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

Ⅱ-1. Nuclear Physics

Identification of a millisecond isomeric state 129 Cd via the detectionof internal conversion and Compton electrons†
J. Taprogge,∗1,∗2,∗3 A. Jungclaus,∗1 H. Grawe,∗4 S. Nishimura,∗2 P. Doornenbal,∗2 G. Lorusso,∗2 E. Nácher,∗1
G.S. Simpson,∗5 P.-A. Söderström,∗2 T. Sumikama,∗6 Z.Y. Xu,∗7 H. Sakurai,∗2,∗7 H.Watanabe,2 on behalf of theRIBF-85 and RIBF-60&62R1 collaborations
Isomeric states near nuclear shell closures are a sensitive probe to study the position of single-particle orbitals and their evolution. In the present work wehave studied for the ﬁrst time excited states in 129 Cd
which decay via the emission of γ rays. The neutronrich 129 Cd ions were produced at the Radioactive Isotope Beam Factory (RIBF), identiﬁed and separated
in BigRIPS and then implanted in the active stopper
WAS3ABi, consisting of eight closely packed DSSSD,
which is surrounded by the germanium array EURICA.

counts per 4 keV

300
200

100

400

800
1200
Energy [keV]

1600
T1/2 = 3.6(2) msFig. 1. Energy spectrum of the Si detector in which the129
Cd ions were implanted for all decays observed in
the first 10 ms (black) and from 30 to 40 ms (red) after
the implantation.

21/2

27/27/2+

15/2-5/2+

†
∗1
∗2
∗3
∗4
∗5
∗6
∗7

Condensed from the article in Phys. Lett. B, Vol. 738, 223(2014)
Instituto de Estructura de la Materia, CSICRIKEN Nishina Center
Departamento de Fı́sica Teórica, Uni. Autónoma de Madrid
GSI Helmholtzzentrum für Schwerionenforschung GmbHLPSC, UJF, CNRS, INPG, GrenobleDepartment of Physics, Tohoku University
Department of Physics, University of Tokyo

406 keV
(M1)

13/2-

1/2+
3/2+

Two types of events were recorded by the data acquisition system, the implantation of the ions into theactive stopper WAS3ABi and their subsequent decays.
In the decay of 129 Cd a very short-lived component
in the range of a few milliseconds was observed, much
shorter than the previously reported half-lives of thetwo β-decaying states in 129 Cd1) . The inspection of theenergy spectrum of the Si detector in which the 129 Cdions were implanted for all decays observed in the ﬁrst
10 ms after the implantation (see black curve in Fig. 1 )revealed a peak structure at about 340 keV and an excess of counts below 250 keV. Those structures are notvisible for decays occurring in the time interval of 30 to40 ms after the implantation for which the smooth energy distribution expected for β decays is observed (red353 keV (E3)+1587 keV

01181 keV0curve) in Fig. 1. A comparison to Monte-Carlo simulations indicated that conversion electrons and Compton electrons from transitions following the decay ofa ms isomeric state in 129 Cd lead to the registrationof decay events in the active stopper. The energies ofsubsequent X-rays are summed with a certain chance
to the energy of the Compton electron which shifts thepeak to about 340 keV. Four transitions with energies
of 353, 406, 1181 and 1587 keV were observed in delayed coincidence with the implanted 129 Cd ions andthe half-life of the new isomeric state was determined
to T1/2 =3.6(2) ms from the summed time diﬀerence
distributions between the observed decay events andthe observation of either the 406- or 1181-keV γ rays.
Furthermore, the measured intensities of both γ rays
and conversion electrons were used to tentatively assign an E3 multipolarity to the primary isomeric transition with an energy of 353 keV.

NA-14

11/2-EXPFig. 2. Proposed decay scheme and comparison to the shellmodel calculations NA-14.

The deduced decay scheme is compared to shellmodel calculations employing state-of-the-art realisticinteractions (NA-14, same as in Ref.2) ) in Fig. 2. The
dashed lines represent the predicted positions of lowlying positive parity states as well as states with positive and negative parity in the spin range 17/2 − 23/2.
None of these states is predicted to be isomeric. Basedon this comparison the new state was assigned to havespin and parity (21/2+ ). The experimentally determined reduced transition strength of B(E3) = 0.50(3)W.u. for the (21/2+ ) → (15/2− ) E3 transition is inperfect agreement with the value predicted by the SM
calculations, B(E3) = 0.48 W.u..
References1) O. Arndt et al., Acta Physics Polonica B 40, 437 (2009)2) J. Taprogge et. al., Phys. Rev. Lett. 112, 132501 (2014).

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