Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Fluorescence detection and double resonance spectroscopy using85
Rb beam for the development of OROCHIK. Imamura,∗1,∗2 T. Egami,∗1,∗3 W. Kobayashi,∗1,∗3 M. Sanjo,∗1,∗3 A. Takamine,∗1 T. Furukawa,∗1,∗4T. Fujita,∗1,∗5 D. Tominaga,∗1,∗3 Y. Nakamura,∗1,∗2 Y. Ichikawa,∗1 T. Sato,∗1 H. Nishibata,∗1 A. Gladkov,∗1,∗6
L. C. Tao,∗1,∗7 T. Kawaguchi,∗1,∗3 Y. Baba,∗1,∗3 M. Iijima,∗1,∗3 H. Gonda,∗1,∗3 Y. Takeuchi,∗1,∗3R. Nakazato,∗1,∗3 T. Wakui, ∗1,∗8 H. Ueno,∗1 H. Odashima,∗2 and Y. Matsuo∗1,∗3Nuclear structure study using atomic laser spectroscopy has revealed the nuclear spins and momentsof a wide variety of nuclei with high accuracy in thepast decades. However, in order to apply these methods to nuclei with a production rate of less than 100pps, a highly sensitive method is strongly desired.
To establish such a method, we are developing a laser
spectroscopic method for a smaller number of atoms insuperﬂuid helium (He II) to study their nuclear structure. We call this method OROCHI (Optical RI-atomObservation in Condensed Helium as Ion-catcher). InOROCHI, superﬂuid helium works as an eﬀective stopper for highly energetic ion beams. Most of the immersed ions (nearly 100%) in He II are neutralized in
the stopping process and are stopped as atoms. In addition, the center wavelengths of the absorption andemission spectra of the atoms in He II have an large
diﬀerence because of the eﬀect of the surrounding HeII1) We can detect emitted photons from the stopped
atoms with an extremely low background using wavelength separation. From the features of He II mentioned above, OROCHI should achieve a highly sensitive detection of the ﬂuorescence from atoms.
So far, we have shown the validity of the principle ofour method by observing both laser-MW (microwave)
and laser-RF (radio frequency) double resonance spectra using a 84−87 Rb beam2) However, we also foundthat the reduction of the stray laser light was not suﬃcient. Therefore, the applicable ion beam intensity waslimited. In order to apply OROCHI to rare isotopes
whose production rate is less than 100 pps, we haveupgraded the ﬂuorescence detection system to reduce
the stray laser light more eﬀectively3) . Last December, we conducted an online experiment using an 85 Rbbeam to conﬁrm the validity of our development.
Figure 1(a) schematically shows the experimental
setup. The 85 Rb ions delivered from RIPS were injected into He II and stopped at the center of thecryostat. In order to measure the ion beam intensity,
we counted the number of injected Rb ions using two
∗1
∗2
∗3
∗4
∗5
∗6
∗7∗8RIKEN Nishina Center
Department of Physics, Meiji University
Department of Advanced Science, Hosei University
Department of Physics, Tokyo metropolitan University
Department of Physics, Osaka University
Department of Physics, Kyungpook National University
School of Physics, Peking University
National Institute for Quantum and Radiological Scienceand Technology(QST)- 197 -

photomultiplier tubes (PMTs) and a plastic scintillator. The stopped atoms were subjected to a laser light
whose wavelength corresponds to the Rb D1 line in HeII, namely 780 nm1) The ﬂuorescence from the atoms
was collected through two lenses and focused on the entrance of an optical ﬁber. The exit of the optical ﬁber
was connected to the entrance of a monochoromatorthat was used to select 794-nm emission light. Finally,
the ﬂuorescence was detected using PMT. Figure 1(b)shows the detected photon intensity variation due tothe switching of the ion beam and laser. The ion beam
intensity and the irradiated laser power were approximately 104 pps and 400 mW, respectively. This result
conﬁrms the ﬂuorescence detection using the upgradedsystem and eﬀective background suppression. In addition, we also successfully observed laser-RF double resonance spectra by applying both a switching magnetic
ﬁeld and an RF ﬁeld whose frequency was scanned.
As shown in Fig. 1(c), the detected photon count rate
clearly decreases with the production of the spin polarization when the magnetic ﬁeld is ON. The degree ofthe polarization is approximately 35% which was comparable to that obtained in the previous experiment2)
We are now analyzing these data to estimate thelower limit of the applicable beam intensity.

Fig. 1. Experimental setup and obtained result. (a) Experimental setup. (b) Variation of the detected photon
intensity due to the switching of the ion beam and thelaser. (c) the RF resonance spectrum.

References
1) Y. Takahashi et al. Phys. Rev. Lett. 71, 1035 (1993).
2) X. F. Yang et al. Phys. Rev. A 90, 052516 (2014).
3) W. Kobayashi et al. in this report.