Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 52 (2019)

Absolute optical absorption cross-section of Rb atoms injected intosuperfluid helium using energetic ion beams†
K. Imamura,∗1,∗2 Y. Matsuo,∗2,∗3 W. Kobayashi,∗2,∗3 T. Egami,∗2,∗3 M. Sanjo,∗2,∗3 A. Takamine,∗2 T. Fujita,∗2,∗4D. Tominaga,∗2,∗3 Y. Nakamura,∗1,∗2 T. Furukawa,∗5 T. Wakui,∗6 Y. Ichikawa,∗2 H. Nishibata,∗2 T. Sato,∗2
A. Gladokov,∗2,∗6 L. C. Tao,∗2,∗8 T. Kawaguchi,∗2,∗3 Y. Baba,∗3 M. Iijima,∗3 H. Gonda,∗3 Y. Takeuchi,∗3R. Nakazato,∗3 H. Odashima,∗1 and H. Ueno∗2An in-situ laser spectroscopy method utilizing atomic
impurity in superﬂuid helium (He II) has been developed
for the application of nuclear structure study of rareisotopes. The method is named Optical RI-atom Observation in Condensed Helium as Ion-catcher (OROCHI).
The key feature of the technique is that the center wavelengths of the absorption spectra of atoms in He II arelargely blue-sifted from those of emission spectra. Thefeature is signiﬁcantly advantageous for the detectionof ﬂuorescence signals from atoms in He II with ultralow background. Moreover, He II works as an eﬀective
stopper for highly energetic ion beams. The stopping
eﬃciency of He II reaches nearly 100% for the beam energy of several tens of MeV/nucleon. Considering theabove reasons, OROCHI has proven to be an eﬃcientmethod for the study of nuclear structure of low production yield rare-isotopes. We have demonstrated thefeasibility of OROCHI using 84–87 Rb ion beams with up
to 66 MeV/nucleon energy and 104 pps intensity, so far.1)
However, unexpected laser stray light limits the signal
detection sensitivity. To eﬃciently separate the signal
from background, we developed a new ﬂuorescence detection system (FDS) that realizes low background signal detection using double monochromator. The ﬁrst
demonstration of the new FDS was performed using
85
Rb31+ ion beam delivered from RIPS. We deduced theoptical absorption cross-section from the result of ﬂuorescence intensity dependence on beam intensity andapplied laser power.
The experimental setup was the same as that inRef. 2). A 66 MeV/nucleon 85 Rb ion beam was passed
through two aluminum energy degraders to adjust thestopping position in He II. During beam stopping position adjustment using various degraders thickness, weconﬁrmed that the injected ions were stopped within
1 mm injection depth. The energy degraded ion beam
was introduced into He II. Rubidiums that were diﬀusedfrom the observation region with approximately several
hundred miliseconds after stopping in He II were subjected to a laser light from the Titanium sapphire laser(780 nm, ϕ ≈ 2 mm). The laser induced ﬂuorescence(LIF) from Rb atoms was focused on the optical ﬁber
†∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8Condensed from the article in Appl. Phys. Exp. 12, 016502 (2019)
Department of Physics, Meiji University
RIKEN Nishina Center
Department of Advanced Sciences, Hosei University
Department of Physics, Osaka University
Department of Physics, Tokyo Metropolitan University
National Institute for Radiological Science
Department of Physics, Kyungpook National University
School of Physics, Peking University

Fig. 1. Measured fluorescence intensity dependence normalized by injected beam intensity.

bundle with an entrance of 5 × 2 mm2 using the lensunit of the FDS. The LIF guided using the optical ﬁber
bundle was introduced into the double monochromator.
Rb D1 emission light (793 nm in He II) was selectedusing the double monochromator and detected using aphoto-multiplier tube.
The measured LIF dependence on beam intensity andapplied laser power is shown in Fig. 1. The verticalaxis was normalized by the beam intensity and the horizontal axis shows the applied laser power. The observed LIF intensity ILIF was obtained using the equation ILIF = Nobs σ ΦLaser ϵdet , where Nobs , σ, ΦLaser andϵdet , denote the number of atoms in the observation region, optical absorption cross-section, photon ﬂux of thelaser, and detection eﬃciency of FDS, respectively. Nobs
was estimated from the beam intensity Ibeam and diﬀusion time of Rb atom. ΦLaser was proportional to theapplied laser power PLaser . Thus, the equation for normalized LIF intensity ILIF /Ibeam can be written as afunction of PLaser . The value of ϵdet ≈ 10−7 obtained
from the result of experiment using Rb enclosed gas cell
was used for the deduction of σ. As a result of ouranalysis, we deduced σ = 3.59(16) × 10−15 cm2 as theoptical absorption cross-section of Rb in He II. The successful derivation of the quantitative value attributes tothe improvement of detection eﬃciency by two orders ofmagnitude using the new FDS, as compared to previous work.1) We conclude that 200 pps is a suﬃcient ion
beam intensity to detect LIF from Rb atoms in He II inaccordance with our experimental result.
References1) X. F. Yang et al., Phys. Rev. A 90, 052516 (2014).
2) K. Imamura et al., RIKEN Accel. Prog. Rep. 50, 197 (2017).

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