Ⅰ. HIGHLIGHTS OF THE YEAR
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Discovery of 39 Na†
D. S. Ahn,∗1 J. Amano,∗3 H. Baba,∗1 N. Fukuda,∗1 H. Geissel,∗5 N. Inabe,∗1 S. Ishikawa,∗4 N. Iwasa,∗4T. Komatsubara,∗1 T. Kubo,∗1 K. Kusaka,∗1 D. J. Morrissey,∗6 T. Nakamura,∗2 M. Ohtake,∗1 H. Otsu,∗1T. Sakakibara,∗4 H. Sato,∗1 B. M. Sherrill,∗6 Y. Shimizu,∗1 T. Sumikama,∗1 H. Suzuki,∗1 H. Takeda,∗1O. B. Tarasov,∗6 H. Ueno,∗1 Y. Yanagisawa,∗1 and K. Yoshida∗1

Atomic number （Z）

The location of the neutron dripline provides a key
benchmark for advanced nuclear models and theories.
It reﬂects the details of the underlying nuclear structure and interactions, which include the evolution of
the nuclear shell property and associated nuclear deformation. Thus, locating the neutron dripline experimentally provides a signiﬁcant key to understanding the nuclear structure under extremely neutron-richconditions.
In our previous study,1) we searched for the new isotopes 32, 33 F, 35, 36 Ne, and 38, 39 Na to investigate theneutron dripline at ﬂuorine (atomic number Z = 9),neon (Z = 10), and sodium (Z = 11). No events wererecorded for 32, 33 F, 35, 36 Ne, and 38 Na and only one
event was recorded for 39 Na. This enabled us to determine the neutron dripline for ﬂuorine and neon to be
31
F and 34 Ne, respectively, nearly 20 y after 24 O wasconﬁrmed as the dripline nucleus of oxygen (Z = 8).
In this study,2) we conducted a new experiment dedicated to searching speciﬁcally for 39 Na to establishthat it is particle-bound. The new isotope 39 Na has
the mass number A = 3Z + 6, located beyond the previously known most neutron-rich isotope 37 Na, which
was discovered 20 y ago;3) see Fig. 1. It is a strong
candidate to be the dripline nucleus of sodium, and establishing its existence provides a signiﬁcant extension

26Mg

38Mg
35Na

23Na
22Ne

32Ne
27F

19F

39Na

N = 28N = 28

29F

18O

N=N
=2020
17B

N = 16
N=16Stable isotopes
Unstable isotopes
Known dripline

A4 4A ==3Z
3Z+ +Neutron number（N）

39Na

Discoveredin this work

Fig. 1. Section of the nuclear chart indicating the locationof the 39 Na isotope discovered in this study.
†∗1
∗2
∗3
∗4
∗5
∗6Condensed from the article in Phys. Rev. Lett. 129, 212502(2022)RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Department of Physics, Rikkyo University
Department of Physics, Tohoku University
GSI Helmholtzzentrum für Schwerionenforschung GmbH
National Superconducting Cyclotron Laboratory, Michigan
State University- S7 -of the neutron dripline and a benchmark for nuclearstructure calculations as well as nuclear mass models.
It should be noted that 39 Na has the neutron number
N = 28, which is normally a magic number.
The search was conducted at the RIKEN RIBF using projectile fragmentation of an intense 48 Ca beamat 345 MeV/nucleon on a 20-mm-thick beryllium target. The projectile fragments including 39 Na were separated and identiﬁed in ﬂight by the large-acceptance
two-stage fragment separator BigRIPS.4,5) The intensity of the 48 Ca beam was as high as ∼540 particle nA.
The particle identiﬁcation was made at the second
stage of the BigRIPS separator, relying on the combination of time of ﬂight (TOF), magnetic rigidity (Bρ),
and energy loss (∆E) measurements, from which the Zand A/Z values were deduced for each fragment. TheTOF was measured between two thin plastic scintillators installed at the intermediate and ﬁnal foci of thesecond stage. The value of ∆E was measured using
a stack of six identical silicon semiconductor detectors
installed at the ﬁnal focus. The value of Bρ was determined from a position measurement at the intermediate focus using the plastic scintillator that alsomeasured the TOF. The separator setting was tuned
for the optimal transmission of 39 Na. The productionof 36 Ne was also revisited with another separator setting tuned for that of 36 Ne to improve the conﬁdence
level that 34 Ne is the dripline nucleus of neon.
After extensive running, we observed nine events for39
Na and clearly established that the 39 Na nucleus isparticle-bound. Furthermore, no events were observedfor 35, 36 Ne, which is consistent with their particle instability established in our previous work. The measurement enabled us to signiﬁcantly improve the conﬁdence level and hence ﬁrmly determine that 34 Ne is
the dripline nucleus.
The particle stability of 39 Na, established by thepresent discovery, suggests the occurrence of nuclear
deformation in 39 Na, as it induces more stability to
the nuclear binding. 39 Na could be particle-bound as
its ground state is deformed, suggesting the loss of theN = 28 magicity at sodium. This interpretation is supported by the recent state-of-the-art large-scale shellmodel calculation with ab initio eﬀective NN interactions,6) which reproduces the stability of 39 Na as wellas the neutron dripline at ﬂuorine and neon. The calculation reveals that the quadrupole deformation plays
a key role in nuclear binding in this region and thus indetermining the location of the neutron dripline.

Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 56 (2023)

References
1) D. S. Ahn et al., Phys. Rev. Lett. 123, 212501 (2019).
2) D. S. Ahn et al., Phys. Rev. Lett. 129, 212502 (2022).
3) M. Notani et al., Phys. Lett. B 542, 49 (2002).
4) T. Kubo, Nucl. Instrum. Methods Phys. Res. Sect. B
204, 97 (2003).
5) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003(2012).
6) N. Tsunoda et al., Nature 587, 66 (2020).

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