RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

RIKEN Accelerator Research Facility (RARF)– The predecessor of Nishina Center –M. Ishihara ∗1,∗2,∗3
RARF was an organization formed under the authorization of the RIKEN President to help promote scientiﬁc activities using the RIKEN facility of accelerator
complex, which may be called RARF as well. The complex consisted of the central accelerator, RIKEN Ring
Cyclotron (RRC), and its alternate injectors, RIKEN
Linear ACcelerator (RILAC) and the AVF Cyclotronwith K = 70 MeV. Eﬀectively, RARF served as a research center for accelerator-driven science, and thus
can be regarded as the predecessor of Nishina Center.
It was organized upon the completion of RRC by theinitiative of H. Kamitsubo, who was the original leaderof the RRC project. It functioned from 1990 to 2001
under equal partnership among all the RIKEN Laboratories involved in the utilization of the RARF accelerators. Pursuit of a common research program entitled Multidisciplinary Researches on Heavy Ion Science was the central objective of RARF, while it also
took charge of realizing the next-generation facility as
it launched the RIBF project in 1995.
Naturally, the Director of RARF was elected fromamong the associated Chief Scientists. Thus, I served
as the ﬁrst Director during 1990–1997, being succeededby I. Tanihata (1997–2001). During my term of oﬃce
Y. Awaya and Y. Yano served as Deputy Directors, respectively representing the researchers other than thenuclear physicists and the accelerator staﬀ while I represented nuclear physicists.
RARF originally involved 13 RIKEN Laboratories,of which 3 were associated with nuclear physics and 10with other disciplines of science. Among those, the Cyclotron Laboratory led by Y. Yano played a special roleby taking the full responsibility for the construction,
development, and operation of the entire accelerator
facility, except for the part of RILAC that was under
the charge of the relevant group of the Linear Accelerator Laboratory led by M. Kase. The total beam time
was shared almost evenly between nuclear physics andnon-nuclear physics groups; the former primarily usedRRC, while the latter also employed the injector accelerators in their stand-alone mode.
Research activities at RARF were highlighted by
the following principal programs conducted with RRC.
They involved 4 projects for nuclear physics; NP-1)
RIPS project, NP-2) GARIS-IGISOL project, NP-3)
SMART project, and NP-4) ASHURA project, and4 projects for other scientiﬁc disciplines; OS-1) muon
∗1
∗2
∗3

science project named Large Ω, Os-2) atomic physics
project on multiply charged ions, OS-3) nuclear chemistry program by means of rapid chemistry, and OS-4)
radiation biology program via heavy-ion irradiation.
All of these projects were approved in the mid-1980s
and facilitated upon the commissioning of RRC in
1989. It is worth noting that each of these programs
was proposed and conducted under the strong initiative and leadership of the respective Chief Scientists,
symbolizing the spirit and principle of the traditional
RIKEN system of Chief Scientists, which prevailed in
the institutional operation throughout the 1990s. For
some nuclear physics programs, prominent scientists
from outside of RIKEN were invited to serve as Visiting Chief Scientists, and took charge of the programsof their own proposal. This arrangement was realized
upon an oﬃcial request by the Nuclear Physics Forumof Japan to the RIKEN President that the RRC facility
be open for scientists outside of RIKEN. As it turned
out, acceptance of outside researchers was very eﬀective in enlarging the scope of scientiﬁc activities withRARF utilities. Formally, these outside users were afﬁliated to relevant RIKEN Laboratories, being given
the status of Visitors of RIKEN. The other RIKEN
systems of Special Postdoctoral Researcher and JuniorResearch Associate were also useful to attract young
talent from universities.
It is delightful that most of the above-quoted
programs were extremely successful in generating/developing novel sub-ﬁelds of atomic and subatomic science, thereby leaving a great legacy for thecurrent research activities at Nishina Center.
Firstly, the RIPS project (NP-1) led by myself accomplished a major step forward in advancing therising ﬁled of RI-beam (RIB)-driven nuclear physics.
The main vehicle employed was a separator called theRIKEN Projectile-fragment Separator (RIPS), which
produced RIBs of epoch-making intensities. The
achievements of the RIPS project were so great thatthey immediately triggered worldwide enthusiasm forpromotion of RIB science. At RARF, the success of theRIPS project stimulated the promotion of the RIBF
project to serve as the third-generation RIB facility.
Thus, a variety of research ﬁelds cultivated with RIPS
are now in full bloom with BigRIPS at RIBF.
Secondly, the GARIS project of NP-2) led by T. Nomura (INS, the University of Tokyo (UT)) aimed at
the synthesis of super-heavy elements (SHE) by implementing a high-performance Gas-ﬁlled Recoil Ion Separator (GARIS) for the ﬁrst time in the world. This
was the ﬁrst Japanese attempt to look for SHE with

