Silica optical fibers for a detection of X-ray radiation

. The special attention has been paid to visualization and monitoring of harmful radiation to quantify the radiation intensity and prevent the undesired exposition. For these purposes, so called radioluminescent materials are widely exploited. Special attention has been paid to the research of luminescence optical fibers, which can be used for a construction of distributed optical sensors. We present a versatile nanoparticle doping approach to Zn 2+ -doped silica optical fibers. Zinc oxide nanoparticles were applied into porous silica frit by a nanoparticle-doping method providing a preform which was drawn into an optical fiber. The maximum concentration of Zn 2+ ions in the fiber was 0.78 at. % causing the refractive index reached the value of 1.459. The fiber outer diameter was 124.8  m and the fiber core diameter was 14.9  m matching the standard telecommunication dimension. The fibers exhibited a blue radioluminescence showing the emission maximum at 395 nm. The fiber properties make them worthy of investigation as sensing elements of distributed sensors of high energy radiation.


Introduction
The people encounter high-energy radiation in daily life.However, the exposure to high radiation dose usually has a negative impact on human health and it can also destroy electronic devices [1].The special attention has been paid to visualization and monitoring of harmful radiation to quantify the radiation intensity and prevent the undesired exposition.
For these purposes, so called radioluminescent materials are widely exploited [2].Special attention has been paid to the research of luminescence optical fibers, which can be used for a construction of distributed optical sensors [3].
Common RE-doped silica optical fibers are usually used for a construction of distributed radioluminescent sensors.The radioluminescence properties of the fibers can be improved by nanostructuralizing of the fiber core by creating an inorganic nanocomposite of radioluminescent nanoparticles distributed inside the glass matrix [4].
In this contribution we present a versatile nanoparticle-doping approach to Zn 2+ -doped silica optical fibers.We evaluated the chemical and optical properties of the prepared preform and fiber and we evaluated their basic luminescence properties under X-ray excitation.
To prepare optical fiber the modified chemical vapor deposition (MCVD) combined with a nanoparticledoping method was used [6].The ZnO sol was soaked into a porous silica frit prepared by MCVD onto inner side of silica tube.Afterwards the doped silica frit was sintered at 1800 °C and collapsed into a preform.The preform was drawn into a silica optical fiber with the diameter of 125 mm and coated by protective polymer DP-1021 (DeSolite).
A refractive index profile of preform was analyzed on the A2600 refractive index analyzer (PhotonKinetics).The local chemical composition of the preform were measured using SX100 electron microprobe analyzer (Camec).The sample was excited with a 15 keV X-ray beam.Beam current was 60 mA.Radioluminescence spectra were recorded on a 5000M spectrofluorometer (Horiba Jobin Yvon) equipped with TBX-04 single photon counting detector.The spectra were excited by the X-ray tube (Seifert Gmbh) operating with a voltage 40 kV.

Results and discussion
To maintain the waveguiding properties of optical fibers, it is necessary to achieve a uniform distribution of dopants in the core of the preform.The concentration distribution of Zn 2+ ions in the preform and refractive index profile are demonstrated in Fig. 1.The concentration of Zn 2+ ions was symmetrically distributed around the central preform axis.At the boundary between the deposition silica tube and the doped preform core the concentration of Zn 2+ ions was equal to zero increasing to the maximum value of 0.78 at.%.The central part of the preform showed a depleted region where the concentration of Zn 2+ ions fall down to 0.6 at.%.The existence of the depleted region is usually attributed to the partial evaporation of dopants during the processing of the preform.The refractive index profile followed the concentration of Zn 2+ ions.The refractive index raised up from the value of 1.457 typical for pure silica glass to the maximum value of 1.459.The preform core exhibited strong emission under Xray excitation.The recorded radioluminescence spectrum is shown in Fig. 2. The envelope of the emission peak consisted of three minor peaks located at 298, 360 and 395 nm.The overall maximum of the emission was at 395 nm.The emission intensity gradually decreased with increasing emission wavelength.Beside the radioluminescence signal, the secondary peak located at 520 nm was presented in the recorded spectrum.However, this peak was identified as an artefact of the setup.

Fig. 2. Luminescence spectrum of the preform core under X-ray excitation
The preform was drawn into the optical fiber.The optical microscope image of the cross-section of the fiber is shown in Fig. 3a.The fiber size fitted the standard telecommunication dimensions.The fiber outer diameter was 124.8 m and the fiber core diameter was 14.9 m.The light field waveguided through the fiber corresponded to the refractive index profile of the preform exhibiting the brighter ring around a darker spot in the middle of the fiber core.Fig. 3b shows the zoom on the fiber core under X-ray illumination.The fiber core emitted a blue corona, proving that the radioluminescent properties of the preform did not disappear after the fiber drawing.However, the quantification of the observed results requires a modification of the experimental set-up, which is currently underway.

Conclusions
A versatile approach to the preparation of radioluminescent silica optical fibers was demonstrated.Zinc oxide nanoparticles were applied into porous silica frit by a nanoparticle-doping method providing a preform which was drawn into an optical fiber.The fiber outer diameter was 124.8 m and the fiber core diameter was 14.9 m matching the standard telecommunication dimension.The preform core and prepared optical fiber exhibited blue luminescence under X-ray excitation.The fiber properties make them worthy of investigation as sensing elements of distributed sensors of high energy radiation.
This work was supported by the Czech Science Foundation by the project No. 23-05507S., 05032 (2023)

Fig. 1 .
Fig. 1.Concentration profile Zn 2+ ions in the preform core (left scale), and refractive index profile of the preform (right scale).

Fig. 3 .
Fig. 3. Optical microscope image of the fiber cross-section.a) The overall fiber image showing the dimensions and the light field distribution in the fiber core.b) Zoom on the fiber core under X-ray excitation showing the blue radioluminescence.