RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-1. Nuclear Physics

Study of Neutron-Proton Correlations & 3N Forces in 12 C
H. N. Liu,∗1,∗2 J. Lee,∗1,∗3 H. Otsu,∗1 Y. Kondo,∗4 S. Takeuchi,∗1 Y. Togano,∗4 H. Baba,∗1 N. Chiga,∗5P. Doornenbal,∗1 N. Fukuda,∗1 J. Gibelin,∗6 J. Hwang,∗7 T. Isobe,∗1 T. Kawabata,∗8 N. Kobayashi,∗9T. Kobayashi,∗5 K. Kusaka,∗1 S. Leblond,∗6 Y. Matsuda,∗8,∗13 M. Matsushita,∗10 R. Minakata,∗4T. Motobayashi,∗1 T. Murakami,∗5 K. Muto,∗5 T. Nakamura,∗4 N. Nakatsuka,∗8 A. Navin,∗11 M. Nishimura,∗1S. Ogoshi,∗4 J. Ohnishi,∗1 S. Sakaguchi,∗12 H. Sakurai,∗1 M. Sasano,∗1 H. Sato,∗1 Y. Satou,∗7 K. Sekiguchi,∗5Y. Shimizu,∗1 T. Sumikama,∗5 Y. Wada,∗5 K. Yoneda,∗1 and J. Zenihiro∗1∗1RIKEN Nishina Center
School of Physics and State Key Laboratory of Nuclear
Physics and Technology, Peking University∗3 Department of Physics, The University of Hong Kong
∗4 Department of Physics, Tokyo Institute of Technology∗5 Department of Physics, Tohoku University∗6 LPC, Caen
∗7 Department of Physics, Seoul National University∗8 Department of Physics, Kyoto University∗9 Department of Physics, The University of Tokyo∗10 CNS, The University of Tokyo∗11 GANIL, CEA/DSM-CNRS/IN2P3∗12 Department of Physics, Kyushu University∗13 CYRIC, Tohoku University∗27h26Charge Number Z

The study of neutron-proton (np) correlations in nuclei is very important to understand the nuclear structure. Direct two-nucleon knockout reactions oﬀer apowerful tool for quantitative measurements of the np correlations in N = Z nuclei.1,2) The measured inclusive two-nucleon knockout cross sections3,4) show
signiﬁcant enhancement of np (T = 0 & 1) over nn
and pp (T = 1) correlations in 12 C. The shell-model
calculation for the two-nucleon overlaps within the pshell can reproduce the inclusive cross sections for likenucleon pair removal, but underestimates the np-pairremoval cross section by approximately a factor of two.1) This discrepancy implies insuﬃcient descriptionof the T = 0 np interactions in the shell-model wave
functions. A recent calculation using the no-core shellmodel that exploits modern chiral eﬀective ﬁeld theory NN+3N interactions2) suggests that the ﬁnal-stateexclusive np-knockout cross sections from 12 C to theT = 0 states can provide an immediate test of the np
correlations (particularly in their T = 0 component) and three-nucleon forces. To investigate the natureof these forces, we therefore measured γ-residue coincidence to extract the ﬁnal-state-exclusive np- andpp-removal cross sections from 12 C to 10 B and 10 Be.
The experiment was carried out at RIBF. The secondary 12 C beam (presently not available as the primary beam for SRC-use experiments) was produced byfragmentation of an 18 O primary beam at 250 MeV/uusing a 5-mm thick Be target. The ions of interest
were selected using the BigRIPS fragment separator5)
by measuring the energy loss (∆E ) and time of ﬂight(TOF) with plastic scintillators at the intermediate foci F3 and F13. The incident angle and position of thebeam on a 1.879-g/cm2 thick Be target were determined by two drift chambers, BDC1 and BDC2. The
target was surrounded by the DALI2 γ-ray spectrom-10102B105Be1043
1.6

1.8

2

2.2

Mass-to-Charge Ratio A/Z

2.4

2.6

1Fig. 1. Particle identification after the secondary target.

eter.6) One hundred and ﬁfty-nine crystals of DALI2 were employed with an azimuthal angular coverage
from 25◦ to 154◦ . The mid-target energy of 12 C wasapproximately 190 MeV/u. Reaction products weretransported to the SAMURAI spectrometer7) and identiﬁed with the Bρ-∆E -TOF method. The magnetic rigidity Bρ was determined from the measured
positions at the forward drift chambers, FDC1 andFDC2. ∆E and TOF were measured by using the plastic scintillator hodoscopes, HODF and HODP. Fig. 1
shows the particle identiﬁcation of reaction residues.
It should be noted that the reaction channels of interest were measured simultaneously because of the large
acceptance of SAMURAI.
The preliminary ratio of inclusive np- to pp-removal
cross sections was obtained, which is consistent withthe previous result within errors.3) Currently, the γray spectrum analysis is ongoing. Further, the partial
cross sections will be extracted.
References
1) E. Simpson and J. A. Tostevin: Phys. Rev. C 83, 014605(2011).
2) E. Simpson et al.: Phys. Rev. C 86, 054609 (2012).
3) J. M. Kidd et al.: Phys. Rev. C 37, 2613 (1988).
4) D. L. Olson et al.: Phys. Rev. C 28, 1602 (1983).
5) T. Kubo et al.: Prog. Theor. Exp. Phys. 2012, 03C003(2012).
6) S. Takeuchi et al.: Nucl. Instr. Meth. A 763, 596 (2014).
7) T. Kobayashi et al.: Nucl. Instr. Meth. B 317, 294(2013).

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