Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Q-moment measurement of isomeric state of 99 Zr
using spin-aligned beam
Y. Shinohara,∗1,∗2 Y. Ichikawa,∗1,∗2 S. Go,∗2 H. Nishibata,∗1,∗2 S. Ando,∗1,∗2 K. Asahi,∗2 H. Baba,∗2N. Fukuda,∗2 G. Georgiev,∗2,∗3 A. Gladkov,∗2 K. Imamura,∗2 K. Kishimoto,∗1,∗2 R. Lozeva,∗2,∗3 M. Mukai,∗2M. Niikura,∗2 Nurhaﬁza M. Nor,∗2,∗4 A. Odahara,∗2,∗4 Y. Shimizu,∗2 M. Si,∗2,∗3 K. Stoychev,∗3 H. Suzuki,∗2M. Tajima,∗2 A. Takamine,∗2 H. Takeda,∗2 S. Takeshige,∗2,∗5 M. Tanaka,∗2 Y. Togano,∗2 H. Ueno,∗2T. Wakasa,∗1,∗2 W. Yamashita,∗1,∗2 H. Yamazaki,∗2 M. Yoshimoto,∗2 and J. M. Daugas∗2,∗6The 99 Zr nucleus with a neutron number N = 59 is
located at the border of a region where a sudden onset of ground-state deformation occurs for Zr isotopes
between N = 58 and 60.1) This change has been described as a quantum phase transition (QPT) with theneutron number as a control parameter. The 99 Zr nucleus closest to the critical point of the QPT has anisomer (99m Zr) with a spin parity of 7/2+ at 252 keV.
A recent measurement of its magnetic moment showeda surprising result. Namely, the 7/2+ state is not of
single-particle-like nature but rather collective.2) To
conﬁrm this collectivity directly, measurement of thequardupole (Q) moment of the same isomer was conducted.
The experiment was conducted at BigRIPS at RIBF
in April 2022. The two-step fragmentation schemewith the momentum-dispersion matching technique3,4)
was employed to produce a highly spin-aligned beamof 99 Zr. First, 100 Zn was produced by a ﬁssion reactionof a 345-MeV/nucleon 238 U beam on a 9 Be target witha thickness of 1.29 g/cm2 . The secondary 100 Zr beam
was impinged to a second target of wedge-shaped aluminum with a mean thickness of 0.810 g/cm2 and awedge angle of 2.65 mrad, placed at the momentumdispersive focal plane F5. The 99 Zr nuclei, including
those in isomeric state 99m Zr, were produced throughone-neutron removal from 100 Zr. The tertiary 99 Zrbeam was subsequently transported to F7 while matching the momentum dispersion of 99 Zr in F5–F7 to thatof 100 Zr in F3–F5. F7 slits with a width of ±10 mm
were used to extract the region around the center ofthe momentum distribution for 99 Zr.
Prior to the Q-moment measurement, the magnitude of spin alignment realized in 99m Zr was checkedby the time-diﬀerential perturbed angular distribution(TDPAD) methods. The TDPAD apparatus, placed atF8, consisted of a dipole magnet, Cu crystal stopper,
Ge detectors, and plastic scintillator. The dipole magnet provided a static magnetic ﬁeld of B0 = 0.200 T.
99m
Zr was implanted into the Cu stopper, and γ rays
were detected with four Ge detectors placed on a plane
∗1
∗2
∗3
∗4
∗5
∗6

perpendicular to B0 at angles of ±45 and ±135 degrees
with respect to the beam axis. A plastic scintillator
with a thickness of 0.1 mm was placed upstream of thestopper to provide the time-zero trigger.
The R(t) ratio, which represents the change of the γ
ray’s anisotropy synchronized with the spin precession,associated with the 130-keV γ ray deexciting 99m Zr wasobtained, as shown in Fig. 1. Preliminarily, the magnitude of spin alignment realized in this measurement
was found to be approximately 10%.
0.2

Preliminary
R(t)

0.1

0-0.1

-0.2
200

400600

800

1000

1200

Time (ns)Fig. 1. R(t) ratio associated with the 130-keV γ ray deexciting 99m Zr under a static magnetic ﬁeld. The magnitude of spin alignment was deduced from the amplitudeof the ocscillation.

For the Q moment measurement, the interaction between the Q moment and electric-ﬁeld gradient in asingle crystal of zyrconium metal was used to apply
the TDPAD method. The zyrconium crystal was set
so that the crystal axis was perpendicular to both thebeam axis and detector plane. The 99 Zr beam collimated to 10 mm ϕ was implanted into the stopper
crystal. Three Ge detectors were arranged at angles of
0 and ±90 degrees with respect to the beam axis. Data
analysis on the Q-moment measurement is in progress.
References

Department of Physics, Kyushu University
RIKEN Nishina Center
IJCLab, CNRS/IN2P3, Université Paris-Saclay
Department of Physics, Osaka University
Department of Physics, Rikkyo University
Institut Laue-Langevin

1) P. Campbell et al., Phys. Rev. Lett. 89, 082501 (2002).
2) F. Boulay et al., Phys. Rev. Lett. 124, 112501 (2020).
3) Y. Ichikawa et al., Nat. Phys. 8, 918 (2012).
4) Y. Ichikawa et al., Nat. Phys. 15, 321 (2019).

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