Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Coexisting Single-Particle and Collective Structures in 137 Sb
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,∗5and the EURICA RIBF-85 collaboration∗5β, β-n decay of137

Sn70

137
–100

200

400

500 600 700
Energy (keV)∗3∗4∗5*(0 )-0βIβn (%)

225.8(1/2 + , 3/2 + )34

5.6(5/2 + )>5.5(7/2 + )<44800137

220.72283.4

00<78SbFig. 2. Level scheme of 137 Sb obtained in the current work.

signed from the β and β-n feeding intensities to levels
in 136,137 Te. In particular the observed feeding to spin(2+ ) − (6+ ) levels in 136 Te agrees best with a (7/2+ )ground-state spin assignment. It is important to notethat the ﬁrst excited state in 137 Sb receives no direct
feeding from the β-n decay of 138 Sn, allowing a spin of(5/2+ ) to be assigned.
The experimental level scheme has been comparedto the results obtained from shell-model calculations3)
and level systematics. The calculations predict a setof four low-lying states with spins of 1/2+ , 3/2+ , 5/2+ ,and 7/2+ all within an energy range of 0–400 keV, withother states in this spin range at energies >700 keV. As
three low-lying states in 137 Sb appear to be fed directlyfrom β decay there is considerable unseen populationfrom the γ decay of higher lying levels.
The wavefunctions of the four lowest-lying states of137
Sb are calculated to be diﬀerent in nature. For
example the 7/2+ state has a predominantly singleparticle character, whereas the 1/2+ and 3/2+ states
are more fragmented, with seniority-3, and higher, couplings making up >70% of the wavefunction. The latter is a sign of the breakdown of independent singleparticle behavior and emergening collectivity. Indeed
the theoretical results point towards the coexistenceof single-particle and weakly collective states near theground state of 137 Sb.

The structure of the nucleus is governed by a delicate
interplay between single-particle and collective states.
Currently it is unclear how collective modes develop in
nuclei in very neutron-rich regions far from stability,
which are diﬃcult to access experimentally. Collectivity occurs when the integrated proton-neutron interaction is able to overcome the pairing force. Experimental measurements have shown that reduced pairing was
found to be necessary to correctly describe the structures of 136 Sn1) and 72,74 Ni,2) which possess signiﬁcantseniority (broken pair) mixing. It is therefore interesting to search for the development of collective modesin nuclei east of 132 Sn. The very neutron-rich nucleus
137
Sb consists of a single valence proton coupled to136
Sn and its structure is most appropriate for testing
the proton–neutron part of shell-model eﬀective interactions. A decay scheme of this nucleus has been constructed via β- and β-n decay spectroscopy data taken
at RIBF, RIKEN during the EURICA campaign.
The experimental spectrum obtained is presentedin Fig. 1. It was possible to construct a ﬁrst level
scheme using γ − γ coincidences, along with the β-ndecay data of 138 Sn, shown in Fig. 2. The ground statespin of the parent 137 Sn nucleus was determined fromthe β-n feeding of known spin (4− ) and (2− ) levels in
136
Sb. The 137 Sb ground-state spin of (7/2+ ) was as∗2Sb4)

300

Fig. 1. γ-ray spectrum obtained from the β-decay of 137 Sn
in the current work.

∗1nIβ(%) log ft

1000-00)20

138

0.3(630T1/2 = 151(33) ms
Qβn = 7094(600) keV 138
Sn = 2230(1070) keV + Sn

83.
4(140Sn

T1/2 = 254(38) ms
Qβ = 10270(400) keV
Sn = 3620(50) keVβ625.1 +668.6 *

50(7/2 )137.9 +
172.9 *
207.0 *

60

43.4 *

Number of Counts per keV

80

137

83.8 +

90

References

School of Engineering, University of the West of Scotland
LPSC, Grenoble
IEM, CSIC Madrid
Departamento de Fı́sica Téorica,Universidad Autonoma deMadrid
RIKEN Nishina Center

1) G. Simpson et al., Phys. Rev. Lett. 113, 132502 (2014).
2) H. Grawe et al., Nucl. Phys. 704, 211 (2002).
3) L. Coraggio et al., Phys. Rev. C 87, 034309 (2013).

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