Ⅱ-9. Instrumentation
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Measuring the stopping position of the energetic radioactive Rbbeams in superﬂuid helium
M. Ito,∗1,∗2 K. Imamura,∗1 S. Akimoto,∗1,∗2 A. Takamine,∗1 K. Kikuchi,∗1,∗2 R. Mitsuyasu,∗1,∗2 T. Miwa,∗1,∗2A. Gladkov,∗1 M. Tajima,∗1 S. Go,∗1 M. Mukai,∗1 H. Endo,∗1,∗2 S. Sasamori,∗1,∗2 S. Takahashi,∗1,∗2Y. Fukuzawa,∗1,∗2 M. Hase,∗3 K. Kawata,∗1,∗4 A. Kitagawa,∗5 T. Wakui,∗5 H. Ueno,∗1 and Y. Matsuo∗1,∗2
We are developing a laser spectroscopy techniquecalled OROCHI (Optical RI-atom Observation in Condensed Helium as Ion-catcher) to study the nuclear
structure of short-lived and low-yield radioisotopes
produced at accelerator facilities.1) In OROCHI, ion
beams are injected into superﬂuid helium (He II),
where the ions are slowed down, stopped in a small
volume, and neutralized with nearly 100% eﬃciency.
Nuclear spins/moments are derived from the measurements of the Zeeman/hyperﬁne splitting energy using
the laser-radio frequency (RF)/laser-microwave (MW)double resonance. The feasibility of this method formeasuring the hyperﬁne splitting energy with an accuracy of six orders of magnitude was demonstrated
in oﬀ-line experiments conducted with Cs atoms.2)
Therefore, this method is expected to be promisingfor short-lived unstable nuclei applications.
In previous online experiments,1) our group observed laser-RF double resonance spectra for 84–87 Rb
ion beams provided by the RIKEN RIPS at∼60 MeV/nucleon. In the future, we plan to conduct
measurements on low-yield unstable nuclear atoms,
such as Ag, which are provided at energies as highas ∼300 MeV/nucleon at the RIKEN BigRIPS facility.
To demonstrate the applicability of our method to highenergy beams, we conducted experiments at the QSTHIMAC SB2 beamline, which can deliver a beam inexcess of 350 MeV/nucleon. Among the single-valenceelectron alkali atoms, Rb is one of the atoms whosebehavior in superﬂuid helium is well studied. Furthermore, 84 Rb is estimated to be produced with the highest yield with a primary beam of 84 Kr according to theLISE++ simulation. This is why we employed 84 Rb as
the target atom in our experiments. Because the number of atoms stopped in the observation region in He
II is small, a LIF (laser-induced ﬂuorescence) detection
mechanism with high eﬃciency is needed. Therefore,
the accurate measurement of the beam stopping position at HIMAC SB2 beamline is one of the importantdevelopments for the future double resonance measurement of atoms with unstable nuclei.
So far, in FY2019, we measured the beam yields of ahigh-energy 84 Rb beam at the HIMAC SB2 beamline
∗1∗2
∗3∗4
∗5RIKEN Nishina Center
Department of Advanced Sciences, Hosei University
National Institute for Materials Science (NIMS)
Center for Nuclear Study, University of Tokyo
Institute for Quantum Medical Science, National Institutes for Quantum and Radiological Science and Technology(QST-iQMS)- 105 -using liquid nitrogen as a stopping material to estimate the number of stopped atoms in the possible LIF
observation region.3) Then, in FY2021, we successfully
observed the LIF of 84 Rb atoms in superﬂuid helium(He II) at the HIMAC SB2 beamline.4)
In FY2022, the number of measurement points wasincreased compared to the beam experiment conductedin FY2021 to more precisely determine the beam stopping position. Two measurement methods were performed as shown in Fig. 1. (1) measurement of thenumber of injected ions that penetrated the plastic
scintillator placed in the center of the cryostat in normal ﬂuid liquid helium and (2) measurement of LIF
from atoms stopped in superﬂuid helium in the cryostat. A taper-ampliﬁed laser light5) was used for theirradiation of the cryostat center with a cross section
size of 3 mm × 6 mm. These two types of measurements were performed while varying the thickness ofthe Al degrader every 100 µm to optimize the degrader
thickness for eﬃcient LIF collection from 84 Rb atoms.
We are currently in the process of data analysis.

Fig. 1. Schematics of RI beam introduction to helium at
HIMAC SB2 beamline.

References
1) X. F. Yang et al., Phys. Rev. A 90, 052516 (2014).
2) K. Imamura et al., Hyperﬁne Interact. 73, 230 (2015).
3) K. Tsubura et al., RIKEN Accel. Prog. Rep. 53, 132(2020).
4) K. Tsubura et al., RIKEN Accel. Prog. Rep. 55, 91(2022).
5) R. Mitsuyasu et al., in the abstract of 25th Congress of
the International Commission for Optics (2022).