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Abstract
A novel tapered fiber-optic radiation sensor (TFRS) based on cerium (Ce) and terbium (Tb) co-doped YAG scintillation crystals is demonstrated for the first time. Using the CO2 laser-heated method, a Ce/Tb:YAG crystal is well embedded into silica glass cladding without any cracks. The scintillation light emitted from the YAG scintillation crystal can be directly coupled into the derived silica optical fiber by the tapered region. The loss of the derived optical fiber is 0.14 dB/cm, which is one order of magnitude lower than the 1.59 dB/cm of the YAG crystal in the TFRS. Subsequently, strong photo- and radio-luminescence of Tb3+ (5D4→7F5) ions in TFRS are achieved under ultraviolet light and high-energy ray excitation, respectively. In particular, a prominent remote radiation response of the TFRS is presented under excitation by γ-rays through fusion splicing with multimode optical fibers. The response is approximately four times larger than that of a plastic scintillation fiber (BCF-12) sensor. Furthermore, the results possess high stability as well as a good linearity between the radiation dose rate and the response intensity. The TFRS in combination with an all-silica fiber system is a promising candidate for remote radiation detection.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber-optic radiation sensors (FRSs) allow radiation detection in difficult to access or dangerous areas, so they can be used in harsh fields like, such as, close to nuclear reactors, energy exploration, and industrial irradiation [1–4]. This mainly benefits from the optical fiber’s immunity to electromagnetic and chemical interferences, as well as its high-temperature and high-pressure resistance. Typically, these sensors can be placed hundreds of meters from the photoelectric conversion devices. Owing to their miniature dimensions and light weight, a particularly interesting application can be also found in diagnostic or radiotherapy irradiations for careful control of the dose imparted to the patient [5,6]. Compared to conventional optical fibers-based radiation sensors, intensive attention has been paid to plastic scintillation fibers because of their excellent characteristics like easy shaping and low cost [7,8]. In addition, inorganic scintillators have been attached or embedded at the ends of polymethylmethacrylate (PMMA) plastic and silica optical fibers in FRSs to improve their detection sensitivity. Such FRSs perform satisfactorily in remote and real-time radiation monitoring and radiotherapy [9–12]. However, they suffer from the limitations of aging and operation only at room temperature. Additionally, cerium (Ce) or terbium (Tb)-doped silica scintillation optical fibers fabricated using the powder-in-tube (PIT) [13–16] or rod-in-tube (RIT) [17] techniques are emerging as promising and attractive scintillation sensors for remote and real-time radiation sensing in harsh environments owing to their low loss. Nevertheless, the main drawback of scintillation fibers based on silica glass materials is their lower light output. Although Ce/Tb co-doped YAG crystal materials have excellent scintillation properties [18–21], it is difficult to realize remote radiation sensing due to their short crystal length, high loss, and strict growth conditions. Splicing techniques between YAG crystal fibers and silica optical fibers [22] have recently become popular in remote radiation detection and high-power lasers. However, there are many cracks at the interface between the YAG crystal and the silica fiber because of the materials with dissimilar thermal expansion and melting points. Consequently, the splicing point between YAG crystal and the silica glass is very fragile. Thus, an efficient connection between crystal scintillators and silica optical fibers is still challenging to achieve.
In this work, a tapered fiber radiation sensor (TFRS) based on a Ce/Tb:YAG scintillation crystal is fabricated using the RIT method in a CO2-laser drawing tower. Strong photo-luminescence (PL) and radio-luminescence (RL) are obtained in the TFRS due to the presence of the YAG crystal, which is confirmed by the X-ray diffraction and scanning electron microscopy analyses. Furthermore, the transmission loss and coupling efficiency of the YAG scintillation crystal and derived optical fiber are systemically investigated in terms of the remote RL responses of the TFRS under γ-ray excitation.
