II-9. Instrumentation

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Oﬄine collinear laser spectroscopy of zirconium IIM. Tajima,∗1 A. Takamine,∗1 H. Iimura,∗2,∗1 M. Wada,∗3 H. A. Schuessler,∗4,∗1 Y. Matsuo,∗5,∗1 and H. Ueno∗1

∗1
∗2
∗3
∗4
∗5

RIKEN Nishina Center
Nuclear Science and Engineering Center, JAEA
Wako Nuclear Science Center (WNSC), IPNS, KEK
Department of Physics and Astronomy, Texas A&M University
Department of Advanced Science, Hosei University

The laser spectroscopy of atomic transitions is sensitive to nuclear structures in ground states. The measurement of isotope shifts and hyperﬁne splittings bycollinear laser spectroscopy (CLS) is a powerful tool forstudying radioactive isotopes (RIs) systematically alongisotope chains. Charge radii, quadrupole deformation,electromagnetic moments, and spins have been extensively measured at ISOL-type facilities. We are preparing for the CLS of RIs at the gas-cell decelerator facility,
SLOWRI. Our main targets include refractory elements
near the N = Z line in the intermediate mass region,
where experimental data are not suﬃcient because of theexperimental diﬃculties at ISOL-type facilities. Barium
isotopes have been measured as a ﬁrst oﬄine demonstration of CLS.1) This report describes the next step
wherein an oﬄine measurement of zirconium ions wasperformed.
Zirconium ions were produced from a disk-shaped
metal target by using pulsed-laser ablation. A Qswitched Nd:YAG laser (Coherent, Minilite II) was usedwith the time width of 5 ns at the repetition rate of 10 Hz
at the maximum. Ion beams were extracted at ∼10 keV
and mass separated so that a selected isotope could betransported to a photon detection region.1) We chosethe transition 4d2 (3 F )5p z4 G◦5/2 ↔ 4d2 (3 F )5s a4 F3/2 ,which is at 357 nm, and the strongest transition from theatomic ground state.2) A continuous-wave Ti:Sapphirelaser (M-square, SolsTis) pumped by a solid-state laser(Verdi, V10) was used to obtain the fundamental wavelength of 714 nm and a commercial second harmonics
generator with a LBO crystal (M-square, ECD-X) wasused. The fundamental wavelength was stabilized by awavelength meter within 10 MHz (FWHM); the power
was 0.5 mW at the detection region. The time-of-ﬂight(TOF) of the ions was measured by a channeltron (Photonis, Magnum 5901) at the downstream of the beamline. We found that the TOF of the resonant ions wereconcentrated within 50 μs. Therefore, a time gate forthe photon detection by a PMT was set to 50 μs for thetiming of the pulsed laser irradiation to suppress background counts.
Figure 1 shows the observed spectra with Voigt ﬁtting
curves. Since 90, 92, 94 Zr+ and 91 Zr+ were measured at
separate times and diﬀerent wavelengths were available
between them, the energy of ion beams was tuned to13.6 keV for 90, 92, 94 Zr+ and 10.6 keV for 91 Zr+ . The
background count rates were suppressed by a factor of1000 compared to that of the continuous measurement.
In the future, we plan to suppress backgrounds further      !"#$%  Fig. 1. Observed resonance spectra of Zr+ at 357 nm withthe time gate of 50 μs. Magenta lines represent the Voigt
ﬁtting curves.

with a shorter ion bunch by performing CLS with a multi
reﬂection TOF mass spectrometer3) where the bunch
length of 40 ns is possible. Although all 12 peaks were
not fully resolved for 91 Zr+ , hyperﬁne coeﬃcients forboth upper and lower levels were derived from ﬁve isolated peaks; these coeﬃcients were consistent with thosereported in previous measurements.4,5) In addition, thisexperiment is a benchmark for a new lifetime measurement of 93 Zr+ ,6) which is important to accurately evaluate the radioactivity of long lived ﬁssion products innuclear wastes.
References
1) M. Tajima et al., RIKEN Accel. Prog. Rep. 53, 15 (2020).
2) G. Ljung et al., Astron. Astrophys. 456, 1181 (2006).
3) P. Schury et al., Phys. Rev. C 104, L021304 (2021).
4) H. L. Thayer et al., J. Phys. G 29, 2247 (2003).
5) S. D. Rosner, R. A. Holt, J. Phys. B 49, 11501 (2016).
6) A. Takamine et al., in this report.

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