Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Two-neutron removal reaction from 22 C†N. Kobayashi,∗1,∗2 T. Nakamura,∗1 J. A. Tostevin,∗1,∗3 Y. Kondo,∗1 N. Aoi,∗4 H. Baba,∗4 S. Deguchi,∗1J. Gibelin,∗5 M. Ishihara,∗4 Y. Kawada,∗1 T. Kubo,∗4 T. Motobayashi,∗4 T. Ohnishi,∗4 N. A. Orr,∗5 H. Otsu,∗4H. Sakurai,∗4 Y. Satou,∗6 E. C. Simpson,∗3 T. Sumikama,∗7 H. Takeda,∗4 S. Takechi,∗4 S. Takeuchi,∗4K. N. Tanaka,∗2 N. Tanaka,∗2 Y. Togano,∗4 and K. Yoneda∗4We report the ﬁrst measurement of the two-neutron
removal reaction from a 22 C secondary beam at around
240 MeV/nucleon. The experiment was performed at
the RI beam factory in 2009, as detailed in Ref.1) . The
extracted quantities are the inclusive cross section of22
C and the momentum distribution for the charged
residues of 20 C. 22 C is known to be the most neutronrich bound nucleus among C isotopes, whereas 21 C isparticle unbound. Hence, 22 C is pictured as a threebody (20 C + n + n) Borromean system, which may
be useful in deriving the two-neutron halo formationin 22 C.
There is little knowledge about 22 C. Until Gaudefroy et al. performed the mass measurement of 22 C
(i.e., S2n (22 C) = −0.14(46) MeV),2) its experimental
mass was never known. Hence, we followed the 2003
mass evaluation,3) in which the two-neutron separationenergy was 0.42(94) MeV. The ground state of 21 C wasassumed to be produced at a continuum energy of ε∗= 0.30 MeV after neutron removal with a ground-stateseparation energy S1n (22 C) of 0.70 MeV.
Based on the shell model with the WBP eﬀectiveinteraction4) in a psd-model space truncated to allow 0h̄ω and 1h̄ω excitations, three ﬁnal states of 21 C
are predicted below the 20 C ﬁrst neutron thresholdof 2.90 MeV. These states are a 1/2+
1 ground statewith C 2 S = 1.4, a 5/2+
state
at
E
=
1.11 MeV withx
1C 2 S = 4.2, and a 3/2+
stateat
E=2.19MeV withx
1
C 2 S = 0.34. Using these C 2 Ss and an eikonal reactionmodel,5,6) the theoretical inclusive cross section is calculated to be 283 mb, which is in agreement with theexperimental cross section of 266(19) mb.
The measured and theoretical inclusive 20 C parallel momentum distributions (convoluted with the experimental resolution of 27 MeV/c) are compared inFig. 1. The theoretical distribution (solid curve) corresponds to the inclusive (unbound) 21 C momentum
distribution, which is calculated as the weighted sumof the momentum distributions to the individual ﬁnalstates. Prior to this sum being calculated, the neu†
∗1∗2
∗3
∗4
∗5∗6
∗7Fig. 1. Comparison of measured and theoretical inclusive
parallel momentum distributions of 20 C, following twoneutron removal from 22 C on a carbon target at 240
MeV/nucleon. See the text for a description of thecurves.

tron emission recoil broadening for the 20 C residue isincluded for each ﬁnal state according to its ε∗ value,i.e., ε∗ = Ex + 0.30 MeV. The dashed (dot-dashed)
curve shows the contribution of knockout via the 1/2+
121
(5/2+C. Each of two states contributes al1 ) state of
most half of the inclusive one-neutron removal crosssection. The theoretical calculation is in good agreement with the experimental distribution, providing
strong support for the weakly bound ν2s1/2 characterfor the 22 C ground state. This result is consistent withthe result of the recent interaction-cross-section measurement and associated analysis presented in Ref.7) ,which is suggestive of an extended 22 C matter density.
References
1) N. Kobayashi et al., Phys. Rev. C 86, 054604 (2012)
and references therein.
2) L. Gaudefroy et al., Phys. Rev. Lett. 109, 202503(2012).
3) G. Audi, A. H. Wapstra, and C. Thibault, Nucl. Phys.
A729, 337 (2003).
4) E. K. Warburton and B. A. Brown, Phys. Rec. C 46,923 (1992).
5) P. G. Hansen and J. A. Tostevin, Annu. Rev. Nucl. Part.
Sci. 53, 219 (2003) and references therein.
6) E. C. Simpson and J. A. Tostevin, Phys. Rev. C 79,024616 (2009).
7) K. Tanaka et al. Phys. Rev. Lett. 104, 062701 (2010).

Condensed from the article in Phys. Rev. C. 86, 054604(2012)1) .
Department of Physics, Tokyo Institute of Technology
Present address: Department of Physics, University ofTokyoDepartment of Physics, University of Surrey
RIKEN Nishina Center,
LPC-Caen, ENSICAEN, IN2P3-CNRS, Université de Caen
Department of Physics and Astronomy, Seoul National University
Department of Physics, Tokyo University of Science- 27 -