Open Access
Add to BibSonomy
Abstract
Radiation detection technologies have been applied in broad fields such as security inspection, medical diagnosis, environment monitoring and scientific analysis. Fiber-optic radiation detectors exhibit unique advantages including miniaturization, resistance to water, remote monitoring, and distributable detection. However, the low sensitivity and the high limit-of-detection limit its practical applications. Herein we demonstrated high-performance fiber-optic X-ray detectors with scintillating composites consisting of UV glue and uniformly distributed gadolinium oxysulfide (GADOX) powders. The impacts of the length, thickness and GADOX weight ratio of the composite coating upon the detector performance, were systematically investigated in terms of the generation and the coupling efficiency of radio-luminescence. Besides the high-performance scintillator, the scattering loss and the geometric factor greatly affected the detector performance. A higher sensitivity and lower limit-of-detection could be achieved by increasing the GADOX weight ratio and decreasing the thickness simultaneously. The optimal detector with the highest GADOX weight ratio (70%), exhibited a linear sensitivity to the X-ray dose rate within 31-1575 µGyair/s, and a low limit-of-detection of ∼0.26 µGyair/s at a tube voltage of 120 kV. The mechanism discussed here will provide insightful guidance for further development of fiber-optic radiation detectors and these promising results demonstrate the potential applications of fiber-optic detectors.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Radiation detection has appealed great interests in many fields including medical diagnosis and treatment [1–3], scientific research [4,5], environmental monitoring [6,7] and homeland security [8,9]. More urgently, the discharge of radioactive-isotope contaminated water from the Fukushima nuclear power plant and the corresponding permanent impacts on the planet ecology and human health demand the technologies of instant radiation detection and radiation monitoring in water. Fiber-optic radiation detectors have unique advantages such as the miniaturization, the intrinsic immunity to electromagnetic interference, the resistance to liquids, and the capability of real-time, distributable and remote measurement, which are especially attractive for applications demanding high spatial resolution, detection in liquids, and robustness in harsh environments with hazardous radiation. However, fiber-optic X-ray detectors still suffer from the high limit-of-detection (LoD) and the low sensitivity, and are a few years away from commercially available.
The fiber-optic radiation detectors include the intrinsic scheme and the extrinsic scheme [10]. The intrinsic scheme utilizes the decrease of the output intensity of the optical fiber caused by radiation, which is called the radiation-induced-attenuation (RIA) [11]. Larger fiber length and fiber doping are generally required to achieve a greater attenuation and a higher sensitivity [12–14]. However, RIA displays a temperature cross-sensitivity due to the dependence upon the defects in the fiber [10] and a relative short lifetime due to the permanent damage caused by the radiation [15,16]. The extrinsic scheme employs scintillators to convert the ionizing radiation into luminescent light, which is then coupled into optical fibers for detection. Plastic optical fibers (POF) have been widely applied to enhance the light coupling due to the large core diameter [17–20]. The fiber is spliced with a scintillator crystal at the polished end facet [21,22] or directly inserted in a volume of scintillator powders [23]. Besides, an embedded structure where scintillator powders are sealed in a hole at the fiber tip, has also been reported [24]. Reflective coatings such as titanium oxide [25] and aluminum foils [21] are deposited on the scintillators to improve the light collection and the sensitivity of the detector. Recently, fiber tapering was reported to improve the sensitivity, leading to the two-times improvement in the output intensity of the tapered detector [26]. However, fiber detectors utilizing scintillator crystals and powders generally demonstrate low mechanical stability due to the weak binding between the scintillator and the fiber. Therefore, sturdy encapsulation of the crystal or the powder is required to improve the durability and the resistance against environmental disturbances.
Fiber detectors functionalized with scintillating-composite coatings, consisting of scintillator powders and highly-transparent polymers such as PMMA and epoxy resin [20,27], can effectively enhance the mechanical stability and lower the requirements of encapsulation. Nevertheless, fiber-optic detectors based on scintillating composites generally display low sensitivities and high LoDs due to the partial radiation absorption and the low coupling efficiency of the luminescence into the optical fiber. In 2014, Denis McCarthy et al. improved the output intensity of the detector utilizing Gd2O2S: Pr powders with high light yield in the epoxy-based scintillating composite [17]. The performance could be further improved with a homogeneous distribution of scintillator powders. In 2017, Ana I. de Andrés et al. fabricated fiber detectors using the chemically etched fiber tip which greatly enhanced the coupling efficiency [28]. Up to now, no systematic investigations have been performed on the determining factors of the fiber-optic radiation detectors. A thorough understanding of the radio-luminescence generation and the scattering process inside the composite is highly demanded for the performance improvement and further commercialization of fiber-optic detectors.