Director of RARF (1900–1997)
Project Supervisor of RARF/RIBF Project Head Oﬃce(2001–2004)
Chief Scientist of Radiation Laboratory (1984–2000)

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RIKEN Accel. Prog. Rep. 50 (2017)

to the speciﬁc area for hot chemistry. This capability
readily produced useful mixtures of short-lived RIs tobe used to study the features of element circulation ina variety of circumstances. This novel method named“Multi-tracer method” allowed simultaneous injectionof diﬀerent elements into a common host sample, yielding results fairly free from the sample individualities.
Thus, it was widely applied for the study of element
circulation through biological bodies and in diﬀerent
environmental situations.
The radiation biology project led by F. Hanaoka andF. Yatagai established an irradiation system dedicatedto biological samples, which allows irradiation in the
air over a wide ﬁeld of homogeneous dose distribution.
This facility has been used for various purposes including the pilot experiment to validate the irradiation
methods for heavy-ion cancer therapy to be practicallyused at the HIMAC facility at National Institute of Radiological Sciences (NIRS). The irradiation system wasalso used intensively for a project of biological transmutation promoted by S. Yoshida and T. Abe. This
project is still being undertaken in full swing at RRC,
as it has been so successful in generating useful variants of ﬂowers and grains.
The research activities of RARF were also extended
to programs conducted in international collaboration.
Firstly, I. Tanihata launched a set of international programs on RIB physics, for which experiments wereperformed using the FRS facility at GSI, Germanyand the RIBLL facility at IMP, China in collaboration with researchers on site. At RIPS, a Russiangroup from Kurchatov Institute was invited collaboratively to conduct a unique program of synthesizing
super-heavy hydrogen and helium isotopes. These programs, which were signiﬁcantly productive, served wellto spread RIB physics worldwide.
The major undertaking of implanting a RIKEN facility at a foreign institution started when a muon facility was constructed at RAL to produce intense secondary beams of muons by incorporating high-power
proton beams from the ISIS accelerator. The facility
has been in operation since 1995 under the auspicesof the RIKEN Facility Oﬃce at RAL, as originally directed by K. Nagamine. It is a powerful facility open
for outside users, hence widely used internationally.
The central activities have been on material science,
where the method of muon spin rotation, relaxation,
resonance (μSR) was primarily employed to investigate
electron motion or spin correlation in a variety of materials. Uniquely, the facility was also committed toR&D works for the muon-catalyzed d-t fusion.
Another project in international collaboration soon
followed. In 1995, the Radiation Laboratory directedby myself took the initiative to realize the Spin Physics
Program to be conducted with the Relativistic HeavyIon Collider (RHIC) at Brookhaven National Laboratory (BNL). In starting this program, the Radia-

contemporary accelerator facilities. While its campaign with RRC did not succeed in ﬁnding any newelements, it has left an ample set of matured expertise and advanced instrumentation. This has aﬀorded
a strong basis for a later campaign with the upgraded
RILAC facility, resulting in the successful synthesis ofnihonium (Nh). The Ion-Guided Isotope Separator On
Line (IGISOL) project under the leadership of T. Inamura has also left a legacy in the research ﬁeld withslow and stopped RIs, which is currently progressing
with the SLOWRI project at RIBF.
Thirdly, the SMART project of NP-3) was initially
led by H. Ohnuma (Tokyo Institute of Technology)
and later by H. Sakai (UT). It addressed particle spectroscopy with intermediate-energy heavy-ion direct reactions by incorporating a magnetic analyzer namedSMART. The project was drastically reinforced whena spin-polarized deuteron beam was installed throughthe eﬀorts of the Sakai group. This led to the ﬁndingof a signiﬁcant eﬀect of nuclear three-body interaction,
which served to invoke the currently prevalent worldwide campaign to explore the nature and inﬂuenceof higher-order interactions. The current SHARAQ
project under the auspice of Center for Nuclear Study(CNS), UT, is supposed to succeed this tradition ofparticle spectroscopy, which is to be applied to RIBinduced reactions at RIBF.
The ASHURA project led by S. Lee (Tsukuba University) aimed at the investigation of hot/dense nuclei. The destiny of this project was exceptional; itwas terminated before it became ripe because of thethen emerging thrust towards relativistic-energy phenomena.
The projects in the other disciplines were also richin novelty. The Large Ω project of OS-1) led by
K. Nagamine aimed at the promotion of muon-driven
science by establishing a facility of strong secondary
beams of muons. For that purpose, a solenoidal magnet device with a large solid angle was constructed andnamed as Large Ω. After the initial attempt at RRC toproduce negative muons from deuteron-induced reactions, the facility site was moved to ISIS at Rutherford
Appleton Laboratory (RAL), UK, as described later.
The atomic physics project of OS-2) led by Y. Awaya
addressed the investigation of multiply charged ionsof medium-heavy elements, which was an emerging
ﬁeld in those years as promoted by the advent ofintermediate-energy heavy-ion beams. After initial
successes with RRC, an advanced version of the ECR
ion source became the major facility for this project.
The low-energy beam of highly charged ions provided
a unique means to analyze single and multiple electron capture processes, thereby facilitating the spectroscopy of highly excited atomic levels.
The nuclear chemistry project of OS-3) led byF. Ambe made use of the rapid transportation systemof RI products from the RRC bombardment site down

