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

Ⅱ-1. Nuclear Physics

Yrast 6+ Seniority Isomers of 136,138Sn†

The shell model plays a key role in allowing a microscopic description of many of the properties of atomic
nuclei. Its two ingredients are single-particle energies and effective nucleon-nucleon interactions. Experimental studies of semi-magic Sn nuclei beyond thedoubly magic nucleus 132 Sn provide information thatallows the neutron-neutron part of effective interactions for the N = 82 − 126 valence space to be tested
and optimized. More generally, such studies provide
a key benchmark for the methods used to constructeffective interactions in a heavy-mass region far fromstability. Currently there is little experimental dataon the Sn isotopes beyond the N = 82 shell closure,which are difficult to produce and study.
Excited states in the nuclei 136,138 Sn have been investigated by detecting delayed γ-ray cascades using
the EURICA spectrometer1) , which was coupled to
the BigRIPS separator of the RIBF facility. These
exotic nuclei were produced by the in-flight fission ofa 345 MeV/nucleon 238 U beam. Cascades containing
three delayed γ rays each were observed in coincidencewith identified 136,138 Sn ions. The spins of the isomericstates of 136,138 Sn were assigned as (6+ ), in analogy
with a very similar delayed cascade previously reported
for 134 Sn2) .
The energies of the excited states of 134,136,138 Sn
have been compared to the predictions of shell-model
calculations, which used state-of-the-art realistic effective interactions. These calculations used the fullN = 82 − 126 valence space and the effective singleparticle energies were the experimental ones. The experimentally determined level energies of 134,136,138 Sn+
were all well reproduced. The B(E2; 6+
1 → 41 ) values
134,138
were also correctly predicted for
Sn, though thisvalue was more than a factor of 5 away for 136 Sn, as
shown in Fig. 1. Three other shell-model calculations
reported in the literature, using realistic and empirical effective interactions, also failed to reproduce the+136
B(E2; 6+
Sn and are off by at least
1 → 41 ) value fora factor of 2.
†Condensed from the article in Phys. Rev. Lett. 113, 132502(2014)∗1 University of the West of Scotland∗2 SUPA∗3 LPSC, UJF, CNRS, INPG
∗4 Institut Laue-Langevin∗5 RIKEN Nishina Center∗6 IEM, CSIC
∗7 UAM∗8 IPHC∗9 Department of Physics, Tohoku University∗10 Department of Physics, University of Tokyo

B(E2;6+ -> 4+) (e2fm4)

G.S. Simpson,∗1,∗2,∗3 G. Gey,∗3,∗4,∗5 A. Jungclaus,∗6 J. Taprogge,∗6,∗7,∗5 S. Nishimura,∗5 K. Sieja,∗8P. Doornenbal,∗5 G. Lorusso,∗5 H. Sakurai,∗5,∗10 P.-A. Söderström,∗5 T. Sumikama,∗9 Z.Y. Xu,∗10on behalf of the RIBF-85 collaboration
50Exp.
Seniority
Vlowk
Vlowk+mod4030
20
100134136138

AFig. 1. Experimental (black squares) and theoretical re+134−138
Sn.
duced transition rates for 6+
1 →41 γ decays in
The calculations used a realistic Vlow−k interaction(red filled circles), a pairing-modified Vlow−k interaction (blue open circles) and a pure f7/2 seniority scheme(grey curve).

+
The near-constant energies of the (2+1 ), (41 ) and+
134,136,138(61 ) states ofSn are characteristic of dom-inant seniority 2 (one broken pair) excitations. TheB(E2) values of seniority-conserving transitions areexpected to follow the shape of a symmetric positive
parabola, as shown in Fig. 1. The results obtained witha realistic Vlow−k interaction follow a similar pattern
to the seniority 2 scheme. Additional shell-model calculations have been performed which allowed particlehole excitations from the neutron ν0h11/2 and proton
0g9/2 shells to the N = 82 − 126 and Z = 50 − 70
valence spaces, respectively. These allowed the influence of core polarization effects on the transition ratesof the neutron-rich Sn nuclei to be examined. How+136
ever, the B(E2; 6+
Sn was still
1 → 41 ) value for
not correctly reproduced. Reducing the energies of2the ν1f7/2
diagonal and off-diagonal matrix elements+136
by ∼150 keV allowed the B(E2, 6+Sn
1 → 41 ) of
to be correctly predicted. This shift is equivalent toa reduction in the pairing strength. The results using
this pairing-modified Vlow−k interaction are shown inFig. 1. Similar modifications to pairing were necessary
to reproduce the level schemes of 72,74 Ni4) , illustrating the need for additional theoretical efforts on theconstruction of effective interactions.
References
1) P.A. Soderström, et al.: Nucl. Instr. Meth. B 317, 649(2013).
2) A. Korgul, et al.: Eur. Phys. J. A 7, 167 (2000).
3) A. Covello, et al.: J. Phys. Conf. Ser. 267, 012019(2011).
4) H. Grawe, et al.: Nucl. Phys. A704, 211 (2002).

- 28 -完全版2014_本文.indd 28

15/10/16 17:40