II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Radiation resistivity test of an optical fiber for laser cooling of francium
atoms
T. Hayamizu,∗1 K. Nakamura,∗2 T. Aoki,∗3 Y. Kotaka,∗2 H. Nagahama,∗2 S. Nagase,∗2 M. Ohtsuka,∗2,∗4N. Ozawa,∗2 M. Sato,∗3 A. Takamine,∗1 K. S. Tanaka,∗5 H. Haba,∗1 and Y. Sakemi∗2
Francium is an alkali element in the 7th period, andall its isotopes are radioactive with half-lives of up to20 min. The application of francium atoms to highprecision spectroscopy for testing fundamental symmetries has been implemented on a vertical beam line of
the E7 room at the RI Beam Factory. Francium is produced in the 197 Au(18 O, xn)215–x Fr reaction and ﬁnallysupplied as laser-cooled atoms into the spectroscopic region. For Fr production, a 7 MeV/nucleon 18 O6+ beam(target current 18 eµA) propagating through two beryllium foils and helium gas ﬂow is injected into a gold
target positioned at the end of the beam line. However,
the optical experimental region for spectroscopy is exposed to high radiation from the target, which is placeda few meters away. Therefore, the optical experimental
devices should be evaluated for radiation resistivity.
Laser sources were placed in the laser spectroscopy
room in the basement of the RIBF building. A titaniumsapphire laser and external cavity diode lasers supplied
718 nm light beams for the laser cooling of Fr atoms.
These light beams will be transported to the E7 room
via 400 m of optical ﬁber. The optical ﬁber consists ofa polarization-maintaining optical ﬁber (Fujikura SM63PS-U25D) covered by a stainless ﬂexible tube and ﬂameretardant polyvinyl chloride (Nippon Steel Welding &Enginering PICOFLEC). One of the concerns is radiation damage caused by boron, which has a large capture
cross section of neutrons, contained in stress-appliedparts of this optical ﬁber.1) We expected the radiation damage to cause two behaviors: transmission power
drifting and degradation of light polarizability.
To evaluate the radiation resistivity, we performed alight transmission monitoring test using 40 m of thesame model optical ﬁber wired inside the E7 room, during the Fr production beam time (maximum 7.9 eµA).
A part of the optical ﬁber (4.5 m) was stuck with plastic
masking tape on the vertical ﬂat surface of a steel rackapproximately 1.2 m away from the vertical beam line.
The ends of the optical ﬁber were connected to an optical monitoring system set downstairs of the E7 room,
where the radiation level is low. A 685 nm, 1 mW lightbeam from a diode laser was used for the monitoringsystem. The output light power of the diode laser, andeach s- and p-polarized light power from the optical ﬁber
were monitored using power meters. We determined thetransmittance T by the power of the s-polarized output light from the optical ﬁber normalized by the value
∗1
∗2
∗3
∗4
∗5

RIKEN Nishina Center
Center for Nuclear Study, University of Tokyo
Graduate School of Arts and Science, University of Tokyo
Waseda University Honjo Senior High School
Cyclotron and Radioisotope Center, Tohoku UniversityFig. 1. The upper figure shows the integrated neutron doseat the E7 room monitoring post from the beginning ofmeasurement, and the lower figure shows the optical-fibertransmittance.

of the input power monitor. In addition, the radiationdose to the optical ﬁber was estimated using data fromthe E7 room monitoring posts. The peak value of theneutron dose rate was 3.7 mSv/hr, and the integrated
neutron dose at the monitoring posts over the beam timewas 21.6 mSv. Considering only the distance from thetarget, the expected dose of the optical ﬁber was several
times larger than the dose estimated at the monitoringpost.
The data obtained during the beam time are shown inFig. 1. Although it is clear that the cumulative neutrondose and the ﬂuctuation of T in Fig. 1 did not correlate,T gradually reduced by less than 1% during 27 h. Inorder to identify whether this was caused by the irradiation, we carried out followup tests with a low radiation
background condition. By checking the optics used in
the monitoring system, we found that the coupling eﬃciency on the ﬁber couplers may have caused a similar
drift. In conclusion, a signiﬁcant change beyond themeasurement accuracy of the transmitted light power
and light polarization due to the irradiation was not observed with the current experimental setup. Additional
checks using a higher radiation dose are necessary to ensure that this optical ﬁber can be used for a longer beamtime.
Reference1) S. Karasawa et al., IEEE Trans. Nucl. Sci. 34, 1105 (1987).
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