2. Material fabrication
The TFRS, combined with the tapered derived optical fiber, is fabricated using the modified RIT method in a CO2-laser drawing tower. A Ce/Tb co-doped YAG crystal round rod (Lanjing Optoelectronic Technology Co., Shanghai, China) with a diameter of 1.5 mm is used as the raw preform core material, and its length is of 30 mm. The concentrations of Ce and Tb ions are 0.3 and 5.0 mol%, respectively. A home-made silica tube with the outer and inner diameters of 6.0 and 1.8 mm, respectively, is used as the preform cladding material, and its effective length is of 50 mm. The upper end of the silica tube preform is connected to a vacuum pump while fixed to the drawing tower. The maximum negative pressure pumped by the vacuum pump is approximately 2 Pa. The preform is heated directly by the CO2 laser. The ring-profile beam is reflected by a planar mirror at an incident angle of 45° to a circular cone-like mirror. Then, a donut-shaped heating zone is obtained by the reflection of the circular cone-like mirror and the length of the heating zone is about 13.1 mm, as shown in Fig. 1(a). The maximum output power of the CO2 laser is 400 W with the working wavelength at 10.6 µm. The output power of the laser can be precisely adjusted by the laser controller to control the heating temperature of the preform. The heating temperature is monitored in real-time by an infrared camera. During the fabrication process, negative pressure control is simultaneously employed on the silica tube perform to facilitate the formation of a good interface between the YAG crystal core and the silica glass cladding. The detailed fabrication process is demonstrated in our recent work [23]. As the laser power slowly increased, the temperature of the heating area reaches up to approximately 2050 $^\circ \textrm{C}$ whilst the heating region of the preform gets gradually narrower. Owing to the small heating region (13.1 mm) and the proper heating position of the preform, the YAG crystals at the bottom of the preform will be melted in small amounts and encapsulated by the silica glass cladding. Meanwhile, the silica tube preform must be moved down with a moderately slow feeding speed, in order to make the preform viscous enough to meet the drawing requirement owing to the gravity effect. More importantly, the slow feeding speed provides sufficient time for the TFRS to cool down, which is crucial to prevent cracking due to the different thermal expansion coefficients of the YAG core material (approximately 7 × 10−6 K−1) and the silica cladding (approximately 5 × 10−7 K−1) [19]. Finally, a TFRS embedded by Ce/Tb:YAG crystal is obtained, combined with the tapered YAG derived optical fiber, as shown in Figs. 1(b) and 1(c). The length of the tapered region is of ∼23 mm. From the cross-section of the TFRS in Fig. 1(d), it can be observed that the interface between Ce/Tb:YAG crystal material and the cladding is free of cracks. The YAG crystal material is successfully embedded into the cladding silica glass without crack at the bottom of the TFRS. Moreover, the core YAG crystal appears to have a yellow outer layer, which might be caused by the Ce3+ segregation as well as the valence change between Ce3+ and Ce4+ ions during the high-temperature fabrication process of the TFRS. In Fig. 1(e), the diameters of derived optical fiber core and cladding are measured to be 55 and 210 µm, respectively.
Fig. 1. (a) Schematic of the laser drawing for a TFRS. (b) Image of the TFRS with derived optical fiber. (c) Enlarged image of the TFRS. Cross-sections of (d) TFRS and (e) derived optical fiber.
3. Results and discussion
3.1. Structural characterization
To confirm that the YAG crystal still exists at the bottom of the TFRS, as shown yellow part in Fig. 1(c), the crystal phase is characterized by X-ray diffraction (XRD, D/Max 2500/PC, Japan) patterns collected using a Rigaku Miniflex diffractometer equipped with a Cu (Kα) X-ray tube operated at 40 kV and 250 mA with a step size of 0.02° (Rigaku, Japan). The silica claddings of the TFRS and the derived optical fiber are first etched with hydrofluoric acid. Their etched regions are marked with red and purple color in Fig. 1(b). Then, the remaining yellow TFRS core and derived optical core parts are ground into the powder, respectively. The XRD results of TFRS yellow core and derived optical fiber core are presented in Fig. 2(a), respectively. The obtained XRD diffraction peaks coincide well with those standard YAG crystal peaks (JCPDS, #33-0040) in the literature [23,24], which indicates that the YAG crystalline phase is retained at the bottom of the obtained TFRS yellow core. Moreover, only one YAG phase is identified, while no peaks are assigned to other crystalline phases. Meanwhile, a broad amorphous band appears in the derived optical fiber core. It can be caused by the total dissolution of the YAG crystal and the diffusion of SiO2 from the cladding glass to the YAG core region, which has been reported in the literature [25,26]. Moreover, the silicon (Si) composition of derived optical fiber was analyzed by a scanning electron microscope (JSM-7500F field emission SEM, Japan) equipped with an energy dispersive spectrometer (EDS, MX80-EDS, OXFORD, England). The EDS result shows that the elemental Si, Aluminium (Al), Yttrium (Y), Oxygen (O) concentrations are approximately 13.5, 24.5, 22.3, and 31.8 wt% in the core region of derived optical fiber. The obtained results show that the amorphous yttrium aluminosilicate glass has been eventually formed in the derived optical fiber. The derived optical fiber, which has the characteristics of a circular waveguide, might be more suitable for scintillation light transmission and fusion-splicing with transmission multimode optical fibers (MMFs).
