Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Radioactive Isotope Physics Laboratory
H. Sakurai ∗1
The Radioactive Isotope Physics Laboratory was established as one of the institutes laboratory in 2005 whenHiroyoshi Sakurai was appointed as a Chief Scientist.
Since then, the Laboratory has functioned as one of thecore research groups conducting programs at the worldleading heavy-ion accelerator facility, the RIKEN “RI
Beam Factory (RIBF).”
The Laboratory explores exotic nuclear structures anddynamics in exotic nuclei that have never been investigated before, such as those with large imbalanced protonand neutron numbers. Our aim is to develop new experimental techniques utilizing fast radioactive isotope (RI)
beams at RIBF and to discover new phenomena andproperties in exotic nuclei.
Research subjects covered by the Laboratory are theshell evolution in exotic nuclei, the r-process nucleosynthesis, and the equation of state (EOS) in asymmetricnuclear matter.
The shell evolution has been investigated via in-beamgamma spectroscopy and decay spectroscopy. The two
spectroscopy methods complement each other: in-beamgamma spectroscopy is a good method for ﬁnding yrast
states, while decay spectroscopy gives information on
non-yrast states as well. A major experimental site ofin-beam gamma spectroscopy is the “ZeroDegree” spectrometer,1) where reaction products are identiﬁed in coincidence with de-excited gamma-ray measurement at a
NaI array called “DALI2.”2) The ﬁrst spectroscopy atRIBF was organized as a day-one campaign in 2008 forthe island-of-inversion region. The 48 Ca beam was utilized to produce very neutron-rich nuclei in the region.
Not only the high primary beam intensity but also unexpectedly large production cross sections caused surprisingly intense secondary beams, which gave very eﬃcientorganization in managing the day-one programs. Theﬁrst data on 32 Ne were published in PRL in 2009.3) Since
then, multitudes of programs have been conducted under
the SUNFLOWER collaboration. One of the highlights
is “the discovery of a new magic number 34,” which
was published in Nature.4) The SEASTAR collaboration was formed as a subset of SUNFLOWER in 2014
to observe the ﬁrst excited states in even-even neutronrich isotopes, and it ﬁnished its campaign program insummer 2017. Exciting results are being published.5)
Decay spectroscopy at RIBF was started in 2009 as a
day-two campaign by utilizing a primary beam of uranium. A beta-gamma coincidence setup including fourclover Ge-detectors was mounted at the end of ZeroDegree, and neutron-rich nuclei with A ∼ 110 were implanted into active Si strip detectors “WAS3ABi.” Theﬁrst campaign was successfully organized for 2.5 days,and two papers on the shell evolution and the r-process
path were published in PRL.6) The ﬁrst attempt made∗1

Chief Scientist of Radioactive Isotope Physics Laboratory(2005–present)

S-69

a trigger to organize the “EURICA” project, where theGSI RISING setup of Euroball-cluster Ge-detectors wascoupled with RIBF. The EURICA campaign was organized from 2012 to 2016, and about 40 papers including
11 PRL papers have already been published. One of
the highlights is a successful measurement of 110 halflives of neutron-rich nuclei.7) The measurement impacts
the r-process nucleosynthesis scenario. Since 2016, theBRIKEN project of beta-delayed neutron emission probability measurement has been conducted.
The EOS in asymmetric nuclear matter at a densitythat of normal nuclei has been studied with heavy-ion
central collisions by utilizing a large TPC, which wasdeveloped under the SπRIT collaboration.8) In 2016, theTPC was mounted inside the SAMURAI magnet, andphysics runs were organized. A huge amount of data is
being analyzed to obtain π ± yields as well as protonand neutron-ﬂows.
Other activities in the Laboratory were missing-mass
spectroscopy with radioactive isotope beams,9) laser
spectroscopy of radioactive atoms in superﬂuid He,
and Antarctic ice core analysis for supernova explosion
events. At the occasion of a researcher leaving andnew laboratories being created, their research activities were transferred to other Laboratories and otherresearch units in Nishina Center. In 2013, a spallation
reaction study with long-lived ﬁssion products was initiated by the Laboratory for the nuclear waste problem.10)
This activity was also transferred into the other groupin Nishina Center in 2014.
Future projects being initiated by the Laboratory are
the development of detectors such as GAGG crystals,advanced tracking Ge detectors, and silicon telescopes.
Based on these detectors, new programs with decelerated radioactive isotope beams are the future research
directions in the Laboratory.
References
1) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003(2012).
2) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res. A763, 596 (2014).
3) P. Doornenbal, H. Scheit et al., Phys. Rev. Lett. 103,032501 (2009).
4) D. Steppenbeck et al., Nature 502, 207 (2013).
5) N. Paul, A. Corsi, A. Obertelli, P. Doornenbal et al.,Phys. Rev. Lett. 118, 032501 (2017)6) S. Nishimura et al., Phys. Rev. Lett. 106, 052502 (2011);
T. Sumikama et al., Phys. Rev. Lett 106, 202501 (2011).
7) G. Lorusso, S. Nishimura et al., Phys. Rev. Lett. 114,192501 (2015).
8) R. Shane, A.B. McIntosh, T. Isobe et al., Nucl. Instrum.
Methods Phys. Res. A 784, 513 (2015).
9) A. Matta, D. Beaumel, H. Otsu et al., Phys. Rev. C 92,041302 (2015).
10) H. Wang et al., Phys. Lett. B 754, 104 (2016).