II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

N = 32 shell closure below calcium: Low-lying structure of 50 Ar†
M. L. Cortés,∗1,∗2 W. Rodriguez,∗3,∗1,∗4 P. Doornenbal,∗1 A. Obertelli,∗5,∗6 J. D. Holt,∗7,∗8 J. Menéndez,∗9,∗10K. Ogata,∗11,∗12 A. Schwenk,∗6,∗13,∗14 N. Shimizu,∗9 J. Simonis,∗15 Y. Utsuno,∗16,∗9 K. Yoshida,∗16 and theSEASTAR2017 Collaboration
An interesting region to study shell evolution isaround Ca isotopes, where the development of shell closures for N = 32 and N = 34 has been suggested.
The N = 32 sub-shell closure was evidenced by its
relatively high E(2+ ) energy,1) and confirmed by twoproton knockout cross sections2) and mass measurements.3) For the N = 34 shell closure, evidence wasprovided by E(2+ ),4) systematic mass measurements,5)and neutron-knockout cross sections.6) The preservation of the N = 32 shell closure has been determinedin Ti and Cr via spectroscopy, reduced transition probabilities, and precision mass measurements, while for
N = 34, it has been suggested to disappear above Ca.
In contrast, the recent measurement of the E(2+ ) of52
Ar suggests the conservation of the N = 34 shell closure for Z = 18.7) The first spectroscopy of 50 Ar showeda relatively high E(2+ ),8) hinting at the conservation of
the N = 32 shell closure below Ca. A candidate for the4+ state was also reported. No further spectroscopic information is available for this very exotic nucleus. This
work reports low-lying states in 50 Ar.
A beam of 70 Zn with an average intensity of 240 particle nA was fragmented on a Be target. Isotopes
were identified using BigRIPS9) and delivered to the151.3(13)-mm-long liquid hydrogen target of MINOS10)
placed in front of the SAMURAI magnet. Outgoing
fragments were identified using SAMURAI and associated detectors.11) The DALI2+ array,12,13) composed of
226 NaI(Tl) detectors, was used to detect the emittedγ-rays. Doppler-corrected γ-ray spectra were obtainedusing the reaction vertex and the velocity of the fragment reconstructed with MINOS.
Based on the spectra and γγ analysis of theproton- and neutron-knockout, inelastic-scattering, andmultinucleon-removal reactions, the level scheme shown

Fig. 1. Experimental level scheme of 50 Ar.

in Fig. 1 was constructed. The two previously reported
transitions and five new ones were identified. Theoretical level energies and spectroscopic factors for theproton- and neutron-knockout reactions were obtainedwith shell-model calculations using the SDPF-MU interaction, as well as with ab initio calculations using theVS-IMSRG approach. Tentative spin assignments weremade based on the comparison of the calculations andthe experimental results. In both calculations, stateswith J π = 2+ are preferably populated by the reactions, as shown in the figure. In addition, a (3− ) stateis suggested to be populated following the proton inelastic scattering. Both theoretical calculations provide
consistent results and a relatively good agreement withthe experimental data, emphasizing the subshell closure
at N = 32 and strengthening our understanding of shellevolution in this region.
References

Condensed from Phys. Rev. C. 102, 064320 (2020)
RIKEN Nishina Center∗2 INFN-Legnaro∗3 Departamento de Física, Universidad Nacional de Colombia∗4 Departamento de Física, Pontificia Universidad Javeriana∗5 IRFU, CEA, Université Paris-Saclay∗6 Institut für Kernphysik, Technische Universität Darmstadt∗7 TRIUMF∗8 Department of Physics, McGill University∗9 Center for Nuclear Study, The University of Tokyo∗10 Departament de Física Quàntica i Astrofísica, Universitat deBarcelona∗11 RCNP, Osaka University∗12 Department of Physics, Osaka City University∗13 ExtreMe Matter Institute (EMMI)∗14 Max-Planck-Institut für Kernphysik∗15 Institut für Kernphysik and PRISMA Cluster of Excellence,Johannes Gutenberg-Universität∗16 Advanced Science Research Center, JAEA
†1) A. Huck et al., Phys. Rev. C 31, 2226 (1985).
2) A. Gade et al., Phys. Rev. C 74, 021302 (2006).
3) F. Wienholtz et al., Nature 498, 346 (2013).
4) D. Steppenbeck et al., Nature 502, 207 (2013).
5) S. Michimasa et al., Phys. Rev. Lett. 121, 022506 (2018).
6) S. Chen et al., Phys. Rev. Lett. 123, 142501 (2019).
7) H. N. Liu et al., Phys. Rev. Lett. 122, 072502 (2019).
8) D. Steppenbeck et al., Phys. Rev. Lett. 114, 252501 (2015).
9) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003(2012).
10) A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
11) T. Kobayashi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 294 (2013).
12) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res.
A 763, 596 (2014).
13) I. Murray et al., RIKEN Accel. Prog. Rep. 51, 158 (2017).

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