Fig. 2. (a) XRD patterns of the derived optical fiber core and TFRS yellow core. The standard diffraction pattern (JCPDS, #33-0040) for YAG crystal is shown in orange color. (b) SEM/EDS line scan of Si through the TFRS cross-section. The inset shows the enlarged cross-section of the TFRS yellow core.
To better understand the binding mechanism between the YAG crystal and the cladding glass, EDS line scan measurements of the Si concentration profile are performed. The EDS measurement of the TFRS reveals a very sharp jump in the Si composition at the interface between the core and cladding, as shown in Fig. 2(b). Moreover, the concentration of Si element in the core region is almost zero. Therefore, it can be concluded that the core YAG crystal is expected to be partially melted after high thermal temperature. Interestingly, the cross-section of the YAG crystal material changes from a circle to a polygon during the high-temperature dissolution process because of the YAG crystal cubic lattice structure, as presented in the inset of Fig. 2(b). There are also no cracks at the interface between the YAG crystal core and the silica glass cladding after the high-temperature treatment, despite the large difference in their thermal expansion coefficients. The results show that CO2 laser heating, combined with the fusion-tapered method, can effectively overcome the cracking problem induced by the large differences between the crystal material and the silica glass. Hence, this technique will be important in the development of compact integrated optical systems based on crystals and glass with significant dissimilarities in their physical and material properties.
3.2. Optical properties
The tapered region coupling efficiency and optical properties of the TFRS are also investigated to demonstrate its potential applications in remote radiation detection. A numerical model for the tapered region of the TFRS is constructed with the beam propagation method (BPM), as presented in Fig. 3(a). The parameters of the tapered shape described in Fig. 1 are used in this simulation. The used light wavelength is 542 nm, which is the main emission light of Tb3+ ions in the TFRS. A single fiber mode HE11 is launched into the tapered core region from its end facet. The refractive indices of the core and cladding of the TFRS tapered region are 1.495 and 1.440, respectively, which are referred to in the literature [26]. The enlarged image and simulation model of the TFRS are also shown in the inset of Fig. 3(a). The calculated coupling efficiency of the tapered region is increased with the increase of the TFRS neck length. The coupling efficiency is estimated to be approximately 74.3% at the tapered region length of 23 mm. Given the omnidirectional scattering phenomenon of the YAG crystal, we limit our analysis on the optical coupling efficiency just from the tapered region to the derived optical fiber, excluding the optical collection efficiency from the YAG crystal scintillator to the tapered region. Based on the improved coupling efficiency of the embedded structures [10,12], it appears that much scintillation light might be coupled into the derived fiber through the tapered region of the TFRS with a small amount of light leakage, as shown in Fig. 3(b).
Fig. 3. (a) The coupling efficiency of TFRS with different lengths of the tapered region. The inset shows the enlarged image (left) and simulation model (right) of TFRS. (b) Optical intensity distribution of a TFRS in x/z plane calculated by BPM. UV-visible absorption spectra of the TFRS (c) and derived optical fiber (d), respectively.
