the He stripper with a thickness of approximately
0.7 mg/cm2, and only the 238U64+ beam is selected by the subsequent bending magnets. As a result, the charge
stripping efficiency is approximately 20%. This means that 80% of the beam is dumped into the beam dump
in the bending magnet.

- 27 -

Figure 3.2.5. Comparison of the current charge conversion scheme using the He gas stripper alone (a) and for
CSR1 (b).
For CSR scheme, the U35+ beam bunches are injected simultaneously into CSR1 with a frequency of 18.25
MHz using the charge exchange injection method. The charge state is converted at the 2-stage stripper (He andN2 stripper) and the energy lost in the charge stripper is recovered by the acceleration cavity. The beams otherthan the selected U64+ beams are circulated and reenter the stripper. The phase spread during the circulation issuppressed by the buncher cavities. The recycling cycles are repeated, and only the U64+ beams arecontinuously extracted using a static magnetic kicker.
The most important property required for CSRs is that all orbit lengths of the orbiting beams (e.g., U59-66+ for
CSR1) are equal (isometricity). Forming such isometric orbits by eight special bending magnets, the bunch
structure of the orbiting beams can be maintained. The spatial shape of bunchs for all charge states are also
matched at the stripper by controlling the oscillation of the beams using a charge-independent focusing system(quadrupole-magnet stations). We have performed lattice designs and beam simulations of CSR1 as shown in
Fig. 3.2.6 and confirmed that the bunch structure and shape at the stripper can be maintained and matchedalmost perfectly. The results of beam calculations considering emittance growth due to the stripper show thatthe charge conversion efficiency up to 80% can be obtained for CSR1.

Figure 3.2.6. Basic design works for CSR1.
In parallel, we have been developing elemental technologies to realize CSR1. The magnets, in particular, areimportant development elements that determine the feasibility of CSR1, and has been proceeded as shown in
Fig. 3.2.7. The 3D magnetic field calculations have been almost completed for the eight main bending magnets,
which define isometric orbits. The mechanical design including trim coils for fine tuning of the magnetic field,
is also progressing sequentially. Concerning about the quadrupole-magnet station, which is the key tocontrolling beam oscillation, the magnetic field calculations, prototype fabrication and magnetic fieldmeasurements have been completed for the quadrupole magnets and beam extraction bending magnets, which
are the main components of the quadrupole-magnet station. Although this is an unprecedented magnet
configuration, we have confirmed that the required performance can be obtained, and we are already well
prepared to move on to the fabrication of the actual magnets.
- 28 -

Figure 3.2.7. Magnet design and development works for CSR1.
Other necessary tasks have been almost completed, including calculation of interactions in the stripper and2-stage stripper design, RF cavities design and optimization, beam position monitor (BPM) design calculations[3.2.12], design of beam transport (BT) systems before and after CSR1, and their beam-matching methods(Fig. 3.2.8).

Figure 3.2.8. Component design works for CSR1.

- 29 -

As mentioned above, a technical advisory committee (TAC) meeting was held on June 15-16, 2023. We have
had strong support and 20 recommendations (R1-20) from TAC in moving forward with the project with CSR1.
The items of the 20 recommendations can be categorized as follows.
1. Recommendations on how to proceed with the project (R1-2, R20).
2. Recommendations related to essential system design (R4-6).
3. Recommendations related to understanding beam interaction (R8, R10, R12).
4. Recommendations on beam tuning (R3, R7, R14).
5. Recommendations for detailed mechanical design and prototype fabrication (R9, R11, R13, R15-19).
The recommendations in category 1 are very reasonable, and we will proceed with the upgrade plan inaccordance with the recommendations. Category 2, the system-related recommendations, is the most important,and for R4, we have already completed a study for a 10-charge circulation. The number of circulating charges
can be adapted to meet budgetary requirements. We have also decided to place dedicated steerers for closed
orbit distortion (COD) compensations in accordance with the R5 recommendation. In category 3, especially
R10 is important and will be studied by a dedicated research team. For the category 4, we will build a realistic
beam simulator of CSR1 according to R7. Concerning about category 5, mechanical design and prototyping
have been concentrated on the main magnets, but will be progressed for all components in sequence. The
concentration of recommendations on category 5 indicates that the development of this unprecedented device,
CSR1, is entering a phase of actual fabrication in all directions.
CSR2 and modification of injection system for IRC
As mentioned above, we have already started the detailed design and element development of CSR1, and it
is possible to design CSR2 with the same equipment configuration as CSR1. For CSR2, we are also proceedingwith the determination of basic parameters, lattice design and orbit calculation, and have shown that CSR2 is
feasible with the same instrument design as CSR1.
Prior to the introduction of CSR2, the IRC injection radius will be modified to be variable within a range of
5 %. Currently, the injection energy of IRC is 10% lower than the extraction energy of fRC due to energy loss
at the second stripper. Since the CSR is a machine with equal injection and extraction energies, these energies
need to be matched in the use of CSR2.
We note that the 10% energy difference is too large even for the current acceleration scheme. This is due tothe lack of data on the equilibrium charge of the uranium beam during the design and construction phase of
the RIBF. In the current scheme, a thicker stripper than the optimum thickness to obtain U86+ required for IRC
acceleration is used, which leads to unnecessary emittance growth of the beam. The data obtained at the RIBF
shows that the U86+ can be obtained with an energy loss of about 5%. The energy loss required for non-uranium
acceleration is also less than 5%. If the injection radius of the IRC can be made variable to accommodate
injection with energy loss within 5%, CSR2 can be used in the future and strippers with optimal thickness canbe used in the current acceleration scheme without CSR2. This is important to reduce the heat load on the
stripper and improve beam quality. The feasibility of the moving mechanism of the IRC injection devices(MIC1, MIC2 and EIC) to change the injection radius has been confirmed, and detailed calculation andoptimization of the injection trajectory is in progress.
RF system upgrade for SRC and fRC
The beam intensity to be extracted from the SRC is planned to be 160 kW in this upgrading project.
Unfortunately, the current RF accelerator for the SRC is not powerful enough to handle this high beam power.
It is necessary to enhance the RF system of the SRC and increase the acceleration voltage Vacc by a factor of
about 2 (~1 MV/unit) before the beam intensity upgrade by CSR2 is completed. The upgrade will increase thebeam turn separation during extraction and suppress the space-charge effect (∝Vacc3) of the beam during
acceleration in the SRC. We can increase the extraction efficiency of the 160-kW beam and suppress the heatload on the beam extraction equipment such as the EDC to an appropriate value. We will modify the inside of
the RF accelerating cavity and install a new RF power amplifier, RF coaxial waveguide, and couplers with apower output of about 500 kW. At the same time, we will improve the extraction system of the SRC such as
EDC. Optimization calculations for the RF accelerator cavity for the SRC are already underway (Fig. 3.2.9).
Furthermore, for the RF system of the fRC, the acceleration voltage will be increased and the frequency will
be changed to suppress the space-charge effect. The current fixed acceleration frequency of the fRC is 54.75- 30 -

MHz, but it can be lowered to 36.5 MHz, the same frequency as that of the later cyclotrons (IRC and SRC), toincrease the phase acceptance. Furthermore, beam frequency conversion from 18.25 MHz to 36.5 MHz can be
performed in CSR1 to fill the beam bunch with 36.5 MHz. This will strongly suppress the space-charge effectin CSR1 and subsequent cyclotrons. Optimization calculations for the RF accelerator cavity of the fRC are
also in progress (Fig. 3.2.9).

Figure 3.2.9. Calculation of new RF cavities for SRC and fRC.
Ion source upgrade
In parallel with upgrade with CSRs, the ion source (28-GHz SC-ECRIS) will be developed to further increase
beam intensity. The ECR plasma is heated by a microwave of approximately 2 kW, which stably produces a
U35+ beam of 150 µA. To obtain higher beam intensities, microwaves power must be increased. As shown in
Fig. 3.2.10, the U35+ beam intensity has not reached its maximum with respect to the microwave power, leavingroom for increasing the beam intensity by enhancing the microwave power. Therefore, we plan to increase theintensity of the U beam by increasing the microwave power (>5 kW) and enhancing the cooling capacity ofthe plasma chamber, which contains the high-temperature and high-density plasma generated by the highpower microwave.

Figure 3.2.10. 28-GHz SC-ECRIS and the output intensity as a function of microwave power.
- 31 -

Furthermore, as the long-term development, we aim to construct a fourth-generation ECRIS (45 GHz, 20
kW) with increase of the microwave frequency and the mirror field strength to obtain higher charged ion beamwith high intensity. Increasing the charge state requires high energy electrons to remove deeply bound electrons
in the ions. To increase charge state of the ion beam, it is necessary to increase the microwave frequency. Since
the microwave frequency (ECR frequency), will be increased, we also develop to increase the strength of themirror field using high-temperature superconductors to optimize the performance of the ECRIS in the future.
Infrastructure upgrade
We are also studying the infrastructure development needed for the upgrade plan, including electric power,
cooling water, and site organization. The total electrical power augmentation required by the upgrade plan isestimated to be approximately 6 MW (Fig. 3.2.11). The current capacity of the special high voltage substationneeds to be upgraded from 25 MW to 30 MW. In coordination with the Facilities Division, the possibility of a
relatively easy expansion by installing cooling fans on the current voltage transformers is being considered.

Figure 3.2.11. Electric power system diagram in the upgrade plan.
Construction Schedule
The construction of the primary accelerator-related devices described above can be divided into threepackages based on the budget size such as CSR1, CSR2 and upgrade of the RF systems. The upgrading of thesecondary beamlines including BigRIPS should also be kept in step. We have an annual plan as shown in Table
3.2.1.
Year
Injection system
upgrade for IRC

1

2

3

design work construction

4

5

design work design work construction construction

installation

ECR-IS upgrade

design work construction construction construction

installation

SRC-RF upgrade

design work construction construction construction

installation

fRC-RF upgrade

design work construction construction

installation

CSR2

design work construction

7

8

9

installation

CSR1

Infrastructure upgrade6

installation
design work design work design work construction construction

Beam intensity

500 pnA

Table 3.2.1. Construction schedule.

- 32 -

installation
2000 pnA

After the detailed design, the construction of the three packages will be completed in about five years, andthe target beam intensity will be achieved three years after the completion of the construction.
The plan will proceed as follows.
Year 2: Start of IRC modification and SRC-RF upgrade.
Year 3: Start of CSR1 construction.
Year 6: Start of CSR2 construction and operation towards 500 pnAYear 9: Operation towards 2000 pnA.
In this plan, each new CSR will increase the beam intensity by a factor of three, and existing equipment will
be upgraded to match the intensity of the beam at each stage.

