II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Observation of anisotropic collective flow of charged particles andneutrons in heavy-ion collisions at beam energies of 400 MeV/nucleon
S. Yamamura,∗1,∗2 S. Nishimura,∗2 T. Isobe,∗2 V. H. Phong,∗2 M. Kaneko,∗2,∗3 T. Murakami,∗3 Y. Nakai,∗2D. Nishimura,∗4 M. Nishimura,∗2 N. Chiga,∗2 and E. Takada∗5
The anisotropic collective ﬂow of protons and neutrons
in heavy-ion collisions is expected to provide experimental information on the equation of state (EOS) of highdensity and neutron-rich nuclear matters because it reﬂects the symmetry energy at the high-density region.1)
The anisotropy is expressed by the distribution functionof particle emissions observed from the reaction plane as()
∑N0dN
=1+2
vn cos nφ ,
(1)
dφ
2πn

where φ, N0 and vn represent the emission angle of theparticles observed from the reaction plane, normalization
constant, and the anisotropic strength, respectively. The
diﬀerence between the v1 (directed ﬂow) of neutrons andof protons is theoretically expected to be highly sensitive
to EOS around target rapidity2) even though only a few
experimental data are available.
A pilot experiment (H355) was conducted at HIMAC
by impinging 400 MeV/nucleon 132 Xe beam on a CsItarget (500 mg/cm2 ) at a high intensity of 106 particles/spill to measure the proton and neutron ﬂows precisely. In this experiment, 32 plastic scintillators (EJ29933, 30 × 55 × 127 mm3 ) were used to identify neutrons
using pulse-shape discrimination. The detectors wereplaced parallel to the beam axis and in a cylindrical conﬁguration to detect particles at target rapidity. Charged
and neutral particles can be identiﬁed by combining
these detectors called NiGIRI (Neutron, ion, and γ-ray
identiﬁcation for radioactive isotope beam) and charged
particle veto scintillators. Figure 1 shows the prototype
NiGIRI array.3) Energies of the detected particles are
deduced from the time-of-ﬂight between the CsI target

Fig. 1. NiGIRI detectors3) (16 arrays 2 layers).
∗1
∗2
∗3
∗4∗5

Department of Physics, University of Tokyo
RIKEN Nishina Center
Department of Physics, Kyoto University
Department of Natural Science, Tokyo City University
NIRSFig. 2. Measured distribution functions dN/φ of charged particles (left), neutrons (center), and γ rays (right). The
spectra were integrated over momentum and impact parameter.

and NiGIRI detectors. Another set of 32 plastic scintillators called the Kyoto-array were used to cover the midrapidity region. The emitted angles of the charged particles were measured for determining the reaction plane.
The plane was determined by the beam axis and Q vector obtained using∑ (cos θk ),(2)
Q=sin θkk

where θk denotes a hit angle of Kyoto-array; the summation is taken over the number of detected particles.
The cross-talk events observed in the Kyoto-array were
mostly rejected in the Q-vector determination to minimize the detector bias.
Emission angles of charged particles, neutrons, and γ
rays with respect to the angle of Q vector were studied
at target rapidity using the data of NiGIRI. Figure 2
shows the measured distribution functions (dN/dφ) ofthe charged particles, neutrons, and γ rays. The results
indicate a clear anisotropy for charged particles and neutrons, and show ﬂatter distribution for γ rays; the results
are expected to indicate collective ﬂow, particularly v1 ,and therefore a pressure gradient is observed in the highdensity region in heavy-ion collisions.
To the best of the author’s knowledge, this experiment
prooves for the ﬁrst time that it is possible to directlycompare the collective ﬂow among light charged ions andneutrons around target rapidity. An upgraded experiment with a higher resolution of the reaction plane detector arrays will be performed at HIMAC and possibly
RIBF in the future.
References

1) B. A. Li, et al., Phys. Rep. 464, 113 (2008).
2) Z. Q. Feng, Phys. Rev. C 85, 014604 (2012).
3) H. Matsuzawa et al., RIKEN Accel. Prog. Rep. 48, 212(2015).
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