It is also important to study the wavelength-dependent absorption spectra of optical fibers if they are used as intrinsic sensors. The absorption spectrum of the YAG crystal in the range of 220 to 500 nm is measured using an ultraviolet (UV) spectrophotometer (UV-2501PC) by cutting the TFRS into 1.0-mm-thick slices, corresponding to the region marked with red color in Fig. 1(b). An aluminum plate with a small 0.5 mm diameter hole is placed in front of the TFRS slice to reduce the spot size and allow light to enter the TFRS core YAG crystal region. The optical transmission property of the derived optical fiber sample, corresponding to the region marked with purple color in Fig. 1(b), is performed using a Deuterium lamp source (DH-2000, Ocean Optics) and a high-resolution spectrometer (HR4000CG-UV-NIR, Ocean Optics) over the wavelength range from 220 to 500 nm. The optical source, emitted from the Deuterium lamp source, is coupled into an ultraviolet transmission fiber with a core diameter of 50 µm. The core of the fiber is smaller than that of the derived fiber, which allows light to enter the core of the derived fiber. Moreover, the cross-section of the TFRS slices and derived optical fiber are all polished to the same optical grade with a polishing film (ADS, Nippon Telegraph and Telephone, Japan). The optimized light sources allow the injected light to be transmitted mainly through the cores of the TRFS slices and the derived fiber.
We firstly present the UV-visible absorption spectra of the YAG crystal in the TFRS and the derived optical fiber. As shown in Figs. 3(c) and (d), the transmission losses of the YAG crystal and derived optical fiber at 500 nm are approximately 1.59 and 0.14 dB/cm, respectively, which is consistent with those of previous reports [27,28]. Compared to the Ce/Tb:YAG crystal in the TFRS, derived optical fiber with much lower loss ensures that the scintillating light can be efficiently transmitted to a large extent. The spectrum of YAG crystal with Ce and Tb ions has two weak absorption bands at 435 and 280 nm, due to the 4f→5d spin-allowed transitions in Tb3+ and Ce3+ ions, respectively. A similar result was also reported in a previous work on Ce/Tb co-doped YAG crystals [23]. However, the absorption peaks of the derived fiber are both shifted to the shorter wavelengths of 300 and 250 nm, respectively. This shift may indicate that the core material of the derived fiber has become glassy [29]. Both the YAG crystal in the TFRS and the derived fiber show different absorption bands caused by the Ce3+ and Tb3+ ions in the visible and UV regions, respectively. These absorption bands indicate the potential RL centers that may be the best wavelengths for the excitation of PLs, as well as for trapping electron-hole pairs. However, the scintillation efficiency of the derived fiber is much lower than that of the TFRS due to the extremely low transfer efficiency of the excitons in the amorphous network.
The PL spectra excited by a Xenon lamp are recorded using a fluorescence spectrometer (FLS980, Edinburgh Instruments, England). The 2D PL and photoluminescence excitation (PLE) spectra map is recorded with excitation and emission mapping steps of 2 nm. The PL and PLE spectra of TFRS are also analyzed and shown in Fig. 4. The main PL and PLE peaks of TFRS appear at 542 and 265 nm, respectively. Under 265 nm excitation, the PL spectrum shows strong fluorescence peaks at 488, 542, 586, and 621 nm caused by the 5D4→7FJ (J=6, 5, 4, 3, respectively) transition of the Tb3+ ions [30,31]. Stark splitting of the Tb3+ emission can be observed in the fluorescence spectrum at 542 nm, which is consistent with the emission splitting of Tb3+ ions in YAG crystal. Except for the 265 nm excitation band of Tb3+ (4f→5d) ions, the characteristic emission peaks of Tb3+ also appear under 320 nm excitation. This peak may be attributed to the energy transfer from Ce3+ to neighboring Tb3+ ions. The effective energy transfer from Ce3+ to Tb3+ ions might improve the scintillating performance of the TFRS. These strong PLE bands observed in the UV range agree with those in the TFRS absorption spectrum. Furthermore, the characteristic excitation/emission wavelengths for Ce3+ ions appear at 450 and 560 nm, which is coincident with the fluorescence properties of Ce3+ ions in YAG crystals [23,27,28]. It can further indicate the retaining of the YAG crystalline phase after the high-temperature fabrication.
