II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Measurement of proton elastic scattering from 132 Sn at
300 MeV/nucleon in inverse kinematics
T. Harada,∗1,∗2 J. Zenihiro,∗3 Y. Matsuda,∗4 S. Terashima,∗5 H. Sakaguchi,∗6 N. Aoi,∗6 H. Baba,∗2 M. Dozono,∗7F. Endo,∗7 S. Enyo,∗3 Y. Fujikawa,∗3 S. Hanai,∗7 S. Hayakawa,∗7 Y. Hijikata,∗3 J. W. Hwang,∗7 N. Imai,∗7K. Inaba,∗3 S. Ishida,∗4 T. Isobe,∗2 T. Kawabata,∗8 S. Kiyotake,∗9 A. Kohda,∗6 H. Kurosawa,∗4 R. Kojima,∗7R. Maeda,∗4 Y. Maeda,∗9 S. Y. Matsumoto,∗3 R. Matsumura,∗1,∗2 B. Mauss,∗2 S. Michimasa,∗7 T. Murakami,∗3D. Nishimura,∗10 T. Nishimura,∗9 K. Nosaka,∗3 S. Ota,∗7 K. Sakanashi,∗3 H. Shimizu,∗7 D. Suzuki,∗2 J. Tanaka,∗2R. Tsunoda,∗7 T. Uesaka,∗2 and K. Yamamoto∗4
The equation of state (EOS) of nuclear matter is expressed as the EOS of the symmetric nuclear matter andthe symmetry energy. Particularly, the symmetry energy is important for understanding astrophysical phenomena, such as neutron stars. The EOS of symmetric nuclear matter is understood from previous experiments on stable nuclei, however there is much less understanding of the the symmetry energy. From many
theoretical studies, it is known that the slope parameter of the symmetry energy is strongly correlated withneutron skin thickness, which is deﬁned as the diﬀerence
between the neutron and proton root-mean-square radii.
In neutron-rich nuclei, the excess neutrons form a neutron skin structure. It is expected that this symmetryenergy can be constrained by determining the neutron
skin thickness from the neutron and proton density distributions.
We employed proton elastic scattering to extract neutron and proton density distributions. For stable nuclei,
we have established a method to extract the proton andneutron density distributions using proton elastic scattering.1) To employ this method to unstable nuclei with
30000

Count

25000
20000
15000
10000
5000
0
2.56

2.58

2.60

2.62

2.64

2.66

2.68

A/Q
Fig. 1. A/Q spectrum of secondary beam including 132 Sn
deduced from position and time-of-flight information atBigRIPS. The peak of 132 Sn is located at A/Q = 2.64
shown in red. The A/Q resolution in r.m.s is 0.058%.
∗1Department of Physics, Toho University
RIKEN Nishina Center∗3 Department of Physics, Kyoto University∗4 CYRIC, Tohoku University∗5 School of Physics, Beihang University∗6 RCNP, Osaka University∗7 CNS, University of Tokyo∗8 Department of Physics, Osaka University∗9 Department of Applied Physics, Miyazaki University∗10 Department of Natural Sciences, Tokyo City University∗2Fig. 2. Kinematical correlations of 132 Sn between scattering
angles θ and kinematic energies of scattered protons Tp .
The red dotted line indicates elastic scattering events between 132 Sn at 303.9 MeV/nucleon and protons.

large asymmetry, we started a new project to measure
the elastic scattering of protons with RI beams (ESPRI)
in inverse kinematics. We developed a recoil proton
spectrometer (RPS), which consists of a 1-millimeterthick solid hydrogen target (SHT2) ), two multi-wire drift
chambers (MWDCs), two plastic scintillators, and fourteen NaI rods. We measure the angle and energy of therecoil protons from the SHT using the RPS. We successfully performed ESPRI measurements for several light
unstable nuclei.3)132
Sn has a larger isospin asymmetry than 208 Pb, andis expected to have a thicker neutron skin thickness. In
Novenver 2019, we performed proton elastic scatteringfrom 132 Sn at 300 MeV/nucleon at the F12 area.4) The
total beam rate was up to 600 kcps, and the purity of132
Sn was 20%. The A/Q spectrum of the secondary
beam including 132 Sn under high intensity is shown inFig. 1. We identiﬁed elastic events of 132 Sn from the correlation of the kinematic energies and recoil angles of thescattered protons with NaI rods and MWDCs as shownin Fig. 2. Data analysis for deducing the excitation energy spectrum of 132 Sn and the angular distribution ofthe cross section is now in progress.
References

1) H. Sakaguchi et al., Prog. Part. Nucl. Phys. 97, 1 (2017).
2) Y. Matsuda et al., Nucl. Instrum. Methods Phys. Res. A
643, 6 (2011).
3) Y. Matsuda et al., Phys. Rev. C 87, 034614 (2013).
4) J. Zenihiro et al., RIKEN Accel. Prog. Rep. 53, 44 (2019).
- 18 -1