Gamma-Spectroscopy around 100Sn

D. Lubos,∗1,∗2 J. Park,∗3 M. Lewitowicz,∗4 R. Gernhauser,∗1 R. Krucken,∗3 S. Nishimura,∗2 H. Sakurai,∗5

H. Baba,∗2 B. Blank,∗6 A. Blazhev,∗7 P. Boutachkov,∗8 F. Browne,∗9,∗2 I. Celikovic,∗4 P. Doornenbal,∗2

T. Faestermann,∗1 Y. Fang,∗10,∗2 G. de France,∗3 N. Goel,∗8 M. Gorska,∗8 S. Ilieva,∗11 T. Isobe,∗2

A. Jungclaus,∗12 G. D. Kim,∗13 Y.-K. Kim,∗13 I. Kojouharov,∗8 M. Kowalska,∗14 N. Kurz,∗8 Z. Li,∗15

G. Lorusso,∗2 K. Moschner,∗7 I. Nishizuka,∗16,∗2 Z. Patel,∗17,∗2 M. M. Rajabali,∗3 S. Rice,∗17,∗2 H. Schaffner,∗8

L. Sinclair,∗18,∗2 P.-A. Soderstrom,∗2 K. Steiger,∗1 T. Sumikama,∗16 H. Watanabe,∗19 Z. Wang,∗3 J. Wu,∗12,∗2

and Z. Y. Xu∗5,∗2

An experiment for studying the superallowedGamow-Teller decay of the doubly magic nucleus 100Snwas performed in June 2013 at the high-resolution sep-arator BigRIPS of the RIBF at the RIKEN NishinaCenter. A 4-mm 9Be target was bombarded with a124Xe beam of 345 MeV/u at intensities up to 36.4 pnAto produce 100Sn and a large cocktail3) of its neighbor-ing nuclei down to neutron numbers N = (Z - 2) byfragmentation. The nuclei were implanted into theWAS3ABi silicon detector that consists of 3 detec-tors with high granularity extended by 10 additionalsingle-sided, seven-fold segmented detectors in a closedstack geometry to absorb the emitted β-particles at amaximum efficiency. This WAS3ABi detector was sur-rounded by 84 Ge- and 18 LaBr-detectors of the 4π-γ-ray spectrometer EURICA.In order to study the branching ratios of the β-decays,derive level schemes of exotic nuclei and determine iso-meric ratios as well as lifetimes of the isomers, a reli-able efficiency calibration has been performed.Owing to the special geometry of the WAS3ABi andthe failure of several Ge-detectors, the calibration wasperformed using a γγ-coincidence method. Cascadingγ-transitions in 98Cd, 94Pd and 96Pd, fed by differentisomers, are available with a large number of counts.For example the delayed γ-emission in 98Cd alwayscontains a complete chain for the energies 147 keV,198 keV, 687 keV and 1395 keV. Thus, the ratio ofcoincident events of a pair of transitions and the totalnumber of events for one of them directly yields theefficiency at the corresponding energy. This method isof great advantage since the radiation, originating inthe implantation region, contains the detector-specificabsorption effects and it does not introduce systematic

∗1 Physik Department E12, Technische Universitat Munchen∗2 RIKEN Nishina Center∗3 TRIUMF∗4 GANIL∗5 Department of Physics, University of Tokyo∗6 CENBG∗7 Institut fur Kernphysik, Universitat zu Koln∗8 GSI Darmstadt∗9 School of Comp., Eng. and Maths., Brighton University∗10 Department of Physics, Osaka University∗11 Institut fur Kernphysik, TU Darmstadt∗12 IES CSIS∗13 Institute for Basic Science∗14 CERN∗15 School of Physics, Peking University∗16 Department of Physics, Tohoku University∗17 Department of Physics, Surrey University∗18 Department of Physics, University of York∗19 Department of Physics, Beihang University

uncertainties that is usually introduced by simulationbased methods. Efficiency calibrations, consistent with

Energy / keV0 200 400 600 800 1000 1200 1400

-ra

y e

ffic

ien

cy

γA

bso

lute

0.02

0.04

0.06

0.08

0.1

0.12

0.14Addback

No addback

Fig. 1. Absolute γ-ray efficiency of the HPGe clusters of

EURICA. γ-rays from isomers of 98Cd, 96Pd and 94Pd,

are used for the fit. Bars indicate the statistical uncer-

tainty of the efficiencies.

previous works4), using this γγ-coincidence method areshown in Fig. 1.In addition, greater precision of known isomer half-lives was attainable owing to high statistics in thisexperiment and preliminary half-lives of 8+ and 12+

isomers in 98Cd were determined as 181+35−25 ns and

228+5−5 ns, respectively. This would lead to better con-

straints on the transition strengths for the test of mod-ern shell models. Previously reported γ-rays in theβ-decay of 100Sn1) were reproduced, allowing an un-precedented βγγ-coincidence analysis for 100In. More-over a new high-spin isomeric state in 96Cd has beenobserved with a half-life of about 200 ns, with a decaybranch into both, the 16+ isomer and the (10+) state,which has a prompt decay cascade to the ground state.Further analysis is underway to finalize experimentalresults and compare these to large-scale shell modelcalculations.