The emission wavelength in the RL process depends on the dopants and ranges from the UV to the infrared region. The YAG crystal lattice vibration plays a key role in the energy transfer from host materials to Ce3+ or Tb3+ ions luminescent centers. The RL spectra are also recorded by the FLS980 with excitation of a Mini-X X-ray tube (Mini-x2, Amptek, US) at Shanghai Institute of Ceramics, China. The fixed X-ray tube voltage and current are 50 kV and 50 µA, respectively. The RL spectrum is shown in Fig. 5, and is very similar to the PL one. The emission line corresponding to the 5D4→7FJ (J = 6, 5, 4, and 3) transitions of the Tb3+ ions is also clearly identified under high-energy ray excitation. Moreover, a small contribution of the Ce3+ 5d→4f transition in the YAG crystal lattice is present from 500 to 600 nm in the RL curve. For comparison, the RL spectrum of a plastic scintillating fiber (PSF, $\phi $=1 mm, BCF-12, SAINT-GOBAIN, France) is also recorded. The emission peak intensity of TFRS is approximately four times larger than that of the PSF (BCF-12). The strong RL indicates that TFRS is a potential candidate for optical fibers in the remote radiation detection.
3.3. γ-ray response
The remote real-time RL responses of the TFRS are recorded using a single-photon detection system comprising a photomultiplier tube (PMT) (H8259, Hamamatsu Photonics, Japan), a photon counter (C8855, Hamamatsu Photonics, Japan), and a laptop computer, as shown in Fig. 6. Calibrated collimated gamma-ray (γ-ray, mean photon energy 1.25 MeV) is emitted from 60Co at Shanghai Institute of Measurement and Testing Technology (SIMTT, Shanghai, China). The γ-ray source is placed in an isolation room, which is separated from the control room by thick protective walls to absorb any scattered γ-ray radiation. The beam size of the γ-ray is approximately 10×10 cm2. The TFRS is placed directly on the holder at a distance of 1.2 m. All the sensors were placed in the same position. The TFRS with the derived optical fiber was directly fusion-spliced to the MMF (L=2 m, $\phi $=0.2 mm), as illustrated in the inset of Fig. 6. The loss at the fusion point is approximately 0.5 dB, and the length of derived optical fiber is approximately 12.0 cm. Therefore, scintillating light can be efficiently transmitted to the PMT through the all-fiber network for the remote radiation detection. Moreover, the PMT can be placed away from the potentially harsh radiation environment owing to the efficient transmission of scintillation light. PMT used here converted the visible light to the electrical signal, which was counted by the C8855. The supporting software developed by Hamamatsu was used to record the data in real-time. The gate time was maintained at the value of 500 ms throughout this experiment. The PSF is also used as a reference sample. The 60Co γ-ray source is highly reproducible and precisely monitored by internal sensors during the operation. All the measurements are performed at room temperature.
Fig. 6. Experimental setup using TFRS for γ-ray radiation sensing. The inset shows the fusion point between the YAG derived fiber and the MMF.
For effective real-time RL dosimeters in practical environments, several requirements must be satisfied, including independence from the dose rate, high stability, and a linear relationship with the absorbed radiation dose. The TFRS and PSF are irradiated using the γ-rays under the same experimental conditions so that their intensities can be fairly compared. The γ-ray source is set to produce a single rectangular pulse with a duration of 80 s. The background noise is first recorded with a TFRS without the embedded YAG crystal (pink dotted line in Fig. 7(a)), which is also known as the stem effect [11,32,33]. After turning on the γ-ray source, the signal increases to the steady-state level immediately. In the meantime, the signal also decreases within a few seconds when turning off the γ-ray, which does not present any afterglow behavior (refer to Visualization 1). The TFRS shows a very strong RL response under the γ-ray irradiation (black dotted line) in real-time. The mean output signal intensities of the pulses are 30306 ± 1103 and 7148 ± 439 photon counts for the TFRS and PSF, respectively. In both cases, the sensors offer excellent stabilities during the irradiation time of 80 s, with a variation coefficient of 3.6% and 6.1% for the TFRS and PSF, respectively. Moreover, the scintillation response intensity of the TFRS is approximately four times larger than that of the PSF, which is similar to the results of the RL spectra.
