Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Nuclear Spectroscopy Laboratory
H. Ueno ∗1
developed a method to produce highly spin-aligned RI
beams through the two-step nuclear reaction combinedwith the dispersion-matching technique 6) using the BigRIPS in-ﬂight RI separator.7) Isomeric-state nuclearmoment measurements are conducted by our group as
its application. To extend these observations to theground states of extremely rare RIs, we are now attempting to develop a new method and apparatus for
producing spin-polarized RI beams at RIBF.

The Nuclear Spectroscopy Laboratory was started in2013 in at Nishina Center. In the group, ﬁve full-time
researchers are working as of 2016 together with theChief Scientist, including three Research Scientists inthe ﬁelds of nuclear physics, atomic physics, and materials science as key persons in each ﬁeld of study. The
group has been conducting various spectroscopic studies of radiation and ﬂuorescence emitted from sloweddown or stopped radioactive-isotope (RI) beams produced at the RIBF facility, aiming at the measurementof electromagnetic nuclear properties such as spin, isotope shift, and electromagnetic nuclear moments. In
this report, we introduce these studies by roughly categorizing them into three.

The second category comprises nuclear laser spectroscopic studies for stopped/slowed-down RI beams.
We have been conducting system development for nuclear laser spectroscopy from the following two approaches in order to realize experiments for rare isotopes at RIBF. One is collinear laser spectroscopy
for a large variety of elements using slowed-down RI
beams produced via a projectile-fragmentation reaction, which can be achieved only by the universal lowenergy RI-beam delivery system, SLOWRI. Thus far,nuclear laser spectroscopy has intensively been conducted at the CERN-ISOLDE facility, taking advantage of quite a narrow momentum spread of low-energy
RI beams, although there is an element limitation. The
SLOWRI facility, under installation, can deliver suchhigh-quality low-energy RI beams without the elemental limitation because of the diﬀerence in the RI production method. We plan to start new measurementsof nuclear laser spectroscopy based on this advantageous feature.
The other approach is a new method utilizing superﬂuid helium (He II) as a stopping medium of energetic
RI beams, in which the characteristic atomic properties of ions surrounded by superﬂuid helium enables us
to perform unique nuclear laser spectroscopy. RI ionstrapped in He II are known to exhibit a characteristic excitation spectrum signiﬁcantly blue-shifted compared with the emission one. Consequently, the background derived from the excitation-laser stray light,
which often causes serious problems in measurements,
can be drastically reduced. This enables us to measure a Zeeman or hyperﬁne structure splitting with ahigh sensitivity by applying the laser-RF/MW double
resonance method to the trapped atoms.8) These R&Dstudies are conducted under the supervision of Professor Y. Matsuo (Hosei Univ.), Senior Visiting Scientistof our Laboratory.

The ﬁrst category comprises nuclear-structure studies utilizing spin-oriented RI beams produced in inﬂight nuclear reactions. This research has been conducted successively for generations, and new techniques were developed at the accelerators in RIKEN.
A spin-polarized RI beam at RIKEN was ﬁrst produced in late 1970 by the experimental group led byM. Ishihara, who was a member of the Cyclotron Laboratory at that time, based on the low-energy quasielastic scattering reaction of a heavy-ion beam from the160-cm Cyclotron.1) After the beam energy reachedE/A ∼ 100 MeV with the installation of the RIKEN
Ring Cyclotron (RRC), the method was improved inearly 1990 by Ishihara and K. Asahi et al. in the Radiation Laboratory. The method makes good use of properties of the projectile-fragmentation reaction that isdominant at this beam energy.2,3) Here, the RIKEN
projectile-fragment separator RIPS 4) played an important role. This method enabled the production of
spin-polarized RI beams far from the β-decay stabilityline. Thus, systematic ground-state nuclear-momentmeasurements of light unstable nuclei were conductedfrom the late 1990s to the 2000s in the Applied Nuclear Physics Laboratory led by Asahi, based on thetechnique of fragmentation-induced spin polarization
combined with the β-ray-detected NMR method. For
instance, the isospin dependence of eﬀective charges
was ﬁrst observed in these studies.5) Then, the RIBF
facility started operation at the end of 2006. Primarybeams at RIBF are delivered typically at E/A = 345
MeV. At this energy, we have a great advantage inthe production yield of RI beams, but we simultaneously encounter diﬃculties in producing spin polarization owing to the properties of the fragmentation reaction. Instead of spin polarization, we have newly∗1

Chief Scientist(2013−present)ofNuclear

Spectroscopy

The last category comprises the application of RI
and heavy-ion beams as a probe for condensed matterstudies. The microscopic material dynamics and properties have been investigated through the deduced internal local ﬁelds and the spin relaxation of RI probes

Laboratory

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RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

based on various spectroscopic studies, aiming at utilization of the RIBF facility in a wide range of researchﬁelds of, in particular, materials science. We havealready conducted β-NMR/nuclear quadrupole resonance (NQR) spectroscopy, in-beam Mössbauer spectroscopy, and the γ-ray time-diﬀerential perturbed angular correlation (γ-TDPAC) spectroscopy in collaboration with Univ. of Electro-Comm., ICU, Tokyo
Univ. of Sci., Osaka Univ. among the others. Amongthem, we note that in-beam Mössbauer spectroscopyusing a 57 Mn RI beam was developed in the Nuclear
Chemistry Laboratory led by F. Ambe.9)
Recently, we started studies on diamond superconductivity by high-density doping through heavyion implantation as well as low-density, low-ﬁeld oxygen NMR/NQR studies utilizing 21 O as an RI
probe. They are partly conducted together with several Chief-Scientist Laboratories outside the Nishina
Center under the framework of a RIKEN competitive
interdisciplinary-research budget program.
References
1) K. Sugimoto, N. Takahashi, A. Mizobuchi, Y. Nojiri,
T. Minamisono, M. Ishihara, K. Tanaka, H. Kamitsubo, Phys. Rev. Lett. 39, 323 (1977).
2) K. Asahi et al., Phys. Lett. B 251, 488 (1990).
3) K. Asahi et al., Phys. Rev. C 43, 456 (1991).
4) T. Kubo et al., Nucl. Instrum. Methods Phys. Res. B
70, 309 (1992).
5) H. Ogawa et al., Phys. Rev. C 67, 064308 (2003).
6) Y. Ichikawa et al., Nature Phys. 8, 918 (2012).
7) T. Kubo et al., Nucl. Instrum. Methods Phys. Res. B
204, 97 (2003).
8) X.F. Yang et al., Phys.
(2014).
Phys. Rev.
Rev. AA 90,90, 052516,052516 (2014).
9) M. Nakada et al., Bull. Chem. Soc. Jpn. 65, 1 (1992).

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