[3.2.1]

J. Wei et al., Progress towards the Facility for Rare Isotope Beams, in Proceedings of 2013 North American

Particle Accelerator Conference (NA-PAC’13), Pasadena, CA, U.S.A., September 2013, pp. 1453-1457.

[3.2.2]FAIR Baseline Technical Report. vol. 2, (2006).

[3.2.3]

D. Jeon, Status of the RAON accelerator systems, in Proceedings of the 4th International Particle Accelerator

Conference (IPAC2013), Shanghai, China, May 2013, pp. 3898-3900.

[3.2.4]

J.C. Yang et al., Introduction of HIAF project, in Proceedings of the 4th International Particle Accelerator

Conference (IPAC2013), Shanghai, China, May 2013, WEOBB103.

[3.2.5]

H. Imao et al., Charge Stripper Ring for cyclotron cascade, in Proceedings of the Twenty-first International

Conference on Cyclotrons and their Applications (CYC2016), Zurich, Switzerland, September 2016, pp. 155-159.

[3.2.6]

H. Imao et al., Development of gas stripper at RIBF, in Proceedings of the 9th International Particle

Accelerator Conference (IPAC2018), Vancouver, BC, Canada, April 2018, pp. 41-46.

[3.2.7]

H. Imao, Charge Stripper Ring Plan at RIKEN RIBF, J. Particle Accelerator Society of Japan, Vol. 17, No. 4,

2020. 262.

[3.2.8]

H. Imao, Charge Stripper Ring for RIKEN RI Beam Factory, J. Inst. (2020) 15 P12036.

[3.2.9]

H. Imao, Status update of the Charge Stripper Rings project, J. Inst. (2023) 18 P03028.

[3.2.10] Q. Zhao, Multi-charge-state beam dynamics in FRIB, in Proceedings of the 54th ICFA Advanced Beam
Dynamics Workshop on High-Intensity, High Brightness and High Power Hadron Beams (HB2014), East Lansing, MI,
U.S.A., November 2014, TUO1LR01.

[3.2.11] H. Okuno et al., Present status of and recent developments at RIKEN RI beam factory, J. Phys.: Conf. Ser.
1401 (2020) 012005.

[3.2.12]

Patent Pending, JPN 2023-128268 (Aug 7, 2023).

- 33 -

4

In-flight separator BigRIPS
The BigRIPS separator [KUB12], which consists of 6 dipoles and 14 superconducting triplet quadrupole
magnets (STQ) [KUS04], is a tandem-type RI-beam production and separator device (Figure 4.1). Various
kinds of RI-beams over a wide Z range in the nuclear chart are produced by the in-flight fission of a 238U beamor projectile fragmentations of 238U (70 pnA), 124Xe (140 pnA), 78Kr (600 pnA), 70Zn (400 pnA), 48Ca (500pnA), and 18O (500 pnA) beams, where the nominal intensities currently reached are shown in the parentheses.
These primary beams are accelerated up to 345 MeV/u by the accelerator complex including the SRC and areirradiated to the Be production targets at F0, the entrance of BigRIPS, to produce RI beams. The RI beams are
separated in the first stage equipped with an aluminum degrader by the B-E-B method [FUK13]. Recently,
another separation method based on B-Q-B has been developed to remove contaminants of fission
fragments for high-Z RI beams produced by the projectile fragmentation of a 238U beam. The RI beams arethen identified on an event-by-event basis in the second stage from F3 to F7 by the TOF-B-E method. The
particle identification (PID) relies on plastic scintillators for TOF measurements, position-sensitive parallel
plate avalanche counters (PPACs) [KUM01] for deducing the magnetic rigidity B by the track reconstruction,and the multi-sampling ionization chamber MUSIC [KIM05] to measure the energy loss E. In the standard
ion optics, the momentum resolution of the first and second stages are 1260 and 3440, respectively, whichenable us to perform new-isotope search experiments even close to the Z = 70 region, where the fully- andpartially-stripped ions are very close each other in the PID plots (Z vs. A/Q). Since the operation initiated in
2007, we have discovered a total of 156 new isotopes and made more than 500 beam settings to supply RI
beams to the users of RIBF.
Other than the RI-beam production/separation, BigRIPS is also used as a spectrometer for a few experiments.
One example is the spectroscopy of pionic atoms, in which the dispersion matching at F5 was achieved bychanging the ion optical modes in the primary beam line and BigRIPS [TAK20]. Another example is reaction
cross-section measurements. The reaction targets of carbon were installed at F5 and the PID was performed in
F3-F5 and F5-F7, respectively, to assign the reaction channel.

Figure 4.1 Layout of the BigRIPS separator (left) and a birdview of BigRIPS from upstream of F5 (right).
In this section, we will describe the achievement and future upgrade plans of BigRIPS. In Section 4.1, theachievements and performances of BigRIPS, such as new isotopes, since 2007 to today are summarized. In
Section 4.2, the upgrade plans for BigRIPS and spectrometers are shown.

4.1

Programs FY2007 – 2022

4.1.1 Discovery of new isotopes
Since 2007, we have discovered 156 new isotopes in a wide-Z range from 11Na to 68Er. Among them, 132 RIs
were produced by the in-flight fission of the 238U beam. The other 24 RIs were produced by the projectile- 34 -

fragmentation reaction of the 124Xe (10), 78Kr (4), 70Zn (9), and 48Ca (1) beams, where the numbers in theparentheses denote the number of new isotopes produced. The colored squares in Figure 4.2 show the newisotopes produced from each primary beams. In addition to these results, about 40 more new isotopes are
currently under analysis.

Figure 4.2 New isotopes discovered at the RIBF since 2007. In total, 156 new isotopes were reported. The
numbers in the parentheses denote the number of new isotopes produced by the respective primary beams.

4.1.2 Secondary beam production
Since 2007, a total of 514 settings were made for 250 experiments, including PAC approved experiments(EXP), machine studies (MS), and director’s discretionary allocations (DA). Figure 4.3 shows the chart of the
RIs set as the central particle in the beam settings. Among them, the light-blue circles show the RIs supplied
to users with the PID information confirmed by the isomer-tagging technique, while the dark-blue the RI beams
supplied without the confirmation by the isomer-tagging technique. The most frequently visited regions
include; light RIs near the neutron-dripline region produced from the 18O or 48Ca beams, the “island of
inversion” around 40Mg, and the regions near magic numbers, such as Ca, Ni, and Sn isotopes. The heaviest
setting made with the PID confirmation is for an 68Er beam. This beam was used for the cross-section
measurement and new-isotope search.
Efforts to produce RIs beyond Z = 70 by the projectile-fragmentation reaction with the 238U beam are recently
made in several programs of MS or DA. The magenta circles show the RIs used in the MS programs performed
to test the beam production of RIs with high Z (~90).

- 35 -

Figure 4.3 RI-beam settings of BigRIPS since 2007. All PAC approved experiments (EXP), machine studies(MS) and director’s discretionary allocations (DA) are considered. The settings without the PID confirmationby the isomer-tagging technique (blue) and the settings for the RI beam production tests (magenta) were also
shown. Note that the total number of the data plotted in this figure is less than the total number (514) of thesettings, since some RIs were produced multiple times.

4.1.3 Development of high-performance beamline detectors
To improve the PID performance in the second stage of BigRIPS, especially toward the region for high Z RIs
with Z > 70, high-performance beamline detectors are now developing.
A. The first one is a Xe-gas ionization chamber [YOS22] based on MUSIC [KIM05] to improve the Z
resolution for high-Z RIs, especially for the region of Z > 70. In the energy range of the RIBF (~250 MeV/u
at F7), charge-state fluctuations are not negligible for such heavy-Z RIs. This leads to fluctuations of theenergy loss and deteriorates the Z resolution of MUSIC. With the conventional P10 gas (Ar:CH4 90:10),
the ions with Z < 50 stay fully stripped over the course of the travel inside MUSIC. On the other hand, thehigher Z ions fluctuate its charge state (fully-stripped, H-like, etc.). Ions of 70Yb for example change thecharge state twice in average inside MUSIC according to simulation studies. As this is not frequent enoughto reach the charge-state equilibrium, the average charge does not converge within the travel distanceinside the chamber. As a result, the average charge and the associated energy loss significantly fluctuate
event-by-event. To minimize this effect, it is effective to choose a gas, which leads to the charge-state
equilibrium condition as quickly as possible. We chose a Xe-based gas mixture (Xe:CH4 70:30) for thisaim. This gas changes the charge state of Yb ions about 50 times while traveling inside MUSIC. The
equilibrium is thus established more quickly and the average charge converges in a shorter distance.
The arrangement of the charge sampling electrodes (12 anodes and 13 cathodes) is shown in Figure 4.4.
The electrodes are arranged 2 cm apart inside the total length of 60 cm. The gas pressure is 620 Torr forboth Xe-based gas and P10. The Z resolution with the Xe-based gas was evaluated to be about 0.35 instandard deviation (S.D.) for RIs with Z = 86 - 89 at 250 MeV/u. This is much better than the performance- 36 -

with P10-gas evaluated to be about 0.50 in S.D. The improvement of the Z resolution is clearly seen in thecomparison of the PID plots (in Z vs. A/Q) with P10 and the Xe-based gas mixture in Figure 4.4.
B. The second development is a Xe-gas scintillator [HIJ23]. This detector is an all-in-one solution for timing,
position, and energy-loss measurements, currently made by different detectors; plastic scintillators, PPACs,
and MUSIC, respectively. Furthermore, the Xe-gas scintillator has an excellent beam rate tolerance formore than 100-kHz RI beams, which is difficult to handle by the present detectors, especially MUSIC.
This performance is thanks to a short decay constant of about 100 ns in the scintillation process of Xe gas.
The lower panel of Figure 4.4 shows a photo of the Xe-gas scintillator. High purity Xe gas without mixing
gas at 2-atm is enclosed in the 9-cm thick gas cell of aluminum with 4-cm openings covered by 125-mm
Kapton foils. The gas cell is coupled to 4 photomultiplier tubes (Hamamatsu R6041-406) interfaced byglass windows of Shin-Etsu Quartz SUPRASIL-P310G, which efficiently transmits the 175-ns wavelength
photons. The typical performances with a 238U beam with Z = 92 are evaluated to be Δt = 74 ps, Δx = 0.47mm, and ΔZ = 0.46, comparable to the current performances with the plastic scintillators (40 ps), PPACs(0.3 mm) and MUSIC with P10 (0.50), respectively. The Z resolution of the Xe-gas scintillator and P10gas MUSIC are summarized in the lower panel of Figure 4.4.
It should also be noted that the detector development and commissioning were performed mainly by agroup of the Kyoto University, closely collaboration with the BigRIPS team. The Xe scintillator project
thus represents a good model of the partnership with universities in the area of detector developments.

