Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Study of unbound oxygen isotopes 25 O and 26 O using SAMURAIY. Kondo,∗1,∗2 T. Nakamura,∗1,∗2 N. L. Achouri,∗3 T. Aumann,∗4 H. Baba,∗2 F. Delaunay,∗3 P. Doornenbal,∗2N. Fukuda,∗2 J. Gibelin,∗3 J. W. Hwang,∗5 N. Inabe,∗2 T. Isobe,∗2 D. Kameda,∗2 D. Kanno,∗1,∗2 S. Kim,∗5
N. Kobayashi,∗1,∗2 T. Kobayashi,∗6,∗2 T. Kubo,∗2 S. Leblond,∗3 J. Lee,∗2 F. M. Marqués,∗3 R. Minakata,∗1,∗2T. Motobayashi,∗2 D. Murai,∗7 T. Murakami,∗8 K. Muto,∗6 N. Nakatsuka,∗8 T. Nakashima,∗1,∗2 A. Navin,∗9S. Nishi,∗1,∗2 S. Ogoshi,∗1,∗2 N. A. Orr,∗3 H. Otsu,∗2 H. Sato,∗2 Y. Satou,∗5 Y. Shimizu,∗2 H. Suzuki,∗2K. Takahashi,∗6 H. Takeda,∗2 S. Takeuchi,∗2 R. Tanaka,∗1,∗2 Y. Togano,∗10,∗13 A. G. Tuff,∗11M. Vandebrouck,∗12 and K. Yoneda∗2Unbound states of the neutron-rich oxygen isotopes
O and 26 O have been studied by the invariant-mass
method by using SAMURAI1) with the aim to elucidate the mechanism of the neutron drip line anomalyin oxygen and fluorine isotopes. Another interesting
topic is the possible two-neutron radioactivity of the26
O ground state, predicted by a theoretical study.2)
Experimentally, only the upper limit of the groundstate energy3,4) and lifetime with a large error5) arecurrently available.
Details of the experimental setup are described in
our previous report.6) Figure 1 shows a mass identification plot of outgoing Z = 8 charged particles observed in the breakup of 27 F on a carbon target. Particle identification is performed by the Bρ-∆E-TOFtechnique. The magnetic rigidity Bρ is determinedby the positions and angles at the entrance and exitof the SAMURAI magnet measured by means of theMWDCs (BDC1,2 and FDC1,2). Combining the Bρ
value with energy loss ∆E and TOF measured by aplastic scintillator hodoscope (HODF), outgoing particles can be clearly identified. The mass resolution∆A = 0.18 (FWHM), corresponding to 13σ separation, is achieved for 24 O.
Figure 2 shows a preliminary decay energy spectrumof 24 O+n observed in the breakup of 27 F. The sharp
peak near the neutron decay threshold corresponds tothe 26 O ground state and the peak at approximately
0.8 MeV corresponds to the ground-state resonance of25
O. Since the obtained statistics is much larger thanthat obtained in the previous experiments,3,4) a better constraint on the 26 O ground-state energy can beobtained. Analysis is currently in progress.

25Fig. 1. Mass spectrum of outgoing Z = 8 particles in thebreakup of 27 F.

Fig. 2. Decay energy spectrum of 24 O+n in the breakup of27F.

∗1Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center∗3 LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3∗4 Institut für Kernphysik, Technische Universität Darmstadt∗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∗13 Present address:Department of Physics, Tokyo Institute ofTechnology∗2References
1) T. Kobayashi et al.: Nucl. Instr. Meth. B. 317, 294(2013).
2) L. V. Grigorenko et al.: Phys. Rev. C. 84, 021303(2011).
3) E. Lunderberg et al.: Phys. Rev. Lett. 108, 142503(2012).
4) C. Caesar et al.: Phys. Rev. C. 88, 034313 (2013).
5) Z. Kohly et al.: Phys. Rev. Lett. 110, 152501 (2013).
6) Y. Kondo et al.: RIKEN. Prog. Accel. Rep. 46, 6(2013).

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