Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Gamma decay of unbound neutron-hole states in 133 Sn†V. Vaquero,∗1 A. Jungclaus,∗1 P. Doornenbal,∗2 K. Wimmer,∗2, ∗3 A. Gargano,∗4 J. A. Tostevin,∗5 S. Chen,∗2, ∗6
E. Nácher,∗1 E. Sahin,∗7 Y. Shiga,∗8 D. Steppenbeck,∗2 R. Taniuchi,∗2, ∗3 Z. Y. Xu,∗9and the NP1306-RIBF98R1 collaboration
The region around the doubly magic nucleus 132 Sn(N = 82 and Z = 50) is of particular interest for nuclear structure investigations. Nuclei with a few nucleons outside this closed-shell core provide direct information about the evolution of nucleon-nucleon correlations, quadrupole collectivity and single-particle energies. In this context, the low-lying states in theneutron-rich nucleus 133 Sn, which consists of a single
neutron coupled to the doubly-magic nucleus 132 Sn,
provide information about the position of the neutron
single-particle orbitals belonging to the N = 82–126
major shell. Neutron single-particle energies of 854,
1367, 1561, and 2002 keV for the 2p3/2 , 2p1/2 , 0h9/2
and 1f5/2 orbitals, respectively, relative to the 1f7/2orbital, have been established combining the information from both β decay and (d, p) neutron-transfer experiments.1,2) The neutron single-hole states in 133 Sn
are expected to have excitation energies above Sn =2.402(4) MeV and to decay via neutron emission mediated by the strong interaction.
In an experiment performed in April 2015 at theRI Beam Factory (RIBF), excited states in the nucleus 133 Sn were investigated by in-beam γ-ray spectroscopy. These states in 133 Sn were populated knocking out a neutron from a slightly heavier nucleus,
134Sn, at relativistic energies. The exotic nuclei to beinvestigated were produced by the in-ﬂight ﬁssion of a345 MeV/nucleon 238 U beam with an average intensityof 15 pnA, impinging on a 4-mm thick Be target. In theBigRIPS in-ﬂight separator, the Bρ-∆E-Bρ method
was used in order to select and identify a secondary
beam of 134 Sn. The identiﬁed 134 Sn ions then impinged
with a kinetic energy of 165 MeV/nucleon on a 3-mm
thick C target. The γ radiation emitted in the decay ofexcited states was detected using the γ-ray spectrometer DALI2 which was installed surrounding the secondary target. Reaction products from the secondarytarget were identiﬁed using the ZeroDegree spectrometer.3)
Figure 1 shows the Doppler-corrected γ-ray spec†
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9Fig. 1. Doppler-corrected γ-ray spectrum (for γ-ray multiplicity Mγ = 1 after add back) of 133 Sn populated
via one-neutron knockout from 134 Sn. The response
function ﬁt to the experimental spectrum is shown bythe thick red line while the individual components areshown as thin black lines. The background is indicatedas the gray area. The inset shows the high-energy region of the spectrum on a linear scale.

trum measured in coincidence with 134 Sn ions detectedin BigRIPS and 133 Sn nuclei detected in the ZD spectrometer. Besides the known γ rays emitted in thedecay of the single-particle states (transitions at 513,854, 1561, and 2002 keV), clearly additional γ strengthis observed above the neutron separation energy, reaching up to about 5.5 MeV. These excited states are interpreted as neutron-hole states that are populated following the knock-out of a neutron from the closed N =50–82 shell of the 134 Sn projectile ion. These neutronhole states are expected to decay via neutron emission
because they are situated far above the neutron separation energy. However, the ability of γ-ray emission
to compete with neutron decay is explained taking into
account the structure of the initial and ﬁnal states andthe resultant wave-function overlap. Our study raises
the question whether, due to nuclear structure eﬀects,
the γ-ray emission may play a much more signiﬁcantrole than generally assumed in the decay of highly excited states populated following β decay in the region
southeast of 132 Sn.

Condensed from the article in Phys. Rev. Lett. 118, 202502(2017)
Instituto de Estructura de la Materia, CSICRIKEN Nishina Center
Department of Physics, University of Tokyo
Istituto Nazionale di Fisica Nucleare
Department of Physics, University of Surrey
State Key Laboratory of Nuclear Physics and Technology,
Peking University
Department of Physics, University of Oslo
Department of Physics, Rikkyo University
Department of Physics, University of Hong Kong

References
1) P. Hoﬀ et al., Phys. Rev. Lett. 77, 1020 (1996).
2) K. L. Jones et al., Nature (London) 465, 454 (2010);
Phys. Rev. C 84, 034601 (2011).
3) N. Fukuda et al., Nucl. Instrum. Methods Phys. Res. B
317, 323 (2013).

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