Measurement of the hyperfine splitting of 133 Cs atoms in superfluid helium

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1 Hyperfine Interact DOI /s z Measurement of the hyperfine splitting of 133 Cs atoms in superfluid helium K. Imamura T. Furukawa X. F. Yang Y. Mitsuya T. Fujita M. Hayasaka T. Kobayashi A. Hatakeyama H. Ueno H. Odashima Y. Matsuo Springer International Publishing Switzerland 2014 Abstract We have been developing a new nuclear laser spectroscopy method named OROCHI (Optical RI-atom Observation in Condensed Helium as Ion-catcher). OROCHI utilizes superfluid helium (He II) not only as an efficient stopping medium of highly energetic ions but also as a host matrix of in-situ atomic laser spectroscopy. Using these characteristic of He II, we produce atomic spin polarization and measure Zeeman and hyperfine structure (HFS) splitting using laser-rf (radio frequency) / MW (microwave) double resonance method. From the measured energy splittings, we can deduce nuclear spins and moments. So far, we have conducted a series of experiments using both stable ( 85,87 Rb, 133 Cs, 197 Au, 107,109 Ag) and unstable isotopes ( 84,86 Rb) to confirm the feasibility Proceedings of the 5th Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014) Canberra, Australia, September 2014 K. Imamura ( ) H. Ueno RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama , Japan kimamura@riken.jp T. Furukawa Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo , Japan X. F. Yang School of Physics, Peking University, Chengfu Road, Haidian District, Beijing, , China Y. Mitsuya H. Odashima Department of Physics, Meiji University, Higashi-Mita, Tama-ku, Kawasaki, Kanagawa , Japan T. Fujita Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka , Japan M. Hayasaka Department of Physics, Tokyo Gakugei University, 4-1-1Nukuikitamachi, Koganei, Tokyo , Japan

2 K. Imamura et al. of OROCHI method, especially observing Zeeman resonance and determining nuclear spins. The measurement of HFS splitting of atoms introduced into He II is indispensable to clarify the nuclear properties by deducing nuclear moments as well as the study of nuclear spins. For this purpose, we perform a precision measurement of HFS of 133 Cs atoms immersed in He II using laser ablation technique. In this paper, we describe the result of the experiment. Keywords Laser spectroscopy Superfluid helium 1 Introduction Atomic physics experiments for the determination of ground state properties of radioisotopes are one of the most effective tools for the study of nuclear structure [1]. Especially, laser based techniques provide model independent measurements of nuclear spins, moments, and mean-square charge radii [2]. These techniques have elucidated ground state properties by the precision measurements of atomic sub-level structure such as Zeeman, hyperfine structure (HFS) splittings, and isotope shifts. To extend the applicability of these techniques to rare isotopes, we have proposed and developed a new method that is an in-situ laser spectroscopy technique of neutral atoms in superfluid helium(he II). We call this new method OROCHI (Optical RI-atoms Observation in Condensed Helium as Ion-catcher) [3]. OROCHI utilizes He II both as an effective stopper for highly energetic ion beams and as a host matrix of in-situ laser spectroscopy of atoms. In OROCHI, energetic ions are injected in He II. Injected ions are neutralized during stopping process and trapped as atoms in the observation region. Trapped atoms are driven to spin polarization states using optical pumping. Subsequently, an electromagnetic wave field such as RF (radio frequency) or MW (microwave) is irradiated to spin polarized atoms, then Zeeman and hyperfine structure (HFS) splittings of trapped atoms are measured using double resonance method. From the measured Zeeman and HFS splittings, nuclear spins and moments are deduced respectively. Superfluid helium also has fascinating characteristics as a host matrix of laser spectroscopy [4]. When the atoms are immersed in He II, absorption spectra of atoms in He II are largely blue-shifted and broadened compared with those in vacuum. In contrast to absorption spectra, emission spectra are close to those in vacuum. When atoms are pumped using a laser light whose frequency is tuned to the absorption line and laser induced fluorescence (LIF) photons whose frequency is apart from the laser line are detected, we can achieve nearly background free condition. From the reasons above, we expect that OROCHI is a capable method for studying low-yield and short-lived nuclei. T. Kobayashi RIKEN Center for Advanced Photonics, 2-1 Hirosawa, Wako, Saitama , Japan A. Hatakeyama Department of Applied Physics, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo , Japan Y. Matsuo Department of Advanced Sciences, Hosei University, Kajino-cho, Koganei-shi, Tokyo , Japan