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Research Activities 1991–2016

duct of many collaborative works both at GANIL andRARF and of a series of Japan-France Joint Symposium (1987–1994) on Heavy-ion Science. Long-lasting
collaboration with INFN has been conducted similarly
since 1991.
RARF as well pursued cooperation with domestic
institutions. Most importantly, intensive cooperation
with CNS was started in 2000 when it moved in theWako campus of RIKEN so that its researchers could
readily join the utilization of the RARF facility. Nowadays this collaboration has grown so far as to practicejoint operation of RIBF.
The research programs/projects described thus far
were started before the mid-1990s. On approaching andentering the 21 century, the conﬁguration of RARF
members was considerably changed with the additionof newly assigned Chief Scientists in return for the retirement of most of the Chief Scientists originally involved. Nevertheless, most of the research enterprises
mentioned above have remained to be the central involvements in the following years to come not only for
RARF but also for Nishina Center.
RARF was also committed to the realization of thenext-generation facility. Thus, the RIBF project waslaunched in 1995 under the auspices of RARF, while
the construction of RIBF was performed under theleadership of Y. Yano, who led the newly organizedsystem called Cyclotron Center. To coherently promote the construction of RIBF and the daily operation of RARF, a new organization named RARF/RIBF
Project Head Oﬃce was operated through the ﬁscal years of 2001–2004. It eﬀectively accommodated
the entire system of the old RARF, while I. Tanihata (2001–2003) and T. Motobayashi (2003–2005)
served as the Director of the Accelerator-based Research Group. It was closed in 2005 when a transient system functioning similarly to RARF was created within the framework of the RIKEN Frontier Research System. Finally, RIKEN Nishina Center forAccelerator-Based Science (RNC) was started in 2006,and Y. Yano became its ﬁrst Director. Incidentally,
RIBF was commissioned in the same year.

tion Laboratory has strongly contributed ﬁnanciallyto the installation of Siberian snakes and spin rotators at RHIC, which are essential for the accelerationand manipulation of spin-polarized protons. It has
then been committed to play a major role in the Spin
Physics Program conducted in the framework of thePHENIX Collaboration. The primary aim of this program was to elucidate the so-called spin puzzle by clarifying the role of gluons in making up the nucleon spinof 1/2. Experimental runs with RHIC polarized proton beams started in 2001 when the Radiation Laboratory accepted a new Chief Scientist, H. En’yo. Afterseveral runs on double-helicity asymmetry, the integrated contribution of gluons over the accessible rangeof Bjorken’s x was found to be compatible with thequark contribution. Thus, the quest for the remainingcontributions, possibly from smaller-x gluons and/orfrom the parton orbital angular momenta, has provided a strong cause for proposing the next-generation
US accelerator facility, Electron Ion Collider (EIC).
The RIKEN BNL Research Center (RBRC) was created in 1997 originally to support the RIKEN activities
for the Spin Physics Program, while its objective wassoon extended to nursing young talents of QCD theorists under the strong initiative of T. D. Lee, the ﬁrst
Director of RBRC.
Promotion of international cooperation on the basisof institutional agreement was another consistent policy undertaken by RARF. In this context the 1st agreement was eﬀectively formed with Institute of Modern
Physics (IMP), Lanzhou, China as early as 1979 forcollaboration on accelerator issues. A series of JapanChina Joint Symposium on Accelerators for Nuclear
Physics and Their Applications thus realized through
1980–1996 greatly contributed to establishing close relations between the relevant communities of the twocountries. In 1988 the agreement was formalized also
to cover activities on nuclear physics. Since then acouple of postdoctoral researchers from IMP were regularly invited to stay and work at RARF through 1990s.
It is delightful to see that many of those colleagues are,nowadays, playing leading roles in the Chinese community of nuclear physics.
The 2nd agreement was established with a US institution, Cyclotron Laboratory, Texas A&M University(TAM) in 1982, while the collaborative experimental
program using the TAM facility was already initiatedin 1978. The success of this fairly early engagement
stimulated later collaboration with several European
institutions, including IN2P3 (France), INFN (Italy),
GSI (Germany), Flerov Laboratory and Kurchatov Institute (Russia). Among those the agreement withIN2P3 started in 1985 provided a unique scheme ofcooperation, essentially allowing participation of any
nuclear physicists in both the countries. Indeed it
has developed a blessed tradition of close partnership between the two communities, through the con-

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