Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

One-neutron knockout reaction of 17 C on a hydrogen target at 70
MeV/nucleon†
Y. Satou,∗1 J.W. Hwang,∗1 S. Kim,∗1 K. Tshoo,∗1 S. Choi,∗1 T. Nakamura,∗2 Y. Kondo,∗2 N. Matsui,∗2Y. Hashimoto,∗2 T. Nakabayashi,∗2 T. Okumura,∗2 M. Shinohara,∗2 N. Fukuda,∗3 T. Sugimoto,∗3 H. Otsu,∗3
Y. Togano,∗3 T. Motobayashi,∗3 H. Sakurai,∗3 Y. Yanagisawa,∗3 N. Aoi,∗3 S. Takeuchi,∗3 T. Gomi,∗3M. Ishihara,∗3 S. Kawai,∗4 H.J. Ong,∗5 T.K. Onishi,∗5 S. Shimoura,∗6 M. Tamaki,∗6 T. Kobayashi,∗7Y. Matsuda,∗7 N. Endo,∗7 and M. Kitayama∗7
Much of our knowledge on the quantum nature ofatomic nuclei comes from the studies of nuclear reactions. Among the various collision processes, thenucleon knockout reaction is recognized as one of themost sensitive tools for spectroscopy, especially of unstable nuclei. The knockout residue produced by removing a nucleon from a fast-moving beam particle iseﬃciently observed in inverse kinematics by a detector placed in the forward hemisphere. The removed
nucleon(s) will be selected democratically from thevalence space, allowing the states with unique, often
rarely accessible, conﬁgurations to be populated in thisprocess. The ﬁnal state in the residue is identiﬁed bytagging de-excitation γ rays and by observing decay
neutrons and constructing the invariant mass. For onenucleon knockout case, the momentum spread of theresidue reﬂects the Fermi motion of the nucleon suddenly removed, and is sensitive to its orbital angularmomentum (the l value). The cross sections leading
to the individual ﬁnal states relate to the occupancy
of single-particle orbits, providing a link to understandthe details of the nuclear structure.
This study aims at exploring the unbound states in16
C through an application of the one-neutron knockout technique to a 17 C beam. This is done by focusing
on searching the lowest-lying cross-shell transitions,whose location reﬂects the shell gap between the p andsd orbits. The experiment was performed at the RIPS
facility of RIKEN using the setup given in Refs.1,2) .
The 17 C beam was produced from a 110-MeV/nucleon
22Ne beam, which impinged on a Be target. The secondary target was pure liquid hydrogen contained ina cylindrical cell. The average energy of 17 C at themiddle of the target was 70 MeV/nucleon. The target
was surrounded by a NaI(Tl) scintillator array. Thefragment was bent by a dipole magnet behind the target, and was detected by a plastic counter hodoscope.
The neutrons were detected by plastic scintillator arrays placed ∼5 m downstream from the target. The†

∗1
∗2
∗3∗4
∗5
∗6∗7Fig. 1. Relative energy spectra for the (a) 1 H(17 C,15 C+n)and (b) 1 H(17 C,15 C(5/2+ ; 0.74 MeV)+n) reactions.

relative energy (Erel ) of the ﬁnal system was calculated
from the momentum vectors of the charged fragment
and the neutron.
Fig. 1 shows the Erel spectra for the (a)1H(17 C,15 C+n) and (b) 1 H(17 C,15 C(5/2+ ; 0.74 MeV)+n)reactions. Shown in the inset of Fig. 1 (b) is the energy spectrum for γ rays emitted from 15 C. Fig. 1 (a)was used in a ﬁtting analysis to extract the resonanceparameters.
Two new states at 5.45(1) and 6.28(2) MeV were
populated together with a known state at 6.11 MeV.
For the 5.45-MeV state, an attempt was made to deduce the l value of the knocked-out neutron from thep|| distribution associated with the unbound residue.
This, together with a comparison in terms of the measured and calculated knockout cross sections, has led toa spin-parity assignment of 2− for this state. Possible
spins and parities have been suggested for the otherstates, bringing about an advanced understanding ofthe level scheme of 16 C. The energy of the ﬁrst 2−
state was adequately reproduced by the standard shellmodel calculation using the WBT interaction without
invoking modiﬁcations to the residual interaction.

Condensed from the article in Phys. Lett. B 728, 462 (2014)
Department of Physics and Astronomy, Seoul National University
Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center
Department of Physics, Rikkyo University
Department of Physics, University of Tokyo
Center for Nuclear Study, University of Tokyo
Department of Physics, Tohoku University

References
1) Y. Satou, et al.: Phys. Lett. B 660, 320 (2008).
2) K. Tshoo, et al.: Phys. Rev. Lett. 109, 022501 (2012).

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