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

Oﬄine collinear laser spectroscopy of barium II toward measurement ofRIs at SLOWRI facility
M. Tajima,∗1 A. Takamine,∗1 Y. Sasaki,∗2,∗1 T. Asakawa,∗2,∗1 H. Iimura,∗3,∗1 M. Wada,∗4 H. A. Schuessler,∗5,∗1Y. Matsuo,∗2,∗1 and H. Ueno∗1∗1∗2
∗3
∗4
∗5

RIKEN Nishina Center
Faculty of Science and Engineering, Hosei University
JAEA
Wako Nuclear Science Center, KEK
Department of Physics and Astronomy, Texas A&M University- 15 -100000

138Ba+50000

137Ba+10000

Count rate of fluoresence signal [cps]The laser spectroscopy of atomic transitions is sensitive to nuclear structures in ground or metastablestates. In particular, the measurement of isotope shifts
and hyperﬁne splittings by collinear laser spectroscopy
has been a powerful tool to study RIs. Charge radii,
quadrupole deformation, and electromagnetic moments
have been elaborately measured at ISOL-type facilities
such as ISOLDE at CERN. However, the available nuclei are limited by chemical properties and lifetime with
the ISOL technique. On the other hand, the gas cell decelerator facility, SLOWRI,1) which is being developed
in RIKEN, will supply low-emittance and low-energy
RI beams including refractory elements and short-lived(≳10 ms) nuclei, which are diﬃcult to obtain from theISOL-type facility. We are preparing for collinear laser
spectroscopy by taking advantage of slow RI beams delivered from SLOWRI.
As a test of the system for collinear laser spectroscopy,
we performed the spectroscopy of barium ions. Barium was chosen because its ion production is relatively
easy and an existing dye laser is capable of inducing oneof the strong transitions, 5d 2 D5/2 -6p 2 P3/2 (614 nm).
A surface ionization source was constructed to obtain
Ba+ based on the design previously used at JAEA.2)
Barium oxide was heated up to approximately 2000 K
and a few nA of Ba+ were constantly produced. It was
extracted at 10.5 keV, focused by electrostatic lenses,isotope-separated by a magnet, and focused again toenter an observation region. A continuous-wave (CW)dye laser (Coherent 899 Ring Laser) with Kiton Red
pumped by a solid-state laser (Verdi V10, 6 W) was irradiated collinearly with the Ba+ beam. The power was∼0.3 mW in the observation region. The laser wavelength was locked at 614.10433 nm by utilizing feedback
signals from a wavelength meter (HighFinesse WS-U10). The full width at half maximum (FWHM) of thelaser frequency was ∼5 MHz. In the observation region,
the velocity of ions was changed using a set of electrodes.
When the velocity was tuned according to the Doppler
shift, the ion was at resonance, and spontaneous emission from 6p 2 P3/2 to 6s 2 S1/2 (455 nm) successively occured. This ﬂuoresence signal was detected by a photomultiplier (Burle 8850). The quantum eﬃciency of thephotomultiplier was about 20% at 455 nm, but the efﬁciency was less than 1% at 614 nm. Therefore, this

5000

15000

136

Ba+

10000
5000

135

Ba+

6000
4000
2000

4000

134

Ba+

2000

900

1000

1100

1200

Scaning voltage [V]

1300

1400

Fig. 1. Resonance spectra for the isotopes 134–138 Ba+ showing the transition 5d 2 D5/2 -6p 2 P3/2 .

measurement is almost free of background counts due tostray laser light. Figure 1 shows the count rate of theﬂuoresence signal as a function of the voltage applied to
the Doppler-shift-tuning electrodes. Isotope shifts andhyperﬁne splittings for 135, 137 Ba+ with the nuclear spin
I = 3/2 were successfully observed.
Trajectory simulations suggest that the resonancespectrum becomes asymmetric due to misalignment of
the laser and ion beams, which can be a main systematic error. Detailed analysis is ongoing. We plan to introduce a ﬁne-tuning system of alignment before online
measurement.
References

1) M. Wada, Nucl. Instrum. Methods Phys. Res. B 317, 450(2013).
2) S. Ichikawa et al., Nucl. Instrum. Methods Phys. Res. A274, 259 (1989).