Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Excited states of 136−138Sb from β decay
J. Keatings,∗1 G. Simpson,∗2,∗1 A. Jungclaus,∗3 J. Taprogge,∗4 S. Nishimura,∗5 P. Doornenbal,∗5 G. Lorusso,∗5P. -A. Söderström,∗5 T. Sumikama,∗5 J. Wu,∗5 Z. Y. Xu,∗5 T. Isobe,∗5 H. Sakurai,∗5 andthe EURICA RIBF-85 collaboration∗5The properties of nuclei close to the doubly magicSn, with N ≥ 82, have some of the highest impact
on the final r-process abundances1,2) . Studies of thenuclear structure of the nuclei 135−139 Sb can therefore
provide important input into r-process calculations, especially as the exact site of this astrophysical reaction
is presently unclear. The nucleus 135 Sb is the mostneutron-rich one currently whose excited states havebeen studied via β decay3) .
The Sb nuclei have one valence proton with respect
to the Z = 50 shell closure hence their nuclear structure can provide a sensitive test of the neutron-protonpart of the effective interactions used in shell-model
calculations. The predicted structure of these nuclei
can vary considerably, depending on the effective interaction used. Recently it has been shown that weakerneutron pairing may be necessary to reproduce the features of the Sn nuclei beyond N = 824) . An interesting consequence of weak pairing is the appearanceof low-lying seniority-4 neutron states in 137 Sb which
has been predicted to cause a rapid compression andconsiderable mixing, of the low-lying excited states inthis nucleus5) . A consequence of this is that levels
with πs1/2 and πd3/2 components predicted at energies above 2 MeV in 135 Sb may be found at energies
below 500 keV in 137 Sb.
In order to investigate the structure of the nuclei
136−138
Sb in-flight fission of a 345-MeV/nucleon 238 U
beam was performed at RIBF. Ions including 135−139 Sn
were selected using the BigRIPS spectrometer and implanted in the WAS3ABI stopper, situated at the F11
focal plane. The high segmentation of the WAS3ABIstopper allows detected β decays to be correlated withidentified and implanted ions. The EURICA Ge array
was used to detect any γ rays emitted following theβ decay of 135−139 Sn ions. Use of ion–β − γ and ion–β − γ − γ coincidences has allowed new level schemes
to be established for the nuclei 136−138 Sb.
When adding neutrons beyond the N = 82 shell closure the probability for β−n emission increases rapidly
to values of > 50 %. Therefore performing only an ion–β − γ coincidences is not sufficient to assign a detected
γ ray to a particular nucleus. Common transitions
emitted from parent isotopes with masses A and A + 1
can be assigned to a daugher nucleus with mass A.
Gates set on strongly produced transitions along with200∗1∗2∗3∗4∗5150

100173

Number of counts per keV

1325000200

400

600
800
Energy (keV)

1000

1200

1400Fig. 1. Spectrum of γ rays obtained from the β and β − ndecay of 137 Sn.

observed γ decays in grand-daugher nuclei in the same
event allow masses to be assigned too. This is thanks
to the high granularity and efficiency of the WAS3ABIand EURICA arrays.
An example of such analysis is shown in Fig. 1, whichshows a portion of the γ-rays detected following theβ and β − n decay of 137 Sn. The 173-keV transition
has previously been assigned to 136 Sb via isomer spectroscopy6) .
Analysis of the decays of the nuclei 136−138 Sn andhence first level schemes for the isotopes 136−138 Sb isnearing completion. Comparisons with the predictionsof shell-model calculations will also aid in assigningspins.
References
1) S. Brett et al: Eur. Phys. J. A 48, 184 (2012).
2) G. Lorusso et al: Phys. Rev. Lett. 114, 192501 (2015).
3) J. Shergur et al: Phys. Rev. C 72, 024305 (2005).
4) G. S. Simpson et al: Phys. Rev. Lett. 113, 132502(2014).
5) L. Coraggio et al: Phys. Rev. C 87, 034309 (2013).
6) M. Mineva et al: Eur. Phys. J. A 11, 9 (2001).

University of the West of Scotland
LPSC, Grenoble
IEM, CSIC Madrid
Universidad Autonoma de Madrid
RIKEN Nishina Center- 26 -