RIKEN Accel. Prog. Rep. 48 (2015)

Ⅰ. HIGHLIGHTS OF THE YEAR

Laser-RF double resonance spectroscopy of 84−87 Rb isotopes trappedin superfluid helium†
X.F. Yang,∗1,∗2 T. Furukawa,∗1,∗3 T. Wakui,∗1,∗4 K. Imamura,∗1,∗5 T. Fujita,∗1,∗6 Y. Mitsuya,∗1,∗5M. Hayasaka,∗1,∗7 Y. Ichikawa,∗1 Y. Ishibashi,∗1,∗9 H. Shirai,∗1,∗8 T. Suzuki,∗1,∗8 Y. Ebara,∗3 A. Hatakeyama,∗10M. Wada,∗1 T. Sonoda,∗1 Y. Ito,∗1 T. Kobayashi,∗11 S. Nishimura,∗1 M. Kurata-Nishimura,∗1 Y. Kondo,∗1,∗8K. Yoneda,∗1 H. Ueno,∗1 T. Shinozuka,∗4 T. Shimoda,∗6 K. Asahi,∗1,∗8 and Y. Matsuo∗1,∗12


Laser spectroscopy measurements of various isotopes
have provided a number of valuable results, such as
nuclear spins, moments, and charge radii. However, tofurther study exotic atoms that are far from stability, higher eﬃciency and higher resolution are strongly
required to overcome certain experimental limitations(low yield, limited beam time, large contaminations,and so on). Therefore, a new method called OROCHI
was developed for laser spectroscopy measurements ofradioactive isotopes (RIs) in superﬂuid helium (He II)using a small and controllable number of atoms.1) In
this method, using He II as the trapping medium forthe energetic ion beam and matrix for trapped atoms,
we aim to systematically determine the nuclear spins and moments of RIs with a low yield. This measurement is based on the observation of Zeeman andhyperﬁne structures by optical pumping and the double resonance method. Recently, we have succeededin trapping, polarization, and laser spectroscopy measurements of 84−87 Rb isotopes in He II.
A general introduction to the experimental principle and method has been presented elsewhere.2) In thisexperiment, both stable 85,87 Rb and unstable 84,86 Rb
energetic ions produced from the RIPS were counted
accurately and implanted into He II. The number ofatoms injected into He II was on the order of 104 pps for the current setup. Using the trapping position
control system, we conﬁrmed the precision of the trapping site (around 1 mm) of atoms in He II by detecting
laser-induced ﬂuorescence (LIF).3) On the basis of thetrapping volume (π × 5 × 5 × 1 mm3 ) of atoms in HeII (ion beam spot size: ϕ ≈ 10 mm), and the observation range (5 × 2 × 2 mm3 ) of the LIF detection system, we estimated that more than 20% of the trapped
atoms were used for laser spectroscopy measurements.
From the number of injected ions and detected LIF
photons, the number of detectable LIF photons from
one injected atom was estimated to be approximately








Fig. 1.: LRDR spectra of Rb isotopes trapped in He II. The
red solid line is the fitting function, as described in Ref.4)

0.2 - 0.3.
After the precise trapping of 84−87 Rb in He II, theatoms were optically pumped and polarized with a polarized laser light. In addition, laser-RF double resonance spectra of the 84−87 Rb atoms were measured
by monitoring the LIF signal as a function of the scanned external magnetic ﬁeld with a ﬁxed-frequency
transverse RF ﬁeld applied to the atoms, as shownin Fig. 1. Relatively high spin polarization (above40%) was achieved for the 84−87 Rb isotopes,4) which
was estimated from the diﬀerence in the LIF intensity between the linearly and circularly polarized laser.
The resonance peaks provide information regarding theZeeman splitting of atoms in a magnetic ﬁeld. Fromthese Zeeman resonance peaks, nuclear spin values for
84m,84−87
Rb isotopes were determined with reasonableaccuracy, after eliminating the eﬀect of the backgroundmagnetic ﬁeld.
In conclusion, we performed the eﬀective trapping
and laser spectroscopy measurements of 84−87 Rb isotopes with a controllable number of atoms in He II
using the newly developed method. The achievementof high trapping eﬃciency and high spin polarizationand the determination of the nuclear spins of RIs in
He II suggest the potential application of this method
to the study of various exotic particles in He II.

†Condensed from the article in Phys. Rev. A 90, 052516(2014)∗1 RIKEN Nishina Center∗2 Nuclear and Radiation Physics Section, KU Leuven∗3 Department of Physics, Tokyo Metropolitan University∗4 Cyclotron Radioisotope Center, Tohoku University∗5 Department of Physics, Meiji University∗6 Department of Physics, Osaka University∗7 Department of Physics, Tokyo Gakugei University∗8 Department of Physics, Tokyo Institute of Technology∗9 Department of Physics, University of Tsukuba∗10 Department of Applied Physics, Tokyo University of Agriculture and Technology∗11 Laser Technology Laboratory, RIKEN∗12 Department of Advanced Sciences, Hosei University

References
1) T. Furukawa et al.: Hyp. Int. 196, 191 (2010).
2) T. Furukawa et al.: Nucl. Instrum.Methods B 317, 590(2013).
3) X. Yang et al.: Nucl. Instrum.Methods B 317, 599(2013).
4) X. Yang et al.: Phys. Rev. A 90, 052516 (2014).

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