Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

RIPS Project: Pursuit of the RIB Physics Program with RIPS
M. Ishihara ∗1,∗2
RIPS is designed to be a separator of incoming projectile fragments, delivering the resultant RIB at the exit
channel. It consists of two 45◦ dipoles (D1 and D2) supplemented with twelve quadrupoles and four sextupoles.
The system is essentially composed of two sections in cascade with their relevant focuses. The ﬁrst section with
D1 gives rise to a dispersive focus at F1 and analyzes themagnetic rigidity of fragments. The second section with
D2 compensates for the dispersion of the ﬁrst section andgives rise to a doubly achromatic focus at F2. These two
sections combined, with a wedge-shaped energy degrader
placed at F1, constitute a doubly achromatic spectrometer, separating the projectile fragments with respect totheir A and Z (or q).
In designing RIPS, special precautions were taken toachieve the maximum intensities for the outcoming RIB.
The large angular and momentum acceptances taken as
5 msr and 6%, respectively, are essential for this purpose. Also important was the adoption of a large valueof 5.76 Tm for the maximum magnetic rigidity, which
well exceeds the value of 3.5 Tm of RRC. Such a large
bending power allows the use of the highest possible energy of the primary beam even for the production offragments with very large magnetic rigidity. This is useful to drastically enhance the yield of neutron drip-line
nuclei like 11 Li, since the production rate of RIB varies
with the incident energy Eint of the primary beam asαEint, where α  3.3. Thus, RIPS was constructed to
become an epoch-making deliverer of RIBs by aﬀording
beam intensities stronger than the preceding facilities by
3–4 orders of magnitude.
Such intensities of RIPS beams served tremendously to
expand the scope of the applicable experimental methods and hence of the research subjects to be addressed.
Most importantly, the RIPS beams were strong enoughto easily facilitate observation of a variety of direct reactions, the cross sections of which are typically of theorder of several 10 mb. As closely described later thisfeature greatly helped to promote in-beam studies of nuclear structure and nuclear astrophysics, thereby opening
up a wide research ﬁeld of RIB physics.
Moreover, the capability of RIPS was extended to deliver not only ordinary RIBs but also special types of
RIBs, namely spin-polarized RIBs and RIBs of isomers.
Spin-polarized fragments are produced in the course of
the projectile-fragmentation process. Hence, spin polarized RIBs can be readily obtained through RIPS.6) The
spin-polarized beams thus produced provided a uniquetool to determine the nuclear moments of unstable nuclei
by means of an NMR method incorporating the β-decay
process.

The RIB Physics Program with RIPS was initiated in
1984, when I took the oﬃce of Chief Scientist of the Radiation Laboratory. The primary objective of the job
was to cultivate a novel ﬁeld of nuclear physics using
the RIKEN Ring Cyclotron (RRC) facility,1) which wasthen under construction since 1980 to be completed in
1989. RRC is an intermediate-energy heavy-ion facility, following predecessors such as NSCL, MSU at Michigan and GANIL at Caen. Since we were late comers, I
was keen to select a research objective well distinguishedfrom those of the preceding facilities, where studies of
hot and/or dense nuclei were most intensively pursued.
Our subject thus chosen was the study on exotic nuclei
far from the line of stability by means of a radioactive
ion beam (RIB), which was a research ﬁeld still in its
infancy in the 1980s. The major vehicle to drive ourprogram was an in-ﬁght projectile-fragment separator,
RIKEN Projectile-fragment Separator (RIPS),2) which
emerged as the world strongest RIB deliverer, marking
the advent of the second-generation RIB facility.
This program was proven to be so successful that ittriggered and enhanced worldwide enthusiasm towards
the promotion of RIB science. Indeed, the RIPS program
has attained many of the ﬁrst accomplishments in termsof identifying unique scientiﬁc agendas on exotic nuclei
and of cultivating novel methods for RIB experiments.
These accomplishments altogether served to constitutethe universal framework for the research conduct on RIB
physics through the years that followed. At RIKEN, thesuccess of the RIPS project became the cause for promoting the project of RIKEN RI-Beam Factory (RIBF),
which was commissioned in 2006 to serve as the worldleading facility for the third-generation RIB science.3)
1

