Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Measurement of the hyperfine structure of 197Au atomin superfluid helium
T. Fujita,*1 T. Furukawa,*2 K. Imamura,*3, *4 X. F. Yang,*3, *5 Y. Mitsuya,*4 M. Hayasaka,*6 T. Sagayama,*6S. Kishi,*6 T. Kobayashi,*7 H. Ueno,*3 T. Shimoda,*1 and Y. Matsuo*8
We have developed a new laser spectroscopic techniquecalled Optical Radioisotope atom Observation inCondensed Helium as Ion-catcher (OROCHI) forinvestigating the structure of exotic nuclei.1) In this method,
we observe atomic Zeeman splitting (ZMS) and hyperfinesplitting (HFS) by using optical pumping andlaser-microwave (MW) double resonance spectroscopy insuperfluid helium (He II) to derive nuclear spins andelectromagnetic moments. The characteristic opticalproperties of atoms in He II, for example, blue-shifted andconsiderably broadened absorption spectra, enables us toapply the optical pumping technique to several elements.
Recently, we performed a series of on-line experiments byusing energetic (up to 66 MeV/u) 84-87Rb beams from Riken
Projectile-fragment Separator (RIPS), and confirmed thefeasibility of the OROCHI method.2) Furthermore, we
succeeded in producing a large atomic spin polarization(>80 %) of 197Au by means of optical pumping in He II by
the laser light of the fourth harmonics of a LD-pumped
pulsed Nd:YLF laser (263.5 nm, 3 kHz). Subsequently, weplan to measure the spins and moments of neutron-deficient
Au isotopes possessing interesting structures.3)
As the first step, we measured the HFS of a stable 197Au
atom in an off-line experiment. Fig. 1 shows theexperimental apparatus. An open-topped cubic quartz cell ina cryostat is fully filled with He II. The produced Au atoms
are introduced into He II by using laser sputtering of thesample material with two pulsed lasers4). We observed theintensity of Laser Induced Fluorescence (LIF) by means ofa photomultiplier tube (PMT) through a monochromator forwavelength selection, and performed the laser-MW double
resonance spectroscopic measurements (MW power:typically 1 W).
Fig. 2 shows an HFS resonance spectrum of 197Au in HeII. In fact, the observed HFS resonance frequencies wereshifted because of the Zeeman interaction with the appliedmagnetic field. Then, we derived the HFS with thezero-magnetic field effect from two HFS resonancefrequencies measured by employing opposite polarization
directions of the pumping laser, + and −, respectively, forcancelling the shift due to the Zeeman effect. The deduced
HFS in this study was consistent with the literature value ofthe HFS of 197Au in vacuum (with an accuracy of 0.5 %).

The obtained value was slightly different from the literature
value as well as the values for 133Cs and 85,87Rb.1) The shift
is due to the pressure from surrounding helium atoms.
However, the slight shift can be neglected in the discussion
regarding the structure of nuclei.
The successful HFS measurement indicates the feasibilityof future measurements for neutron-deficient nuclei of Au
by using the OROCHI method. In the near future, we plan
to propose experiments with exotic Au isotopes.

*1

Department of Physics, Osaka University
Department of Physics, Tokyo Metropolitan University*3
RIKEN Nishina Center*4
Department of Physics, Meiji University*5
School of Physics, Peking University*6
Department of Physics, Tokyo Gakugei University*7Laser Technology Laboratory, RIKEN*8Department of Physics, Hosei University

Fig. 1. Experimental apparatus.

Fig. 2. HFS resonance spectrum of 197Au in He IIwith + polarized pumping laser light.
References*21) T. Furukawa et al.: Hyp. Int. 196, 191(2010).
2) T. Furukawa et al.: Nucl. Instrum. Meth. B 317, 590 (2013).
3) H.-J. Kluge and W. Nörtershäuser : Spectrochim. Acta Part B
58, 1031(2003).
4) T. Furukawa et al. : Phys. Rev. Lett. 96, 095301 (2006).

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