Figure 4.4 Photos of the Xe-gas ionization chamber (top left) and the Xe-gas scintillator (top right). The PID
plots (Z vs. A/Q) with P10 gas (bottom left) and Xe:CH4 70:30 (bottom right) both at 620 Torr are compared.
The energy resolution of the Xe scintillator is also compared to P10 MUSIC (bottom right). The figures aretaken from Refs. [HIJ23, YOS22].

4.2

Upgrade plan

Heavy element RIs (Z > ~70) will be the grand challenge in a long-term perspective of the RIBF. The projectile
fragmentation reaction of a 238U beam is crucial to produce RIs in the heavy element region. The upgrade
project of the core facilities of the RIBF thus consists of two major items, one focused on the accelerator- 37 -

complex to maximize the beam intensity of 238U at 2 pμA by constructing a pair of CSRs (Chapter 2) and theother the BigRIPS separator and spectrometers to maximize the RI beam availabilities and the platform forreaction studies, described in this section. The BigRIPS separator will thus be upgrade to fully handle the highpower U beam of 2 pμA. In addition, the purity will be a defining quality in RI beams of heavy element RI,
where the separation is challenging. The separation power of BigRIPS will thus be strengthen. An excellent
platform for reaction studies with heavy element RI beams will be a key to secure advantages of the upgraded
RIBF. The SAMURAI and ZeroDegree spectrometers will be upgraded for this aim.
In this section, we will first describe the RI-beam intensities expected for the primary beam intensity of 2 pμA
delivered by the upgraded accelerator complex. The second part will be focused on the upgrade plan of the
BigRIPS separator and spectrometers equipped to the secondary beam delivery ports.

4.2.1 Evaluation of RI-beam intensities with 2-pμA primary beams.
The primary-beam intensities will increase to 2 pμA after the upgrade of the accelerator complex based on apair of charge-stripper rings CSRs. The intensities of RI beams were evaluated over the whole domain of thenuclear chart for the primary beams of 2-pμA at 345 MeV/u (Figure 4.5). The isotopes denoted by colors are
expected to have intensities higher than 10-5 particles per second (pps), which corresponds to about 1 particle
per day. Listed below are examples of possible experiments at the respective intensities:-

10-5 pps; new-isotope searches.

-

10-4 pps; β-decay lifetime measurements.

-

10-3 pps; β-delayed γ-ray spectroscopy.

-

10-2 pps; mass measurements by MRTOF-MS / R3-

10-1 pps; in-beam γ-ray spectroscopy for the first 2+ state of even-even RIs.

-

1 pps; in-beam γ-ray spectroscopy for other channel or reaction cross-section measurements.

Candidates of new isotopes are found outside the boundary of currently known nuclei (red line). A total of
about 290 new isotopes are expected to be discovered. Among them, about 160 isotopes are neutron-rich nuclei
produced by the in-flight fission of the 238U beam. They are spread over the region of Z = 30 – 60 (~30 isotopes),
the region of Z = 60 – 70 (~30 isotopes), and Z > 70 (~100 isotopes). At the proton-rich side, about 110 isotopes
are produced by the projectile fragmentation reaction of the 238U beam. Another 20 are produced from otherprimary beams, such as 124Xe or 70Zn. The majority of new isotopes (~210) are thus concentrated in the highZ region from Z = 70 to 92.
Other than the primary beam species listed above, we also evaluated the RI beam intensities for the primarybeam of Pb, so far not accelerated at the RIBF. At very proton-rich and neutron-rich regions, the beam
intensities of Pb do not seem to have distinct advantages over the projectile fragmentation reaction of 238U. In
practice, the purity could be easier to handle with a Pb beam without contaminants of fission fragments.
The beam intensities were evaluated by LISE++ under the conditions described below:-

The currently available primary beams, namely 18O, 48Ca, 70Zn, 78Kr, 86Kr, 124Xe, and 238U, are considered.
The beam energy is 345 MeV/u. The intensity is set to 2 pμA.

-

RI-beam production reactions are the projectile fragmentation reaction for all primary beams. In-flightfission is considered for the 238U beam.

-

The production targets are made of beryllium with thicknesses equivalent to 0.35 in d/R.

-

The measured cross sections were considered. If not available, the cross-section formula of EPAX3.1a wasused for the projectile fragmentation reactions. For the in-flight fission, in the region far from the stability,
the cross sections are assumed to decrease by 1/50 with one neutron added. Inside or near the stability line,
the three-fissile model in the LISE++ ver. 8.4.1 was used.
- 38 -

-The transmission for the fission fragments of a 238U beam. The losses due to momentum spreads and to
charge state spreads are taken into account independently. For the momentum spreads, the transmission isdefined as a function of A. It is fixed to be 5% for A < 45 and 65% for A > 195, and is gradually increased
from the former to the latter. For the charge states, the transmission is expressed as a function of Z. It is set
to be 100% for Z < 45 and 3% for Z > 65, and to constantly decrease in between.

-

The transmission for the projectile fragmentation reactions. For the momentum spreads, the momentum
distributions are calculated with the Goldhaber model. Note that the resulting spread sizes are multiplied
by a factor of 1.2 to better reproduce the distributions calculated by LISE++. For the charge states, the sameprocedure for the fission fragments is adopted.

-

The loss due to the in-flight decay is taken into account. When the lifetime is close to or shorter than the
TOF of F0 to F7 (typically 450 ns), the intensity decreases as RIs decay in flight. This effect is importantfor neutron-deficient RIs. The lifetime data were taken from the JAEA chart of nuclide [No127-06.pdf(aesj.or.jp)].

-

The transmission loss by the exit beam dump is not included. Sometimes, the orbits of the primary beamand the RI beam are very close to each other. In such cases, a significant transmission loss results. Thisissue is not observed for very exotic RI beams.

Figure 4.5 RI beam intensities evaluated for the primary beam intensity of 2,000 pnA. The color code denotes
the expected intensity in the unit of particles per second (pps). The primary beam species are indicated by the
red squares. The boundaries of the known nuclei (red lines) and the predicted driplines by the KTUY mass
formula [KOU05] (black lines) are shown.

4.2.2 Upgrade plan for BigRIPS
The upgrade plan has three objectives as summarized below and in Figure 4.6.
(1) Upgrade of the BigRIPS first stage to handle high-power primary beams: The sustainable, efficient andsafe operation of BigRIPS will be ensured for the high-power uranium beams. For this aim, the tolerance
for radiation and heat loads will be improved for the first stage components irradiated by primary beams.
In addition, to minimize the beam downtime, a remotely-controlled auto-exchanging system will be- 39 -

developed for the maintenance and replacement works of these components.
(2) Upgrade of the BigRIPS first stage for high beam purities: The separation of RI beams is challenging in
the high-Z region from Z = 70 to 92 produced by the projectile fragmentation of the 238U beam. The
separation power of BigRIP will be improved by implementing newly-developed superconducting quartet
quadrupole magnets (SQQ) and an RF deflector (RFD) in the first stage.
(3) Upgrade of SAMURAI and ZeroDegree Spectrometer: For reaction studies with heavy element RI beams,
the PID performance after the secondary reaction is crucial. The momentum resolution of the ZeroDegree
Spectrometer will be improved. The SAMURAI spectrometer will also be upgraded for a higher
acceptance of incoming RI beams and for an alternative dispersion matching optics.

Figure 4.6 Summary of the upgrade plan for the BigRIPS and spectrometers in the secondary beam lines. Notethat the layout of the four-bend ZeroDegree Spectrometer is conceptual.

Upgrade items are described for each purpose below:(1) Upgrade of the BigRIPS first stage to handle high-power primary beams
A. The first stage with a production target and beam dump experiences the highest radiation and heat loads
in BigRIPS. An improved resilience against radiation and heat loads is essential for the sustainable andefficient RI beam production by BigRIPS under the irradiation of a high power uranium beam. The current
BigRIPS can accept only up to 500 pnA of the 238U beam. For the final goal of 2 pμA, we will develop a
new production-target system, beam dump and F1 slits.
The new target system will be a rotational carbon plate with a diameter of 1 m, which is much larger thanthe present beryllium target of 30 cm diameter with the intensity limit at 1 pμA. The new exit beam dump
will be in a drum shape and made of copper-alloy of 30 cm in diameter. The drum is rolling underirradiation. The rotating surface will dissipate the heat more efficiently compared to the current fixed beam
dumps of copper alloy, arranged in a V shape, with a limit of 500 pnA. The same rolling-drum structure
will be adopted for the F1 slits. These slits will help intercept the primary beams in the F1 chamber, when
the orbit of the primary beam is very close to the RI beam of interest.
- 40 -

Figure 4.7 Simulation of the heat load with the beam dump with the rolling drum structure.
B. For the safety operation, a remotely-controlled auto-exchanging system will be developed for the beamline components of the first stage. Currently, all works at the first stage are made manually. The beamneeds to be stopped until the radiation level becomes low enough to access the vault of the first stage. For
example, to change energy degraders at F1, the downtime of BigRIPS is typically 12 to 24 hours. Whenthe primary beam intensity reaches 2 pμA, a longer downtime will be required, causing an inefficient
operation. If replacement works or light repair works can be performed remotely and without entering thevault, the downtime can be greatly minimized. We will thus develop an automated exchanging system forthe beam line components of the first stage, such as the production targets or energy degraders, together
with a remote monitoring system. In addition, radiation-shield concrete blocks will be added to handle theradiation for the beam intensity up to 2 pμA.

(2) Upgrade of the BigRIPS first stage for high beam purities
A. To improve the beam separation power for the high-Z region and/or more neutron-rich region, themomentum resolution of the first stage will be improved from the current value of Δp/p = 1/1260 to about
1/3000. Such a high momentum resolution could be obtained by replacing the STQ01 and STQ04 with
Superconducting Quartet Quadrupole magnets (SQQ). The momentum resolution is mostly governed by
the D/M ratio (D; dispersion. M; magnification). By using the current STQs, it is hard to change D or M
independently to vary the D/M ratio. SQQ01 provides a more degree of freedom to the ion optics design,
thus greatly helping improve the D/M ratio. For the symmetry of the first stage layout, STQ04 is also
replaced by SQQ. The optimization of the ion optics with SQQs is underway.
To increase the transmission of the RI beams, new F1 energy degraders will be developed. The material ofwill be chosen under the consideration that a much heavier material than aluminum, such as gold, may be
useful for a faster charge-state equilibration for high Z beams. We will also design a better degrader shape.
The momentum-compression mode, for instance, will be considered in the optimization of the shape.
B. For further separation, a new RF deflector (RFD) will be installed at F2.5, the middle point between F2and F3. The RFD applies a vertical electric field at a frequency of 36.5 MHz, which transmits ions ofinterest passing at the moment of zero field and deflects unwanted contaminants with a different TOF. To
evaluate the effects of the RFD, simulations were performed by using the existing experimental data of
BigRIPS. The trajectories of ions recorded in the data are deflected in the software by a virtual RF field.
The size of deflection is calculated based on the experimental TOF values from F0 to F2.5 with respect tothe virtual RF phase, optimized for the transmission of the ions of interest. Figure 4.8 shows the
improvement of the purities for the 100Sn setting with a 124Xe primary beam (top) and the N = 126 (202Os)
setting with a 238U beam (bottom). In both cases, the purity is improved by a factor of six to seven with the
RFD. Importantly, the latter case demonstrates that the RFD is useful not only for RI beams in the protonrich region, as often expected, but also for the neutron-rich high-Z region, where the TOF filter serves toreject the majority of contaminants created by fission products of the 238U beam. Together with other few
case studies, we found that the RFD improves the purity from several factors to one oder of magnitude.
The specifications required for the RFD are summarized in Table 4.1.