In this paper, we employed scintillating composites consisting of gadolinium oxysulfide (GADOX) and UV-glue as the coating on the cladding-stripped fiber core, by virtue of their impressive light yield and outstanding transparency, respectively. Moreover, the fast curing of UV-glue guarantees the homogeneous distribution of GADOX powders in the matrix and the uniform coating thickness even at a high GADOX weight ratio. The impacts of the coating length, the coating thickness, and the coating composition on the detector performances are investigated in terms of light generation and light coupling. A mathematical model was developed to understand the physical process of the fiber-optic detectors. Parameters are optimized to improve the luminescence intensity and the light coupling efficiency. All the endeavors contribute to a high sensitivity and a low detection limit of ∼1.2 µGyair/s at an X-ray tube voltage of 35 kV when the weight ratio of GADOX is 70% and the coating thickness is ∼329 µm. Furthermore, the potential of medical applications such as the localized dosimeter for radiotherapy of this fiber-optic detectors is also explored. It exhibits an even lower detection limit of ∼0.26 µGyair/s under the illumination of high-energy X-ray of a medical computed-tomography scanner at a tube voltage of 120 kV. This high-performance fiber-optic X-ray detector displays potentials as a candidate of remote and real-time X-ray detection for many applications.
2. Materials and methods
2.1 Preparation of the scintillating composite
The GADOX powders (Gd2O2S: Tb) and the UV glue (epoxy acrylate) were purchased from Hefei Kejing Ltd. and Shenzhen KSIMI Ltd., respectively. They were mixed and then stirred for 10 min to form a uniformly-dispersed mixture. Then the mixture was placed under vacuum to remove the air before the preparation of the composite films. The weight ratio of the GADOX powder varied from 10% to 70%.
2.2 Fabrication of the fiber-optic X-ray detector
The CK-40 POFs with a core diameter of 980 µm were purchased from Mitsubishi chemical corporation and the black jackets were peeled off before the device fabrication. The exposed fiber tips were first immersed in acetone for 1 min until the polymer cladding swelled and could be easily removed from the fiber core. The fiber tips were cleaned in DI water under ultrasonication for 10 s and then in ethanol for 10 s. After drying in air, the exposed fiber cores were dipped in the mixture of the GADOX and UV-glue for 5 s and lifted up slowly from the mixture within 10 s. The composite coatings were immediately illuminated with UV light for 60 s until the fully solidification.
2.3 Material characterization
XRD patterns were obtained using BRUKER D8 ADVANCE X-ray diffractometer utilizing the Kα ray of Cu. The scanning angle ranges from 10° to 80°, with a step of 0.02°. The transmittance of the composite films was measured using U-3900H UV-Vis spectrometer (HITACHI), and the reflectance were measured using QE-R3001 quantum efficiency measurement system (ENLITECH). The steady-state PL spectra were obtained from FLS 980 (Edinburgh Instruments) with an exciting wavelength of 244 nm. The time-resolved PL spectrum of GADOX powders was acquired via the time-correlated single photon counting system using a 405-nm pulsed picosecond laser. The thicknesses and surface roughness of the composite films and the diameters of the fiber-optic detectors were determined using LEXT OLS5000 3D Measuring Laser Microscope (OLYMPUS).
2.4 X-ray detection
The detecting system is composed of an X-ray tube (Oxford, Apogee 5500 Series), a fiber-optic detector, an optical spectrometer (QE Pro, Ocean Optics), and a computer. The voltage of the X-ray tube was set as 35 kV. The fiber detector was placed around 30 cm below the X-ray emitting window. The X-ray dose rate was adjusted by tuning the tube current, and calibrated using a Radcal Accu-Dose+ dosimeter. The fiber-optic detector was connected to the optical spectrometer through a SMA-905 connector. The integration time was set as 20 s to increase the signal intensity. The demonstration of medical applications was carried out using a high-energy X-ray tube (Hamamatsu, L8121-03) of a CT scanner. The integration time was set as 1 s to increase the responding instantaneity.
