Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

In-beam γ-ray spectroscopy of 34,36,38Mg: Merging the N = 20 and
N = 28 shell quenching†
P. Doornenbal,∗1 H. Scheit,∗1,2 S. Takeuchi,∗1 N. Aoi,∗1 K. Li,∗1,2 M. Matsushita,∗1,3 D. Steppenbeck,∗1H. Wang,∗1,2 H. Baba,∗1 H. Crawford,∗4 C.R. Hoffman,∗5 R. Hughes,∗6 E. Ideguchi,∗7 N. Kobayashi,∗8Y. Kondo,∗8 J. Lee,∗1 S. Michimasa,∗7 T. Motobayashi,∗1 H. Sakurai,∗1 M. Takechi,∗1 Y. Togano,∗1R. Winkler,∗9 and K. Yoneda∗1†
∗1
∗2
∗3
∗4
∗5∗6
∗7
∗8
∗98038Mg
Mγ <460656(6)401360(20)Counts / 25 keV

The neutron-rich 10 Ne, 11 Na, and 12 Mg isotopes arelocated within a region known as the “Island of Inversion” and form one of the most notable regions of sudden shell structure change. Abnormally high masses
were discovered for 31,32 Na, leading to the presumptionthat the νf7/2 orbitals intrude into the sd shell orbitals,
thereby quenching the N = 20 shell gap. Later theoretical works predicted, however, that not the entireorbitals are inverted but ν(sd)−2 (f p)2 (2h̄ω) configurations are lowered so much in energy that they form
the ground states for 10 ≤ Z ≤ 12, 20 ≤ N ≤ 22 nuclei
instead.
The N = 28 magic number is originally formed by
the large νf5/2 –νf7/2 spin-orbit splitting but is also
known to vanish, as seen in the large deformation aris1,2)
ing for 42. Initially believed to be two isolated re14 Si
gions, we show in this letter that the N = 20, 28 shellquenching is interlinked via the neutron-rich magnesium isotopes, thereby forming a new connected large
area of deformation in the Segré chart.
Key information on the shape of a nucleus can beobtained for even-even nuclei from the energy of the++first excited 2+ state E(2+1 ), the first 4 state E(41 ),++and their E(41 )/E(21 ) ratio, R4/2 . Previous studies
revealed a low excitation energy of 660(6) keV for the362+
Mg and suggest that the “Island of In1 state inversion” stretches at least to neutron number N = 24for the magnesium isotopes and thus beyond its originally proposed boundaries3) . In the present study,+the experimental knowledge of the E(2+1 ) and E(41 )38
is extended to the N = 26 nucleus Mg via one- andtwo-proton removal reactions..
A primary beam of 48 Ca with an average intensityof 70 particle nA and an energy of 345 MeV/nucleon
was impinging on a 15 mm thick rotating Be target located at the BigRIPS fragment separator’s entrance. Secondary beams were selected and purified
via the Bρ − ∆E − Bρ method, and identified with2000500

1000

1500
2000
Energy (keV)

Fig. 1. Doppler corrected γ-ray energy spectrum in coincidence with 38 Mg detected in BigRIPS and ZeroDegree.

the ∆E − Bρ − TOF method. The rate for 39 Al and40
Si isotopes transported through BigRIPS was 75 and3000 pps, respectively. The secondary beams were incident on a 2.54 g/cm2 thick carbon secondary target,
which was surrounded by the DALI2 spectrometer4) .
Reaction residues from the secondary target were identified by the ZeroDegree Spectrometer, applying again
the ∆E − Bρ − TOF method.
Two γ-ray transitions were observed in 38 Mg fromthe 1p and 2p knockout channels after correcting forthe Doppler shift, as shown in Fig. 1, which were+++attributed to the 2+
1 → 0gs and the 41 → 21 de36
cays. In Mg, following a different reaction channel, asecond transition was observed and attributed to the+34
4+
Mg known values were
1 → 21 decays, while fordetermined with higher accuracy5). Almost constantR4/2 ratios of 3.14(5), 3.07(5), and 3.07(5) were obtained for 34,36,38 Mg at N = 22, 24, 26, close to theideal value of 3.33 for a rigid rotor. The values werein agreement with state-of-the art shell model calculations and suggested that the N = 20 and N = 28
shell quenching merge for the neutron-rich magnesium
isotopes.

Condensed from the article in Phys. Rev. Lett. 111, 212502(2013)RIKEN Nishina Center
School of Physics, Peking University
Department of Physics, Rikkyo University
Nuclear Science Division, Lawrence Berkeley National Laboratory
Physics Division, Argonne National Laboratory
Department of Physics, University of Richmond
CNS, University of Tokyo
Department of Physics, Tokyo Institute of Technology
NSCL, Michigan State University

References1) B. Bastin et al., Phys. Rev. Lett. 99, 022503 (2007).
2) S. Takeuchi et al., Phys. Rev. Lett. 109, 182501 (2012).
3) A. Gade et al., Phys. Rev. Lett. 99, 072502 (2007).
4) S. Takeuchi et al., RIKEN Acc. Prog. Rep. 36, 148(2003).
5) K. Yoneda et al., Phys. Lett. B, 233 (2001).

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