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

g-Factor of the 99 Zr isomer: monopole evolution in the shape-coexistingregion†F. Boulay,∗1,∗2,∗3 G. S. Simpson,∗4 Y. Ichikawa,∗2 S. Kisyov,∗5 D. Bucurescu,∗5 A. Takamine,∗2 D. S. Ahn,∗2K. Asahi,∗2,∗6 H. Baba,∗2 D. L. Balabanski,∗2,∗7 T. Egami,∗2,∗8 T. Fujita,∗2,∗9 N. Fukuda,∗2 C. Funayama,∗2,∗6T. Furukawa,∗2,∗10 G. Georgiev,∗11 A. Gladkov,∗2,∗12 M. Hass,∗13 K. Imamura,∗2,∗14 N. Inabe,∗2 Y. Ishibashi,∗2,∗15T. Kawaguchi,∗2,∗8 T. Kawamura,∗9 W. Kim,∗12 Y. Kobayashi,∗16 S. Kojima,∗2,∗6 A. Kusoglu,∗11,∗17 R. Lozeva,∗11S. Momiyama,∗18 I. Mukul,∗13 M. Niikura,∗18 H. Nishibata,∗2,∗9 T. Nishizaka,∗2,∗8 A. Odahara,∗9 Y. Ohtomo,∗2,∗6D. Ralet,∗11 T. Sato,∗2,∗6 Y. Shimizu,∗2 T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2 L. C. Tao,∗2,∗19 Y. Togano,∗6D. Tominaga,∗2,∗8 H. Ueno,∗2 H. Yamazaki,∗2 X. F. Yang,∗20 and J. M. Daugas∗1,∗20.03
0.02
0.01

R(t)

A rapid change of the ground-state shape has been
known to occur in neutron-rich Zr isotopes between thespherical 98 Zr and deformed 100 Zr.1) This change has
been described as a quantum phase transition (QPT)
with the neutron number as a control parameter. The
99
Zr nucleus closest to the critical point of the QPT has
an isomer (99m Zr) with a spin parity of 7/2+ at 252 keV.
The interaction between the πg9/2 and νg7/2 orbitals has
been thought to be important for this QPT to occur,2)
and the 7/2+ state may be key to this mechanism. In
the present study, the nature of 99m Zr was investigatedthrough the magnetic moment.
The magnetic-moment measurement was performedat the BigRIPS at RIBF. 99m Zr was produced and spinaligned via the abrasion-ﬁssion of a 345-MeV/nucleon
238
U beam impinged on a 100-µm-thick 9 Be target. The
g-factor of 99m Zr was measured by the time-diﬀerential
perturbed angular distribution (TDPAD) method. Figure 1 shows the R(t) ratio evaluated using γ rays of
130 keV and 122 keV by assuming pure M 1 and E2 transitions, respectively, where a degree of spin-alignment of1.5(4)% was extracted. The g factor of 99m Zr determined
was determined as |g| = 0.66(4); thus, the magnetic moment is |µ| = 2.31(14) µN .
This value is far from the Schmidt value of gfree =+0.425 for the νg7/2 orbital, indicating that 99m Zr is
not in a pure (νg7/2 )1 state. A comparison of the exper-0-0.01
-0.02
-0.03
200

400

600

800

1000

1200

1400

Time (ns)Fig. 1. R(t) ratio associated with two γ rays.

7/2+7/2+252 |μ|=2.31(14)μN
B(E2)=1.16(3) W.u.

3/2+

122 |μ|=0.42(6)μN
B(M1)=0.0102(3) W.u.
0 μ=-0.930(4)μN

1/2+Exp.

441 μ=2.08μN

B(E2)=2.66 W.u.

3/2+
1/2+

B(M1)=0.0109 W.u.
30 μ=0.38μN
0 μ=-1.29μN

IBFM-1Fig. 2. Comparison between experimental and theoreticalvalues. The IBFM-1 calculation is based on the transition probabilities among the lowest three states and themagnetic moments of the lowest two states.3)

Condensed from the article in Phys. Rev. Lett. 124, 112501(2020)∗1 CEA, DAM, DIF∗2 RIKEN Nishina Center∗3 GANIL, CEA/DSM-CNRS/IN2P3∗4 LPSC, CNRS/IN2P3, Université Joseph Fourier Grenoble 1∗5 IFIN-HH∗6 Department of Physics, Tokyo Institute of Technology∗7 ELI-NP, IFIN-HH∗8 Department of Advanced Sciences, Hosei University∗9 Department of Physics, Osaka University∗10 Department of Physics, Tokyo Metropolitan University∗11 IJCLab, CNRS/IN2P3, Université Paris-Saclay∗12 Department of Physics, Kyungpook National University∗13 Department of Particle Physics, Weizmann Institute∗14 Department of Physics, Meiji University∗15 Department of Physics, University of Tsukuba∗16 Department of Informatics and Engineering, University ofElectro-Communications∗17 Department of Physics, Istanbul University∗18 Department of Physics, University of Tokyo∗19 School of Physics, Peking University∗20 Intsituut voor Kern-en Stralingsfysica, K. U. Leuven†

imental values with the results of the interacting bosonfermion model (IBFM-1), as shown in Fig. 2, suggeststhat this state is strongly mixed with the main composition being νd5/2 . Furthermore, we found that monopole
single-particle evolution changes signiﬁcantly with theappearance of collective modes.4,5)
References

1) P. Campbell et al., Phys. Rev. Lett. 89, 082501 (2002).
2) P. Federman, S. Pittel, Phys. Lett. B 69, 385 (1977).
3) P. Spagnoletti et al., Phys. Rev. C 100, 014311 (2019).
4) Y. Tsunoda et al., Phys. Rev. C 89, 031301(R) (2014).
5) T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).

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