RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Overview of activities of the NUSPEQ group in CNS
S. Shimoura ∗1
at the end of FY2008. The NUSPEQ group a played
major role in the developments and research activities.
The basic idea of the SHARAQ project is to use the RI
beams as probes for the direct reactions, in which a variety of isospin (T ), internal energy (mass excess), andspin (S) diﬀerent from those of stable beams exist. We
focused on the exothermic charge-exchange reaction ofRI beams, in which the internal energy of the beam
particle is used for exciting the target nucleus with avery small momentum transfer.
In the SHARAQ project, we developed tracking detectors for the high-resolution beamline 13) and the focal plane of the SHARAQ spectrometer. We also contributed to the development of the DAQ system including SHARAQ.14)
The ﬁrst result from the SHARAQ project was theisovector spin-monopole resonances in 90 Zr and 208 Pb
by using the (t, 3 He) reaction at 300 A MeV.15) The
recent highlight is the possible candidate of the tetraneutron resonance immediately above the 4n thresholdobtained by using the 4 He(8 He, 8 Be) reaction at 190 A
MeV 16) (Fig. 1).
Another program in the SHARAQ project is the direct measurement of the mass. In order to obtain very
good timing information, we developed diamond detectors having ∼ 10 ps resolution.17) The masses ofnuclei around 54 Ca were simultaneously measured anddetermined with 160, 250, and 990 keV resolution for55, 56, 57
Ca to clearly demonstrate N = 34 magicity inthe Ca isotopes.18)
Parallel to the SHARAQ project, the NUSPEQ
group joined the SEASTAR and the EURICA collaboration. Isomer spectroscopy of heavy neutron-rich nuclei was performed, several results of which have already been published.19)

Varieties in the properties of nuclei far from stability
give rich information on the nuclear structure in a wide
area of the nuclear chart. Experiments using radioactive isotope (RI) beams in the last few decades haveshown exotic phenomena such as neutron halos, softcollective excitations, and changes of magic numbers.
We are realizing that the nuclear structure should bestudied as a function of the proton and neutron numbers (Z and A) as well as of the excitation energies(Ex ).
Based on this consideration, the NUSPEQ a) groupin CNS started the in-beam spectroscopy of exotic nuclei with direct reactions of RI beams at several tens of
MeV per nucleon in FY2000. In FY2003, we extended
the studies to approach high-spin states by using fusion
reactions of RI beams at a few MeV per nucleon. Major parts of the studies before FY2006 were performedat the RIPS facility.
The ﬁrstly developed equipment was a liquid helium
target,1) which enables measurements of α-induced reactions on exotic nuclei. An array of position-sensitive
Ge detectors, GRAPE, was developed for measuring
in-ﬂight γ decay with high resolution.2) For totalenergy measurements of projectile-like ejectiles, an array of NaI(Tl) detectors was also developed.
By using the developed equipments combined withinvariant mass and/or in-beam γ-ray spectroscopies,
the following studies were performed: (1) (α, α ) reactions for studying isoscalar responses of 14 O 3) andcluster states in 12 Be.4) (2) shell evolution in neutronrich ﬂuorine 5) and boron 6) by using proton-transfer
reactions (α,t), and (3) electric quadrupole excitationof neutron-rich nuclei by Coulomb excitations.7) As aby-product of the experiment using the 12 Be beam, theisomeric 0+ state was discovered by analyzing cascade
γ-ray decays in ﬂight, and its lifetime was measured,
which shows the disappearance of N = 8 magicity.8)
For high-spin study, we developed a low-energy 46 Ar
beam by using a set of energy degraders.9) By using the 46 Ar beam and the GRAPE array, a fusion reaction 9 Be(46 Ar, 5n)50 Ti was measured to study
high-spin states of 50 Ti.10) The superdeformation in40
Ar was also found for the ﬁrst time by using the26
Mg(18 O, 2p2n)40 Ar reaction at the JAEA tandem facility.11)
In FY2005, CNS started the SHARAQ b) project 12)
in order to extend research opportunities in the RIBF
and completed the construction of the spectrometer∗1a)b)

Director of Center for Nuclear Study (CNS), the Universityof Tokyo (2016–present)
NUclear SPectroscopy for Extreme Quantum system
Spectroscopy with High-resolution Analyzer of RadioActive
Quantum beamFig. 1. Schematic image of the tetra-neutron experiment.

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Research Activities 1991–2016RIKEN Accel. Prog. Rep. 50 (2017)

In FY2015, we have initiated the OEDOc) project,which aims at the deceleration of intense RI beams
provided in the RIBF. Low-energy RI beams open
opportunities for studying exotic nuclei by using nucleon/pair/cluster transfer, Coulomb excitation, incomplete fusion, and so on. The OEDO system consists of a mono-energetic degrader at a momentumdispersive focus, two sets of quadrupole triplets, andan RF deﬂector. The RF deﬂector works as an opticalmatching element for the time-of-ﬂight dispersion afterthe compression of energy spread after the degrader.
The system was constructed by the end of FY2016.
The author acknowledges all the members in theNUSPEQ group over these 16 years for their excellent
performance in the research activities.
References
1) H. Ryuto et al., Nucl. Instrum. Methods Phys. Res. A555, 1 (2005).
2) M. Kurokawa et al., IEEE Trans. Nucl. Sci. 50 1309
(2003); S. Shimoura, Nucl. Instrum. Methods Phys.
Res. A 525, 188 (2004).
3) H. Baba et al., Nucl. Phys. A 788, 188c (2007).
4) A. Saito et al., Nucl. Phys. A 738, 337 (2004); A. Saitoet al., Mod. Phys. Lett. A 25, 1858 (2010).
5) S. Michimasa et al., Phys. Lett. B 638, 146 (2006).
6) S. Ota et al., Phys. Lett. B 666, 311 (2008).
7) H. Iwasaki et al., Phys. Lett. B 620, 118 (2005); H.
Iwasaki et al., Phys. Rev. C 78, 021304 (2008).
8) S. Shimoura et al., Phys. Lett. B 560, 31 (2003); S.
Shimoura et al., Phys. Lett. B 654, 87 (2007).
9) E. Ideguchi, Prog. Theor. Exp. Phys. 2012, 03C005(2012).
10) M. Niikura et al., Eur. Phys. J. A 42, 471 (2009).
11) E. Ideguchi et al., Phys. Lett. B 686, 18 (2012).
12) T. Uesaka et al., Nucl. Instrum. Methods Phys. Res. B
266, 4218 (2008); T. Uesaka et al., Prog. Theor. Exp.
Phys. 2012, 03C007 (2012); S. Michimasa et al., Nucl.
Instrum. Methods Phys. Res. B 317, 305 (2013).
13) H. Miya et al., Nucl. Instrum. Methods Phys. Res. B
317, 701 (2013).
14) H. Baba et al., Nucl. Instrum. Methods Phys. Res. A616, 65 (2010).
15) K. Miki et al., Phys. Rev. Lett 108, 262503 (2012).
16) K. Kisamori et al., Phys. Rev. Lett. 116, 052501(2016).
17) S. Michimasa et al., Nucl. Instrum. Methods Phys.
Res. B 317, 710 (2013).
18) M. Kobayashi, PhD Thesis (University of Tokyo), Mar.
2017.
19) R. Yokoyama et al., Phys. Rev. C 95, 034313 (2017);
E. Ideguchi et al., Phys. Lett. B 686, 18 (2010); E.
Ideguchi et al., Phys. Rev. C 94, 064322 (2016).

c)

Optimized Energy Degrading Optics for RI beam

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