RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Measurement of proton elastic scattering from 132 Sn
in inverse kinematics
J. Zenihiro,∗1 S. Terashima,∗1,∗2 Y. Matsuda,∗1,∗3 S. Ota,∗4 S. Chebotaryov,∗1 M. Dozono,∗4 T. Harada,∗1,∗10C. Iwamoto,∗4 M. Kobayashi,∗4 A. Krasznahorkay,∗1,∗11 S. Leblond,∗7 T. Lokotko,∗7 Y. Maeda,∗8 S. Masuoka,∗4
M. Matsushita,∗4 S. Michimasa,∗4 E. Milman,∗1 T. Murakami,∗1,∗6 H. Nasu,∗1,∗3 J. Okamoto,∗1,∗3H. Sakaguchi,∗1,∗5 S. Sakaguchi,∗1,∗9 M. Takaki,∗4 K. Taniue,∗1,∗8 H. Tokieda,∗4 M. Tsumura,∗1,∗6 T. Uesaka,∗1O. Wieland,∗12 Z.H. Yang,∗1 Y. Yamaguchi,∗4 and R. Yokoyama∗4ergy resolution of ∼ 500 keV (σ) was achieved.3)
In May 2016, we have performed an ESPRI measurement for 132 Sn (NP1512-RIBF79R1). The 132 Sn
nucleus is a ﬂag-ship doubly magic nucleus which hasa larger isospin assymmetry than that of 208 Pb. The
secondary beam including 132 Sn at 200 MeV/u wasproduced by a 238 U beam at 345 MeV/u. The intensity of the cocktail beam was very high, typically 500
kcps at F3 and 200 kcps at F7. The purity of 132 Sn wasapproximately 25%. Owing to the radiation damage
and the high-rate detections with a heavy and intense
RI beam, we used MWDCs and diamond detectors instead of the conventional PPACs, plastic scintillatorsand ion chambers. We employed a new PI techniqueof the TOF-Bρ-Bρ method which was recently developed.4)
After the BigRIPS seperater the secondary beam
was transported to F8 and ﬁnally to the SHT at F12,
where the ESPRI detectors were installed. Anotherexperimental setup of NP1312-RIBF113 using a CNS
active target (CAT) was also intalled at F8.5) Sinceboth the RIBF79 and RIBF113 experiments requireda high-intensity 132 Sn beam, the same conﬁguration of
the BigRIPS detectors was used. A newly designed
DAQ system was also introduced to achieve a highlive-time ratio even under such a high-intensity beamcondition.6)
Since 132 Sn is known to have an isomeric state of132
Sn (8+ ) with a half life of 2 μs, this state is a possible background of this measurement. Four LaBr(Ce)detectors were installed around a beam stopper to investigate the isomer tagging system by measuring γ
rays emitted from the isomeric state.7)
The experiment has been successfully performed.
Analysis to deduce the excitation energy spectrum andthe angular distribution of cross sections is now inprogress.

The neutron skin thicknesses of medium and heavy
nuclei such as 208 Pb are now known to be strongly
correlated with the nuclear matter equation of state(EOS), especially because of the density dependence of
the isospin-asymmetric term of the EOS, so-called thesymmetry energy. The symmetry energy plays an essential role in the understanding of astorphysical phenomena such as neutron star structures and supernova
explosions. Precise measurements of the neutron skin
thicknesses are very important to constrain the symmetry energy, but are very diﬃcult owing to the model
dependences.
Proton elastic scattering is a powerful tool to study
the ground-state properties of nuclei such as protonand neutron densities, radii, and neutron skin thicknesses. For stable nuclei, thus far, we succeeded in extracting the model-independent neutron density distributions and neutron skin thicknesses from proton elastic scattering at 300 MeV.1) For the systematic studyof neutron skin thicknesses with large isospin asymmetry, the measurements of proton elastic scattering fromunstable nuclei as well as stable ones are necessary. For
this purpose we launched a new experimental project
to measure the elastic scattering of protons with RI
beams (ESPRI) based on the missing mass technique.
To measure recoil protons scattered by unstable nuclei with a wide range of momentum transfers (1 ∼ 3fm−1 ), we developed a new device, recoil proton spectrometer (RPS), which consists of a 1-mm-thick solidhydrogen target (SHT),2) two multi-wire drift chambers (MWDC), two plastic scintillators, and fourteen
rods of NaI(Tl) calorimeters, which cover an angularrange from 65◦ to 85◦ in the laboratory system. At
two heavy-ion facilities, GSI and HIMAC, we have already tested the performance of RPS and successfully
performed ESPRI measurements for several unstable
nuclei such as 9 C, 20 O, and 66,70 Ni. An excitation en∗1RIKEN Nishina CenterBeihang University∗3 CYRIC, Tohoku University∗4 Center for Nuclear Study, University of Tokyo∗5 RCNP, Osaka University∗6 Department of Physics, Kyoto University∗7 Department of Physics, University of Hong Kong
∗8 Department of Applied Physics, University of Miyazaki∗9 Department of Physics, Kyushu University∗10 Department of Physics, Toho University∗11 MTA ATOMKI, Debrecen∗12 Istituto Nazionale di Fisica Nucleare sezione di MilanoReferences∗21) S. Terashima et al.:77, 024317 (2008); J.
al., Phys. Rev. C 77,
Phys. Rev. C 82,82, 044611 (2010).
Zenihiro et al.:al., Phys.
2) Y. Matsuda
Instr.
Meth.
Phys.
Res. Res.
A 643,Matsudaetetal.:al.,Nucl.
Nucl.
Instr.
Meth.
Phys.
A6 (2011).
643,6 (2011).
3) Y. Matsuda et al.:Phys. Rev. C 87,87, 034614 (2013).
al., Phys.
4) S. Ota et al.:
RIKEN Accel.
Accel. Prog.
Prog. Rep.
Rep. 49,49, 15 (2016).
al., RIKEN
5) S. Ota et al.:In thisthis report.
report.
al., In
6) H. Baba et al.:In thisthis report.
report.
al., In
7) S. Terashima et al.:al., In this report.

- 54 -