Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Robustness of the N = 34 shell closure: First spectroscopy of 52 Ar
H. N. Liu,∗1,∗2 Y. L. Sun,∗1 A. Obertelli,∗3,∗1,∗4 P. Doornenbal,∗4 H. Baba,∗4 F. Browne,∗4 D. Calvet,∗1F. Château,∗1 S. Chen,∗5,∗6,∗4 N. Chiga,∗4 A. Corsi,∗1 M. L. Cortés,∗4 A. Delbart,∗1 J-M. Gheller,∗1 A. Giganon,∗1A. Gillibert,∗1 C. Hilaire,∗1 T. Isobe,∗4 T. Kobayashi,∗7 Y. Kubota,∗4,∗8 V. Lapoux,∗1 T. Motobayashi,∗4I. Murray,∗9,∗4 H. Otsu,∗4 V. Panin,∗4 N. Paul,∗1 W. Rodriguez,∗10,∗4 H. Sakurai,∗4,∗12 M. Sasano,∗4D. Steppenbeck,∗4 L. Stuhl,∗8 Y. Togano,∗11,∗4 T. Uesaka,∗4 K. Wimmer,∗12,∗4 K. Yoneda,∗4 N. Achouri,∗13O. Aktas,∗2 T. Aumann,∗3 L. X. Chung,∗14 F. Flavigny,∗9 S. Franchoo,∗9 I. Gašparić,∗15,∗4 R. -B. Gerst,∗17J. Gibelin,∗13 K. I. Hahn,∗18 D. Kim,∗18,∗4 T. Koiwai,∗12 Y. Kondo,∗19 P. Koseoglou,∗3,∗16 J. Lee,∗6 C. Lehr,∗3B. D. Linh,∗14 T. Lokotko,∗6 M. Maccormick,∗9 K. Moschner,∗17 T. Nakamura,∗19 S. Y. Park,∗18,∗4 D. Rossi,∗3E. Sahin,∗20 D. Sohler,∗21 P-A. Söderström,∗3 S. Takeuchi,∗19 H. Toernqvist,∗16 V. Vaquero,∗22 V. Wagner,∗3,∗4S. Wang,∗23 V. Werner,∗3 X. Xu,∗6 H. Yamada,∗19 D. Yan,∗23 Z. Yang,∗4 M. Yasuda,∗19 and L. Zanetti∗3∗1

IRFU, CEA, Université Paris-Saclay
Department of Physics, KTH∗3 Institut für Kernphysik, TU Darmstadt∗4 RIKEN Nishina Center∗5 Department of Physics, Peking University∗6 Department of Physics, The University of Hong Kong
∗7 Department of Physics, Tohoku University∗8 CNS, The University of Tokyo∗9 IPN Orsay, CNRS, Univ. Paris-Sud, Univ. Paris-Saclay∗10 Universidad Nacional de Colombia, Sede Bogotá, Facultadde Ciencias, Departamento de Física
∗11 Department of Physics, Rikkyo University∗12 Department of Physics, The University of Tokyo∗13 LPC, Caen
∗14 INST Hanoi∗15 Ruđer Bošković Institute, Zagreb∗16 GSI Helmoltzzentrum Darmstadt∗17 Institut für Kernphysik, Universität zu Köln
∗18 Department of Physics, Ewha Womans University∗19 Department of Physics, Tokyo Institute of Technology∗20 Department of Physics, University of Oslo
∗21 MTA Atomki∗22 Instituto de Estructura de la Materia, CSIC
∗23 Institute of Modern Physics, Chinese Academy of Sciences∗2- 45 -23
Atomic number Z

It is now well known that magic numbers are not universal across the nuclear landscape and that new shellclosures may emerge in exotic nuclei. For example, anew subshell closure at N = 34 has been predicted forneutron-rich nuclei.1) On the experimental side, the systematics of the E(2+1 ) of Ti isotopes show no evidence
for the existence of the N = 34 shell gap.2) Recently, the54
Ca was measured to be ∼0.5 MeV smallerE(2+1 ) of
than that of 52 Ca.3) This drop was attributed to thelarger ground state correlation energy of 52 Ca, and theresults were interpreted as conﬁrming the N = 34 magic
number in Ca isotopes. For 52 Ar, no spectroscopic information has been measured; however, its E(2+1 ) waspredicted to be the highest among Ar isotopes with N> 20.4) The spectroscopy of 52 Ar thus oﬀers a uniquechance to explore the robustness of the N = 34 subshellclosure and pin down the mechanism of its emergence.
The measurement of 52 Ar was performed at theRIBF as part of the third campaign of the SEASTARprogram.
The fast radioactive beam containing
53K, amongst other products, was produced by fragmentation of a ∼220 pnA 70 Zn primary beam at345 MeV/nucleon on a 10-mm thick Be target. The
constituents were identiﬁed using the BigRIPS frag-104

22103

2120102

1952

Ar
1018
17
2.5

2.6
2.7
2.8
Mass-to-charge ratio A/Q

2.9

1Fig. 1. Particle identification after the secondary target.

ment separator with the ∆E-TOF-Bρ method. The
incident beam, magnetically centered on 53 K, was impinged on a 150-mm thick MINOS5) liquid hydrogentarget to induce proton-removal reactions. The recoil
protons were detected by the MINOS TPC tracker5) toreconstruct the reaction vertex. The MINOS eﬃciency
was measured to be 90(5)%. The kinematic energy andintensity of the 53 K beam in front of the target were∼240 MeV/nucleon and 1.0 pps, respectively. The reaction residues passed through the SAMURAI6) magnet
with a central magnetic ﬁeld of 2.7 T, and were identiﬁed by a 24-element plastic hodoscope and two forward drift chambers. Figure 1 shows the particle identiﬁcation of the reaction residues. The de-excitation
γ rays from the reaction residues were measured by
the upgraded DALI2+ array,7) which consists of 226
NaI(Tl) crystals. The preliminary Doppler-corrected
γ-ray spectrum of 52 Ar was obtained, and a clear (2+1→ 0+1 ) candidate peak was found. Evidence for othertransitions in 52 Ar requires further analysis.
References

1) T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001).
2) S. N. Liddick et al., Phys. Rev. Lett. 92, 072502 (2004).
3) D. Steppenbeck et al., Nature (London) 502, 207(2013).
4) D. Steppenbeck et al., Phys. Rev. Lett. 114, 252501(2015).
5) A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
6) T. Kobayashi et al., Nucl. Instrum. Methods B 317, 294(2013).
7) I. Murray et al., in this report.