Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

Location of the neutron dripline at fluorine and neon†
D. S. Ahn,∗1 N. Fukuda,∗1 H. Geissel,∗5 N. Inabe,∗1 N. Iwasa,∗4 T. Kubo,∗1,a K. Kusaka,∗1 D. J. Morrissey,∗6D. Murai,∗3 T. Nakamura,∗2 M. Ohtake,∗1 H. Otsu,∗1 H. Sato,∗1 B. M. Sherrill,∗6 Y. Shimizu,∗1 H. Suzuki,∗1H. Takeda,∗1 O. B. Tarasov,∗6 H. Ueno,∗1 Y. Yanagisawa,∗1 and K. Yoshida∗1The location of the neutron dripline has been a longstanding issue, and it is only known up to oxygen (atomicnumber Z = 8) so far. No conﬁrmed extensions of thedripline above Z = 8 have been made for nearly 20 years,
since 24 O (neutron number N = 16) was determined to
be a dripline nucleus.
In order to investigate the neutron dripline for higherZ elements such as ﬂuorine (Z = 9), neon (Z = 10), andsodium (Z = 11), we conducted a search for the new isotopes 32, 33 F, 35, 36 Ne, and 38, 39 Na at the RIKEN RIBF.
We attempted to observe isotopes beyond the line of themass number A = 3Z + 4, where the most neutron-richisotopes known so far are located. The search was performed using the projectile fragmentation of an intense48
Ca beam at 345 MeV/nucleon on a 20-mm-thick beryllium target. The projectile fragments were separated
and identiﬁed in ﬂight by the large-acceptance two-stage
fragment separator BigRIPS.1,2) The 48 Ca-beam intensity was as high as ∼450 particle nA (∼ 3 × 1012 particles/s).
Particle identiﬁcation was performed 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 of the fragments were deduced.3) The TOF wasmeasured between two thin plastic scintillators installedat the intermediate and ﬁnal foci of the second stage.
The ∆E was measured using a stack of four identical silicon semiconductor detectors installed at the ﬁnal focus.
The Bρ was determined from a position measurement at
the intermediate focus using the same plastic scintillator.
The measurement was conducted with two Bρ settingsof the BigRIPS separator, the central particle of which
was tuned to be 33 F and the middle of 36 Ne and 39 Na,
respectively. In these settings, their neighboring isotopes
also had a reasonably good transmission.
a)After extensive running, we observed no events forF, 35, 36 Ne, and 38 Na and one event for 39 Na, implying that these unobserved isotopes were particle unbound. In order to assess the conﬁdence level of the implication, production cross sections were systematicallymeasured as a function of mass number, and the data
were compared with the predictions from the EPAX 2.1532, 33

†
∗1∗2
∗3
∗4
∗5
∗6aCondensed from the article in Phys. Rev. Lett. 123, 212501(2019)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
Corresponding author-7-Fig. 1. Section of the nuclear chart showing the location of
the neutron dripline determined in the present work.

systematics4) and the Qg systematics.5) These systematics allowed us to extrapolate the observed cross sections
and, hence, estimate the expected yields of the unobserved isotopes. The comparison of no observed eventswith the expected yields revealed that the existence ofbound 32, 33 F, 35, 36 Ne, and 38 Na was excluded with highconﬁdence levels. Thus, we concluded that the mostneutron-rich and heaviest bound nuclei, corresponding
to the dripline nuclei, are 31 F and 34 Ne for ﬂuorine andneon, respectively. The location of the neutron dripline
has thus been extended up to Z = 10 for the ﬁrst timein nearly 20 years, as shown in Fig. 1.
The location of the neutron dripline is sensitive to theunderlying nuclear structure of neutron-rich exotic nuclei, such as the evolution of nuclear shell structure andoccurrence of nuclear deformation, which are caused by
the characteristic eﬀects of complex nuclear forces.6–8)
We compared the present results with various nuclear
mass and structure models, and we found that no models could consistently account for the nuclear binding of
neutron-rich nuclei in the present Z region.
The present results provide new keys to understandingthe nuclear stability at extremely neutron-rich conditions
and also present a new challenge to nuclear mass models
as well as nuclear structure theories and calculations.
References

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2) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003(2012).
3) T. Ohnishi et al., J. Phys. Soc. Jpn. 77, 083201 (2008).
4) K. Sümmerer, B. Blank, Phys. Rev. C 61, 034607 (2000).
5) O. B. Tarasov et al., Phys. Rev. C 75, 064613 (2007).
6) D. S. Ahn et al., Phys. Rev. Lett. 123, 212501 (2019) andreferences therein.
7) T. Otsuka, Phys. Scr. T 152, 014007 (2013).
8) T. Otsuka et al., arXiv:1805.06501v5 [nucl-th] 2 Jan 2020.