II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Development of ion trap system for the neutralizer toward production ofspin-polarized RI beam using atomic beam magnetic resonance method
K. Imamura,∗1 A. Takamine,∗1 and H. Ueno∗1
Spin-polarized radioactive isotope (RI) beams havebeen used for the measurements of nuclear electromagnetic moments and spins to investigate nuclear
structure. In many experiments, the polarization of nuclear spin is generated via a projectile fragmentation reaction. Although the reaction mechanisms have been
well studied so far, the achievable spin polarization istypically as low as several percentage points.1) Laser
optical pumping can realize spin polarization of greaterthan a few tens of percentage points, but the method iselement-limited. A highly eﬃcient and universal method
for producing spin-polarized RI beams is desired. For
this purpose, we are developing a spin-polarized beam
production apparatus using the atomic beam magnetic
resonance (ABMR) method. In the method, the spinpolarization of neutral atomic beams is generated by
two-step spin selection using multi-pole magnets andmagnetic resonance. The eﬃcient production of a thermal energy neutral RI atomic beam is signiﬁcantly difﬁcult from a technical perspective, although the ABMR
method itself is one of the conventional methods for investigating spin-related nuclear properties of nuclei near
the stability region.2) To overcome this diﬃculty, we propose a new ion neutralizer which, combines ion trappingwith the laser cooling technique. The main feature of
the neutralizer is that it traps and cools RIs producedby nuclear reactions to decrease the beam energy.
In the system, we ﬁrst prepare laser-cooled ions in alinear Paul trap. RI ions of several eV are introduced tothe trap region of the laser-cooled ions and trapped using He buﬀer gas cooling. The trapped RI ions are sympathetically cooled down through Coulomb interaction
between laser-cooled ions and the RI ions. After cooling,a neutralization gas (e.g., N2 , NH3 , and so on) is blown
against the RI ions. Subsequently, neutral atoms areproduced by charge-exchange interaction. For the eﬃcient extraction of the neutral atoms, we apply a swing
ﬁeld in the beam-propagation direction to move trapped

Fig. 2. Mass spectrum obtained downstream of the linear
Paul trap.

RI ions back and forth prior to the neutralization.
In order to develop the proposed neutralizer, we initiated an oﬄine R&D experiment using Rb by constructing a linear Paul trap system and quadrupole mass ﬁlter(QMF) for selecting ions that are introduced to the trap
region. The experimental setup for the R&D experimentis shown in Fig. 1. Rb ions emitted from a surfaceionization ion source were guided to a linear Paul trap
after mass selection by a QMF. The electrodes of thetrap were assembled at the head of the cryostat, which
is used for realizing a suﬃcient high vacuum to conduct the ion trapping and laser cooling experiment. We
achieved a vacuum of ∼10−8 Pa in the current setup.
Two gas lines were prepared near the center part of thetrap. One is used for introducing He as the buﬀer gas toperform buﬀer gas cooling. The other is used for introducing a neutralization gas. A channel electron multiplier (CEM) detector was placed downstream of the trap
to detect transported ions.
Toward the Rb ion trap experiment, we conducted anion transportation test. Figure 2 shows a mass spectrumof the ions transported. The vertical and horizontal axes
correspond to ion counts and RF voltages applied to the
QMF electrodes, respectively. Three peaks correspondto singly charged K, Rb, and Cs. According to thisresult, we conﬁrmed that suﬃcient Rb ions to test ion
trapping are guided from the ion source to the trap. We
will proceed to the next step of Rb ion trapping. We
will then start a neutralization test in the near future.
References

Fig. 1. Schematic diagram of the setup.
∗1

RIKEN Nishina Center

1) K. Asahi et al., Phys. Lett. B 251, 488 (1990).
2) N. F. Ramsey, Molecular Beams (Oxford University
Press, New York, 1956).

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