3 Measurement of the hyperfine splitting of 133 Cs atoms So far, we have successfully shown the feasibility of OROCHI method to achieve high degree of spin polarization of stable isotopes of 85,87 Rb( 50 %), 133 Cs( 90 %), 197 Au( 80 %), and 107,109 Ag( 80 %) which are introduced into He II using laser ablation technique [3, 5]. We succeeded in deducing nuclear spins from measured Zeeman splittings of alkali and alkali-like atoms. In parallel to the experiments with stable isotopes, we have conducted a series of experiments using Rb ion beams delivered from RIPS (RIKEN Projectile fragment Separator) in RIKEN Nishina Center [6]. We successfully observed Zeeman resonance of the injected Rb atoms. For further development of OROCHI, measurement of HFS of injected atoms is necessary to deduce nuclear moment using this technique. In early work of Kyoto university group, Takahashi, et. al. observed HFS transition of 133 Cs atoms in He II in 1995 [7]. The reported HFS value in He II was found to be about 0.6 % larger than that in vacuum. This difference can be interpreted as the effect of pressure from surrounding helium atoms. In other word, surrounding helium atoms compress the electron cloud of introduced atoms. However, the accuracy of transition frequency in the previous work was somewhat limited, because HFS resonance was observed by scanning the applied magnetic field which was used to produce spin polarization whereas the applied MW frequency was fixed. To know the effect of surrounding helium more precisely, the experiment to measure HFS splitting of 133 Cs atoms in He II by sweeping MW frequencies is more preferable. Therefore we perform an experiment in such a manner by using laser ablation method as a beam injection scheme. We here report the detail of our experiment and the obtained result. 2 Experimental setup The schematic diagram of experimental setup is given in Fig. 1. This experiment is conducted in a cryostat (Oxford Co. Ltd.). Liquid He II is filled in an open-topped quartz cubic cell ( mm 3 ). A solid Cs sample (φ = 10 mm, t = 5 mm) is placed at about 1 cm above the surface of liquid He II. A pulsed Nd:YAG laser (wavelength: 532 nm repetition rate: 10 Hz, pulse width: 10 ns) is focused on a solid Cs sample to produce clusters, ions, and atoms. Since liquid He II surface has the potential barrier of which height is about 1 ev, atoms can not be introduced into He II. Only produced clusters can overcome the potential barrier. Immersed clusters are dissociated to atoms using a pulsed femtosecond Ti:Sapphire (Ti:S) laser (Spectra physics, model: Hurricane) focused on the observation region. Cesium atoms, which are produced using two pulsed lasers and introduced into He II, are subjected to a irradiation of the pumping laser (CW Ti:S laser, Coherent Co. Ltd, model: 899, laser beam diameter φ = 1 mm, power = 100 mw) light to produce spin polarization. The pumping laser wavelength is tuned to the Cs D1 line in He II (876 nm). Laser induced fluorescence from the Cs atoms is focused on a monochromator (JASCO, model: CT25-C, grating dimension: mm, number of grooves: 1200 lines/mm) using three lenses. Monochromator is tuned to the D1 emission line in He II (892 nm) and fluorescence is detected by a cooled photomultiplier tube (HAMAMATSU, model: R636-10). The pumping laser light is switched from linear to circular polarization using an electric optical modulator (EOM) to confirm the degree of spin polarization. To produce spin polarized atoms, a static magnetic field of several gauss is applied to the observation region using a pair of Helmholtz coils (50 turns, φ = 120 mm). Microwave radiation whose frequency corresponds to the HFS of 133 Cs atom is applied to the spin polarized atoms to observe HFS resonance. Microwave generated by an oscillator (ANRITSU, model: MG-369B) is amplified and transmitted to two directions through a directional coupler.