Features of RIPS

The production method of RIB by means of in-ﬂight
projectile fragments was introduced in the mid-1980s,
and pioneering works with such RIBs were performed at
Bevalac, LLNL to determine the radius of exotic nuclei 4)
and then at LISE, GANIL mainly to synthesize newisotopes.5) The ﬁnding of the neutron halo by the former work was particularly fascinating, as it revealed theexotic nature of nuclei very far from the line of stability. While these earlier studies were much stimulative,
the fairly weak beam intensities inherent to those ﬁrstgeneration facilities severely hampered further extension
of RIB physics. Thus, the RIPS project was aimed at
drastically expanding the research territory by introducing reinforced RIB of unprecedented intensities. RIPS
was also charged to facilitate the unique capability of
producing spin-polarized RIB.6)∗1∗2

Chief Scientist of Radiation Laboratory (1984–2000)
Professor, the University of Tokyo (1986-2000)

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RIKEN Accel. Prog. Rep. 50 (2017)2

Research Activities 1991–2016

Research goals with RIPS-RIB

In undertaking the RIPS project, we have chosen threemajor subjects on nuclear physics.7) Firstly our focus
was directed to reveal and explore exotic phenomena of
nuclear structure, which are to be unique to unstable
isotopes.8) Thus, we placed our research emphasis on extremely neutron-rich nuclei, which are uniquely characterized by i) isospin asymmetry with a large excess ofneutrons and ii) loose binding of valence neutrons. As iswell recognized nowadays, the former feature gives rise
to exotic bulk phenomena such as shell evolution resulting in appearance/disappearance of new/classical magic
numbers, while the latter gives rise to the peripheral
phenomena of neutron halo or skin. Such dilute neutron matter may enhance the cluster formation of thedi-neutron. In general, decoupling of valence neutrons
from the core may be another key property of extremely
neutron-rich nuclei.
Direct reactions have been essential in exploring suchexotic features because each variety of direct reactions
will provide a sensible probe for a speciﬁc property of
nuclear structure. Hence, various direct reactions atintermediate energies were combined to probe broad
aspects of exotic nuclear properties. Among others,
Coulomb excitation/dissociation, nuclear inelastic scattering, nucleon-removal reactions, and exchange reactions were most frequently employed.
The second primary subject concerned nuclear astrophysics. Nucleosynthesis in stellar environments often
occurs via proton/neutron radiative capture processes
involving targets of unstable nuclei. Hence, we focusedon the determination of stellar reaction rates for thoseradiative processes by measuring cross sections of theireﬀectively inverse reactions, i.e., Coulomb dissociation
processes. Proton-induced radiative capture reactions
occurring in the p-p chain and CNO cycle were chosen
to be the primary objectives.
Finally the unique asset of RIPS, spin-polarized RIB,
has facilitated the pursuit of the third primary subject;
determination of nuclear moments of unstable nuclei,
which are hardly accessible otherwise. The research programs and achievements on these three subjects will beseparately described in Sections 4, 5, and 6, respectively.
3

Cultivation of experimental methods for RIB

Most of our experiments on the ﬁrst and second subjects as deﬁned above were conducted by observing RIBinduced direct reactions. When performing such in-beamexperiment, one observes the reaction process in inversed
kinematics, where the incoming projectile of RIB serves
as the nucleus to be characterized while the target nucleus serves as a probing particle. One will then encounter several diﬃculties characteristic of RIB made of
in-ﬂight fragments: the wide energy spread and the deteriorated emittance in addition to the poor luminosity.
Among others, the wide energy spread of RIB is most

S-10

disturbing since in-beam spectroscopy generally requires
the precise determination of excitation energy Ex of the
ﬁnal state of interest, while the missing mass method,which is standard with the primary beam, does not properly work with the poor energy resolution of RIB.
To manage this problem, we have developed the following two methods for the in-beam RIB experiment:
one is to measure the gamma rays de-exciting the boundﬁnal states 9) and the other is to determine the invariant
mass of the unbound ﬁnal states via the measurement of
4 momentum vectors of all the particles emitted in theﬁnal channel.10,11) Nowadays, these two methods havebecome prevalent as the standard experimental methods
for RIB physics. The poor luminosity is partly overcomeby the useful features of the intermediate-energy RIB,
as the use of a thick target is allowed and a full solid
angle can be readily covered by the detector assembly
by virtue of the strong forward focusing of the outgoingparticles owing to Lorenz boost.
4

