RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Seniority isomer in 128 Pd†

Counts / 1 keV

H. Watanabe∗1,∗2 and EURICA U-beam collaboration in 2012
The level structure of the very neutron-rich nucleus
Pd has been studied for the ﬁrst time. Neutron-richnuclei below 132 Sn were produced using in-ﬂight ﬁssionof a 238 U86+ beam at 345 MeV/nucleon impinging on
a 3-mm-thick beryllium target. The primary beam intensity ranged from 7 to 12 pnA during the experiments. The nuclei of interest were separated by theBigRIPS separator and the following ZeroDegree spectrometer. The identiﬁed particles were implanted intoa highly segmented active stopper named WAS3ABi1) ,which consisted of eight double-sided silicon-strip detectors (DSSSD) stacked compactly. Each DSSSD had
a thickness of 1 mm with an active area segmented into
sixty and forty strips (1-mm pitch) on each side in thehorizontal and vertical directions, respectively. The
DSSSDs also served as detectors for electrons following β-decay and internal conversion processes. Gammarays were detected by the EURICA spectrometer2) ,which consisted of twelve Cluster-type detectors, eachof which contained closely packed seven HPGe crystals.
Figure 1 shows a γ-ray energy spectrum measured
in delayed coincidence with 128 Pd ions. Four γ rays atenergies of 75, 260, 504, and 1311 keV have been unambiguously observed. These γ rays are found to be inmutual coincidence and exhibit consistent time behavior. Therefore, we conclude that they proceed througha single cascade originating from one isomeric state. A
least-squares ﬁt of the summed gated time spectra of
the isomeric-decay transitions yields T1/2 = 5.8(8) µs
half-life, as shown in Fig. 1. The relative intensities ofthese isomeric γ rays are in agreement within experimental errors, except for the 75-keV transition thatis expected to be highly converted. The total internalconversion coeﬃcient for the 75-keV transition derived
from a comparison with the 1311-keV γ-ray intensity is2.6(17), which is consistent with the theoretical valueof 3.88 for an E2 multipolarity.
On the basis of the above arguments on the observed
γ transitions, the level scheme of 128 Pd is proposed
as displayed in Fig. 1, where the spin and parity of
the 5.8-µs isomeric state at 2151 keV is assigned as
J π = 8+ . The spin-parity assignment for the levels andthe ordering of the transitions between the isomer andthe ground state are based on a close resemblance tothe yrast level energies below the analogous 8+ isomers
in 130 Cd3) and 96 Pd (N = 50)4) . A transition strengthof B(E2; 8+ → 6+ ) = 0.22(3) W.u. can be obtained
from the measured half-life of the 2151-keV isomeric128†∗1∗2260.1

2075.1504.4

1311.41000200(8 + )(6 + )(4 + )

400

600

Eγ [keV]

800

1000

1200

1400

2151 5.8(8) µs
75.1 2076
260.1
1816
504.4

T 1/2=5.8(8) µs

13111311.4

Counts / 2000 ns(2 + )

10

10200

400

600

800

1000

Time [25 ns/ch]

0+012846

Pd 82

Fig. 1. Gamma-ray spectrum measured in coincidence with
128
Pd ions within 0.15 − 25 µs (top), level scheme of128
Pd (bottom left), and sum of time distributions ofthe 260-, 504-, and 1311-keV γ rays in 128 Pd (bottomright).

state.
The excitation energies of the J π = 2+ − 8+ statesin 128 Pd are comparable to those in 130 Cd3) . The constancy of level energies is characteristic of the seniority
scheme, where seniority υ counts the number of nucleons that are not in pairs coupled to spin zero. In thecase of an n-particle (or n-hole) system in a single-jshell, the level energies with identical J π and υ areindependent of n. Such energy properties are also visible for the even N = 50 isotones from Mo (Z = 42) toCd (Z = 48), in which the yrast J π = 2+ − 8+ levels
consist of the same multiplet that involves predominantly valence protons in the πg9/2 orbital with υ = 2.
Since the single-proton levels in the Z = 28 − 50 shell
are nearly identical in the 132 Sn and 100 Sn regions, it
is expected that the level properties exhibited by theN = 82 isotones are similar within the valence proton
space to those in the case of N = 50. Therefore, theexcited states in 128 Pd can be interpreted in terms ofthe υ = 2 conﬁguration of the πg9/2 subshell.
References
1) S. Nishimura: Prog. Theor. Exp. Phys. 03C006 (2012).
2) P.-A. Söderström et al.: Nucl. Instrum. Methods B 317,649 (2013).
3) A. Jungclaus et al.: Phys. Rev. Lett. 99, 132501 (2007).
4) W. Kurcewicz et al.: Z. Phys. A 308, 21 (1982).

Condensed from the article in Phys. Rev. Lett. 111, 152501(2013)
IRCNPC, School of Physics and Nuclear Energy Engineering, Beihang University
RIKEN Nishina Center- xv -