RIKEN Accel. Prog. Rep. 50 (2017)

Ⅰ. HIGHLIGHTS OF THE YEAR

Nucleus 26 O: A barely unbound system beyond the drip line†∗1,∗2 R.
∗1,∗2 S.
∗1,∗2 N. A. Orr,∗3∗3 N. L. Achouri,∗3Y.
Tanaka,∗1,∗2
R. Minakata,
Minakata,∗1,∗2S. Ogoshi,
Ogoshi,∗1,∗2Y. Kondo,
Kondo∗1,∗2 T. Nakamura,∗1,∗2 R. Tanaka,
∗4,∗5
∗2
∗3
∗2
∗2
∗3
T. Aumann,
H. Baba, F. Delaunay, P. Doornenbal, N. Fukuda, J. Gibelin, J. W. Hwang,∗6∗2N. Inabe, T. Isobe,∗2 D. Kameda,∗2 D. Kanno,∗1,∗2 S. Kim,∗6 N. Kobayashi,∗1,∗2 T. Kobayashi,∗7,∗2T. Kubo,∗2 S. Leblond,∗3 J. Lee,∗2 F. M. Marqués,∗3 T. Motobayashi,∗2 D. Murai,∗8 T. Murakami,∗9 K. Muto,∗7T. Nakashima,∗1,∗2 N. Nakatsuka,∗9,∗2 A. Navin,∗10,∗13 S. Nishi,∗1,∗2 H. Otsu,∗2 H. Sato,∗2 Y. Satou,∗6Y. Shimizu,∗2 H. Suzuki,∗2 K. Takahashi,∗7 H. Takeda,∗2 S. Takeuchi,∗2 Y. Togano,∗5,∗14 A. G. Tuﬀ,∗11M. Vandebrouck,∗12 and K. Yoneda∗2The unbound nucleus 26 O has been investigated using invariant-mass spectroscopy following aone-proton removal reaction from a 27 F beam at
201 MeV/nucleon. The ground state of 26 O has recently been found to be barely unbound with respect
to two-neutron emission – by 53 keV (1σ upper limit)
in an intermediate energy reaction study1,2) and by
120 keV (upper limit with a 95% conﬁdence level) at
high energies.3) The 2+
1 state has yet, however, to belocated. It may be noted that Ref.3) claimed the existence of a level at 4.2 MeV, which could be a protonhole state, although the statistics were limited.
The 27 F secondary beam was produced byprojectile fragmentation of 48 Ca (∼140 pnA) at
345 MeV/nucleon. It was puriﬁed using BigRIPS andtransported to a secondary target of carbon (thickness
1.8 g/cm2 ). The decay products, 24 O and neutron(s),were measured in coincidence using the spectrometer
SAMURAI.4) In addition to the measurements madeof 26 O with the 27 F beam, data were also taken for
one-proton removal from a 26 F beam leading to 25 O.
The obtained relative energy spectrum of 25 O wasﬁtted with a d-wave Breit-Wigner line shape, following
the prescription of Ref.3) , after taking into account theexperimental response function. In practice this wasdone using a complete simulation of the setup basedon GEANT4 and employing the QGSP INCLXX physics
model for the neutron interactions in NEBULA. A resonance energy of 749(10) keV and a width of 88(6) keV
were deduced.

26

Counts/50keV

O(0+)

50

26

O(2+)

00

123
4E fnn (MeV)

56Fig. 1. Three-body decay energy spectrum of 26 O reconstructed from 24 O and two neutrons in the one-proton
removal reaction from 27 F.

Turning now to 26 O, the ground-state resonance wasfound to lie only 18±3(stat)±4(syst) keV above thethreshold (Fig. 1). In addition, a higher level, which
is most likely the ﬁrst 2+ state, was observed for theﬁrst time at 1.28+0.11−0.08 MeV. On the other hand, noresonance-like structure was observed at higher energies as reported in Ref.3) . Comparison of the 26 O(2+1)
energy with theory suggests that three-nucleon forces,
pf -shell intruder conﬁgurations, as well as an appropriate treatment of the continuum are key elements
to understanding the structure of the heaviest oxygenisotopes.

†Condensed from the article in Phys. Rev. Lett. 116, 102503(2016)∗1 Department of Physics, Tokyo Institute of Technology∗2 RIKEN Nishina Center∗3 LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3∗4 Institut für Kernphysik, Technische Universität Darmstadt∗5 ExtreMe Matter Institute (EMMI) and Research Division,GSI∗6 Department of Physics and Astronomy, Seoul National University∗7 Department of Physics, Tohoku University∗8 Departiment of Physics, Rikkyo University∗9 Department of Physics, Kyoto University∗10 GANIL, CEA/DSM-CNRS/IN2P3∗11 Department of Physics, University of York∗12 Institut de Physique Nucléaire, Université Paris-Sud,IN2P3CNRS∗13 Present address: GANIL CEA/DRF-CNRS/IN2P3∗14 Present address: Department of Physics, Tokyo Institute ofTechnology

References
1) Z. Kohley et al.: Phys. Rev. C 91, 034323 (2015).
2) E. Lunderberg et al.: Phys. Rev. Lett. 108, 142503(2012).
3) C. Caesar et al.: Phys. Rev. C 88, 034313 (2013).
4) T. Kobayashi et al.: Nucl. Instrum. Methods. Phys. Res.
Sect. B 317, 294 (2013).
5) T. Nakamura, Y. Kondo et al.: Nucl. Instrum. Methods.
Phys. Res. Sect. B 376, 156 (2015).
6) C. R. Hoﬀman et al.: Phys. Rev. Lett. 100, 152502(2008).

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