Study on exotic nuclear structure of extremely neutron-rich nuclei

The experimental studies on exotic nuclear structurein the earlier stage were primarily performed in two regions of neutron-rich nuclei, one being around N = 8
isotones including 11 Li and 12 Be and the other aroundN = 20 isotones including 32 Mg. Both of the regions
involve phenomena of signiﬁcant shell quenching and ofneutron halo as described below.
4.1 Study on halo nuclei
The neutron halo phenomenon was ﬁrst indicated for11
Li by the large interaction radius determined via themeasurement of interaction cross section.4) To further
explore the nature of the phenomenon, we employed
Coulomb dissociation as a probing reaction, as ﬁrst applied to the study on 11 Be.10) In this process, halo nuclei
are strongly excited via the E1 transition, leading to thecontinuum ﬁnal states at low excitation energies. Thisso-called soft dipole excitation turned out to be a powerful tool for characterizing the wave function of the halo
nucleon, which is supposedly well decoupled from thecore part of the nucleus. In fact, the decay spectrum, as
observed in the single-neutron halo nucleus 11 Be with theinvariant mass method, represents the square of Fouriertransform of the wave function times radius. Hence thespectrum shape is dictated by the neutron separation energy Sn , with the peak appearing around Ex ∼ 8/5Sn .
The cross section is proportional to the square of theasymptotic normalization coeﬃcient, which is gigantic
for a halo neutron. Thus, the strength may well exceed
the Weisskopf unit, yielding the cross section of several
100 mb.
This feature of huge cross sections for low excitationenergies serves as a clear signature of any neutron halo
nuclei. In fact, later experiments singly exploiting thisfeature newly identiﬁed several halo nuclei, includingResearch Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)19

C, 31 Ne, and 37 Mg. With complementary experiments,
C was ascribed to the s-wave halo like 11 Be, while 31 Ne
and 37 Mg in the pf-shell region were ascribed to the pwave halo. It is worth noting that the appearance of thep-wave halo is caused by deformation of those nuclei.12)
For the case of a two-neutron halo such as of 11 Li,
the occurrence of a di-neutron cluster may be conceived.
This phenomenon can be identiﬁed by ﬁnding an enhanced strength of soft dipole excitation, since the total
strength of the E1 excitation for the case of the twoneutron halo represents the square of the radius vectorof the center of gravity of the pair of neutrons. Indeed,
the observed strength for 11 Li clearly exceeded the upper limit for the two uncorrelated neutrons, disclosing
occurrence of a di-neutron cluster in the halo area of
dilute density.13)
19

4.2

Disappearance of classical magic numbers as
signatures of shell evolution
This quest was initially addressed to nuclei aroundN = 8 as stimulated by the notion of the N = 7
anomaly.14) As pointed out by I. Talmi et al., the supposedly higher-lying neutron level of the 2s1/2 + orbit
gradually lowers with respect to the 1p1/2 − orbit as theneutron excess increases along N = 7 isotones, evencrossing over at 11 Be. To clarify the nature of this phenomenon, we ﬁrst studied the level scheme of an N = 9
isotone, 14 B, by observing the β decay of 14 Be.15) A
large-acceptance detector assembly for β-γ spectroscopy
was used together with a high-eﬃciency TOF detector
array for delayed neutrons. It was conﬁrmed that theevolution of level crossing with increasing neutron excess
also occurs in N = 9 isotones, supporting the conjecturethat N = 8 magicity is quenched in extremely neutronrich isotones.
The systematic study on shell evolution was strongly
activated when we introduced a novel method of inbeam γ ray spectroscopy,9) by which the behaviors of theﬁrst excited 2+ state (2+1 ) and other prominent low-lying
states of even-even nuclei may be easily assessed. The
ﬁrst experiment of this category was performed on 32 Mg
by incorporating the intermediate-energy Coulomb excitation process (I.E. Coulex) at Ein /A = 49.2 MeV. The
use of Coulex at such a high energy was a challenging
attempt at that time since there had prevailed a strong
prejudice that the dominance of Coulex over nuclear excitation for the case of E2 transition could be only assured when the incident energy was taken well below the
Coulomb barrier. As is revealed, Coulex well dominates
even at intermediate energies when a target nucleus ofa heavy element such as Pb is employed. Thus, the E2+
strength, B(E2; 0+g.s. → 21 ), and the excitation energy++of the 21 state, Ex (21 ), were unambiguously determinedfor 32 Mg. The enhanced B(E2) and the reduced Ex (2+1)
together provided clear evidence of enhanced collectivity, assuring the quenched magicity of N = 20 in thisextremely neutron-rich isotope.
In-beam γ-ray spectroscopy with intermediate-energy

