RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Exploring the N=16 sub-shell closure: level structure of 22 C andsearch for 21 B
N. Orr for
the SAMURAI36
and R3B-NeuLAND
Collaborations
SAMURAI36
and R3B-NeuLAND
Collaborations
The investigation of the most neutron-rich dripline
nuclei is one of the principal themes of nuclear structure physics. These studies have, however, been limited for the most part to the He, Li and Be isotopes
owing to the diﬃculty in producing beams of heavier nuclei at the dripline. With the advent of theRIBF and the coupling of the BigRIPS separator tointense 48 Ca beams, the path has been opened to exploring structure at and beyond the neutron dripline
up to Z≈12. In the present work, measurements aimed
at elucidating the evolution of shell structure in themost neutron-rich isotopes of B and C were undertaken. These measurements are a natural extension ofthe NP1106–SAMURAI04 experiment which explored
16,18
B and 21 C1) and observed unbound 20 B2) for theﬁrst time, and took advantage of recent developmentsin the 48 Ca primary beam intensity and the enhanced
neutron detection capabities oﬀered by the couplingof the large-area fast neutron arrays NeuLAND3) andNEBULA4) .
The principal goal of the NP1512–SAMURAI36 experiment was to investigate the level structure of 22 C
and, in particular, locate the ﬁrst 2+ state. In parallel,
an attempt was made to extend our recent observationof 20 B to search for 21 B. The nuclei in question arelocated in a region of considerable interest in termsof the evolution of shell-structure far from stability.
Speciﬁcally, they lie at the N=16 sub-shell closure below doubly-magic 24 O. In terms of the evolution of thesingle-particle orbits, the O isotopes exhibit magicity
at N=14 and 16. When protons are removed, the N=14
gap is predicted to disappear and the neutron s1/2 andd5/2 single-particle orbitals become degenerate5) . Suchbehaviour is expected to be at the origin of the formation of the s-wave halo neutron conﬁguration in 22 C6) ,rather than the naive shell model d-wave valence neutron occupation. Moreover, spectroscopy of 21 C, performed within the context of SAMURAI04, indicatesthat the physical 5/2+ and 1/2+ levels are inverted1) .
The measurements were accomplished using theSAMURAI facility7) , which comprised the superconducting 7 Tm spectrometer coupled to the neutron array NEBULA, which was complemented by elements ofNeuLAND (four so-called “double-planes” each composed of two crossed layers of 250 cm long 5×5 cm2scintillator modules). In addition, the newly completed CATANA CsI(Na) γ-ray multi-detector8) wasmounted around the secondary target position. Thesetup was identical to that employed in the SAMURAI27 experiment9) which was run in tandem with thepresent measurements. It is worthwhile noting that

this experiment represented the beam commissioningof CATANA and the on-line analysis of well-know γrays, such as the 1.6 MeV line arising from the decayof the 18 C 2+
1 state, indicated that the detector andelectronics performed as expected.
Primary beam intensities as high as some 800 pnA
were provided and allowed secondary beam rates ofup to ∼700 pps of 23 N (over an order of magnitude higher than available during SAMURAI04) at∼270 MeV/nucleon to be delivered by the RIBF. Single and two-proton removal reactions on the 2 g/cm2
secondary carbon target will be analysed in order to establish the level structure of 22 C and search for 21 B. As
22C was also present in the secondary beam (∼35 pps),
eﬀorts will be also made to investigate the populationof the excited states via inelastic scattering. In thecase of both the 22 C exited states and 21 B, beam velocity “core” fragments (20 C and 19 B) plus two-neutron
events will be employed to reconstruct the corresponding invariant mass spectra from the measured momenta.
References
1) S. Leblond, PhD Thesis, Université de Caen (2015);
https://tel.achives-ouvertes.fr/tel-01289381.
2) F.Miguel Marqués et al.: JPS Conf. Proc. 6, 020002(2015).
3) http://www.fair-center.de/ﬁleadmin/fair/experiments/
NUSTAR/Pdf/TDRs/NeuLAND-TDR-Web.pdf
4) T. Nakamura, Y. Kondo: Nucl. Instr. Meth. B376 1(2015).
5) M. Stanoiu et al.: Phys. Rev. C78, 034315 (2010).
6) Y. Togano et al.: Phys. Lett. B761 412 (2016).
7) T. Kobayashi et al.: Nucl. Instr. Meth. B317, 294(2013).
8) Y. Togano et al.: RIKEN Accel. Prog. Rep. 48, 207(2015).
9) T. Tomai et al.: in this report.

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