II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Neutron intruder states and collectivity beyond N = 50 towards 78 Ni
F. Flavigny,∗1 M. Górska,∗2 and Zs. Podolyák∗3 for the RIBF196 and HiCARI Collaborations

∗1
∗2
∗3

104

34
33

103

32

Z

The recent spectroscopy of 78 Ni,1) together with
the identiﬁcation of shape coexistence just below theN = 50 shell closure for 80 Ge2) and 79 Zn,3) indicates that deformed intruder conﬁgurations could
play a crucial role in low-energy structure properties in this region and towards the limits of the nuclear chart. Such conﬁgurations are predicted to originate from multiparticle-multihole excitations4) above
the N = 50 and Z = 28 shell gaps pushed down inenergy due to neutron-proton correlations, which enhance quadrupole collectivity. Quantifying how collectivity develops near 78 Ni is crucial because it inﬂuences binding energies and the drip-line location5)
with consequences for nucleosynthesis calculations relying on these inputs. Because these states involve
many-particle excitations, their theoretical description
is challenging, and identifying them experimentally is,thus, of prime interest to constrain models. So far,very few experimental indications of these conﬁgurations have been obtained in 80 Ge and 79 Zn, but no direct evidence of these conﬁgurations exist for N = 50
or above in more exotic species.
During the RIBF196 experiment in November 2020,
we sought to identify and characterize for the ﬁrst timesuch 2p-1h intruder states in 83 Ge and 81 Zn which are
the last two odd-even N = 51 isotones above 79 Ni. Todo so, we used neutron knockout from 84 Ge and 82 Zn,
both having two neutrons in the s1/2 d5/2 valence space
above N = 50. This direct reaction allows the removalof one of the neutrons from the quasi-full g9/2 orbital
below N = 50 to selectively populate 9/2+ intruder
states based on a ν(g9/2 )−1 (s1/2 d5/2 )+2 conﬁgurationand extract their spectroscopic factors.
In addition, the ﬁrst spectroscopy of low-lying levels in 82, 84 Zn6) recently indicated that magicity wasstrictly conﬁned to N = 50 in 80 Zn with the onset ofdeformation developing towards heavier Zn isotopes.
To reproduce these ﬁndings, it was demonstrated thatstate-of-the-art shell model calculations needed to include suﬃcient valence orbitals above N = 50 (fullgds valence space) and to allow the breaking of the78
Ni core. Simultaneously to the study of the intruder
states mentioned above, these low-lying states in 82 Zn
were populated using proton removal from 83 Ga, andtheir lifetimes will be studied by line-shape analysis.
To reach these scientiﬁc goals, we performed an experiment in which a primary beam of 238 U with a mean
intensity of 60 particle nA at 345 MeV/nucleon collided with a 4-mm thick 9 Be primary target at the102

3130

1029
2.55

2.6

2.65

2.7

1

2.75

A/Q
Fig. 1. Preliminary particle identification of the secondary
beam in BigRIPS. Black, red and purple ellipsoids indicate 84 Ge, 83 Ga, 82 Zn respectively, see text for details.

object point of the BigRIPS separator. The secondary
beam was puriﬁed using Al degraders at the F1 andF5 dispersive planes (8- and 2-mm thick). The secondary cocktail beam containing approximately 61%,
7.4% and 0.2% of 84 Ge,83 Ga and 82 Zn, respectively, ataveraged rates of approximately 6100, 740, and 20 s−1
impinged for 2.5 days on a 6-mm-thick 9 Be target at253, 248, and 242 MeV/nucleon. Event-by-event identiﬁcation of projectiles and reaction residues in termsof the atomic number (Z) and mass-to-charge ratio(A/Q) was achieved using the TOF-Bρ-∆E methodin both the BigRIPS and ZeroDegree spectrometers.
A preliminary particle identiﬁcation plot in BigRIPS
without higher-order optical corrections is shown inFig. 1. De-excitation γ-rays of the neutron and proton
knockout residues 83 Ge,82 Zn and 81 Zn were detectedusing the HiCARI germanium array (described separately7) ) surrounding the secondary reaction target. A
0.5-mm-thick lead shield was placed around the beam
pipe surrounding the target to reduce the rate of lowenergy atomic background in the detectors. Knownand unknown transitions in 83 Ge were clearly identiﬁed online. The statistics obtained in this neutronknockout channel will allow clear identiﬁcation of thestates populated and their possible intruder character
after careful analysis of the feeding pattern.
References

1) R. Taniuchi et al., Nature 569, 53 (2019).
2) A. Gottardo et al., Phys. Rev. Lett. 116, 182501 (2016).
3) X. F. Yang et al., Phys. Rev. Lett. 116, 219901 (2016).
4) K. Heyde, J. L. Wood, Rev. Mod. Phys. 83, 1467 (2011).
5) J. Erler et al., Nature 486, 509 (2012).
6) C. Shand et al., Phys. Lett. B 773, 492 (2017).
7) K. Wimmer et al., in this report.

LPC Caen, CNRS/IN2P3
GSI Helmholtzzentrum für Schwerionenforschung GmbH
Department of Physics, University of York

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