Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Structure of 18 B
S. Leblond,∗1 S. Ogoshi,∗2 R. Minakata,∗2 J. Gibelin,∗1 F. M. Marqués,∗1 N. A. Orr,∗1 Y. Kondo,∗2T. Nakamura,∗2 R. Tanaka,∗2 N. L. Achouri,∗1 T. Aumann,∗3 H. Baba,∗4 F. Delaunay,∗1 P. Doornenbal,∗4N. Fukuda,∗4 J. W. Hwang,∗5 N. Inabe,∗4 T. Isobe,∗4 D. Kameda,∗4 D. Kanno,∗2 S. Kim,∗5 N. Kobayashi,∗2T. Kobayashi,∗6 T. Kubo,∗4 J. Lee,∗4 T. Motobayashi,∗4 D. Murai,∗7 T. Murakami,∗8 K. Muto,∗6T. Nakashima,∗2 N. Nakatsuka,∗8 A. Navin,∗9 S. Nishi,∗2 H. Otsu,∗4 H. Sato,∗4 Y. Satou,∗5 Y. Shimizu,∗4H. Suzuki,∗4 K. Takahashi,∗6 H. Takeda,∗4 S. Takeuchi,∗4 Y. Togano,∗10 A. G. Tuff,∗11 M. Vandebrouck,∗12K. Yoneda∗4Counts

The investigation of the light neutron-rich dripline
nuclei, including in particular those exhibiting halos,
is a central theme of nuclear structure physics. In thepresent work a series of measurements, aimed at elucidating the structure of the two heaviest candidate
two-neutron halo systems, 19 B and 22 C1–3) , and theassociated unbound sub-systems 18 B and 21 C, the level
schemes of which are critical to the defining the 17 Bn and 20 C-n interactions for three-body models, havebeen undertaken. In addition to being of direct importance to halo physics, 18,19 B and 21,22 C are of considerable interest in terms of the evolution of shell-structure
far from stability as they span the N=14 and 16 subshell closures below doubly-magic 22,24 O.
The measurements were accomplished using theSAMURAI spectrometer4) coupled to the large area
neutron array NEBULA5) and were performed as partof the first phase of SAMURAI experiments. The analysis to date has concentrated on the fragment+neutronchannels and, in particular, 17 B+n which is known toexhibit a strongly interacting virtual s-wave thresholdstate6) . Beyond the intrinsic physics interest notedabove, a well defined threshold state provides an idealmeans to validate the calibration and analysis procedures.
In addition to populating 18 B via proton removalfrom 19 C (which should populate almost exclusively swave strength), the complementary probe of neutron
removal from a 19 B beam has been investigated. Figure 1 shows the reconstructed 17 B+n invariant mass(or relative energy) spectra for the two reactions. As
may be clearly seen the proton removal populates avery narrow threshold structure, the form of which isconsistent with the s-wave virtual state deduced by

C(19B,17B+n)

1500
1000
500
0
4000

C(19C,17B+n)

3000
2000
1000
00

123

Relative energy (MeV)

Fig. 1. Preliminary results for the 17 B+n relative energyspectra obtained for proton and neutron removal reactions at 240 MeV/nucleon.

Spyrou et al.6) . The neutron removal, however, in addition to the threshold peak shows clear evidence forthe population of a state or states in the region of 0.5–
1 MeV.
The further analysis of these preliminary results iscurrently underway as are the data sets for the analogue reactions populating 21 C.
The research descibed here forms part of the thesis work of S. Leblond who acknowledges the support
provided in terms of a 6 month RIKEN Nishina Center
IPA fellowship in 2013.

LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3Department of Physics, Tokyo Institute of Technology∗3 Institut für Kernphysik, Technische Universität Darmstadt∗4 RIKEN Nishina Center∗5 Department of Physics and Astronomy, Seoul National University∗6 Department of Physics, Tohoku University∗7 Departiment 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, Orsay∗1∗2References1) K. Tanaka et al.: Phys. Rev. Lett. 104, 062701 (2010).
2) N. Kobayashi et al.: Phys. Rev. C83, 054604 (2012).
3) L. Gaudefroy et al.: Phys. Rev. Lett. 109, 20503(2012).
4) T. Kobayashi et al.: Nucl. Instr. Meth. B 317, 294(2013).
5) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 45, 131(2012); http://be.nucl.ap.titech.ac.jp/~nebula
6) A. Spyrou et al.: Phys. Lett. B 683, 129 (2010).

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