Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Searching optimum measurement conditions of the laser-microwave
double resonance for the atoms stopped in superfluid helium
M. Sanjo,∗1,∗2 K. Imamura,∗2,∗3 A. Takamine,∗2 T. Fujita,∗2,∗4 T. Egami,∗1,∗2 D. Tominaga,∗1,∗2W. Kobayashi,∗1,∗2 Y. Nakamura,∗2,∗3 T. Wakui,∗2,∗5 T. Furukawa,∗2,∗6 H. Ueno,∗2 and Y. Matsuo∗1,∗2We have developed a laser spectroscopic method
named OROCHI (Optical RI-atoms Observation in Condensed Helium as Ion catcher) aiming at nuclear structure studies of unstable nuclei with low production yields
and short lifetimes. In OROCHI, highly energetic ion
beams are eﬃciently caught as neutralized atoms in superﬂuid helium (He II) owing to its high density. In
addition, the absorption wavelength of atoms in He II issigniﬁcantly blue-shifted due to the eﬀect of surrounding He atoms while the emission wavelength is almost
the same as that in vacuum. This enables us to detectphotons emitted from the atoms with low background by
removing the excitation-laser stray light. Consequently,
we can measure Zeeman and hyperﬁne structure (HFS)
splittings with a high sensitivity by applying laser-RFand laser-microwave (MW) double resonance methods
to the atoms in He II.
So far, we have succeeded in observing the laser MW
double resonance (LMDR) signals for 104 particles per
second (pps) Rb ion beams with an energy of 66 A
MeV.1,2) However, the LMDR signal intensities were insuﬃcient to realize measurements of the HFS splittingfor lower intensity ion beams of less than 103 pps. In
general, it is expected that the higher-power laser andMW irradiations lead to higher resonance peak heights inLMDR spectroscopy. In order to estimate the optimum
measurement conditions, it is necessary to experimentally and quantitatively investigate the laser and MW㻸㻹㻰㻾㻌㻵㼚㼠㼑㼚㼟㼕㼠㼕㼑㼟㻌㼇㼏㼜㼟㼉㻌㻝㻚㻠㼤㻝㻜㻡㻝㻚㻞㼤㻝㻜㻙㻝㼉㻌㻸㼍㼟㼑㼞㻌㼑㼤㼏㼕㼠㼍㼠㼕㼛㼚㻌㼜㼞㼛㼎㼍㼎㼕㼘㼕㼠㼥㻌㼇㼟㻢㻢㻢㻜㻞㼤㻝㻜㻠㼤㻝㻜㻢㼤㻝㻜㻝㼤㻝㻜㻡㻤㼤㻝㻜㻠㻢㼤㻝㻜㻠㻠㼤㻝㻜㻠㻞㼤㻝㻜㻠㻜㻜㻝㻜㻞㻜㻟㻜㻠㻜㻡㻜㻞㼤㻝㻜㻡㻠㼤㻝㻜㻡㻢㼤㻝㻜㻡㻤㼤㻝㻜㻡㻝㼤㻝㻜㻢㻌㻸㼍㼟㼑㼞㼜㼛㼣㼑㼞㻔㼙㼣㻕㻸㼍㼟㼑㼞㻌㼜㼛㼣㼑㼞㻌㼇㼙㼃㼉㻸㼍㼟㼑㼞㻌㼑㼤㼏㼕㼠㼍㼠㼕㼛㼚㻌㼜㼞㼛㼎㼍㼎㼕㼘㼕㼠㼥㻌㼇㼟㻙㻝㼉㻌㻜㻝㻚㻞㼤㻝㻜㻡㻸㻹㻰㻾㻌㼕㼚㼠㼑㼚㼟㼕㼠㼕㼑㼟㻌㼇㼏㼜㼟㼉㻌㻤㼤㻝㻜㻢㻡㻢㻜㻹㼃㻌㼜㼛㼣㼑㼞㻌䠖㻢㻚㻜㻌㼃㻌䠖㻟㻚㻡㻌㼃㻌䠖㻝㻚㻥㻌㼃㻌㻡㻝㼤㻝㻜䠖㻜㻚㻝㻡㻌㼃㻌㻤㼤㻝㻜㻠㻢㼤㻝㻜㻠㻠㼤㻝㻜㻠㻞㼤㻝㻜㻠㻜㻜㻝㻜

㻞㻜
㻟㻜
㻠㻜
Laser power (mW)㻸㼍㼟㼑㼞㻌㼜㼛㼣㼑㼞㻌㼇㼙㼃㼉㻌㻡㻜㻢㻜

Fig. 1. Laser power dependence of LMDR intensities for different MW powers. The upper and lower ﬁgures show
the result using Rb cells containing He buﬀer gas of
1.3 × 104 Pa and 1.0 × 105 Pa, respectively.
∗1
∗2
∗3
∗4
∗5
∗6

Department of Advanced Science, Hosei University
RIKEN Nishina Center
Department of Physics, Meiji University
Department of Physics, Osaka University
QST
Department of Physics, Tokyo Metropolitan Universitypower dependence of LMDR signal intensities. As a pilot study, we measured the laser power dependence of
LMDR signal intensities for diﬀerent MW powers using
a glass cell containing Rb vapor with He buﬀer gas (“Rbcell”).
We irradiated the Rb atoms in the cell with a circularly polarized laser light (wavelength: 794 nm), andapplied a magnetic ﬁeld to the atoms, parallel to thelaser axis, in order to generate and maintain spin polarization. On scanning the MW frequency, we observeda resonant peak at the MW frequency corresponding tothe HFS splitting in the ground state of 85 Rb. We measured the LMDR signal intensities (deﬁned as resonantpeak heights in LMDR – background) for diﬀerent laser
powers when the MW powers were 6.0 W, 3.5 W, 1.9 W,
and 0.15 W.
Figure 1 shows the laser power dependence of LMDR
intensities for diﬀerent MW powers with two diﬀerent Rbcells containing 1.3 × 104 Pa and 1.0 × 105 Pa He buﬀergas. The stronger the laser and MW we applied, thehigher the LMDR intensities we observed. However, theintensities were saturated at a certain laser power. The
higher He buﬀer gas pressure case required higher laser
power for saturation. This means that the laser excitation cross section was decreased because of the eﬀect ofpressure broadening. The absorption spectral line width
in 1.3 × 104 Pa He gas is of the order of 1 GHz and thatin He II is of the order of 1 THz. Therefore, the laser
excitation probability is estimated to decrease by the order of 103 in He II similar that in to Ref. 3). We canroughly estimate that the saturation of LMDR signal intensities needs 103 times more laser power density in He
II compared to that in He gas. In the previous online
experiment, we performed the experiment with 100 mW
laser power (diameter: 2 mm), which corresponds to alaser excitation probability of 1.68×103 s−1 . Our present
result implies that this value was too low to saturate theLMDR signal intensities, and that it was in a linearly
increasing region at a low laser power, as depicted inFig. 1.
The laser power can be increased without increasing
the laser stray light by using the newly developed detection system.4) Now, we can irradiate atoms with up
to 1 W laser by using a Ti:Sa laser. By using a laser
power of 1 W, the LMDR intensities are expected to be
increased by a factor of 10.
References

1) X. F. Yang et al., Phys. Rev. A 90, 052516 (2014).
2) K. Imamura et al., JPS Conf. Proc. 6, 030115 (2015).
3) S. I. Kanorsky et al., Phys. Rev. B 49, 3645 (1994).
4) W. Kobayashi et al., RIKEN Accel. Prog. Rep. 50, 198(2017).
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