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reactions is subject to a huge Doppler eﬀect. We thus
used an array of highly segmented NaI(Tl) γ-ray detectors named DALI to facilitate the Doppler shift correction. The use of the NaI(Tl) detector was helpful to increase the photo-peak eﬃciency, while its relatively poor
energy resolution suﬃced to observe the spectrum withsparse peaks owing to the stringent selectivity inherent
to intermediate-energy direct reactions. In-beam γ-ray
spectroscopy with I.E. Coulex thus initiated soon became the most popular method worldwide to explore thephenomena of shell evolution.
In-beam γ-ray spectroscopy with RIB marked anotherbig step forward when we studied 12 Be by incorporating the proton inelastic scattering for the ﬁrst time.16) It
was immediately recognized that the (p, p’) reaction isequipped with several precious features to reinforce themethod of in-beam spectroscopy. Firstly, the number ofatoms per given target thickness in mg/cm2 is huge forthe proton target of A = 1, leading to the tremendous
enhancement of event rate. Another indispensable feature of the (p, p’) reaction is that it provides a diﬀerent
measure of transition strength, which is the deformation
length δ. To make it more usable, the global optical potential is available for the proton scattering to be readily
employed for DWBA analysis. The lowered Ex (2+1 ) and+enhanced δ2 (0+
→2
)
thus
obtainedwere
clear
signag.s.
1tures of the strong collectivity of 12 Be, conﬁrming theoccurrence of quenched magicity for this neutron-rich
N = 8 isotone. For 12 Be, the ﬁrst 1− state 17) and thesecond 0+ state 18) were also found close to the groundstate. This implies the near degeneracy of 2s1/2 + and1p1/2 − orbits, further supporting the quenched magicity.
The (p, p’) reaction aﬀords additional merits: it+strongly excites the ﬁrst 4+
1 state as well as 21 , in contrast to the case of I.E. Coulex, thereby providing uniqueinformation on nuclear collectivity. This feature was ﬁrst
exploited in the study of 62 Cr to assure its strong deformability.19) It is also worth noting that the (p, p’) process occurs through nuclear interaction, while Coulex occurs through electromagnetic interaction. Accordingly,
the former process is more sensitive to the neutron component of the transition matrix, while the latter to theproton component. Thus, the combination of these tworeactions enables us to determine the proton and neutron
matrixes separately. This feature was exploited, e.g., toclarify the nature of the strongly hindered E2 strengthof 16 C.20) It was revealed that the eﬀective charges ofthe valence neutrons are strongly suppressed, indicating
their decoupling from the core nucleons.21)
Finally, the nucleon-removal reaction was introduced
as the third probe reaction, as ﬁrst used for the studyof 34 Mg.22) It was particularly useful to broaden the accessible region of in-beam spectroscopy towards larger
neutron excess. The in-RIB γ-ray spectroscopy thus developed has been intensively employed till today to explore the features of shell evolution over the broad regionof the nuclear chart. So far, such endeavors worldwide

RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

have been successful to conﬁrm the occurrence of shellquenching at N = 28 23) as well N = 8 and 20, while suchquenching only occurs in the isotones located very close
to the neutron drip line. In contrast, no signature of shellquenching was found for the heavier classical magic numbers of N = 50 and 82, even at extremely neutron-richisotones such as 78 Ni 24) and 128 Pd.25) For these systematic studies on shell evolution, the 2015 Nishina Memorial Prize was awarded to T. Motobayashi and H. Sakurai
on behalf of the international colleagues.
5

Study on nucleosynthesis in the cosmos:
Determination of astrophysical reaction rates
via RIB-induced Coulomb dissociation