- 41 -

Figure 4.8 Simulation of the RF deflector as a TOF filter of the contaminants for the 100Sn beam (top) and for
the N = 126 beam (bottom). The experimental PID plot (Z vs. A/Q) obtained without the RF deflector (left) arefiltered by TOF (right) to reflect the effects of the RF deflector. For the N = 126 setting, the purity was evaluatedfor the neutron-rich region with A/Q > 2.65. For reference, 204Pt77+ with Z = 78, N = 126 is indicated.
Year of installation
Cavity enclosure diameter (cm)Cavity enclosure height (cm)Electrode width (cm)Electrode length (cm)Electrode gap (cm)Electric field
RF frequency (MHz)Max voltage (kV)Max electric field (kV/m)
Max power (kW)Costs (1,000 JPY)

RIPS
2002
80
230
12
70
4
vertical12 ~ 19
150
3,750
20
~80,000

OEDO
2016
150
298
40
120
20
horizontal
18.25
＞350
1,750
40
~200,000

BigRIPS RFD
<120
~150
~15
100
~12
vertical
36.5
720
6,000
170 ~ 200
~500,000

Table 4.1 Summary of the specitications of the new RF deflector for BigRIPS together with the data of theexisting RF deflectors at RIPS [YOS] and OEDO [MIC19].
(3) Upgrade of SAMURAI and ZeroDegree Spectrometer
A. The SAMURAI spectrometer will be upgraded for the better acceptance of RI beams. The STQ04 of thefirst section will be replaced by newly-developed SQQ04 for the purpose of high-resolution optics as
described above. The STQ04 will be relocated to the downstream of F12 in the SAMURAI beam line. By
this modification, the beam-spot size at F13 (SAMURAI target position) will be half in the normal
achromatic optics. For instance, cryogenic target systems, often having a small opening of the order of 1
cm, will benefit from the smaller beam profile. An additional advantage of this modification is the new- 42 -

dispersion matching optics (Figure 4.9). By using this mode, the unreacted secondary beams from the
SAMURAI target can be focused into one point and isolated from reacted ions after the SAMURAI dipole
magnet. A small dump can intercept the beam without shadowing the acceptance of the focal plane
detectors for reacted ions. This facilitates, for instance, high beam rate experiments.

Figure 4.9 Ion optics calculation of the dispersion matching mode of SAMURAI
B. High-resolution ZeroDegree spectrometer with four-bend structure will be constructed. For reactions ininverse kinematics, the particle identification of reaction residues after the target is essential to determine
the reaction channel. However, the momentum resolution of the current ZeroDegree with the two-bend
structure (two bending dipole magnets) is not enough for heavy element RI beams. The present momentum
resolution of 1,380 in the standard optics (Large Acceptance Achromatic mode) is much worse comparedto the second stage of the BigRIPS with 3,440. To solve this problem, the four-bend structure is an effective
solution as adopted for the BigRIPS second stage. For this aim, the two-bend ZeroDegree will be relocatedto the east area of SAMURAI and be extended with another two-bend sector. This provides the same levelof momentum resolution (~3,000) as the BigRIPS second stage and also doubles the flight length. In
addition, a timing detector with a good time resolution (~5 ps) will be developed to further improve the
TOF resolution. In total, the A/Q resolution will be improved from the current value of ~0.1% to ~0.03%,
which achieves a similar performance with the second stage of BigRIPS.

4.2.3 Schedule and resources
The timeline of the BigRIPS upgrade is coordinated in harmony with the schedule of the accelerator complex
upgrade (Table 4.2). The project is divided into two phases; the phase one covers the development of the CSR1
from Year 1 to 5 and the phase two the development of CSR2 from Year 3 to 8. In the first phase, the beam
intensity is planned to reach 500 pnA in Year 6. With this intensity, the radiation and heat loads will stay
manageable with the current BigRIPS. The efforts of the first phase will thus be focused on the developmentof a pair of SQQ and the RFD for high separation power. During the second phase, the BigRIPS first stage will
be upgraded for high power beams and be preapared to receive the full beam intensity of 2,000 pnA in Year 8.
In parallel, the ZeroDegree will be upgraded to the four-bend structure. The total cost related to the BigRIPS
upgrade is estimated to be about 5.1 billion JPY.
The BigRIPS upgrade project will be undertaken by the BigRIPS team of the Research Instruments Group.
The team currently has 10 members consisting of one PI, six permanent and three fixed-term members. Fivemembers over 50s assisted the construction and commissioning of RIBF, while other five members under 40s
are the main workfoce for the RI-beam production and development of beam line detectors at the current RIBF.
The young staff members will drive the upgrade project with strong supports from the senior staff members.
The manpower should be reinforced to undertake both the upgrade task packages and the daily duties, such as
the BigRIPS tuning, maintenance, trouble-shootings or data analysis. We plan to hire five new permanent staff- 43 -

members under 40. Among them, two will be involved in the first phase (focused on the SQQs and RFD) andshould be recruited urgently. Each recruit will be assigned a different responsibility in the upgrade
workpackages, so that they are visible in the project and developed as an expert of a certain technology. They
will also be assigned a related daily duty to assist the opearaiton and to smoothly slide from the development
phase into the operation phase in future.
Other related developments, such as for beam line detectors, will be undertaken with close collaborations withother teams in the Research Instruments Group and research groups in universities (as in the model case of the
Kyoto University commitment to the development of the Xe scintillation detector).

Year

1

2

3

Injection system upgrade for IRC

designconstruction

installation

4

5

CSR1

designdesignconstruction

construction installation

ECR-IS upgrade

designconstruction

construction

construction installation

SRC-RF upgrade

designconstruction

construction

construction installation

designconstruction

construction installation

designconstruction

installation

fRC-RF upgrade
Infrasstructure upgrade

6

7

8

CSR2

designdesigndesignconstruction

construction

installation

BigRIPS first stage
for high power beam

designdesigndesignconstruction

construction

installation

construction

construction

installation

BigRIPS first stage
for high purity beamSpectrometer upgrade

designdesignconstruction
design9

construction installation
designBeam intensity

design500 pnA

2000 pnA

Table 4.2 Schedule of the RIBF upgrade. The work packages describted in this section are highlighted byyellow.

4.3

References[FUK13] N. Fukuda et al., Nucl. Instrum. Methods Phys. B 317B, 323 (2013).
https://doi.org/10.1016/j.nimb.2013.08.048[HIJ23] Y. Hijikata et al., Nucl. Instrum. Methods Phys. B 541, 333 (2023).
https://doi.org/10.1016/j.nimb.2023.05.039[KIM05] K. Kimura et al., Nucl. Instrum. Methods Phys. A 538, 608 (2005).
https://doi.org/10.1016/j.nima.2004.08.100[KOU05] H. Koura et al., Prog. Theor. Phys. 113, 305 (2005).
https://doi.org/10.1143/PTP.113.305[KUB12] T. Kubo et al., Prog. Theo. Exp. Phys. 2012, 03C003 (2012).
https://doi.org/10.1093/ptep/pts064[KUM01] H. Kumagai et al., Nucl. Instrum. Methods Phys. A 470, 562 (2001).
https://doi.org/10.1016/S0168-9002(01)00804-X[KUS04] K. Kusaka et al., IEEE Transactions on Applied Superconductivity 14, 310 (2004).
https://doi.org/10.1109/TASC.2004.829102[MIC19] S. Michimasa et al., Prog. Theo. Exp. Phys. 2019, 043D01 (2019).
https://doi.org/10.1093/ptep/ptz007[TAK20] H. Takeda, Y.K. Tanaka et al., Nucl. Instrum. Methods Phys. B 463, 515 (2020).
https://doi.org/10.1016/j.nimb.2019.03.016[YAM04] K. Yamada et al., Nucl. Phys. A 746, 156 (2004).
https://doi.org/10.1016/j.nuclphysa.2004.09.064- 44 -

[YOS22] M. Yoshimoto et al., presentation at EMIS2022 (2022).
https://indico.ibs.re.kr/event/469/contributions/3675/attachments/3379/3935/83%20Masahiro%20Yoshimoto.pdf- 45 -

5

Experimental devices
In this chapter, we will present the instrumentation at the RIBF, its evolution since the first beam in 2007 topresent and the future projects envisioned at the upgraded RIBF.

5.1

Programs FY2007 – 2022

The RIBF encompasses a fast secondary beamline based on the SRC-BigRIPS facility and a slow beamlinewith the AVF or RRC. The fast secondary beamline delivers RI beams produced and sepearated by the BigRIPS
separator. Fast RI beams at the nominal energy range of 200 ~ 300 MeV/u create great opportunities forreaction studies in inverse kinematics. A variety of reactions at this energy range such as knockout reactions
or charge exchange reactions are instrumental, for instance, in probing the magicity or in investigating the tetra
neutrons. Compared to the normal kinematics, the inverse kinematics has an important advantage that thebeam-like reaction residues can be momentum-analyzed and identified by a magnetic spectrometer
downstream of the reaction target. Spectrometers are key to differentiate the reaction channel, identify the final
quantum states, or investigate the kinematical correlations. Therefore, the fast secondary beamline of the RIBF
has three different delivery ports, equipped with the ZeroDegree Spectrometer [KUB12], the SAMURAI
spectrometer [KOB13] and the SHARAQ/OEDO spectrometer [MIC19, UES12], the last serving also as an
injector to the storage ring R3. The three spectrometers with distinct characteristics offer strong platforms forversatile reaction studies; ZeroDegree for the high particle-identification (PID) resolution, SAMURAI for thewide acceptance, and SHARAQ/OEDO for the ultrahigh momentum resolution or energy degrading RI beams.
The instrumentation of a variety of radiation devices around these spectrometers has been pivotal to exploit
the full capacity of RI beams provided by BigRIPS. The evolution in types and in performances over the courseof years has been behind a wide range of achievements of the RIBF. It should also be noted that most of the
projects are carried out on a wide and organized international collaboration basis. The Nishina Center iscommitted to a total of 41 MoUs with institutions in 19 countries.

