I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

In-gas-cell laser resonance ionization spectroscopy of 196, 197, 198 Ir†
M. Mukai,∗1,∗2,∗3 Y. Hirayama,∗3 Y. X. Watanabe,∗3 S. Schiﬀmann,∗4,∗5 J. Ekman,∗6 M. Godefroid,∗5 P. Schury,∗3Y. Kakiguchi,∗3 M. Oyaizu,∗3 M. Wada,∗3 S. C. Jeong,∗7 J. Y. Moon,∗7 J. H. Park,∗7 H. Ishiyama,∗1 S. Kimura,∗1H. Ueno,∗1 M. Ahmed,∗2 A. Ozawa,∗2 H. Watanabe,∗8 S. Kanaya,∗9 and H. Miyatake∗3
The experimentally measured nuclear quadrupole deformation parameters of mercury, gold, platinum, andiridium approach the value for the spherical shape withincreasing neutron number.1) However, for iridium isotopes, the ﬁnite-range droplet model (FRDM)2) predicts
a shape transition from prolate (A ≦ 196) to oblate(A ≧ 197) at A = 197. We have investigated the evolution of nuclear structure of iridium isotopes, 196–198 Ir(Z = 77, N = 119–121), with in-gas-cell laser ionization spectroscopy as the ﬁrst step of a systematic studyof nuclear structures for nuclei around N = 126 at theKEK Isotope Separation System (KISS).3) In the presentwork, we report the magnetic dipole moments (µ), meansquare charge radii, and nuclear quadrupole deformation
parameters (β2 ), as determined from hyperﬁne structure(HFS) measurements.
Neutron-rich iridium isotopes were produced using multi-nucleon transfer reactions of a 198 Pt target (12.5 mg/cm2 ) and 136 Xe beam (9.4 MeV/nucleon,
50 particle-nA).4) The target-like fragments were thermalized and neutralized in a gas cell ﬁlled with puriﬁed
Ar gas of ∼1 atm3) and re-ionized element-selectivelyby using a laser resonance ionization technique at theexit of the gas cell. The excitation transition of 5d7 6s24F9/2 → 9/2◦ (λ1 = 247.587 nm) was chosen for thespectroscopy, and the excitation laser was produced bya dye laser pumped by an excimer laser. The mass number was chosen using a dipole magnet with a mass resolving power of A/∆A ∼ 900. The mass-analyzed ionswere transported to a decay station, which consists ofa tape transport device, a multi-segmented proportionalgas counter, and four clover-type germanium detectors.
The extraction of each nucleus was conﬁrmed throughthe measurement of β-decay half-life. The HFS spectra
were measured by counting β-rays as a function of excitation laser frequency, as shown in Fig. 1. The measured
spectra were ﬁtted using the Voigt function for each transition between HFSs of ground and excited states to determine the µ and isotope shift values.
The µ value of 197 Ir (I π = 3/2+ ) was determined to be†∗1∗2∗3∗4∗5∗6∗7∗8∗9Condensed from the article in Phys. Rev. C 102, 054307 (2020)
RIKEN Nishina Center
Graduate School of Sciences and Technology, University ofTsukuba
Wako Nuclear Science Center (WNSC), Institute of Particle
and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK)
Department of Physics, Lund University
Spectroscopy, Quantum Chemistry and Atmospheric Remote
Sensing (SQUARES), Université libre de Bruxelles
Materials Science and Applied Mathematics, Malmö University
Rare Isotope Science Project, Institute for Basic Science
International Research Center for Nuclei and Particles in theCosmos, Beihang University
Department of Physics, Osaka University

Fig. 1. Spectrum of 197 Ir ion counts as a function of the excitation laser frequency with the best ﬁt (red solid line).
The black dotted line indicates the ﬁtted backgroundlevel. The spectrum was measured at an argon gas pressure of 74 kPa.

+0.27+0.10−0.03 µN . In the case of the unknown spin nuclei
196
Ir (I π = (0− )) and 198 Ir, the results of chi-square
ﬁtting strongly indicate that their nuclear spin are I > 0with ﬁtted µ values of +(0.31–0.36) µN (I = 1–3) for196
Ir and +(0.13–0.26) µN (I = 1–3) for 198 Ir.
The quadrupole deformation parameter was deduced
from the measured isotope shifts while assuming anaxial-deformed nuclear charge distribution and theoretical electronic factor of the excitation transition fromlarge-scale atomic calculations.5) The deduced |β2 | values were 0.06(2) and 0.07(2) for 197 Ir and 198 Ir, respectively.
The experimental µ values were compared with calculated µ values based on the strong coupling model.6,7)
The comparison shows good agreement in the µ value
for 197 Ir with the assumption of prolate deformation andsuggests I = 1− or 2− for 196 Ir and I = 2− for 198 Ir.
References

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Phys. A 342, 1 (1992).
2) P. Möller et al., At. Data Nucl. Data Tables 59, 185 (1995).
3) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res. B353, 4 (2015); Y. Hirayama et al., Nucl. Instrum. Methods
Phys. Res. B 412, 11 (2017).
4) Y. X. Watanabe et al., Phys. Rev. Lett. 115, 172503 (2015).
5) S. Schiﬀmann, M. Godefroid, J. Quant. Spectrosc. Radiat.
Transf. 258, 107332 (2020).
6) I. Hamamoto, Phys. Rev. C 99, 024319 (2019).
7) A. Bohr, B. R. Mottelson, Mat. Fys. Medd. K. Dan. Vidensk. Selsk. 27, 16 (1953).

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