3. Results and discussion
The fiber-optic detection system setup and the structure of fiber-optic probe used in this work and the working principle of the detector are demonstrated in Fig. 1. Under the illumination of X-ray, the scintillator coating on the fiber core shown in Fig. 1(b) is activated and emits light, which propagates along the optical fiber and is detected by the spectrometer through the SMA-905 connector. The optical spectrum of the radio-luminescence is recorded by the spectrometer, and the peak intensity is measured as the function of the dose rate of X-ray. The absorbed radiation energy and the corresponding radio-luminescence intensity are determined by the by the content of GADOX scintillators in the coating. As shown in Fig. 1(c), the luminescent light could escape out of the coating and transmit into the air either directly or after the scattering of GADOX powders, causing losses of the luminescent signal. The luminescent light could also travel into the fiber core directly or after scattered by GADOX powders, and then contributes to the output signal. Meanwhile, luminescent light might escape out of the fiber core and cause the signal loss when propagating along the fiber due to the removal of the cladding. Therefore, the coupling efficiency is affected by the coating length, the coating thickness, and the weight content of GADOX powders. Overall, both the material factor (weight ratio) and the structural parameters including the coating thickness and length, should be optimized to improve the sensitivity and the LoD of the detector.
Fig. 1. (a) Schematic illustration of the fiber-optic X-ray detecting system. (b) Illustration of the detector structure. (c) Physical processes in fiber detector.
GADOX scintillators are chosen in this work for the high light yield. The structural and optical characterizations of GADOX powders and the composite films are shown in Fig. 2. The X-ray diffraction (XRD) patterns in Fig. 2(a) coincide well with the PDF card of gadolinium oxysulfide, demonstrating the high purity of the purchased powders. The scanning electron microscope (SEM) image in Fig. 2(b) shows that the grain size of the GADOX powders is several microns, indicating the probable strong Mie scattering on the incident light [29]. The photoluminescence (PL) spectrum in Fig. 2(c) displays multiple luminescent peaks between 450 nm and 650 nm originating from the transitions of doped Tb3+ ions, and the strongest peak appears at 544 nm corresponding to the 5D4-7FJ transition [30]. Corelating the PL spectrum with the UV-Vis absorption spectrum, it can be drawn that GADOX powders have nearly no self-absorption, implying the potential high-performance in x-ray detection. The excitation spectrum is obtained by monitoring the 544 nm emission, which has the strongest excitation wavelength at 244 nm and a broad profile in the wavelength range of 250-320 nm. The luminescence lifetime is 562 µs via fitting the decay curve of the time-resolved photoluminescence spectrum in Fig. 2(d), which is decently short for X-ray detection. Overall, the strong luminescence, the absence of self-absorption, and the decently-short lifetime of luminescence, make GADOX a promising candidate for high-performance X-ray detection.
Fig. 2. Characterizations of GADOX powders and calculation of the minimum thickness of the GADOX/UV-glue composites to fully absorb X-ray with a photon energy of 35 keV. (a) XRD patterns; inset: optical photograph of GADOX powders. (b) SEM image of GADOX powders. (c) Steady-state photoluminescence spectrum, UV-Vis absorption spectrum, and the excitation spectrum of GADOX/UV-glue composites. (d) Time-resolved photoluminescence spectrum labeled with the lifetime obtained from the exponential decay fitting. (e) X-ray emission spectrum of a tungsten tube with a voltage of 35 kV calculated using SpekPy [31]. (f) Minimum required thicknesses of GADOX/UV-glue composite films with different weight ratios to attenuate the X-ray intensity to 1% under the tube voltage of 35 kV, the inset gives the mass attenuation coefficients of GADOX corresponding to different X-ray photon energies from 10−3 to 102 MeV.
