Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Isomer spectroscopy of 92, 94 Se in the SEASTAR 2015 experiment353531034.5
102Z

34
34

33.5

zd_pidc_ces_oc
Entries
Mean xMean y
RMS x
RMS y

33
33

9492

Se

Se

10

2.72

A/Q

2.74

2.76

1Fig. 1. ZeroDegree PID spectrum and 2D cuts of 92, 94 Se.
∗1
∗2
∗3
∗4
∗5
∗6

Institut für Kernphysik, Technische Universität Darmstadt
GSI, Darmstadt
RIKEN Nishina Center
CEA, Saclay
School of Computing, Engineering and Mathematics, University of Brighton
IFIN-HH, Bucharest- 61 -

20

00200

400

600

11(2)
1249

1100

1200

898 85(6)1000

1065 19(2)968 13(2)Counts per 4 keV

600

0800

1000

800

Energy (keV)

1200

1180 10(3)

40400

50

822 91(12)Se

642 82(13)

200

94

752 10(3)0539 100(5)0637 53(4)713 39(3)100

470 100(9)67 21(4)Counts / 2 keV

200495 85(13)Seh94s1000

Entries

595

Mean

425.4

RMS

234.9

1200

Fig. 2. Delayed γ-rays correlated to 92 Se and 94 Se. The energy (black) and eﬃciency-corrected intensity (blue) ofthe transitions are shown next to the peaks.

with each other. The isomeric half-lives were obtained
from the exponential ﬁt of the time spectrum obtained
after adding events of individual transitions, leading toT1/2 = 15.7(7) µs for 92 Se and T1/2 = 0.68(5) µs for 94 Se.
For 92 Se, the isomeric decay occurs because of anE2 67-keV transition with a decay strength of approximately 1 W.u. The comparison between BrICC calculations and experimental intensities allowed the multipolarity assignment. In the context of Nilsson schemes ofstate-of-the-art beyond mean-ﬁeld (BMF) theories, theisomeric decay likely corresponds to a 9− →7− transition
between two oblate quasi neutron states. For 94 Se anoblate K-isomer is suggested, decaying via a 7− →6+
1 E1transition with an energy of 495 keV. The spin quantum
numbers were tentatively assigned assuming the simplest
high-K quasiparticle couplings near the Fermi surface, inboth cases near β∼−0.24, involving the 11/2[505] level.
A prolate-to-oblate transition between 90−94 Se56−60 ispredicted by BMF theories recently used for the neighbouring 98, 100 Kr.6) It seems to be supported by the experimental systematics and the decay pattern between
the low-lying levels observed during the isomeric decay.
References

1.356063e+08
2.695
34.84
0.01533
0.2575

2.7

92
300

Counts / 2 keV

We report on the analysis of the isomer spectroscopyof 92, 94 Se conducted during the SEASTAR 2015 campaign of experiments.1) Neutron-rich nuclei were produced via the in-ﬂight ﬁssion of a 345-MeV/nucleon 238 U
primary beam colliding with a 9 Be target. The fragments were selected with BigRIPS, previously tuned toproduce a radioactive beam centered around 95 Br. Thissecondary beam impinged on a 99(1)-mm LH2 targetof the MINOS device placed at F8, producing neutronrich selenium nuclei via knockout reactions. The ejectiles were transported through the ZeroDegree spectrometer, where particle identiﬁcation was conducted via
the TOF-Bρ-∆E measurement to deduce the mass-tocharge ratio (A/Q) and atomic number (Z). The obtained PID histogram is shown in Fig. 1.
The nuclei were implanted on the silicon layers of theAIDA stopper placed at the end of the ZeroDegree spectrometer. The existence and decay of isomeric statesof the implanted ions were studied using the EURICA
array comprised of HP Ge detectors.2,3) The obtained
spectra for delayed γ rays are shown in Fig. 2. All
the reported transitions for these isotopes4,5) were observed. For 92 Se, new transitions were found at 67, 353
and 1252 keV. For 94 Se, the isomeric state was observed
for the ﬁrst time, with new transitions having energies
of 495, 822, 752, and 1180 keV.
Several analysis steps such as add-back reconstruction, background subtraction, eﬃciency and time-walkcorrection, and γγ coincidences have been conducted toidentify the γ rays emitted after the isomeric decay andtheir placement in the level schemes, which have beenextended. In both nuclei, a single isomeric state has
been found because the half-lives for the individual transitions, measured from their time spectra, are consistent

bckgnd 351
353
429 38(4)503 94(6)C. Lizarazo,∗1,∗2 P.-A. Söderström,∗1,∗2,∗3 V. Werner,∗1 N. Pietralla,∗1 P. Doornenbal,∗3 S. Nishimura,∗3A. Obertelli,∗1,∗3,∗4 F. Browne,∗3,∗5 C. R. Niţă,∗6 for the SEASTAR and EURICA collaborations

1) P. Doornenbal et al., RIKEN Accel. Prog. Rep. 49, 35(2016).
2) P.-A. Söderström et al., Nucl. Instrum. Methods B, 317,649 (2013).
3) S. Nishimura et al., Prog. Theor. Exp. Phys. 2012,03C006 (2012).
4) S. Chen et al., Phys. Rev. C 95, 041302(R) (2017).
5) D. Kameda et al., Phys. Rev. C 86, 054319 (2012).
6) F. Flavigny et al., Phys. Rev. Lett. 118, 242501 (2017).