4 K. Imamura et al. (a) (b) Fig. 1 A schematic diagram of setup is given in a. The inner part of the cryostat is shown in b Main part of the transmitted MW is applied to atoms using a dipole antenna. The other part is necessary for the measurement of the MW power. To evaluate transmitted MW power, not only applied MW power but also reflected MW power are monitored using MW power meters (Hewlett-Packard Co. Ltd, model: 84811A) installed at the other branches of the directional coupler. The variation of LIF intensity is recorded during MW frequency sweep to obtain spectra. For the accurate MW frequency measurement, the output MW frequency is changed step by step (so called step-sweep mode) at a certain frequency step (typically a few khz) from start to stop frequencies. Note that MW frequencies are phase-locked at each steps. 3 Result and discussion Figure 2 shows the observed HFS spectra of 133 Cs in He II. In this experiment, the resonance frequencies of the HFS resonance are shifted depending on the polarization direction of the pumping laser light. We observe the both case of using σ + and σ circularly polarized pumping laser light. The corresponding transition of each polarization is F = 4, m F = +4 F = 3, m F =+3 for σ + pumping and F = 4, m F = 4 F = 3, m F = 3 for σ pumping, respectively. Applied static magnetic field is 3.20(2) G during the measurement and applied MW power is about 3W at the exit of the directional coupler. The fluctuation of the baseline in the σ case is caused by the time variation of the number of atoms in the observation region. This variation of the number of atoms originates from large shot-to-shot fluctuation of the number of clusters generated by laser ablation. The resonance frequencies of two spectra are calculated by fitting to the Lorentzian function. By averaging two frequencies, we obtained preliminarily value of the HFS splitting of 133 Cs in He II as (2) MHz. By scanning the MW frequency in step mode and phase-locking at each frequency, the measurement accuracy is successfully improved one order of magnitude compared to the previous work. The known value in literature is (4) GHz deduced from hyperfine coupling constant A in [7]. In this work, measurement accuracy is limited mainly by the fluctuation of the applied magnetic field. The overall accuracy of this technique can be improved one or two orders of magnitude by installing the stabilized power supply for magnetic field. Although, the deduced A constant value using this method has uncertainty due to the pressure effect from surrounding He atoms, this pressure effect is considered to be the same for the electron cloud of the isotopes. Because nuclear moment values are calculated by taking the ratio of A constant between known and unknown isotopes, the uncertainty of A constant due to the pressure effect could be canceled.

5 Measurement of the hyperfine splitting of 133 Cs atoms Fig. 2 HFS resonance spectra obtained in this experiment. The left side figure is the case of σ + polarization laser light. In case of σ polarization is given in right side figure. The red curves in both figures show the fitting result using Lorentz function 4 Summary In summary, we have been developing a new laser spectroscopy technique that is called OROCHI. In OROCHI, He II is utilized both as an effective stopper for accelerated ion beam and as a host matrix of in-situ laser spectroscopy. Owing to high trapping efficiency of He II and optical proprieties of atoms in He II, we expect that OROCHI is one of the effective methods to study nuclear structure of low-yield exotic nuclei. To investigate information of nuclear structure, it is necessary to measure both Zeeman and HFS splittings of atoms using double resonance method. Although the feasibility of OROCHI for Zeeman splitting measurement is successfully shown in a series of experiments, the measurement of HFS was relatively scarce. In this work, we perform precision measurement of the HFS splitting of 133 Cs atoms in He II using laser ablation technique before we conduct experiment with beam injection scheme. By using the MW frequency scanning in step mode and phase-locking, we successfully improved measurement accuracy one order of magnitude compared to the early work [7] that used magnetic field scanning with the fixed MW frequency. In future, we will apply this technique to other spices such as 85,87 Rb, 197 Au, 107,109 Ag to confirm applicability of our method. Furthermore, to apply OROCHI to lowyield exotic nuclei, we will upgrade OROCHI system and conduct systematic measurement of HFS using Rb beams. Acknowledgments This work was partly supported by KAKENHI (Grant-in-Aid for Scientific Research). On of the author (K I) is grateful to kind support of RIKEN Junior Research Associate Program. References 1. Blaum, K., et al.: Phys. Scripta T152, (2013) 2. Cheal, B., Flanagan, K.T.: J. Phys. G Nucl. Partic. 37, (2010) 3. Furukawa, T., et al.: Hyperfine Interact. 196, 191 (2010) 4. Moroshkin, P., et al.: Low Temp. Phys. 32, 981 (2006) 5. Furukawa, T., et al.: Phys. E. 43, 843 (2011) 6. Furukawa, T., et al.: Nucl. Instrum. Meth. B 317, 590 (2013) 7. Takahashi, Y., et al.: Z. Phys. B Con. Mat. 98, 391 (1995)