RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Structure of 19 C studied by one-neutron knockout at SAMURAI
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∗2Department 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∗1- xix -600d σ/dE (mb/MeV)The structure of light nuclei near the neutron dripline has been studied extensively over the past decades.
With development in experimental technology, heavier nuclei have emerged as a major focus, and a large
number of relevant experiments for neutron-rich carbon or more massive nuclei have been conducted. The
present work aims to expore neutron-unbound states of
19
C by one-neutron knockout. A study exploiting inbeam γ-ray spectroscopy for 19 C concluded that 3/2+
11)
and 5/2+
A recent report on the1 states are bound.
inclusive measurement of one-neutron knockout crosssection, however, argued that the experimental knockout cross section from 20 C to 19 C did not support the2)In
existence of the 5/2+
1 state below the threshold.
this reserach, the invariant mass measurement in inverse kinematics was carried out, which allows us toelucidate the issue of boundedness of the 5/2+
1 state.
The experiment was performed at RIBF. BigRIPS
produced a 20 C secondary beam with an energy of280 MeV/nucleon. The beam intensity was 170 cps
at the target position with a momentum acceptanceof ∆P/P = ±3%. The beam impinged on a carbon
target with a thickness of 1.8 g/cm2 , which produced19C via the one-neutron knockout. The trajectory ofthe beam was determined with two drift chambers(BDCs). Charged fragments from the reaction wereseparated using SAMURAI3) , and their energy loss
and time of flight were measured using a hodoscope,
which is an array of plastic scintillators. Their momenta were reconstructed from their trajectories withtwo drift chambers (FDCs) placed before and after themagnet. Momenta of decay neutrons were measured

400

200

000.2

0.4

0.6

Relative Energy (MeV)

0.81Fig. 1. Relative energy spectrum for the 18 C + n unboundsystem (filled circles). The black solid line is the resultof the fit; the blue dash-dot line is the assumed background; the red dashed line is the extracted resonance.

with NEBULA using the TOF method.4) The experimental scheme is identical to that of Ref.5) .
A preliminary spectrum for the relative energy of theC(20 C,18 C + n) reaction is shown in Fig. 1. Geometrical acceptance was estimated with a Monte-Carlo simulation. The observed resonance close to the thresholdis consistent with the result of Thoennessen et al.6)A preliminary value for the cross section populating
this state was derived, which seems to corroborate theargument that the 5/2+
1 state is unbound. A quantitative analysis will be carried out to obtain (1) the position of the resonance and (2) the angular momentumof the knocked-out neutron by examining the parallel
momentum distribution of the knockout residue.
References
1) Z. Elekes et al.: Phys. Lett. B 614, 174 (2005).
2) N. Kobayashi et al.: Phys. Rev. C 86, 054604 (2012).
3) T. Kobayashi et al.: Nucl. Instr. Meth. B 317, 294(2013).
4) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 45, 131(2012).
5) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 46, 6 (2013).
6) M. Thoennessen et al.: Nucl. Phys. A 912, 1 (2013).