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

Ⅱ-1. Nuclear Physics

Structure at and beyond the dripline in the vicinity of the
N=14 and 16 sub-shell closures
S. Leblond,∗1 Y. Kondo,∗2 J. Gibelin,∗1 F. M. Marqués,∗1 T. Nakamura,∗2 N. A. Orr,∗1 S. Ogoshi,∗2R. Minakata,∗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,∗12
and K. Yoneda∗4∗1

LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3
Department 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

22C + C → 20C + n + X

Counts / 100 keV

200

150

100

5000

800

1700

2223N +C→2045C+n+X

600

Counts / 50 keV

The structure of the neutron-rich nuclei in the vicinity of the N=14 and 16 sub-shell closures have attracted considerable interest in recent years. In particular, 19 B and 22 C represent the heaviest canididate two-neutron halo systems1–3) whilst also spanning
N=14 and 16 below doubly-magic 22,24 O. In terms ofthe most neutron-rich oxygen isotopes, the behaviour
beyond 24 O, with the ultimate goal of investigating
28O, has taken on renewed interest in the light of recent
improvements in sophisticated structure modelling4,5) .
The present report describes the status of the analysisof data acquired during a series of three experiments,
undertaken with these goals in mind, as part of theinitial phase of operation of the SAMURAI spectrometer6) coupled to the large area neutron array NEBULA7) and DALI28) NaI array.
One of the systems of particular interest in termsof modelling 22 C and the evolution of the ν2s1/2 andν1d5/2 levels is 21 C9) , the unbound sub-system of Borromean 22 C. In the present work we have attemped
to populate 21 C via both neutron and proton removalfrom 22 C and 22 N respectively, whereby it is expectedthat the different valence neutron configurations of theprojectile ground states should result in the population of correspondingly different final states. Figure 1
shows the reconstructed 20 C+n invariant mass (or decay energy) spectra for the two reactions. As may beseen, the selectivety of the two reactions is markedlydifferent, with neutron removal from 22 C exhibiting inparticular a rather narrow structure close to 1.5 MeV.
We note that in comparison to the featureless, but very
limited statistics spectrum of Ref.10) , the proton removal from 22 N shows here a very pronounced struc-500
400300200
10000

123Edecay (MeV)

45Fig. 1. Preliminary results for the 20 C+n decay energy
spectra obtained for neutron (upper) and proton (lowerpanel) removal reactions at 240 MeV/nucleon. The dotted line, normalised at high decay energy, represents anevent-mixed estimate of the corresponding uncorrelateddistribution.

∗2ture at around 1 MeV.
The further analysis of these preliminary results iscurrently underway, including the extraction of the20C+n momentum distribution for the neutron removal channel, which will shed light on the angularmomentum of the removed neutron and hence the character of the corresponding 21 C states. Analysis of thedata acquired for two-proton removal from 23 O is alsoin progress with the initial results exhibiting a decay- 42 -

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RIKEN Accel. Prog. Rep. 48 (2015)

estimated, which are in line with the results of earlier studies undertaken elsewhere employing the same
reaction12,13) .
Work is currently proceeding to finalise the analysis of the data acquired for single-proton removal from27
F to populate 26 O. To date the 24 O+2n decay energy
spectrum has been successfully reconstructed after implementation of neutron cross talk rejection procedures
based on kinematic conditions. The spectrum exhibitsa feature at around 1.3 MeV, which represents the firstobservation of an excited (presumably 2+ ) state in 26 O,
together with a very sharp peak at threshold corresponding to the ground state13,14) . A detailed analysis is being performed to determine the characteristics(energies and widths) of both states. The considerablyhigher statistics obtained in the present work should
allow for a much more precise determination of theground state energy. In the near future the data acquired for the two-proton removal from 27,28 Ne will
also be exploited in the hope of providing further insight into the structure of 25,26 O.
We note that in an earlier report15) , intensities werequoted for the secondary beams employed for the investigation of 25,26 O. For reference, for future experiments, Table 1 lists updated determinations of theintensities.
The work described here relating to the investigation of 21 C forms part of the PhD thesis of S. Leblond(LPC-Caen).

Fig. 2. Preliminary results for the 24 O+n relative energy
spectrum obtained for proton removal from 26 F at 212
MeV/nucleon. The solid curve shows the results of a fit
assuming a single resonance (see text)

Beam

Energy(MeV/nucleon)∆P/P(%)

Intensity(kpps/100pnA)

27

241
212
212
240±3±3±0.6±0.6

1.0
29
2.5
14F
Ne
26F
27Ne
28

References

Table 1. Intensities, in terms of the 48 Ca primary beam
(354 MeV/nucleon) intensity (pnA), for the secondary
beams employed to investigate 25,26 O. The momentum
aecceptance of the BigRIPS separator is noted in eachcase as is the mean secondary beam energy.

1) 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. Otsuka et al.: Phys. Rev. Lett. 105, 032501 (2010).
5) G.R. Jansen et al.: Phys. Rev. Lett. 108, 242501(2012).
6) T. Kobayashi et al.: Nucl. Instr. Meth. B 317, 294(2013).
7) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 45, 131(2012); http://be.nucl.ap.titech.ac.jp/~nebula8) S. Takeuchi et al.: Nucl. Instr. Meth. B A763, 596(2014).
9) G. Jansen et al.: Phys. Rev. Lett. 113, 142502 (2014).
10) S. Mosby et al.: Nucl. Phys. A909, 69 (2013).
11) T. Nakamura et al.: Phys. Rev. Lett. 96, 252502(2006).
12) C.R. Hoffman et al.: Phys. Rev. Lett. 100, 152502(2008).
13) C. Caeser et al.: Phys. Rev. C88, 034313 (2013).
14) E. Lunderberg et al.: Phys. Rev. Lett. 108, 142503(2012).
15) Y. Kondo et al.: RIKEN Accel. Prog. Rep. 46, 6(2013).

energy spectrum similar to that observed in singleproton removal from 22 N.
Data was also acquired for reactions of 19 B and 22 C
beams on a Pb target with the goal of deducing theE1 strength functions for the two-neutron dissociation
channel. This should enable further constraints to beplaced on the halo structure of both nuclei as well as
insight into the halo neutron spatial configurations11) .
The analysis of the data is on going, with very preliminary results indicating that both nuclei exhibit considerable near threshold strength.
In the case of the heavy oxygen isotopes, the analysis of the data obtained for proton removal from a 26 F
beam to populate 25 O is in its final stages. Figure 2
shows the preliminary results obtained for the 24 O+n
decay energy spectrum, including a comparison withthe best fit results for an =2 resonance. A resonanceenergy of 0.75 MeV and width of 0.09 MeV have been

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