Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Progress report of Gamow–Teller giant resonance studies at RIBF
M. Sasano,∗1 J. Yasuda,∗2 L. Stuhl,∗1,6 R.G.T. Zegers,∗3 H. Baba,∗1 W. Chao,∗1 M. Dozono,∗1 N. Fukuda,∗1N. Inabe,∗1 T. Isobe,∗1 G. Jhang,∗1,13 D. Kamaeda,∗1 T. Kubo,∗1 M. Kurata-Nishimura,∗1 E. Milman,∗1T. Motobayashi,∗1 H. Otsu,∗1 V. Panin, ∗1 W. Powell, ∗1 M. Sako, ∗1 H. Sato,∗1 Y. Shimizu,∗1 H. Suzuki,∗1T. Suwat, ∗1 H. Takeda,∗1 T. Uesaka,∗1 K. Yoneda,∗1 J. Zenihiro,∗1 T. Kobayashi,∗1,4 T. Sumikama,∗1,4T. Tako,∗4 T. Nakamura,∗5 Y. Kondo,∗5 Y. Togano,∗5 M. Shikata,∗5 J. Tsubota,∗5 K. Yako,∗6 S. Shimoura,∗6S. Ota,∗6 S. Kawase,∗6 Y. Kubota,∗6 M. Takaki,∗6 S. Michimasa,∗6 K. Kisamori,∗6 C.S. Lee,∗6 H. Tokieda,∗6
M. Kobayashi,∗6 S. Koyama,∗7 N. Kobayashi, ∗7 H. Sakai,∗1 T. Wakasa,∗2 S. Sakaguchi,∗2 A. Krasznahorkay,∗8T. Murakami, ∗9 N. Nakatsuka, ∗9 M. Kaneko, ∗9 Y. Matsuda, ∗10 D. Mucher, ∗11 S. Reichert, ∗11 D. Bazin,∗3and J.W. Lee∗12spectrometer such as SAMURAI8) . Since last year, wehave been working on the upgrade of WINDS to reduce background events in the detectors. The primary
goal of the upgrade plan is to eliminate background
events due to gamma rays arising from the environment by the so-called neutron-gamma discrimination
method. Such gamma rays are considered not to besynchronized with the reaction timing, and therefore
they have a ﬂat distribution in time. The contribution of such gamma rays is enhanced in the region of
forward scattering angles in the center-of-mass system,
because a wide TOF range having background events
uniformly distributed in time is compressed to a narrow phase space in that region. The method is being
tested using a prototype low-energy neutron detector
made from a novel plastic scintillator material for application of the neutron-gamma method. Details of thetest are also given in Ref.9) .

Among the collective modes1) , the Gamow–Teller(GT) giant resonance is an interesting excitation mode.
It is a 0h̄ω excitation characterized by the quantumnumber changes in orbital angular momentum (ΔL =0), spin (ΔS = 1), and isospin (ΔT = 1), and it isinduced by the transition operator στ . In the stable
nuclei in medium or heavier mass regions (A > 50),
the collectivity in this mode exhibits the GT giant
resonance (GTGR), which provides information thatis critically important for understanding the isovector
part of the eﬀective nucleon-nucleon interaction2) andthe symmetry potential of the equation of state3) .
We have been rapidly expanding the domain of
GTGR studies at RIBF in the nuclear chart, since ourdevelopment of a new method to study GT transitions
on unstable nuclei via the charge-exchange (CE) (p, n)reaction with RI beams4,5) . An experiment at RIBF
was performed in March 2014 to extract the GT andspin-dipole (SD) transition strengths over a wide excitation energy range covering their giant resonances on
the key doubly magic nucleus 132 Sn6) . This is an essential step toward establishing comprehensive theoreticalmodels for nuclei located between 78 Ni and 208 Pb. The
data analysis of this experiment has been ﬁnalized and
we are preparing a draft for publication. In 2016, theexperimental program for 48 Cr and 64 Ge was reevaluated and approved by the NP-PAC meeting. A study
on these nuclei will provide a unique opportunity toelucidate the role of isoscalar pairing in nuclei along
the N=Z line.
From an experimental point of view, we employ alow-energy neutron detection system, WINDS7) , to detect recoil neutrons produced via the (p, n) reaction
in inverse kinematics, in combination with a magneticReferences
1) M. N. Harakeh and A. M. van der Woude: Giant Resonances (Oxford University Press, Oxford, 2001).
2) S. Fracasso and G. Colo, Phys. Rev. C72, 064310(2005).
3) P. Danielewicz, Nucl. Phys. A 727, 233 (2003).
4) M. Sasano et al., Phys. Rev. Lett. 107, 202501 (2011).
5) M. Sasano et al., Phys. Rev. C 86, 034324 (2012).
6) J. Yasuda et al., in this report.
7) K. Yako et al., RIKEN Accel. Prog. Rep. 45 (2012).
8) T. Kobayashi et al., Nucl. Inst. Meth. B 317, 294-304(2013).
9) L. Stuhl et al., in this report.

∗1RIKEN Nishina Center
Department of Physics,Kyushu University∗3 NSCL, Michigan State University∗4 Department of Physics, Tohoku University∗5 Department of Physics, Tokyo Institute of Technology∗6 CNS, University of Tokyo∗7 Department of Physics, University of Tokyo∗8 MTA, Atomki∗9 Department of Physics, Kyoto University∗10 Department of Physics, Konan University∗11 Department of Physics, Technical University Munich∗12 Department of Physics, Korea University∗2- 47 -