Nucleosynthesis in the cosmos is supposed primarily tooccur in stellar burning processes that involve very-lowenergy nuclear reactions with unstable nuclei. Thus, determination of reaction rates of relevant processes should
provide clues in deciding among the various possible scenarios on cosmic evolution. In this context, we set up
a novel program to measure the cross section of stellar
reactions of radiative capture, which is the most popular
process in star burning. For this purpose, we followed thesuggestion 26) by G. Baur and C. A. Bertulani that one
should measure the cross sections of Coulomb dissociation with an intermediate-energy RIB, which eﬀectively
serves as the inverse reaction for the radiative capture
process directly feeding the ground state of the residual
nucleus. The measurement of this high-energy reaction
instead of the original very-low energy reaction tremendously enhances the experimental eﬃciency by allowing athicker target and a wider detection acceptance as well as
aﬀording a greatly increased cross section. These eﬀects
together contribute to increase the overall eﬃciency by
about 6 orders of magnitude, well compensating for the
disadvantage of using RIBs with much reduced intensity.
The method was ﬁrst applied to determine the reaction
rate of the 13 N(p, γ)14 O reaction,11) a key reaction to ignite the hot CNO cycle of hydrogen burning in stars. The
observation of Coulomb dissociation of 14 O by means of
the invariant mass method revealed the relevant resonance of 1− located at a decay energy of approximately
545 keV with a radiative width of Γγ = 3.1 ± 0.6 keV.
This marked the ﬁrst successful experiment on the astrophysical reaction rate ever performed with RIB.
The astrophysical study with I.E. Coulex was then addressed to the 7 Be(p, γ)8 B reaction, a key process in the
production of high-energy solar neutrinos by the 8 B β +decay. To determine the E1 strength free from the disturbing E2 contaminant, the ﬁrst experiment using the
RIPS 8 B beam 27) was complemented by another experiment performed using RIB of a higher incident energy
at FRS-GSI.28) The S17 value ﬁnally determined was
S17 = 20.6±1.2 (exp.)±1.0 (theo.), slightly smaller thanthe values recently obtained directly from the low-energy
bombardment of protons on a 7 Be target.
The Coulomb dissociation method was later appliedS-12

not only to proton-capture reactions but also to aneutron-capture process, 14 C(n, γ)15 C, one of the key reactions in the Big-Bang nucleosynthesis.29)
6

Study on nuclear moments as determinedwith spin-polarized RIB

Our tradition of producing spin-polarized unstable nuclei with heavy-ion reactions started as early as the mid1970s, when we obtained spin-polarized 12 B isotopes via
the 100 Mo(14 N, 12 B)X reaction at Ein = 90 MeV.30)
However the polarizability could be meager in theintermediate-energy projectile fragmentation since thegrazing angle of such a high-energy reaction tends toapproach zero. To examine this aspect, we performed apilot experiment at GANIL in the late 1980s to ﬁnd that12
B produced in 18 O projectile fragmentation at Ein /A= 60 MeV indeed exhibited a sizable spin orientation.31)
Encouraged by this ﬁnding, a novel production scheme ofspin-polarized RIB was developed.6) RIPS was used forthis purpose as well, through which the spin-polarized
projectile fragments of speciﬁc nuclear species were separated and collected. To favorably collect spin-polarizedfragments, two supplemental procedures were combined.
One is to collect fragments emitted at angles away fromzero. This was facilitated with a magnetic swinger placed
upstream of the production target, which was used to tilt
the direction of the incoming projectile. Secondly, themomentum bin for the in-ﬂight fragment was selected
at the intermediate dispersive focal plane so that thenet polarization may be optimized. The magnitude offragment spin polarization thus obtained is signiﬁcantly
large for the case of single nucleon removal, while it decreases rather rapidly as the number of removed nucleons
increases.
For the measurement of g-factor, the spin-polarized βunstable fragments were implanted into the host material, which preserves the spin orientation under an externally applied magnetic ﬁeld. Meanwhile, the adiabatic
fast-passage NMR method for spin reversal was applied
to the implanted nucleus to ﬁnd the relevant Larmor frequency and hence the g-factor of interest. The spin precession occurring across the Larmor frequency was monitored by observing asymmetric emission of β rays withrespect to the spin-polarization axis. Spin-polarized unstable nuclei were also employed for the measurement of
Q-moment, where a sequence of NMR procedures wasapplied to the fragments implanted into a host material
with an adequate electric-ﬁeld gradient.
Highlights in the earlier phase of the program involve
ﬁrstly the determination of g-factor of 17 C 32) and secondly of Q-moments of 15, 17 B isotopes.33) The former result unambiguously conﬁrmed the anomalous spin-parityof 3/2+ for this nucleus of (d5/2 )3 dominance. The latter result revealed extremely suppressed neutron eﬀective charges of less than 0.1 as compared to the ordinary magnitude of ∼0.6. It was concluded that the valence neutrons of those extremely neutron-rich isotopes

Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

are strongly decoupled from the core part of the nuclei.
The campaign of this program has been continued till
today. As of 2010, g-factors and Q-moments were newly
determined, respectively, for 14 and 10 unstable isotopes.
7

Collaborators

As described above, the RIPS project was carried out
under the auspices of the Radiation Laboratory. On
the other hand, the project has covered such a broad
scope of nuclear research that a large number of collaborators from outside of the Radiation Laboratory werealso involved. Among others, T. Kubo from the Linear
Accelerator Laboratory should be noted as the primarycontributor to the construction/operation of RIPS. Regarding the pursuit of research programs, the following
three groups from outside of RIKEN played a major rolein the initial stage of the project: 1) a group from the
University of Tokyo (UT) led by myself and assisted by
S. Shimoura, 2) a Rikkyo University group led by T. Motobayashi, 3) a group from Tokyo Institute of Technology(TIT) led by K. Asahi, and 4) a Kyushu University groupled by Y. Gono. Among others, group 1) played a specialrole by directing the entire program jointly with the Radiation Laboratory. In the later stage, 5) a group from
TIT led by T. Nakamura, 6) a group from the Science
Faculty, UT, led by H. Sakurai, and 7) a group from CNS,
UT, led by S. Shimoura joined to expand the scope of
the program. Meanwhile, Radiation Laboratory ﬁnishedits commitment to this program when I retired from theLaboratory in 2000. Instead, three RIKEN Laboratories
with their respective Chief Scientists, 8) T. Motobayashi,
9) K. Asahi, and 10) H. Sakurai, were newly created topursue RIB-related nuclear physics. The Laboratory of9) was eﬀectively succeeded by 11) the current Laboratory led by H. Ueno.
The research program on halo nuclei was originallyimplanted by group 1) and dramatically developed by
the leadership of group 5) in the following stage. The
program incorporating the in-RIB γ-ray spectroscopy
has been most prosperous. It was originally started bygroups 1) and 2) and later involved many other groups6), 7), 8), and 10). In particular, group 2) was creditedwith the construction of the principal device, a γ-ray
Detector Array for Low Intensity radiation (DALI). The
research program on astrophysical nucleosynthesis was
also initiated by groups 1) and 2), and later conductedprimarily by groups 2) and 8). The research programwith spin-polarized RIB was started by groups 1) and3), and later taken over by groups 3) and 9) and ﬁnallyby the current group 11). Group 4) was committed to achallenging program of developing and utilizing the socalled isomer beam. It was proven that such a beam isuseful in studying, e.g., properties of excited bands built
on isomeric states.34)
Finally, RIPS has been so useful that many researchgroups other than the above joined in the utilization of
the facility. Among others the Linear Accelerator Lab-

S-13

oratory group led by I. Tanihata set up a unique program on nucleosynthesis to exploit the extremely intense
RIPS beam of 11 Li. This program, which was performedin collaboration with a group from Kurchatov Instituteled by A. Korsheninnikov, has succeeded in the synthesisof e.g., “super heavy” helium isotope, 10 He.35) Anothermajor commitment was made by a group from Osaka
University led by T. Minamisono and K. Matsuta, who
intensively worked on spin-dependent β-decay issues bymeans of spin-polarized RIB.
Altogether, the number of researchers who have participated in the RIPS-based programs is greater than 250.
In particular, the participation of graduate students fromvarious universities was signiﬁcant. As of January 2008,
the RIPS project resulted in the awarding of 52 Ph.D.
degrees.
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