References1) C. Hinke et al.: Nature, 486, 341 (2012).2) B. S. Nara Singh et al.: Phys. Rev. Lett. 107, 172502

(2011).3) D. Lubos, RIKEN Accel. Prog. Rep.: 47 (2014)4) K. Steiger: PhD thesis TU Munchen (2010)

Yrast 6+ Seniority Isomers of 136,138Sn†

G.S. Simpson,∗1,∗2,∗3 G. Gey,∗3,∗4,∗5 A. Jungclaus,∗6 J. Taprogge,∗6,∗7,∗5 S. Nishimura,∗5 K. Sieja,∗8

P. Doornenbal,∗5 G. Lorusso,∗5 H. Sakurai,∗5,∗10 P.-A. Soderstrom,∗5 T. Sumikama,∗9 Z.Y. Xu,∗10

on behalf of the RIBF-85 collaboration

The shell model plays a key role in allowing a micro-scopic description of many of the properties of atomicnuclei. Its two ingredients are single-particle ener-gies and effective nucleon-nucleon interactions. Ex-perimental studies of semi-magic Sn nuclei beyond thedoubly magic nucleus 132Sn provide information thatallows the neutron-neutron part of effective interac-tions for the N = 82− 126 valence space to be testedand optimized. More generally, such studies providea 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 136,138Sn have been in-vestigated by detecting delayed γ-ray cascades usingthe EURICA spectrometer1), which was coupled tothe BigRIPS separator of the RIBF facility. Theseexotic nuclei were produced by the in-flight fission ofa 345 MeV/nucleon 238U beam. Cascades containingthree delayed γ rays each were observed in coincidencewith identified 136,138Sn ions. The spins of the isomericstates of 136,138Sn were assigned as (6+), in analogywith a very similar delayed cascade previously reportedfor 134Sn2).The energies of the excited states of 134,136,138Sn

have been compared to the predictions of shell-modelcalculations, which used state-of-the-art realistic ef-fective interactions. These calculations used the fullN = 82 − 126 valence space and the effective single-particle energies were the experimental ones. The ex-perimentally determined level energies of 134,136,138Snwere all well reproduced. The B(E2; 6+1 → 4+1 ) valueswere also correctly predicted for 134,138Sn, though thisvalue was more than a factor of 5 away for 136Sn, asshown in Fig. 1. Three other shell-model calculationsreported in the literature, using realistic and empiri-cal effective interactions, also failed to reproduce theB(E2; 6+1 → 4+1 ) value for

136Sn and are off by at leasta 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

0

10

20

30

40

50

134 136 138

B(E

2;6+ ->

4+ ) (

e2 fm4 )

A

Exp.Seniority

VlowkVlowk+mod

Fig. 1. Experimental (black squares) and theoretical re-

duced transition rates for 6+1→4+

1γ decays in 134−138Sn.

The calculations used a realistic Vlow−k interaction

(red filled circles), a pairing-modified Vlow−k interac-

tion (blue open circles) and a pure f7/2 seniority scheme

(grey curve).

The near-constant energies of the (2+1 ), (4+1 ) and(6+1 ) states of 134,136,138Sn 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 positiveparabola, as shown in Fig. 1. The results obtained witha realistic Vlow−k interaction follow a similar patternto the seniority 2 scheme. Additional shell-model cal-culations have been performed which allowed particle-hole excitations from the neutron ν0h11/2 and proton0g9/2 shells to the N = 82 − 126 and Z = 50 − 70valence spaces, respectively. These allowed the influ-ence of core polarization effects on the transition ratesof the neutron-rich Sn nuclei to be examined. How-ever, the B(E2; 6+1 → 4+1 ) value for 136Sn was stillnot correctly reproduced. Reducing the energies ofthe ν1f2

7/2 diagonal and off-diagonal matrix elements

by ∼150 keV allowed the B(E2, 6+1 → 4+1 ) of 136Snto be correctly predicted. This shift is equivalent toa reduction in the pairing strength. The results usingthis pairing-modified Vlow−k interaction are shown inFig. 1. Similar modifications to pairing were necessaryto reproduce the level schemes of 72,74Ni4), illustrat-ing the need for additional theoretical efforts on theconstruction of effective interactions.

References1) P.A. Soderstrom, 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).

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Ⅱ-1. Nuclear Physics RIKEN Accel. Prog. Rep. 48 (2015)RIKEN Accel. Prog. Rep. 48 (2015) Ⅱ-1. Nuclear Physics

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