RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-1. Nuclear Physics

Study of neutron-unbound states of 19 C at SAMURAI

The neutron-unbound region of the nuclear levelstructure has been investigated extensively in recentyears, especially for nuclei near the neutron drip line.
In this report, we provide the result of the experiment
to study neutron-unbound states in 19 C by usin a oneneutron knockout reaction. The report contains theoutcome of the succeeding research following the report in the previous year.1)
The experiment was performed at the RIBF facility.
A 20 C secondary beam at 280 MeV/nucleon separated
by BigRIPS impinged on a 1.8 g/cm2 -thick carbon target. Neutron-unbound states of 19 C produced by oneneutron knockout decays into a 18 C ion and a neutron.
18
C was identified and its momentum was measured
using the SAMURAI spectrometer2) with the help of
its experimental appratus. Arrays of plastic scintillators for neutron detection (NEBULA) were used tomeasure the momenta of decay neutrons.3) The experimental setup is identical to that of Ref.4) .
A preliminary relative energy spectrum for the system of 18 C + n is shown in Fig. 1 with the result offitting analysis. Acceptance was corrected for considering the effect of the geometry of the detector system, which was estimated using a Monte-Carlo simulation. Error bars are statistical ones. Briet-Wigner
shape functions were used as response functions generated using a simulation code taking the detector resolution into account. The background consists of a
Boltzmann-type distribution for non-resonant continuum and a distribution for uncorrelated 18 C + n pairsreconstructed using the event mixing technique. A different scale from 0.5 MeV is taken in the y-axis be∗1Department of Physics and Astronomy, Seoul National University∗2 RIKEN Nishina Center∗3 LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3∗4 Department of Physics, Tokyo Institute of Technology∗5 Institut für Kernphysik, Technische Universität Darmstadt∗6 Department of Physics, Tohoku University∗7 Department of Physics, Rikkyo University∗8 Department of Physics, Kyoto University∗9 GANIL, CEA/DSM-CNRS/IN2P3∗10 ExtreMe Matter Institute (EMMI) and Research Division,GSI∗11 Department of Physics, University of York∗12 Institut de Physique Nucléaire, Université Paris-Sud,
IN2P3-CNRS∗13 Present address:Department of Physics, Tokyo Institute ofTechnology

dσ/dErel (mb/MeV)

J.W. Hwang,∗1,∗2 Y. Satou,∗1 S. Kim,∗1,∗2 N. A. Orr,∗3 T. Nakamura,∗4,∗2 Y. Kondo,∗4,∗2 N. L. Achouri,∗3T. Aumann,∗5 H. Baba,∗2 F. Delaunay,∗3 P. Doornenbal,∗2 N. Fukuda,∗2 J. Gibelin,∗3 N. Inabe,∗2 T. Isobe,∗2D. Kameda,∗2 D. Kanno,∗4,∗2 N. Kobayashi,∗4,∗2 T. Kobayashi,∗6,∗2 T. Kubo,∗2 S. Leblond,∗3 J. Lee,∗2F. M. Marques,∗3 R. Minakata,∗4,∗2 T. Motobayashi,∗2 D. Murai,∗7 T. Murakami,∗8 K. Muto,∗6 N. Nakatsuka,∗8T. Nakashima,∗4,∗2 A. Navin,∗9 S. Nishi,∗4,∗2 S. Ogoshi,∗4,∗2 H. Otsu,∗2 H. Sato,∗2 Y. Shimizu,∗2 H. Suzuki,∗2K. Takahashi,∗6 H. Takeda,∗2 S. Takeuchi,∗2 R. Tanaka,∗4,∗2 Y. Togano,∗10,∗13 A. G. Tuff,∗11M. Vandebrouck,∗12 and K. Yoneda∗2600

50x 12404003020200

10000.5

11.5

22.5

33.5
4Erel (MeV)

0Fig. 1. Preliminary relative energy spectrum for the system of 18 C + n (filled circles). The black solid line is
the result of the fit; the dashed lines are for the two resonances; the dot-dashed line is the assumed background.
(see details in the text)

cause of the significant threshold resonance. Three resonances were observed at Erel = 36±1 keV, 0.84±0.02
MeV and 2.33 ± 0.01 MeV, corresponding to the statesof Ex = 0.62(9) MeV, 1.42(9) MeV, and 2.91(9) MeV,
respectively, based on the formula: Ex = Erel + Sn ,where Sn is the one-neutron separation energy of 19 C
(0.58(9) MeV5) ). Note that all values are preliminary.
While the first and second resonances are consistent+with the 5/2+
1 and 5/22 states reported by the knock6)
out experiment and the inelastic scattering measurement7) , respectively, the third one was observed for thefirst time in the present study. A next step of analysis
is planned to obtain the parallel momentum distribution of the knockout residue to investigate spins andparities of the observed states.
References
1) J. W. Hwang et al.: RIKEN Accel. Prog. Rep. 47, xix(2014).
2) T. Kobayashi et al.: Nucl. Instr. Meth. B 317, 294(2013).
3) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 45, 131(2012).
4) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 46, 6 (2013).
5) M. Wang et al: Chin. Phys. C 36, 1603 (2012).
6) M. Thoennessen et al.: Nucl. Phys. A 912, 1 (2013).
7) Y. Satou et al.: Phys. Lett. B 660, 320 (2008)- 44 -完全版2014_本文.indd 44

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