RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-1. Nuclear Physics

Rotational level structure of sodium isotopes
inside the “Island of Inversion” ††∗1∗2∗3∗4∗5∗6Condensed from the article in Prog. Theor. Exp. Phys.
2014, 053D01 (2014).
RIKEN Nishina Center
State Key Laboratory of Nuclear Physics and Technology,
Peking University
Advanced Science Research Center, Japan Atomic Energy
AgencyDepartment of Physics, Rikkyo University
Center for Nuclear Study, University of Tokyo
Department of Physics, Tokyo Institute of Technology400a)

300100Na, Mγ <45033Mγ < 3020040Mγ = 3201000600700800

Counts / 25 keV040b)

34c)

35Na, Mγ <43020100Counts / 25 keV

The study of neutron-rich Ne, Na, and Mg nuclei
around the breakdown of the N = 20 neutron magic
number, an area in the Segré chart termed “Island ofInversion”1) has provided a wealth of information on
the evolution of nuclear shell structure away from thevalley of β stability. Due to its location in the proximity of the neutron drip-line, accessing the “Island ofInversion” is an experimental challenge. In this paper,
we report on the first γ-ray spectroscopy performed
for the N = 23, 24 sodium isotopes 34,35 Na and a newtransition in 33 Na. For the latter nucleus, previous
measurements suggested that the observed two transi++tions originate from a 7/2+
1 → 5/21 → 3/2g.s. cascade
and the energy ratio was found to be close to an ideal
K = 3/2 rotational band in the strong coupling limit2) .
A 48 Ca beam with an average intensity of 70 particle nA was accelerated by the Superconducting Ring
Cyclotron to 345 MeV/u and incident on a 15 mm
thick beryllium production target. A combination oftwo magnetic dipoles and a 15 mm thick aluminum
degrader was utilized to filter a 36 Mg secondary beamwith the BigRIPS fragment separator3) by applying
the Bρ − ∆E − Bρ method. For further purification,a second aluminum degrader of 5 mm thickness wasinserted at the dispersive focal point of the second BigRIPS stage. After passing BigRIPS, the secondary
beams were incident on 2.54 g/cm2 carbon and 2.13
g/cm2 CH2 (polyethylene) reaction targets, respectively. BigRIPS was operated with its full momentum acceptance of ±3% and the average intensity of36
Mg was 90 particles per second. Gamma-rays emitted in coincidence with the secondary reactions weredetected with the DALI2 array4), which was composedof 186 large-volume NaI(Tl) detectors. The secondary
reaction products were identified with the ZeroDegree
Spectrometer3) .
In the present work, a third γ-ray transition was observed for 33 Na at 760(13) keV in addition to the two
known ones, and forms a doublet with the 7/2+
1 →34
5/2+
decay.
For
theodd-odd
nucleusNa,asin1

Counts / 25 keVP. Doornenbal,∗1 H. Scheit,∗1,∗2 S. Takeuchi,∗1 Y. Utsuno,∗3 N. Aoi,∗1 K. Li,∗1,2 M. Matsushita,∗1,∗4D. Steppenbeck,∗1 H. Wang,∗1,∗2 H. Baba,∗1 E. Ideguchi,∗5 N. Kobayashi,∗6 Y. Kondo,∗6 J. Lee,∗1S. Michimasa,∗5 T. Motobayashi,∗1 T. Otsuka,∗5 H. Sakurai,∗1 M. Takechi,∗1 Y. Togano,∗1 and K. Yoneda∗180Na, Mγ <4 151060

54002000500

1000

500

1000

1500
2000
Energy (keV)

Fig. 1. Doppler corrected γ-ray spectra in coincidence with33Na a), 34 Na b), and 35 Na c). The analysis was restricted to event with aγ-ray multiplicity Mγ of lessthan 4.

gle γ-ray transition was observed at 451(7) keV, while
the energy spectrum of 35 Na exhibited transitions at373(5) and 641(16) keV. The level structure of the oddeven sodium isotopes was found to be well describedby the SDPF-M effective interaction5) .
References
1) E. K. Warburton et al., Phys. Rev. C 41, 1147 (1990).
2) A. Gade et al., Phys. Rev. C 83, 044305 (2011).
3) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003(2012).
4) S. Takeuchi et al., Nucl. Instr. Meth. A 763, 596 (2014).
5) Y. Utsuno et al., Phys. Rev. C 60, 054315 (1999).

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