II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Single particle structure of semi-magic 129 Ag82
T. Parry,∗1 Zs. Podolyák,∗1 M. Górska,∗2 M. Armstrong,∗2 and A. Yaneva∗2 for the RIBF189 Collaboration
The shell structure is one of the fundamental ingredients in the understanding of mesoscopic systems,
which includes the atomic nucleus. One crucial question in nuclear physics is how the quantum orbitals
evolve as the number of protons and neutrons change.
The ordering of the nuclear orbitals can change drastically in neutron-rich nuclei. In extreme cases, traditional magic numbers can disappear and new ones canemerge. Such changes were already observed for lightnuclei, and are predicted for middle-mass ones. The
evolution of orbitals and shell gap has obvious consequences on the r-process nucleosynthesis, which is responsible for the production of half of the nuclei heavier than iron. The abundances of nuclei with magic
neutron numbers are enhanced due to their lower neutron capture probabilities. Thus the A ∼ 130 r-process
abundance peak is the consequence of the N = 82 neutron shell closure. The unknown evolution of the shellstructure in this region is one of the main sources ofnuclear physics uncertainty in r-process calculations.
One has to rely on theoretical models whose predictions for regions far oﬀ stability diverge signiﬁcantly.
129
Ag is a singly magic N = 82 nucleus. With three
protons holes below 132 Sn it is neutron-rich, and any
experimental information to be obtained on its structure is directly applicable for the understanding of theinﬂuence of the N = 82 nuclei on the r-process path.
Using proton knockout from 130 Cd single proton statesin 129 Ag can be populated.
The experiment was performed in November
2020.
For an approximate length of 3 days a345 MeV/nucleon beam of 238 U with an average inten-350

Counts/2 keV

300

100

500

104
132

Sn
103

13149

Z

In
13048Cd

102

47
1046

2.6

2.62

2.64

2.66

2.68

2.7

2.72

2.74

1Fig. 1. Particle identification in BigRIPS with N = 82 nuclei including 130 Cd required for the main expected production channel of 129 Ag.

∗2700
Energy (keV)

800

9001000sity of around 60 particle nA impinged on a 4 mm 9 Betarget. Fission fragments were separated and identiﬁedin ﬂight with the BigRIPS separator centred on 130 Cd.
Identiﬁcation was achieved by using the Bρ-∆E-Bρ
technique on an individual particle basis. A preliminary BigRIPS particle identiﬁcation plot is shown inFig. 1. This secondary radioactive beam impinged onanother target of 6 mm 9 Be located at F8. The newgermanium HiCARI array1) was located around thissecond beryllium target and used to detect the emitted γ rays with high resolution. The reaction products
were identiﬁed in the ZeroDegree spectrometer, centred on 129 Ag, using the same methods as in BigRIPS.
The obtained data set is under analysis. As an early
indication of the ability of the HiCARI array to achieve
in-ﬂight gamma-ray spectroscopy the isotope 128 Cd
was selected to conﬁrm the array was functioning asintended. 128 Cd was selected due to its high yield andits known structure.2) A preliminary Doppler corrected
γ-ray spectrum of 128 Cd populated in one-proton removal from 131 In showing the 4+ and 2+ gamma rays
is presented in Fig. 2. Note that using β values determined for individual particles, as well as updated
calibrations, will result in a much better improved energy resolution.
References

1) K. Wimmer et al. (HiCARI Collaboration), in this report.
2) A. Scherillo et al., Phys. Rev. C 70, 054318 (2004).

A/Q∗1600Fig. 2. Doppler corrected γ-ray spectrum of 128 Cd as
measured by the full HiCARI. Clearly visible are theknown2) 646 keV 2+ to 0+ and 785 keV 4+ to 2+ transitions.

5150200150

10552250Department of Physics, University of Surrey
GSI Helmholtzzentrum für Schwerionenforschung GmbH-9-1