Ⅲ-1. Atomic & Solid State Physics (Ion)
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Development of a spin polarized RI atomic beam apparatus the using
atomic resonance method
K. Imamura,∗1 Y. Fukuzawa,∗1,∗2 A. Takamine,∗1 S. Go,∗1 A. Gladkov,∗1 M. Tajima,∗1 and H. Ueno∗1
We are currently developing a spin-polarized radioactive isotope (RI) atomic beam apparatus using theatomic beam magnetic resonance (ABR) method to apply RIs to nuclear structure study and material sciences. The apparatus mainly consists of the following
two parts: i) An RI ion neutralizer based on a radio frequency (RF) trap and laser cooling technique for generating a low-energy RI atomic beam. ii) An ABR apparatus consisting of a sextupole magnet, dipole magnet, quadrupole magnet, and RF cavity for producing
a highly polarized RI atomic beam.1) In our method,
RI and laser-cooled ions are simultaneously trapped ina linear Paul trap at ﬁrst. Then, the energy of trapped
RI ions is sympathetically decreased by the Coulombinteraction between laser-cooled and trapped RI ions
(sympathetic cooling).2) After cooling the RI ions, anion neutralization gas is blown against the trapped ions.
An RI atomic beam produced by the above neutralization process is delivered to the ABR apparatus. In theABR apparatus, spin polarization is achieved by a combination of double spin selection using a magnetic ﬁeld
gradient generated by a sextupole and quadrupole magnet and single spin ﬂip by magnetic resonance with an
RF cavity assembled in a uniform magnetic ﬁeld generated by a dipole magnet. In the development process,establishing highly eﬃcient RI ion neutralization and
RI atomic beam transport method is crucial because
the ABR method itself is a well-established techniquein the ﬁeld of measuring nuclear electromagnetic moment of stable isotopes. In this study, we report on anoﬄine laser cooling experiment conducted using singly
charged 88 Sr ions toward sympathetic cooling for developing the neutralizer.
Figure 1 shows (a) the schematic of the setup and(b) corresponding energy levels of the 88 Sr+ ion in theexperiment. Laser pulses from a Nd: YAG laser (wavelength: 1064 nm, repetition rate: 1 Hz, pulse duration: 5 ns, intensity: ∼2.5 mJ) were focused on aSrTiO3 plate (width × hight × thickness = 10 × 10× 0.5 mm3 ) using a convex lens with a mm focal lengthof 350 mm. The 88 Sr+ ions produced by laser ablation
were trapped at the center electrode (denoted as “C”
in Fig. 1) in our linear Paul trap. For ion trapping, weintroduced He as the buﬀer gas whose pressure was approximately 2.0 × 10−2 Pa for buﬀer gas cooling. After200 s of buﬀer gas cooling, the buﬀer gas was quickly
pumped out and pressure was maintained in the rangeof 10−6 –10−7 Pa. To produce laser-cooled 88 Sr+ ions,
we prepared two lasers. One was the cooling laser with
∗1
∗2RIKEN Nishina Center
Department of Advanced Science, Hosei University

Fig. 1. (a)The schematic of the experimental setup. (b) The
related energy levels of Sr ion in the experiment.

awavelength of 422 nm corresponding to the D1 (5S1/2 –5P1/2 ) transition of the 88 Sr+ ion. The other was therepump laser (wavelength: 1092 nm) used to pump
88 +Sr ions de-excited in the 4D3/2 metastable state
to the 5P1/2 excited state. We irradiated the trappedions with both lasers in a co-propagation conﬁguration. The optical paths of the lasers were spatially combined using a dichroic mirror (Thorlabs, DMLP650).
The laser powers of the cooling laser and the repump
laser were 1.5 mW and 5 mW, respectively. The laser
induced-ﬂuorescence (LIF) emitted from 88 Sr+ ions wascollected using a convex lens with a focal length of100 mm and detected using a photo-multiplier tube
(Hamamatsu photonics, H16721-50). In order to reduce the background stray light, an interference ﬁlter
with a bandwidth of 435 ± 20 nm (Edmund optics, Fluorescence band-pass ﬁlter) was mounted in front of thePMT. We obtained the LIF spectra by scanning thelaser frequency of the cooling laser. So far, we have
conﬁrmed the onset of laser cooling eﬀect by observing the narrowing of the line width and change of line
shape under diﬀerent RF voltages applied to the trap
electrodes. Further optimization of the experimental
parameters, such as the applied laser power, number oftrapped ions, and so on, is under investigation.
Along with the development of the neutralizer, an
RF cavity is being developed. We have designed theRF cavity, whose resonance frequency is approximately
3 GHz, as a prototype. Currently, preparations to evaluate the performace of the cavity using an Rb vapor
enclosed glass cell is in progress now.
References
1) H. T. Duong et al., Nucl. Instrum. Methods Phys. Res.
A 325, 465 (1993).
2) D. J. Larson et al., Phys. Rev. Lett. 57, 70 (1986).

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