Figure 5.1 Large-scale equipement at the current RIBF.

- 46 -

'07

'08

'09

'10

'11

'12

'13

'14

'15

'16

'17

'18

'19

'20

'21

'22

ZeroDegree
DALI2
SEASTARHiCARIEURICA
BRIKENIDATENSAMURAINEBULA
NeuLANDNEBULA+HIME
WINDS
CATANATOGAXISTRASSE
SpiRIT TPC
SHARAQ
OEDO
GRAPE
TiNA
SAKURA
R3
SCRIT
ESPRI
CAT
Pol. H target
Pol. D beam
Tritium targetCRIB
GARIS IIGARIS IIIKISSSLOWRI RFGCSLOWRI PALISMRTOF-MS (GARIS)MRTOF-MS (KISS)MRTOF-MS (ZDS)

Figure 5.2 History of instrumentation at the RIBF since 2007 to present. The installation (blue), commissioning(yellow), and operation (green) phases are shown for each device. The large-scale equipment is highlighted
by the light blue.
The slow beamline is hosting various studies of low-energy RI beams that complementary to the fast beamline. For the superheavy element studies, two spectrometers, GARIS II [KAJ13] at E6 and GARIS III at
SRILAC, are operating. The CRIB facility [YAM23], coordinated by the CNS, is another in-flight RI beamfacility that is capable of producing light RI beams at beam energies from 1 to 10 MeV/u. The HyperECR ion
source and the liquid nitrogen cooled production target enable the maximum intensity of 3 × 108 pps. The KISS
facility [WAT18], recently constructed by KEK, is an initiative to produce heavy element RIs in the region- 47 -

from N = 126 to actinides, which has been difficult to access, by using deep-inelastic scattering.
Besides the heavy-ion-accelerator based instrumentation, a unique apparatus such as SCRIT [OHN13] forelectron-RI scattering is also available.
In this section, major instrumentation projects since the arrival of the RIBF are overviewed.

5.1.1 ZeroDegree Spectrometer
The ZeroDegree [KUB12] has been a base for in-beam gamma-ray spectroscopy experiments. The NaI(Tl)scintillator array DALI2 [TAK14] is a standard device for in-beam γ-ray spectroscopy. The excellent detection
efficiency (about 36% for 1 MeV γ rays at 100 MeV/u), while providing an energy resolution of 9% and apeak-to-total ratio of 0.54, assisted the findings of the first 2+ states in even-even nuclei at the extreme,
including 54,56,58Ca, 78Ni, 52Ar, 36,38,40Mg, 110Zr, 96,98Kr or 42Si. The coupling with the MINOS liquid hydrogentarget system [OBE14] enhanced experimental yields for proton knockout reactions with a maximum liquidhydrogen thickness of 15 cm. The SEASTAR project (2014 to 2017) based on the MINOS-DALI2 setup was
hosted at the ZeroDegree for the RUN1 (2014) and RUN2 (2015). The first initiative to use position-sensitive
and/or tracking germanium clusters was taken in the HiCARI project (2019 to 2021). The HiCARI arrayconsists of eight Miniball clusters from CERN Isolde, two GRETINA clusters of the LBNL and RCNP, andfour Clover detectors of the IMP.

β-decay experiments have been carried out by combining in-house and outside high-performance detectors on
a strong international collaboration basis. With the setup being stationed at the final focal plane F11 of the
ZeroDegree, a vast amount of RI beams have been successfully measured over a wide region of the nuclear
chart. The first large-scale project EURICA (2011 to 2016) [SOD13] was focused on β-delayed γ-ray
spectroscopy by combining the position-sensitive Si detectors array WAS3ABi for the RI-beam implantationand β-ray radiations and the Ge detectors array EUROBALL (efficiency of about 25% at 200 keV). The next
BRIKEN project (2017 to 2021) [TOL19] with a high-efficiency neutron detectors array (about 67%) is more
dedicated to neutron emission probabilities Pn. β-delayed neutron measurements were performed for about 200
nuclei (with the Pn values for 83 nuclei reported). The IDATEN project [LEE23] is currently organized to run
from 2023 onwards (until the expected completion around 2026), which couples WAS3ABi to 84 LaBr3(Ce)detectors (efficiency of about 18% at 200 keV) for fast timing γ-ray measurements. The future programs also
include (1) the TATAKI project based on the NaI(Tl) array DTAS for total decay energy measurements, (2) the
coupling of the β-decay station to the MRTOF mass spectrograph, (3) the proton radioactivity studies by- 48 -ACTAR TPC, and a few other projects under development.

5.1.2 SAMURAI spectrometer
The wide-acceptance and multifold detection capabilities, including neutrons, of the SAMURAI spectrometer(since 2012) [KOB13] assisted a variety of reaction studies on both bound states and resonances over a rangeof nuclei from light RIs to medium-mass fission fragments.
The large opening at zero degrees of SAMURAI enables an efficient detection of neutrons. The neutron
scintillation counters array NEBULA [KON20] is the core device for this purpose. It consists of two layers of
120 plastic scintillator bars, which facilitate the multifold detection of neutrons. Several efforts have been
made to achieve higher efficiencies. From 2015 to 2018, the NeuLAND demonstrator of GSI with four double
layers covering 2.5 × 2.5 m was temporarily coupled to NEBULA. In 2022, NEBULA_Plus with additional
two layers of 90 bars was coupled to NEBULA. Both combined, the efficiency is improved to about 60% (from35%) for 1n and 1% (from 0.2%) for 4n. Another neutron counter HIME (since 2016) is also available as acomplementary solution. It has a much more granular construction than NEBULA and thus a better energy
resolution (e.g. 2n relative energy resolution of 25 keV at 1 MeV).
The neutron-counter array WINDS/PANDORA (2011-) [YAS16, STU17], set at finite angles near the reaction
target, is a device optimized for the missing mass spectroscopy by the charge exchange (p,n) reaction. It wasused for studies with 132Sn, 11Li and 48Cr. A new solid scintillation material of Eljen Technology allows for theneutron-gamma discrimination by the waveform analysis. Upgrades are planned for larger solid angles and fora better angular resolution.

Knockout reactions are a powerful probe of nuclear structure at the typical beam energies of 200 to 300 MeV/u
at the RIBF. The development of dedicated detectors was undertaken or is underway. The MINOS liquidhydrogen target system [OBE14] provides a thick hydrogen target equipped with a Time Projection Chamber(TPC), which tracks knockout protons to locate the reaction vertex in a very thick target. The RUN3 (2017) of
the SEASTAR project using MINOS was carried out at SAMURAI to study the level scheme of various
neutron-rich nuclei at the extreme. The CsI(Na) array CATANA [TOG20] (since 2016) is a calorimetric device- 49 -

for knockout protons from the (p,2p) reactions. It consists of 140 crystals (and another 20 crystals in 2023)
equipped with dual gain electronics, which enable the coincidence detection of de-excitation gamma-rays andknockout protons, having different energy ranges. New particle tracker systems, TOGAXI (under the
ONOKORO project) and STRASSE [LIU23], are under construction. Both based on position-sensitive silicon
strip detectors as tracking technologies, TOGAXI equipped with GAGG calorimeters is more adapted to alphaknockout reactions, while STRASSE is coupled to the CATANA calorimeter for (p,2p)/(p,3p) reactions as
main objectives. The two detectors will soon be complete and available for physics experiments.
Another unique instrumentation at SAMURAI is SpiRIT TPC [BAR21] (since 2016) for heavy ion collisionexperiments to investigate the symmetry energy term of the nuclear equation of state. SpiRIT encompasses a
TPC operated under the SAMURAI dipole magnetic field and capable of identifying light ions and pions (massresolution of 15% FWHM for Z = 1 isotopes). The Sn+Sn collision run was performed in 2016 and anotherrun with Xe + Sn is currently prepared for.

5.1.3 SHARAQ-OEDO beamline
The beamline serves as the high-resolution spectrometer SHARAQ [UES12] and as the energy-degrading
beamline OEDO [MIC19]. The SHARAQ and OEDO modes can be switched by reconfiguring thearrangement of beamline magnets.
The SHARAQ beamline (since 2009) is a high resolution beam line optimized to use an RI beam as a reaction
probe. The beam monitoring system based on low-pressure MWDC and diamond detector technologies
realizes high rate tolerance (about 1 MHz), position resolution (about 100 μm FWHM) and time resolution(about 10 ps r.m.s.). Various charge exchange reactions (t,3He), (10C, 10Be+γ), (12N,12C) or (8He,8Be) have beenstudied, best known for the study on a tetra neutron resonance. The SHARAQ beamline is also used for massmeasurements by the TOF technique over the flight length all the way from BigRIPS down to SHARAQ.
The OEDO energy-degrading beam line (2017-), equipped with an angle-tunable energy degrader at FE9 forthe energy compression and with a horizontal RF deflector (RFD) for the focusing, provides energy-degraded
RI beams in-flight down to 10 MeV/u. The first experiment was performed in 2017 to collect low-energy
reaction data at about 20 MeV/u for Long-Lived Fission Products (LLFP), such as 79Se, 93Zr, or 107Pd. The
SAKURA project for (d,p) reactions based on the Si-CsI recoil particle array TiNA was initiated in 2022. The
minimum beam energy of 15 ± 1 MeV/u was so far achieved with a spot size of about 12 mm SD on target.

5.1.4 Rare-RI Ring (R3)
The isochronous storage ring R3 [YAM13] is a device for high-precision mass measurements. It features atrim coil system for high precision isochronism (ΔT/T ~1 ppm) over a wide momentum acceptance (Δp/p~±0.5%), a fast kicker system for the isotope-selectable self-triggered injection and a resonant cavity schottky
system. After the construction (2012 – 2014) and machine studies (2015 – 2017), R3 was put into physics
experiments in the region of 132Sn and neutron-rich Ni isotopes in 2018 and 2021. In parallel, the development
to improve the efficiency and precision continues. Especially the kicker system was upgraded to realize a flat- 50 -top magnetic field for the duration equivalent to one revolution.

Figure 5.3 Schematic layout of the storage ring R3. The figure is taken from Ref. [YAM13].

