Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Measurement of the proton elastic scatteringfrom 132 Sn at 300 MeV/nucleon
J. Zenihiro,∗1 T. Harada,∗2,∗3 N. Aoi,∗4 H. Baba,∗2 M. Dozono,∗5 F. Endo,∗5 S. Enyo,∗1 Y. Fujikawa,∗1S. Hanai,∗5 Y. Hijikata,∗1 J. Hwang,∗5 N. Imai,∗5 K. Inaba,∗1 S. Ishida,∗6 T. Isobe,∗2 T. Kawabata,∗7
S. Kiyotake,∗8 A. Kohda,∗4 R. Kojima,∗5 R. Maeda,∗6 Y. Maeda,∗8 Y. Matsuda,∗6 S. Matsumoto,∗1R. Matsumura,∗2,∗3 B. Mauss,∗2 S. Michimasa,∗5 D. Nishimura,∗9 T. Nishimura,∗8 K. Nosaka,∗6 S. Ota,∗5H. Sakaguchi,∗4 K. Sakanashi,∗7 H. Shimizu,∗5 D. Suzuki,∗2 J. Tanaka,∗2 S. Terashima,∗10 R. Tsunoda,∗5 andT. Uesaka∗2The “neutron skin” structure is an important phenomenon in ﬁnite nuclei and is known to be strongly related with the properties of nuclear matter. In neutronrich nuclei, excess neutrons are pushed out to thesurface region, evolving the “neutron skin” structure.
Many mean-ﬁeld predictions indicate that the thicknessof the neutorn skin (∆rnp ), which is deﬁned as the difference in root-mean-square radii between the protonand neutron, is strongly correlated with the equation ofstate (EOS) of the neutron matter. Neutron-rich doubly magic 208 Pb is a benchmarking nucleus for EOS
studies. Thus far, many theoretical and experimental
studies on the ∆rnp of 208 Pb have been conducted.
Proton elastic scattering is a powerful tool to extract
density information and ∆rnp . In the case of stable nuclei, we have established the method to determine neutron density distributions from proton elastic scatteringat 300 MeV.1) It is natural to expand this study to theunstable region, where large ∆rnp values are expected
because of the large isospin asymmetry. To apply ourmethod to unstable nuclei, we started a new project
to measure the elastic scattering of protons with RI
beams (ESPRI) based on missing mass spectroscopy
in inverse kinematics. We developed a new device, recoil particle spectrometer (RPS), which consists of a
1-mm-thick solid hydrogen target (SHT),2) two multiwire drift chambers (MWDCs), two plastic scintillators,and fourteen NaI(Tl) calorimeter rods. RPS covers awide range of momentum transfers (0.5–2.5 fm−1 ) corresponding to 10◦ –35◦ in the c.m. scattering angles. We
successfully performed ESPRI measurements with RPS
for several light unstable nuclei.3) We also developed a
new method to determine the proton and neutron density distributions from two-energy proton elastic scattering data at 200 and 300 MeV/nucleon. Usually, theproton density distributions can be derived from the nuclear charge distributions determined by electron elasticscattering,4) but at present, little information is avail∗1Department of Physics, Kyoto University
RIKEN Nishina Center∗3 Department of Physics, Toho University∗4 RCNP, Osaka University∗5 CNS, University of Tokyo∗6 CYRIC, Tohoku University∗7 Department of Physics, Osaka University∗8 Department of Applied Physics, Miyzaki University∗9 Department of Natural Sciences, Tokyo City University∗10 School of Physics, Beihang University∗2- 44 -able on the charge distributions of unstable nuclei. Thismethod is based on the diﬀerent energy dependences between pp and pn interactions, which was demonstrated
by the proton elastic scattering data of Zr isotopes at200 and 300 MeV/nucleon.1)
132
Sn is another benchmarking nucleus, which has
a larger isospin asymmetry than that of 208 Pb. InNovember 2019, we successfully completed the ESPRI
experiment for 132 Sn at 300 MeV/nucleon (NP1512RIBF79R1). The ﬁrst part of this experiment at
200 MeV/nucleon was already completed in 2016.5) A
secondary beam around 132 Sn at 300 MeV/nucleon wasproduced by ﬁssion fragmentation with a 238 U beamat 345 MeV/nucleon. In this experiment, the purity of132
Sn was ∼20%, and the total intensity of the cocktailbeam was ∼700 kcps. Since high-intensity and heavy RI
beams cause critical radiation damage to and/or low efﬁciencies of detectors such as plastic scintillators, delayline parallel-plate avalanche counters (DL-PPACs), andion chambers, in place of the standard ones at BigRIPS,
we have used new detectors with good radiation hardnesses and fast timing responses, namely, diamond detectors, MWDCs, strip-readout PPACS (SR-PPACs),and Xe gas scintillators.6) The secondary beam particles were transported to F8 and ﬁnally to the SHT atF12, where RPS was installed. At F8, a CNS activetarget (CAT) was also installed for measurement at aforward scattering angle around 7◦ –10◦ in the c.m. angles. A new high-speed DAQ system, MOCO with parallelized VME (MPV),7) was introduced for the ﬁrst
time, and it worked with a good live-time ratio under ahigh-data-rate condition. During the measurement, weclearly identiﬁed elastic events from the correlation ofrecoil proton energies and angles.
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) H. De Vries et al., At. Data Nucl. Data Tables 36, 495(1987).
5) J. Zenihiro et al., RIKEN Accel. Prog. Rep. 50, 54(2017).
6) J. Zenihiro et al., RIKEN Accel. Prog. Rep. 51, 156(2018).
7) H. Baba et al., in this report.