Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Invariant mass spectroscopy of 17C at SAMURAI
S. Kim,∗1,∗2 Y. Satou,∗1 J.W. Hwang,∗1,∗2 T. Nakamura,∗2,∗3 N. A. Orr,∗4 Y. Kondo,∗2,∗3 J. Gibelin,∗4
N. Kobayashi,∗2,∗3 R. Tanaka,∗2,∗3 R. Minakata,∗2,∗3 S. Ogoshi,∗2,∗3 S. Nishi,∗2,∗3 D. Kanno,∗2,∗3T. Nakashima,∗2,∗3 N. L. Achouri,∗4 T. Aumann,∗5 H. Baba,∗2 F. Delaunay,∗4 P. Doornenbal,∗2 N. Fukuda,∗2N. Inabe,∗2 T. Isobe,∗2 D. Kameda,∗2 T. Kobayashi,∗6,∗2 T. Kubo,∗2 S. Leblond,∗4 J. Lee,∗2 F. M. Marqués,∗4T. Motobayashi,∗2 D. Murai,∗7 T. Murakami,∗8 K. Muto,∗6 N. Nakatsuka,∗8 A. Navin,∗9 H. Otsu,∗2 H. Sato,∗2Y. Shimizu,∗2 H. Suzuki,∗2 K. Takahashi,∗6 H. Takeda,∗2 S. Takeuchi,∗2 Y. Togano,∗10,∗13 A. G. Tuff,∗11M. Vandebrouck,∗12 and K. Yoneda∗2Many properties of nuclei away from the β-stabilityline are important to better understand nuclear processes that control stellar nucleosynthesis and energybalance. These nuclei often exhibit exotic structures.
For example, the appearance of anomalous parity intruder states at a low excitation energy region provides
evidence for the shell-gap quenching and/or large nuclear deformation.
The present study focuses on low-lying negative parity states in 17 C above the neutron decay threshold.
Two β-delayed neutron emission measurements of 17 B
have reported such states: Raimann et al.1) indicatedstates at 2.25(2), 2.64(2), 3.82(5), and 1.18(1) MeV
with no definite spin-parity (J π ) assignment. Achieving higher sensitivity for the β-n-γ coincidence yield,
Ueno et al.2) was successful to locate states at 2.71(2),3.93(2), and 4.05(2) MeV with the suggested J π valuesof 1/2−, 3/2− , and (5/2− ), respectively. This study
aimed to populate these states and to examine theirproperties by the one-neutron knockout reaction of anenergetic beam.
The measurement was performed using the SAMURAI spectrometer3) during the first physics run of
the apparatus. A beam of 18 C at approximately 250
MeV/nucleon provided by BigRIPS at RIBF impinged
on a carbon target with a thickness of 1.8 g/cm2 . The
unbound states in 17 C produced by the one-neutron removal processes subsequently decayed into a 16 C fragment and a neutron. These decay products were detected in coincidence. There should be some background events by the neutrons arising from the oneneutron removal processes; however, such backgroundis expected to be relatively featureless and would notdσ/dE (mb/MeV)

2520

15

1050

1

2345

67

Relative Energy (MeV)

Fig. 1. Preliminary relative energy spectrum of the 16 C +n
unbound system.

affect the resonance. The momentum vector of the 16 C
fragment was determined by (1) position informationin the two drift chambers (FDCs) placed at the entrance and exit of the SAMURAI dipole magnet and(2) energy loss and timing information in the plastic
scintillator hodoscope (HODF). The momentum vector of the neutron was determined using the position
and timing information in the plastic scintillator neutron hodoscope (NEBULA). The energy spectrum of17
C was reconstructed using the invariant mass method
involving the momentum vectors of the fragment andneutron.
A preliminary relative energy spectrum for the 16 C
+ n system is shown in Fig.1. A clear peak structure
was observed at 2 MeV in relative energy, which corresponds to the excitation energy of Ex = 2.7 MeV. This
energy is close to the energy of the first 1/2− state at
Ex = 2.71(2) MeV reported in Ueno et al.2) . To examine the identity of these two states, an analysis isbeing carried out (1) to compare the populating crosssection with the theoretical value based on the corresponding shell-model spectroscopic factor together
with the Glauber model and (2) to extract the orbital
angular momentum of the knocked-out neutron fromthe parallel momentum distribution. A search for theother reported states will also be performed.

∗1Department of Physics and Astronomy, Seoul National University∗2 RIKEN Nishina Center∗3 Department of Physics, Tokyo Institute of Technology∗4 LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3∗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 Department of Physics, Tokyo Institute of Technology

References
1) G. Raimann et al.: Phys. Rev. C 53, 453 (1996).
2) H. Ueno et al.: Phys. Rev. C 87, 034316 (2013).
3) K. Yoneda et al.: RIKEN Accel. Prog. Rep. 45, i (2012).

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