5.1.5 Devices for (in)elastic scattering: SCRIT, ESPRI, and CAT-M
SCRIT [OHN13] is a facility for electron-RI scattering measurements based on the self-confining RI target
technology. After the construction of SCRIT (2009), ISOL(ERIS) (2011) and the electron spectrometer WiSES(2014), the first measurement was performed with stable 132Xe in 2015 to 2016, demonstrating the validity ofthe method. Measurements with radioactive 137Cs isotopes produced online were achieved in 2022. Studies of
Xe isotopes are currently underway. The machine has so far achieved the luminosity of 3.2 ×10 27 /cm2/s (250mA e-beam of 250 MeV, 2.5 ×108 136Xe ions/pulse), which will further increase as the development continues.
The RI production performance is rated to be 3 ×105 cps for 132Sn at 10 W onto a U target of 15 g, which will
be upgraded to 1 kW and 30 g in future.
The ESPRI system [SAK17] is a device dedicated to measure proton elastic scattering of RIs with highprecisions. The neutron and proton density distributions extracted from the elastic data are invaluable
information to constrain the equation of state. The ESPRI telescope for recoiling protons encompasses two
modules of RPS each having an MWDC, a plastic scintillator and seven NaI(Tl) calorimeters. The telescopeis coupled to a solid hydrogen target (SHT) of 1 mm in thickness with an opening of 30 mm in diameter. The
ESPRI system was first used at the RIBF in 2013. It is routinely used for elastic measurements or applied toalpha knockout reactions. The next-generation ESPRI project was recently initiated as part of the ONOKORO
project.
The active target time projection chamber CAT-M [OTA15] is a device of inelastic scattering with RI beams
to measure giant monopole resonances for the nuclear incompressibility. A small prototype with deuteron gas
served to measure the 132Sn(d,d’) reaction in 2016. The CAT-M (2017 - ) has a larger active volume of 28 (W)× 20 (H) × 32 (D) cm3. A permanent magnet of 0.3 T is set in the active volume along the beam path to suppressdelta rays, which is a huge background source in measurements at the RIBF. The signal-to-noise ratio is byabout 400 times better with this magnet.
- 51 -

5.1.6 Spin-polarized target and beam; tritium-loaded Ti target
A spin-polarized proton target system [WAK05] consists of a naphthalene C8H10 target with a thickness of
1 – 4 mm and a diameter of 10 – 24 mm. A special laser pumping technique provides a high polarization (about
20% at 0.1 T and 20% at 0.3 T) under a relatively low magnetic field at around 100 K, thus facilitating thecoupling to detectors for recoiling particles. The target was used for elastic scattering with a 6He beam at
SAMURAI and also for the (p,2p) reactions of O, F isotopes at SHARAQ to investigate the spin-orbit potential
with neutron-rich nuclei. Upgrade efforts to replace the target material from naphthalene to p-Terphenyl(C18H14) are underway to polarize the target at room temperatures without cooling. About 20% polarization is
expected under 0.4 T.
The target system has also been upgraded for a larger opening aperture (±60°). This is intended for the future
study of the three-body nucleon force (3NF) by scattering with a polarized deuteron beam [OKA94]
delivered by the AVF-RRC-SRC acceleration scheme. The beam was used for previous studies on 3NF by nonpolarized proton elastic scattering in the energy ranges from 200 to 300 MeV/u (2009 to 2015). The beam
excels in polarization (about 80%), intensity (100 pnA), and full controllability of the spin axes.
A tritium-loaded Ti target [MIK23] is another unique target developed for the search of tri-neutron by the3
H(t,3He)3n reaction at SHARAQ in 2021. The Ti foil of the targets is loaded with 1.6TBq of tritium, equivalent
to 3.5 mg/cm2, with a high 3H/Ti atomic ratio of 1.54. The tritium thickness is much greater (by about 100
times) than typical Ti-3H targets used in RI beam experiments.

5.1.7 SSRI-PNS (Stop and Slow RI-Precise Nuclear Spectroscopy)
SSRI-PNS is a unified body of collaboration for KISS, SLOWRI, GARIS and MRTOF-MS, each based on
low-energy RI beam techniques at the RIBF.
KISS (KEK Isotope Separation System) [WAT18] is an RI beam production beamline developed and- 52 -

coordinated by KEK. The KISS facility uses deep inelastic scattering to produce neutron-rich RIs in the region
from N = 126 to actinides that are important for r-process but hard to study in conventional ISOL or in-flightfacilities. A combination of an Ar gas cell, laser resonant ionization, and dipole magnet mass separator, allows
to deliver a beam of previously challenging isotopes of refractory elements or of actinide for β-decay and laser
spectroscopy or mass measurements. Since 2012, measurements were performed for 73Ta, 74W, 75Re, 76Os, 77Ir,
or 78Pt isotopes, and most recently for 92U and 93Np, including the new 241U isotope.

Figure 5.4 Schematic layout of KISS.
The SLOWRI project [WAD03] is to provide a low-energy RI beam by the gas catcher technology. It
encompasses two types of gas cell, one the RF-carpet type cryogenic He gas catcher (RFGC) and the other aparasitic Ar gas catcher with resonant laser ionization (PALIS) [SON18]. The RFGC is capable of extracting
ions with less elemental dependence than the ISOL technique. The device is presently used to feed ions to
MRTOF-MS. For a higher stopping efficiency, it is planned to size up the cell (about 1 m length) and increase
the gas pressure (about 300 Torr). PALIS was installed at the F2 terminal of the first stage of BigRIPS in 2015.
The online extraction of RI beams was confirmed in 2021 with alpha emitters produced by BigRIPS. A
quadrupole mass separator (QMS) and TOF spectrometer are under development for the laser ionization.
GARIS is a gas-filled separator for superheavy element studies. The original GARIS-I was used for the
discovery of Nh. GARIS-II [KAJ13] dedicated to hot fusion reactions has been installed in 2009. When the
upgrade construction of the SRILAC started in 2017, GARIS-II was relocated to the E6 vault. The new
SRILAC beamline was equipped with GARIS-III in 2020. The search for Z = 119 by the 51V+248Cm reaction
started in 2018 with GARISS II and is currently underway at the SRILAC + GARIS-III facility. The GARISII was equipped with an MRTOF-MS in 2014. The first high-precision direct mass measurement of asuperheavy nuclide was recently achieved with 257Db. Since the E6 vault will be a site for the CSR1, therelocation of the GARIS-II to elsewhere, including other vaults, is under consideration to keep the current
activity of superheavy mass measurements.
MRTOF-MS (Multi-Reflection Time-Of-Flight Mass Spectrograph) [ROS23] is an instrument for mass
spectroscopy. It consists of a drift tube with a pair of mirror electric fields at both ends, which reflect ions to- 53 -

make multiple laps for the duration of ms range. The mass of an ion is deduced from the TOF over the total
distance traveled. A total of three devices are operating at the fast (ZeroDegree since 2018) and slow (GARISII since 2014 and KISS since 2018) beam lines. The first online application at ZeroDegree was performed
during the HiCARI campaign in 2020 in parasitic experiments. More than 70 masses were measured with themass resolving power better than 500,000. The MRTOF-MS at KISS was instrumental in the recent finding of
new isotope 241U. MRTOF-MS is constantly upgraded for higher performances. Examples include a beta-TOF
detector for decay-correlated mass measurements or an in-MRTOF deflector (IMD), which deflects unwanted
ions by switching on the field at selected times.

Figure 5.5 Schematic layout of the SLOWRI/MRTOF-MS setup at GARIS II (left) and at ZeroDegree (right).

5.2

Upgrade plan

The region of heavy elements will be the new frontier for the upgraded RIBF in a long-term perspective. This
will also be a challenge for the instrumentation. While the existing devices will continuously be upgraded, newinitiatives are envisioned or have already been taken to support, especially, reaction studies in inverse
kinematics.
In-beam γ-ray spectroscopy will stay as an important method for reactions in inverse kinematics with RI beams.
Detectors for γ-ray from fast RI beams constitute an important part of instrumentation. A new scintillators
array of GAGG and/or CeBr3 with a higher energy resolution (5% instead of 9% at 1 MeV) will be developed
to replace the DALI2 array. In addition, the GT-5 project has recently started to develop tracking germanium
detectors with a higher energy resolution (about 2%), position sensitivity (about 3 mm FWHM), and the γ-ray
tracking capability.
Experiments related to isomeric states are a promising area of future studies. The R3 isomer beam filter is aninnovative platform to realize reaction studies with isomeric beams. The isochronous storage ring R3 will serve
to tag and/or physically filter isomeric states based on direct mass measurements by TOF. Ions extracted fromthe ring are delivered in-flight to a reaction station.
Optical model potentials are a fundamental element in reaction analyses. The global potentials currently
available are not calibrated with elastic scattering data of RI beams. The MESA project has been initiated toestablish a global potential for RI beams. The project will deliver a parasitic data taking system for elastic
scattering of RI beams, operated anytime with main physics experiments.
RUNBA is another application of the storage ring technology for reaction studies in future. The storage ringof RUNBA encompasses a reaction target inside. Revolving RI beams in the ring are allowed to hit this target
multiple times during the revolutions. As a result, RUNBA serves as a luminositiy booster. In addition, RI +RI collisions are envisioned by integrating a self-confining RI target of SCRIT.
KISS II is a major upgrade plan from the KISS facility. KISS II will significantly improve the efficiency of
RI beam production and extraction to establish an efficient access to the region from N =126 to the actinide,
which includes the region beyond 238U inaccessible by BigRIPS.
- 54 -

As a common infrastructure to all experimental devices, the data acquisition system will be upgraded to bemuch faster than the present. The expected tolerable trigger rates of about 100 kHz will essentially make it atriggerless system for many cases.

Figure 5.6 Summary of the upgrade plans for the instrumentation. The highlighted instruments are describedin this section.

