II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Precision measurement of ground-state electric quardrupole momentfor neutron-rich 21 O
A. Gladkov,∗1 H. Yamazaki,∗1 Y. Ichikawa,∗1 A. Takamine,∗1 H. Nishibata,∗2 K. Asahi,∗1 K. Kawata,∗1,∗3
K. Imamura,∗1 M. Tajima,∗1 K. Tsubura,∗1 and H. Ueno∗1
The anomalous nuclear properties had been experimentally indicated for the neutron-rich 23 O isotope,such as the formation of halo structure 1) and the presence of the new neutron magic number at neutronnumber 2) N = 16. This originates the interest in theshell evolution in this region of neutron-rich oxygen isotopes. Such information can be directly provided by
the knowledge about the nuclear properties of neighboring isotopes, such as 21 O.
Previously, we have conducted the measurements ofthe electromagnetic moments of 21 O 3) and the groundstate magnetic moment has been successfully determined. However, due to RF tank circuit limitations,the obtained spectrum for the quadrupole moment (Qmoment) measurement was insuﬃcient to make ﬁrmconclusions and required further study. Since then, theexperimental setup was improved and the Q-moment
has been successfully measured.
The present experiment was carried out using theRIPS separator at the RIBF facility. A spin-polarized
beam of neutron-rich 21 O was produced in the projectile fragmentation reaction of a 22 Ne beam at
270 MeV/nucleon on a 185-mg/cm Be target. To ensure polarization, the momentum window and emission angle of the secondary fragments were selectedto be pF = p0 × (0.97 ± 0.03) and θF > 1.5◦ , respectively. Here, p0 is the fragment momentum corresponding to the projectile velocity. The secondary
beam of 21 O was then puriﬁed by the momentum andmomentum-loss analyses and delivered to the β-ray
detected nuclear magnetic resonance (β-NMR) apparatus installed downstream the beam line. The wellestablished method of β-NMR 4) in combination withadiabatic fast passage technique5) was applied to measure quadrupole moment.
The obtained nuclear quadrupole resonance (NQR)
spectra are shown on Fig. 1. It consists of the two series of measurements. The black circles represent themeasurement with ±44 kHz modulation of quadrupole
coupling constant νQ = eqQ/h, where q, Q and h denote the electric ﬁeld gradient of the stopper material,
the Q-moment and the Planck’s constant, respectively.
The spectra are plotted with Aβ P values measured aspeak,a function of deviation from the peak frequency νQwhere Aβ P is the β-decay asymmetry parameter, andP the 21 O nuclear spin polarization. A resonant peakformed by the β rays emitted from 21 O stopped at the∗1∗2∗3Fig. 1. β-NQR spectra of 21 O in TiO2 single crystal. Thefigure shows the results of two series of measurementswith νQ modulation of ±44 kHz (black circles) and±17 kHz (red circles) are shown.

substitution site of TiO2 is clearly identiﬁed in Fig. 1.
Due to the improvement in polarization detection, thesignal-to-noise ratio was greatly increased and a lesspronounced additional NQR eﬀect was detected at alower Q-moment region.
In order to investigate this minor peak in moredetail, a measurement with narrower scan width of±17 kHz was conducted. The result of this measurement is represented on Fig. 1 by the red solid circles. The centroid position of the small peak is consistent with the previous nuclear quadrupole resonance(NQR) measurement of 21 O at RIPS in 2016.3) The
origin of the additional lower-amplitude peak is under analysis. In overall, the two NQR measurements
conﬁrmed the existence of a major peak at higher Qmoment region that could not be detected in 2016 due
to several experimental limitations. The Q-moment
value of 21 O can be then ﬁrmly assigned based on theobtained results. The uncertainty assignment basedon the peak shape analysis and the discussion of theresults in terms of 21 O nuclear structure are work-inprogress.
References

1) C. Nociforo et al., Phys. Lett. B 605, 79 (2005).
2) D. Cortina-Gil et al., Phys. Rev. Lett. 93, 062501(2004).
3) A. Gladkov et al., RIKEN Accel. Prog. Rep. 51, 74 (2018).
4) D. Nagae et al., Nucl. Instrum. Methods Phys. Res. B
266, 4612 (2008).
5) A. Abragam, The Principles of Nuclear Magnetism(Springer, Oxford, 1961).

RIKEN Nishina Center
Department of Physics, Kyushu University
Center for Nuclear Study, The University of Tokyo- 28 -1