Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Structure of Borromean nuclei around N=16 and 20 shells
R. Kanungo,*1 S. Bagchi,*1,*2,*3 Y. Tanaka,*2,*3 H. Geissel,*2,*3 D. Ahn,*4 H. Baba,*4 K. Behr,*2 F. Browne,*5 S. Chen,*5M. L. Cortés,*5 P. Doornenbal,*5 A. Estrade,*4,*6 N. Fukuda,*4 E. Haettner,*2, M. Holl,*1,*7 K. Itahashi,*8 N. Iwasa,*4,*9S. Kaur,*1,*10, S. Matsumoto,*11 S. Momiyama,*12 I. Murray,*5,*13 T. Nakamura,*14 H.J. Ong,*15 S. Paschalis,*16A. Prochazka,*2 C. Scheidenberger,*2,*3 P. Schrock,*17 Y. Shimizu,*4 D. Steppenbeck,*4,*17 D. Suzuki,*5 H. Suzuki,*4S. Takeuchi,*14 M. Takechi,*18 H. Takeda,*4 R. Taniuchi,*12 K. Wimmer,*12 and K. Yoshida*4
information led to an event by event identification of 29F asshown in Fig. 1. A carbon reaction target placed at the F8
focus was surrounded by the DALI2 gamma detectorarray.9) Gamma rays from the de-excitation of thetwo-neutron removal fragment were detected in coincidencewith 27F.
The ZDS was used to transport and identify thefragments or the beam at the final achromatic focus, F11.
The time of flight with scintillators placed at F11 and F8
together with position information is used to determine themomentum distribution from the two-neutron removal
process. At F11 a second carbon reaction target was placed
for measuring the charge changing cross section using multi
sampling ionization chambers10) placed before and after thissecond reaction target. The measured interaction crosssection is expected provide the matter radius of 29F while
the charge-changing cross section will yield a measure of its
proton radius. Charge-changing cross sections weremeasured for the drip-line C and B isotopes as well.

Nuclei near the neutron drip line provide theopportunity to investigate unusual properties that depend on
isospin. The emergence of nuclear halos1),2) and changes ofnuclear shell structure3)-6) are some of the dramatic
unexpected effects resulting from the large neutron/proton
asymmetry. Nuclear halos arise from the one- or twooutermost neutrons forming a low-density extended neutron
surface. Identifying and characterizing the neutron halos
require knowledge of their matter and proton radii.
Information on occupancy of nuclear orbitals sheds light on
the evolution of nuclear shells. Two-neutron halos in
Borromean nuclei have been identified in the p-sd shell in
6
He, 11Li, 14Be, 17,19B and 22C but its occurrence in theconventional pf-shell is still unknown. The breakdown of
the N=20 shell closure has been observed in Mg-Ne
isotopes, and one-neutron halo formation was found in31Ne.7)
In a recent experiment performed at the BigRIPS8) andZeroDegree Spectrometer (ZDS) facilities at the RIKEN RI
Beam Factory we aimed to investigate the structure of theBorromean N=20 drip line nucleus, 29F. 29F nuclei wereproduced via the fragmentation of a 48Ca primary beamwith a Be production target. The BigRIPS fragment
separator and particle detectors were used to separate 29F
from the contaminants. The energy loss in the ionizationchamber placed at the F7 achromatic focus provided Z
identification. Time of flight was measured with scintillator
detectors placed at F3 and F7 foci. Position sensitive PPAC
detectors were used to determine the magnetic rigidity. This䠆1Astronomy and Physics Department, Saint Mary’s University

䠆2GSI Helmholtz Center䠆3Physics Department, Justus-Liebig University

䠆4

Research Instruments Group, RIKEN

䠆5Radioactive Isotope Physics Laboratory, RIKEN

䠆6Central Michigan University

References

䠆7

TRIUMF

䠆8

Advanced Meson Science Laboratory, RIKEN

䠆9Physics Department, Tohoku University

1) I. Tanihata et al., Phys. Rev. Lett. 55, 2676 (1985).
2) P. G. Hansen, B. Jonson, Euro. Phys. Lett. 4, 409 (1987).
3) T. Motobayashi et al., Phys. Lett. B 346, 9 (1995).
4) R. Kanungo, Phys. Scr., T152, 014002 (2013).
5) O. Sorlin, M. –G. Porquet, Phys. Scr. T152, 014003 (2013).
6) D. Steppenbeck et al., Nature 502, 207 (2013).
7) T. Nakamura et al., Phys. Rev. Lett. 103, 262501 (2009).
8) T. Kubo et al.: Nucl. Instrm. Meth. B 204, 97 (2003).
9) S. Takeuchi et al., Nucl. Instrum. & Meth. A 763, 596 (2014).
10) A. Stolz et al., Phys. Rev. C 65, 064603 (2002).

䠆10

Physics Department, Dalhousie University

䠆11Physics Department, Kyoto University

䠆12Physics Department, University of Tokyo

䠆13IPN, Université Paris-Sud, Université Paris-Saclay

䠆14Physics Department, Tokyo Institute of Technology

䠆15Research Center for Nuclear Physics

䠆16Physics Department, University of York

䠆17

Center for Nuclear Study

䠆18

Physics Department, Niigata University

Fig. 1. Particle identification spectrum for detection of 29F.

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