Fig. 7. (a) RL responses of the TFRS and PSF irradiated by γ-ray. (b) TFRS dose-response curve. The inset shows the stability of the RL signal with the increase of irradiation time.
The dose rate is measured in air using a standard ionization chamber (30012, PTW Freiburg GmbH, Germany). In the meantime, the TFRS also works in air. Therefore, we use Gyair to describe the absorbed dose, which means the absorption of 1 J of γ-rays radiation energy per kg of matter in atmosphere [34]. Moreover, the stability of the absorbed dose-response is also investigated over a duration of 600 s under the dose rate of 2.06 mGyair/s of γ-ray irradiation, as shown in Fig. 7(b). The RL response increases with the cumulative irradiation dose, and the absorbed dose starts from 20 mGyair up to several Gyair. Also apparent is that the TFRS provides excellent linearity of the integrated RL response with the dose. The real-time RL response of the TFRS is also shown in the inset of Fig. 7(b). Under the prolonged irradiation, the RL signal remains almost constant. This demonstrates the good stability of the TFRS. A slight signal increase appears with increasing the irradiation time, which might be ascribed to the presence of deep traps acting as storage centers competing with the luminescence recombination centers during the scintillation process and which are progressively filled under prolonged irradiation [35,36]. Once the deep trap is almost completely filled, the RL emission reaches its maximum. Furthermore, linearity might be imperfect at higher dose levels due to this slight signal increase.
The real-time dose rate responses with the variation of the dose rates from the γ-ray source were recorded and are shown in Fig. 8(a). The dose rate is changed from 1.1 to 2.06 mGyair/s. The scintillation intensities also change linearly with the dose rates. The average response photons are calculated and plotted for the different dose rates in Fig. 8(b). The results show an excellent linear dose rate response with R2 = 0.98. Compared to traditional radiation sensors in which scintillators are attached directly to the end face of the transmission optical fiber, the proposed TFRS has many superior properties, such as better integration and longer service life. The radiation dose presented in this work is maintained at the same level as that used in conventional nuclear radiotherapy. The excellent remote radiation detection performance of the TFRS shows its potential practical application in radiotherapy.
Fig. 8. (a) RL response of TFRS irradiated by γ-ray at different dose rates. (b) The radiation dose rate response is plotted against the different γ-ray dose rates.
4. Conclusion
In this work, a tapered fiber-optic radiation sensor based on Ce and Tb co-doped YAG scintillation crystal has been fabricated in the CO2-laser drawing tower using the modified rod-in-tube method. After the high-temperature (approximately 2050 °C) heat treatment, the Ce/Tb co-doped YAG crystal is well embedded into the silica glass cladding without any cracks. The scintillation light, coupled into the tapered region, could be efficiently coupled into the derived fiber through the tapered region with an efficiency of 74.3%. The transmission loss of the derived fiber is 0.14 dB/cm, which is one order of magnitude lower than that of the 1.59 dB/cm for the YAG crystal. Moreover, strong PL and RL of the Tb3+ ions (5D4→7F6,5,4,3) in the TFRS are observed under the 265 nm UV light and high-energy ray excitation, respectively. Through fusion splicing with multimode silica fibers, the RL response intensity of TFRS is almost four times larger than that of the PSF (BCF-12). The variation coefficient of the TFRS sensor is 3.6%, which is lower than the 6.1% variation coefficient of the PSF sensor. The TFRS also exhibits excellent stability and linear response to the radiation dose rate with a R2 value of 0.98. The novel TFRS dosimeter combined with an all-silica fiber sensing network is a promising candidate for remote radiation detection.
Funding
National Natural Science Foundation of China (61520106014, 61635006, 61935002, 61975113); National Key Research and Development Program of China (2020YFB1805800); Higher Education Discipline Innovation Project (D20031); Shanghai Professional Technical Public Service Platform of Advanced Optical Waveguide Intelligent Manufacturing and Testing (19DZ2294000).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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