5.2.1 New-generation scintillator array project
Since advent of the RIBF, the NaI(Tl) based scintillation array DALI2 [TAK14] has been the workhorse forin-beam γ-ray spectroscopy experiments. Due to its modest energy resolution, caused by large opening angles
and intrinsic energy resolution of NaI(Tl) scintillators, long absorption lengths of the scinitllation material, as
well as modest time resolution, the long-term potential is limited. Furthermore, limited available budget makes
low cost alternatives to 4π Ge tracking arrays with superior features, except energy resolution, desirable.
Consequently, a new-generation scintillator array for in-beam γ-ray experiments is being devised for thenear future. Here, the scintillation materials HR-GAGG(Ce) (Gd3(Ga,Al)5O12(Ce)) and CeBr3 have been
identified as the most promising choices. Key advantages for the former include its high density, low radiation
length, and that it’s neither hygroscopic nor self-emissive, while the latter offers a better intrinsic resolution
and extremely fast decay time.
It is envisioned that a hybrid array, composed of HR-GAGG(Ce) and CeBr3 crystals, will be employed at
different experimental stations of the RIBF (ZeroDegree, SAMURAI, SHARAQ), each having different
performance requirements and constraints. In design studies carried out so far, the number of different crystal
shapes is minimized. Only two shapes seem sufficient for an entire array, arranged in a configuration
combining a forward wall with annular rings, as shown in Figure 5.7 (left). Envisioned detector sizes are 25 ×
100 mm3 for the forward wall and 30 × 8 mm3 for the rings. A few prototype HR-GAGG(Ce) crystals havebeen purchased by RIKEN, showing an energy resolution of 4–4.5 % (FWHM) at 1.33 MeV with APD or
SiPM as readout, while about 3 % are expected for the CeBr3 crystals.
Using these numbers, an array of ~1000 crystals yields 5 % energy resolution, 53 % efficiency, and 0.72 P/T
in GEANT4 simulations of a 1 MeV γ ray emitted at 100 MeV/nucleon. These values need to be comparedwith 9 % resolution, 36 % efficiency and 0.54 P/T for DALI2, highlighting the potential of the new γ-ray
calorimeter. Since 2022, a collaboration comprising several European and Asian institutes has formed with theaim of gradual replacement of DALI2+ crystals.
Key experiments to be carried out in the future at the RIBF at intermediate energies involve inelastic scattering- 55 -

on high-Z targets to induce Coulomb excitation, as well as inelastic scattering and quasi-free scattering on
liquid hydrogen. An example of the performance of the array is shown in Figure 5.7 (right) for inelastic
scattering of 32Mg at 50 MeV/nucleon on liquid hydrogen.

Figure 5.7 (left) Layout of scintillation-based γ-ray spectrometer containing 384 HR-GAGG and 624 CeBr3
detectors. The latter are packed in common housing of 2×2 crystals and arranged in anannular configuration
with an inner radius of ~250 mm. (right) GEANT4 simulation of inelastic scattering of 32Mg on liquid hydrogenfor new scintillation array, DALI2+, and the GRETINA setup containing 7 quads. Note that background was
not included into the simulation.

5.2.2 GT-5: Gamm-ray Tracking in R5 for germanium clusters array
The GT-5 project is a new initiative to develop a tracking germanium clusters array for the next generationsof in-beam γ-ray experiments. The array will be a crucial platform for future reaction studies at the upgraded
RIBF that will be steered toward heavier mass regions with a higher level density. The effectiveness of tracking
germanium clusters is demonstrated in the preliminary results of HiCARI, where the GRETINA clusters of
LBNL and RCNP are instrumental to differentiate complex levels in medium-heavy RIs, especially odd nuclei,
and also to measure lifetimes down to 10 ps with fast RI beams at 200 ~ 300 MeV/u.
It is envisioned that the GT-5 array will be coupled to auxiliary devices, such as the knockout particle tracker
STRASSE (for the precise reaction vertex in a thick cryogenic target) or to the new scintillator array. The highvertex resolution of the former can be capitalized with the high position sensitivity of GT-5. The latter provides
a high efficiency for γ-ray detections. The integration of the two arrays will help the γ-γ coincidence analysis
beyond the first 2+ state.
The Nishina Center envisions to procure two quad-type GRETINA clusters under the framework of the TRIP
(Transformative Research Innovation Platform of RIKEN platforms) starting in FY2023 (R5). The first cluster(T1) has been ordered in April 2023 with a delivery date set at the end of March 2024, while the second (T2)
is planned after 5 years. The first cluster will be commissioned in 2024 and used to collect high-quality datawith RI beams to fulfill the mission of the TRIP project. The detector will also be put into physics programs.
A campaign-type project can be considered with other clusters in the RCNP and in other laboratories.

- 56 -

Figure 5.8 GEANT4 simulation of γ-ray interactions in a GRETINA cluster. In this specific event, the γ-ray
interacts multiple times in a crystal by Compton scattering.

5.2.3 R3 isomer beam filter
The R3 isomer beam filter is a new application of the isochronous storage ring to provide isomeric beams
tagged or filtered by the mass-TOF information. The R3 is capable of measuring the mass of an ion from TOF
while revolving inside the ring. Isomeric states are thus tagged by the mass (or excitation energy) information,
after minimum 50 μs of revolution. The fast kicker system (currently having 100 ns for the rise and fall, andalso for the flat-top duration) can handle the extraction of a single ion from the ring. Extracted ions are thendelivered to a reaction station downstream and scattered by a reaction target. The reaction events recorded at
this station will be correlated with the tagging information provided by the R3, thus enabling to sort out theevents associated with an isomeric state. Furthermore, a physical filtering of isomeric states is feasible if the
lifetime is long (> 50 ms). When the revolutions build a sufficient time gap between an isomeric state and theground state, the kicker system can exclusively filter the isomeric state ions by shuting down the kicker field
for the ground state ions.

Figure 5.9 (Left) Concept of the isomer beam filtering with the R3. (right) The limits of isomer tagging by the
TOF-mass measurement method. The curves denote the minimum excitation energies required to differentiate
an isomeric state from the ground state. The known isomer data with lifetimes longer than 50 μs (necessary
for TOF measurements at the R3) are compared to the limits with the present (red line) and upgraded R3 (blue- 57 -

line). For reference, the non-storage measurement for the flight from BigRIPS F3 to SHARAQ S0 (about 500ns) is compared by the dashed line.
With the current performances of the R3, isomers at energies higher than 1 MeV and with a lifetime longer
than 100 ms are only feasible for the filtering. In addition, the extraction efficiency is limited to 1 pps by the
kicker, which constrains the application to reaction studies. To enhance the performance of the isomer beamfilter, two major upgrades are planned. One is to improve the isochronism level. This will help improve themass precision, and thus the excitation energy resolution. The isochronism is currently limited to 3 ppm r.m.s.
by the fluctuation of the magnetic field, which comes from the instability of the power supply current. The
control system of the power supply will be upgraded to eliminate this instability to achieve the isochronouslevel of 1 ppm. The other upgrade is related to the kicker system. A new power supply module will be able todrive the kicker more frequently, improving the extraction efficiency by roughly 100 times (from 1 pps to 100pps). It also enables a more instant rise and fall of the magnetic field (about 30 ns with respect to the present
system with 100 ns) and a shorter flat top duration (about 30 ns compared to 100 ns). A narrower window (~90ns) allows to extract the isomer of interest more efficiently. With these upgrades, it will be possible to extract
an isomeric state (A~125) at 1 MeV for a lifetime down to 10 ms (currently 100 ms). If the lifetime is about
40 ms, the excitation energy range is expected to go down to as low as 300 keV.
Other necessary developments include a vacuum baking system for better vacuum pressures and a moresensitive schottky device.

5.2.4 MESA: Measurement of Elastic Scattering Anytime, anywhere, any beam
The MESA project is an initiative to collect elastic scattering data systematically over the nuclear chart toultimately establish a global optical model potential that can reliably be used for RI beam-induced reactions.
The global optical model potentials presently available are tuned with stable targets data. It is desirable tocalibrate the potentials with data of various RI beams. In addition, it is also interesting in terms of reaction
mechanisms how RIs would alter optical model potentials. To collect systematic data of elastic scattering forvarious RI beams, the idea of MESA is to deploy dedicated setups consisting of telescopes and a thin CH2
target (about 100 μm) to several locations on the secondary beamlines. The MESA stations will always collectelastic scattering data in parallel with the main experiments running downstream of the beam line. The idea ofsuch a parallel data taking existed in the past, but the implementation has been difficult due to the limited
efficiency of data acquisition (DAQ) systems. The new DAQ system described in Section 5.2.7 is expected totremendously improve the efficiency, thus facilitating the parallel data taking.
The project has been initiated in 2023 under the framework of the TRIP. A pilot study is planned in 2023 and2024 by three compact telescopes of the Si-Si-CsI/GAGG three detector layer configuration.

Figure 5.10 MESA prototype for elastic scattering measurements. The prototype consists of a target ladderand three telescopes, each consisting of a pair of Si detectors in stack for ΔE, followed by CsI/GAGG- 58 -

calorimeters.

5.2.5 RUNBA: Recycled-Unstable-Nuclear Beam Accumulator
RUNBA is a storage ring of RI beams to boost the lumnisity for low-enegy reaction studies and a test bench
for RI + RI reaction measurements in future. In a conventional setup, an RI beam delivered by BigRIPS isbombarded onto a reaction target only once, having most of the beam particles left unreacted and discarded.
RUNBA is a solution to enhance the luminosity by revolving RI beams in its ring to allow multiple interactions
with an internal target (1018 /cm2). To address the emittance growth and energy loss due to the interactions withthe target, RUNBA will correct for the deviation from the central trajectory on an event-by-event basis basedon the monitoring of each particle passage position on the target and will also compensate the energy loss by
an RF accelerating field. As a result, RUNBA is expected to revolve beam particles at 1 MHz and realize theluminosity of 1024 /cm2/s even for less than 10 particles. The development is ongoing under the collaboration
with the Kyoto University Institute for Chemical Research. The storage ring sLSR was relocated from Kyoto
to the E21 room of the RIBF to develop a prpototpye. It is planned to deliver RI beams from the ISOL facilityof SCRIT and to measure astrophysical reactions or particle transfer reactions. In future, RUNBA will be
coupled to the SCRIT target to realize RI + RI reaction studies.

Figure 5.11 Schematic layout of RUNBA installed in the experimental vault of SCRIT.

5.2.6 KISS-II
The KISS-II project is aimed at constructing an upgraded facility based on the KISS technologies to expand
the RI production area from the neutron-rich N = 126 region towards actinide isotopes. The KISS-II facility
consists of four key components: a gas-filled superconducting solenoid filter, a large helium gas cell, a variable
mass range separator and MRTOF-MS. Heavy element RIs are produced by deep-inelastic scattering, as it isadopted in KISS. The gas-filled solenoid filter transmits outgoing RIs at large angles into the helium gas cell,while intercepting the primary beam at zero degrees before the cell. This allows to increase the primary a- 59 -

plasma and realizes an extraction efficiency of 1% or higher, which is ten times better than the present Ar gas
cell of KISS. The variable mass range separator can select five different masses at the mass dispersive midfocal plane between the two dipole magnets and transmit various ions to the MRTOF-MS, which identifies theions based on precise mass measurements. The tagged ions are then available for decay spectroscopy. The total
efficacy of KISS II is about 10,000 times higher than KISS. The upgraded performance will open the accessto the region from N = 126 to the actinides previously unavailable for experimental studies, and yet highly
important for the r-process nucleosynthesis.

Figure 5.12 Schematic layout of KISS II.

