II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

RIBF190: Exploring collectivity beyond 78 NiF. Browne∗1 and V. Werner∗2 for the RIBF 190 Collaboration∗1∗2

Atomic number

Recently, it has been indicated that the doubly
magic 78 Ni has competing low-lying spherical andprolate-deformed states.1) In its north-east quadrant,+measurements of E(2+1 ) and E(41 ) of the N = 52, 54
Zn isotopes show deformation of ground-states becomes prominent beyond N = 50.2) Shell model3,4)
and interacting boson model calculations5) that reproduce the available spectroscopic information of N > 50
Se and Ge isotopes predict them as transitioning fromtheir N = 50 sphericity to collective structures. In thecase of the Ge isotopes this takes the form of soft andrigid triaxiality, while the Se isotopes start to exhibit
prolate-oblate shape coexistence. Initial spectroscopy
seems to agree with the models that predict these interesting features.6,7) In addition to the quadrupole degrees of freedom, octupole collectivity may be expected
to be enhanced in the neutron-rich Ge/Se region due totheir proximity to the “doubly octupole magic” numbers, N = 56 and Z = 34
An experiment was performed over 4 days withthree BigRIPS settings to extract the reduced transition probabilities of low-lying states in 84, 86 Ge and86, 88, 90
Se in order to clarify the quadrupole and octupole collectivity of the region. Exotic nuclei wereproduced from the in-ﬂight ﬁssion of a 238 U beam
which was accelerated to 345 MeV/nucleon. Following ﬁssion, the isotopes of interest were selected in theﬁrst stage of BigRIPS using the Bρ-∆E-Bρ techniqueand identiﬁed with the second stage using their Bρ,∆E, and time-of-ﬂight (TOF) values. The BigRIPS
particle-identiﬁcation plots for the three settings ofthe experiment are shown in Fig. 1. Secondary targets of Be (3.8-mm-thick) and Bi (1.1 mm-thick) weresituated at the F8 focal plane to induce nuclear andCoulomb excitations, respectively. Gamma rays emitted from excited states were detected with the HiCARI
array8) which surrounded F8. The trajectory of incoming and outgoing ions with respect to the target
was measured with PPACs. Following the reaction
targets, ions were transported through the ZeroDegree
spectrometer where they were identiﬁed, again using
their Bρ, ∆E, and TOF values. Finally, the ions werestopped in a gas cell and their masses recorded usinga MR-TOF setup.
While oﬀ-line data analysis is yet to begin, the online γ-ray spectra show clearly that the main objectivesof the experiment are attainable. In addition to theCoulomb excitation to the 2+
1 states being observed in
all objective nuclei, some nuclei show 2+
2 excitations
on the heavy target.

8635Se

34
84

33Ge

32
31

Atomic number

2.5

2.55

2.6

2.65

2.7

2.75

8835

Se

34
8633Ge32
31

Atomic number

2.5

2.55

2.6

2.65

2.7

2.75

9035

Se

3488

33Ge

32
31
2.5

2.55

2.6

2.65
2.7
2.75
Mass-to-charge ratio

Fig. 1. On-line particle identification plots for the threeBigRIPS settings employed during the experiment. Labels refer to the identified ions below them.

References

1) R. Taniuchi et al., Nature (London) 569, 53 (2019).
2) C. Shand et al., Phys. Lett. B 773, 492 (2017).
3) K. Sieja, T. R. Rodríguez, K. Koloz, D. Verney, Phys.
Rev. C 88, 034327 (2013).
4) K. Sieja, Private communication (2019).
5) K. Nomura, R. Rodríguez-Guzmán, L. M. Robledo,Phys. Rev. C 95, 064310 (2017).
6) M. Lettmann et al., Phys. Rev. C 96, 011301(R) (2017).
7) S. Chen et al., Phys. Rev. C 95, 041302(R) (2017).
8) K. Wimmer et al., in this report.

RIKEN Nishina CenterIKP, TU Darmstadt-6-1