RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Eﬀective suppression of stray laser light for ﬂuorescence detection
from 85 Rb atoms injected into superﬂuid helium
W. Kobayashi,∗1,∗2 K. Imamura,∗1,∗3 T. Egami,∗1,∗2 M. Sanjo,∗1,∗2 T. Fujita,∗1,∗4 D. Tominaga,∗1,∗2Y. Nakamura,∗1,∗3 A. Takamine,∗1 T. Furukawa,∗1,∗5 T. Wakui,∗1,∗6 H. Ueno,∗1 and Y. Matsuo∗1,∗2
We have been developing a laser spectroscopy
technique named Optical RI-atom Observation inCondensed Helium as Ion-catcher (OROCHI) for theinvestigation of nuclear properties such as nuclear spinsand moments. In OROCHI experiments, we catchhighly energetic ion beams in superﬂuid helium (HeII) to neutralize them and apply laser-rf and lasermicrowave double resonance methods to the stopped
atoms. It is known that the optical absorption spectrum of an atom in He II is greatly blue-shifted compared with the emission spectrum1) . Therefore, thelaser-induced ﬂuorescence (LIF) from the atoms canbe spectrally isolated from the stray laser light that
results in a background in spectra. We have developed
an LIF detection system (LDS) which enables the detection of LIF with an extremely low background. In aprevious online experiment2) , we successfully observed
LIF from 84−87 Rb isotope atoms and double resonancespectra using LDS II3,4) equipped with interferenceﬁlters for wavelength selection (Fig. 1(a)). However,
we observed background signals of ≈103 counts witha laser radiation of 100 mW while the LIF intensity
was also of the same order for the injected beam intensity of the order of 104 pps. This is because the ﬁlter
transmission shifts to shorter wavelengths for oblique
incident light. We require a higher wavelength selectivity for the application of OROCHI to lower-production
yield nuclei.
In order to reduce the background due to stray laser

light more eﬃciently, we developed LDS III (Fig. 1(b)).
It consists of a set of lenses, bundled optical ﬁbers
and a monochromator. The ﬂuorescence light is collected and collimated through lens1 and focused bylens2 onto the entrance of the ﬁbers. It is resolvedin the monochromator according to the wavelengths.
The focal lengths of lens1 and lens2 are optimized for
the LIF collection and transmission. This conﬁguration makes it possible to suppress the stray laser lightthat reaches the monochromator through the ﬁbers bya factor of 10−9 .
In order to conﬁrm the eﬀective reduction of thebackground, we investigated the background inducedby the stray laser light in LDS III. We measured thedependence of a photomultiplier tube (PMT) signal on
the irradiated laser power (Fig. 2). As shown in Fig. 2,
the detected signal attributed to the stray laser light
was four cps per 100-mW laser radiation power. From
this result, we concluded that the stray laser light wasreduced by three orders of magnitude compared withthe case of LDS II and the background was near thelevel of dark counts in the PMT. After conﬁrming thereduction of background counts, we installed LDS III
for the online experiment using a 85 Rb beam delivered from RIPS5) . The evaluation of the applicableion beam intensity in the LDS III is in progress now.

Fig. 2. Background signals due to the stray laser light in
He II as a function of the incident laser power.

Fig. 1. Laser-induced ﬂuorescence detection system (a)LDS II and (b) LDS III.
∗1
∗2
∗3
∗4
∗5∗6RIKEN Nishina Center
Department of Advanced Science, Hosei University
Department of Physics, Meiji University
Department of Physics, Osaka University
Department of Physics, Tokyo Metropolitan University
National Institutes for Quantum and Radiological Scienceand Technology- 198 -References
1) Y. Takahashi et al.: Phys. Rev. Lett. 71, 1035 (1999).
2) X. F. Yang et al.: Phys. Rev. A 90, 052516 (2014).
3) A. Sasaki et al.: RIKEN Accel. Prog. Rep. 44, 204(2011).
4) H. Tetsuka et al.: RIKEN Accel. Prog. Rep. 46, 117(2013).
5) K. Imamura et al.: in this report.