5.2.7 Upgrade of the RIBF data acquisition system
The DAQ system of the RIBF has progressively been upgraded since 2007. A network-distributed system
based on the CAMAC technology was developed in 2007. This system supports the trigger rate up to about 1
kHz. Around 2018, the MPV system was developed for the parallel readout of multiple VME crates. Thissystem can handle the trigger rate up to about 10 kHz.
An upgrade project is currently underway with the completion planned around 2025. The goal is to establish a
DAQ system adapted to the trigger rate of about 100 kHz. With this new system, all data can be recorded up
to the beam intensity of 100 kHz without event selections on a hardware basis. The front-end electronics areunder development in-house since this performance cannot be realized with commercial ADC/TDC modules.
The developing items include (1) a high energy and time resolution system based on a 4GHz ADC (Xilinx
RFSoC) module for plastic and diamond detectors, (2) a medium resolution multi-channel TDC on a FPGA
basis for PPAC and drift chambers, (3) a 125 MHz digitizer system for ion chambers, NaI(Tl) detectors andsilicon semiconductor detectors, (4) a high precision clock distributor, which ensures precise clock
synchronization (about 20 ps in r.m.s.) among sub-DAQ systems deployed in several experimental vaults at
distance. This enables the functionalities that are not feasible with the conventional trigger-based DAQ, forinstance, the determination of TOF based on synchronized clocks, or the free-streaming data collection.

5.3

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6

Role of RIBF upgrade in the regional and international community

The experimental programs accompanied with the RIBF upgrade are internationally unique and competitive
with the newly constructed inflight facility in US, FRIB (https://frib.msu.edu/) and new other facilities underconstruction, FAIR (Germany) (https://fair-center.eu/), RAON (Korea) (https://risp.ibs.re.kr/html/risp_en/)and HIAF (China) (https://hiaf.impcas.ac.cn/hiaf_en/public/). It should be noted that part of programs with the
RIBF upgrade can encourage the nuclear engineering community. From now on these facilities are comparedwith the upgraded RIBF in terms of primary beam energy and intensity, and reaction-study setup.
These facilities are categorized into two parts in terms of heavy-ion accelerator: synchrotron and linac.
The synchrotron inflight facilities are FAIR and HIAF. The FAIR facility is consisted of the S100
synchrotron and super-FRS separator. The maximum energy of heavy ions accelerated at S100 is 2.7 GeV/u
for uranium ions with intensity of 5×1011/cycle. One cycle composes of acceleration period of 1 sec and slow
extraction period. A typical extraction period of 1sec leads to a duty factor of 50%, and hence the intensity is
2.5×1011/sec on average. At present, the RIBF facility has delivered a 345 MeV/u uranium beam with intensityof 6×1011/sec. The intensity will be increased by a factor of 20 at the upgraded RIBF facility to be 1.2×1013/sec,
which is about 50 times higher than that of FAIR. Concerning a spectrometer for analyzing secondary reaction
products, FAIR has only one spectrometer proposed, the R3B spectrometer which is an open-geometry
spectrometer. In general, momentum resolution of such an open-geometry spectrometer is limited up to 0.1%
level, which could be compared with momentum resolution of 0.01% at ZeroDegree and SHARAQ
spectrometers at RIBF.
The new facility, HIAF is under construction in China. One superconducting linac and one synchrotron is
being constructed. The linac coupled with a powerful ion source can accelerate heavy ions up to an energy of
17-22 MeV/u, which are utilized for low-energy nuclear reaction studies. The synchrotron will boost up theenergy of heavy ions injected with the linac, up to a few GeV/u. Energy and intensity of a uranium beam at thesynchrotron is about 1 GeV/u and 5×1011/cycle, respectively. These are the same as those of FAIR. The beams
accelerated at the synchrotron will be utilized for inflight production of RI beams. The physics programs withthe fast RI beams are still under discussion and spectrometers newly constructed are to be funded.
The brand-new facility in US, FRIB started in operation in 2022. The FRIB facility is consisted of a superconducting linac and the ARIS inflight separator. The ARIS separator was uniquely designed to have a large
momentum acceptance and also to have three stages of pre-separation, post-separation and particle
identification. The energy of a uranium beam is about 200 MeV/u, and the goal intensity is 5×1013/sec. The
goal intensity is about 4 times larger than that of the upgraded RIBF. By considering thickness of primary andsecondary targets, total luminosity for reaction study at RIBF is a few times higher than that of FRIB. A newspectrometer system HRS is being constructed and will be completed in 2027. Before HRS completed, theS800 spectrometer can serve for analyzing reaction products, but the magnetic rigidity of S800 is limited up
to 4 Tm; the luminosity for reaction study with neutron-rich RI beams is limited because of the limitation of
the magnetic rigidity.
The RAON facility in Korea is consisted of three sets of accelerators: K70 cyclotron as a driver for ISOL
with a beam power of 70 kW at maximum, two sets of superconducting linac (SCL3 and SCL2). Energy ofheavy ions accelerated at SCL3 is a few 10 MeV/u, and that at SCL2 is a few 100 MeV/u. Two inflight
separators are coupled with SCL3 and SCL2, respectively. The separator coupled with SCL3, “KoBRA” wasdesigned to produce relatively light RI with A<40. The separator for SCL2 beams, “IF” was optimized forfission products with a 200 MeV/u uranium beam with intensity of 5×1013/sec. The goal energy and intensity
are the same as those of FRIB. One of the unique features of RAON is that RIs produced at the ISOL can bepost-accelerated up to a few 10 MeV/u at SCL3 and a few 100 MeV/u at SCL2, and 140Xe at 222 MeV/u withan intensity of 107/sec can produce neutron-rich isotopes with Z<50. Spectrometers coupled with the IF- 62 -

separator are under discussion.
As discussed above, the upgraded RIBF facility will play a leading role in both the regional and international
community, especially for experimental programs with reaction study. The RIBF upgrade aims to maximize
research potentials of inflight RI beams in terms of reaction study as well as a variety of radioactive isotopes
toward a heavy mass region.

- 63 -

7

Cost, Schedule, and Status

The cost and schedule for the RIBF upgrade are shown in Table 7.1. This project is eight-year plan. The first
five years are devoted to achieving 500 pnA intensity of a uranium beam and high purification of the RI beams.
At the end of eighth year, the intensity will reach to 2000 pnA and the BigRIPS will be upgraded for highpower beam. Total cost necessary for the facility upgrade is 22.4 B¥ (224 M$ in case of 1$=100¥). Subtotal
for the accelerator upgrade and the BigRIPS and ZeroDegree upgrade are 17.3 B¥ and 5.1 B¥, respectively.
Additionally required manpower for the accelerator staff and the inflight-separator staff are 12 and 5 permanent
staff members, respectively.
This proposal of RIBF upgrade has been submitted to “Science Council of Japan” (Japanese Academy) andMEXT (Ministry of Education, Culture, Sports, Science and Technology), respectively, under collaboration
with Univ. of Tokyo (CNS), KEK (WNSC), and Osaka Univ. (RCNP). Science Council of Japan is discussing
a long-range plan of academic activities in Japan and defining the role of the RIBF upgrade. MEXT will review
proposals of large-scale project/facility, prioritize the proposals and summarize a short list of promising
proposals as a “roadmap” in 2023. The operation cost including 8-month machine time is requested, too. The
RIBF upgrade proposal has been strongly supported by the nuclear experimental physics community in Japanand RIBF User-Executive-Committee and endorsed by the RIKEN management.
Table 7.1 Cost and Schedule of the RIBF upgradeYear

1

Total Cost

22.4 BYen

Accelerator System

17.3 BYen

2

Injection system upgrade for IRC

designconstruction

CSR1

designdesignECR-IS upgrade
SRC-RF upgrade

installation
installation

designconstruction construction construction

installation

designdesignconstruction

Spectrometer upgrade

6

7

8

9

installation

construction construction construction

construction construction

installation

installation
designdesigndesignconstruction construction

installation

designdesigndesignconstruction construction

installation

construction construction

installation

5.1 BYen

BigRIPS first stage
for high power beam
BigRIPS first stage
for high purity beam

5

designCSR2BigRIPS and ZeroDegree4

construction construction

fRC-RF upgrade
Infrasstructure upgrade3

designdesignconstruction construction
designdesigninstallation
designAchievement Goal
Beam intensity

500 pnA- 64 -

2000 pnA

8

About this document

This document is a report on the RIBF facility upgrade project. It was submitted to the Nishina Center
Advisory Committee for the review meeting held in July 2023.
The report was authored by the following members of the RNC:
Hiroshi Imao, Osamu Kamigaito, Masaaki Kimura, Hiroyoshi Sakurai, Daisuke Suzuki, Hiroshi Suzuki,
Hideki Ueno, and Tomohiro Uesaka.
The future plan outlined in the report also owes much to the members of the RIBF Future Plan Working
Group:The theory subgroup with Yoshihiro Aritomo, Shuichiro Ebata, Tokuro Fukui, Nobuo Hinohara, Wataru
Horiuchi, Futoshi Minato, Takayuki Miyagi, Tomoya Naito, Nobuya Nishimura, Kazuyuki Sekizawa, Hajime
Sotani, Hiroyuki Tajima, Kota Yanase, Kenichi Yoshida, Sota Yoshida, and Masaaki Kimura as a coordinator.
The experiment subgroup with Hidetada Baba, Pieter Doornenbal, Naoki Fukuda, Shintaro Go, Tadaaki
Isobe, Daiya Kaji, Yue Ma, Sarah Naimi, Megumi Niikura, Tetsuya Ohnishi, Masaki Sasano, Tetsu Sonoda,
Toshiyuki Sumikama, Hiroshi Suzuki, Aiko Takamine, Kenichiro Tateishi, Yoshitaka Yamaguchi, andDaisuke Suzuki as a coordinator.
The accelerator subgroup with Taihei Adachi, Yoshihide Higurashi, Yasuto Miyake, Yasuyuki Morita,
Takashi Nagatomo, Takahiro Nishi, Kazutaka Ozeki, Glynnis Mae Quinones Saquilayan, Kenji Suda, Akito
Uchiyama, Kazunari Yamada, and Hiroshi Imao as a coordinator.
The other RNC members who contributed to this report include:
Yuto Hijikata, Hironobu Ishiyama, Koji Morimoto, Shunji Nishimura, Hideaki Otsu, Yohei Shimizu,
Yasuhiro Togano, Ken-ichiro Yoneda, Koichi Yoshida and Masahiro Yoshimoto.
The inputs from the following external colleagues are gratefully acknowledged.
Nobuaki Imai (CNS, University of Tokyo)
Shin’ichiro Michimasa (CNS, University of Tokyo)
Kenjiro Miki (Tohoku University)
Shinsuke Ota (RCNP, Osaka University)
Kimiko Sekiguchi (TITECH)
Yutaka Watanabe (KEK)
Hidetoshi Yamaguchi (CNS, University of Tokyo)
Juzo Zenihiro (Kyoto University)- 65 -