The required minimum thickness of the composite coating to fully absorb X-ray photons can be calculated using the photon energy of X-ray, the mass density and the mass attenuation coefficient of the scintillator, according to the Beer-Lambert law:
where I is the intensity after attenuation, I0 is the initial intensity, µ/ρ is the mass attenuation coefficient, ρ is the mass density, and t is the thickness. The average photon energy of the X-ray tube is calculated based on a Python toolkit named SpekPy provided by Bujila et al. [31]. The anode is set as tungsten and the tube voltage is set as 35 kV. The output spectrum is presented in Fig. 2(e) and the corresponding average photon energy is calculated to be ∼15 keV. For composites, the effective mass attenuation coefficient can be calculated according to simple additivity: where wi and (µ/ρ)i are the weight ratio and the mass attenuation coefficient of the ith component, respectively [32]. The inset of Fig. 2(f) gives the mass attenuation coefficients of GADOX corresponding to different photon energies obtained from the National Institute of Standards and Technology, USA [32]. Accordingly, the effective mass attenuation coefficient of GADOX corresponding to the average energy of X-ray (∼15 keV) is calculated to be 79 cm2/g. The effective mass attenuation coefficient of the UV glue is 3.56 cm2/g, calculated from the measured mass density of 1.35 g/cm3 and the dose rate attenuation from 82.36 µGyair/s to 46.25 µGyair/s after penetrating a cured UV-glue film of 1.2 mm. Assuming that X-ray is fully absorbed when the intensity is attenuated to 1%, the required minimum film thicknesses are calculated and given in Fig. 2(f) as a function of GADOX weight ratio. As expected, a higher weight ratio can effectively reduce the requirement of the full-absorption thickness and guarantee a high efficiency of light generation.The optical properties of the composite have a great effect on the light propagation, hence the impacts of the GADOX weight ratio (w) and thickness (t) of the composite coating on the optical properties (light scattering and transmittance) are investigated. Optical photographs in Fig. 3(a) present the uniform distribution of GADOX powders in the UV-glue matrix. The homogeneous distribution of GADOX powders in the matrix and the uniform coating thickness even at a high GADOX weight ratio can be attributed to the fast curing of UV-glue. The surface roughness of the composite increases with the increase of the weight ratio, as illustrated in Fig. 3(b), probably resulting from the increased number of GADOX clusters with the increase of the weight ratio. The transmittance (T) and reflectance (R) at 544 nm, the wavelength of the strongest emission peak of GADOX, are measured using an integrating sphere and a UV-Vis spectrophotometer, respectively, as given in Figs. 3(c) and 3(d). It is noteworthy that the measured transmittance is smaller than the actual value due to the limited area of the receiving window of the spectrophotometer, which cannot collect all the transmitted light, as demonstrated in the inset of Fig. 3(c). When the thickness of the coating is fixed, the transmittance at 544 nm decreases as the GADOX weight ratio increases. As shown in the optical micrographs, the distance between GADOX powders are larger at the low weight ratio, hence a larger fraction of light transmits directly through the composite without getting reflected or scattered by GADOX powders. Furthermore, the transmittance decreases with the increase of the coating thickness due to the enhanced surface reflectance caused by the increased surface roughness and the enhanced internal reflection/scattering by GADOX powders, which is strongly dependent upon the GADOX content in the composite film, as shown in Figs. 3(e) and 3(f).
Fig. 3. Characterizations of the GADOX/UV-glue composite films with varied weight ratios (10%, 30%, 50% and 70%) and film thicknesses. (a) Optical micrographs showing the uniform distribution of GADOX powders of different weight ratios in the composites. (b) Surface roughness as a function of GADOX weight ratio. (c) Transmittance of composite coatings at 544 nm. (d) Reflectance of composite coatings at 544 nm. (e-f) Dependence of the transmittance and reflectance upon the GADOX content.
The effect of the coating length on the detector performances is investigated using both simulations and experiments. As illustrated in Fig. 4(a), simulations are performed in the Tracepro software and optical fibers with exposed cores (diameter 980 µm) are modeled with varied coating lengths l (0-20 mm). The thicknesses of the cladding and the scintillator coating are both set as 20 µm. According to the data sheet of the POF from Mitsubishi, the refractive indexes of the core and the cladding are set as 1.49 and 1.45, respectively [33]. The default settings of the reflection/transmission properties are applied to each surface in the Tracepro software. The inner surface (the coating/core interface) of the scintillator coating functions as the surface source in the Tracepro software. The emission wavelength is set as 544 nm, which is the characteristic emission wavelength of GADOX. The emission type is set as irradiance and the angular distribution is set as uniform. The total number of emitted rays is proportional to l and can be expressed as:
where n is the number of emitted rays from unit length of the scintillator coating. Principally speaking, the number of emitted rays from the scintillator coating per unit length can be set as an arbitrary constant provided that the number is large enough to eliminate the random error. In our simulation, the value of n is set as 200 in the ray-tracing process. The number of rays transmitted at the end plane of the fiber, i.e., the number of white dots in the black circle in Fig. 4(a), is counted as the output signal. The output signal No increases along with l in the first place and then saturates and there is a minimum saturation length, as shown in Fig. 4(b), which can be explained by the coupling loss during the propagation of the luminescent light in the cladding-free fiber core. When the luminescent light rays reach the interface between the fiber core and the scintillator coating, part of the rays would transmit into the scintillator and then escape, and the remaining rays with a portion of µ (µ<1) would reflect back into the core. Assuming that the mean free path between two reflection events is λ, the eventual output opt can be calculated as follows:Fig. 4. Simulation and experimental results of the detector performances with different coating length. (a) Schematic of the fiber-optic detector with a scintillator coating length of l, the ray-tracing process, and the cross-section of the output plane with each incident ray. (b) Output signal as a function of the coating length. (c) Calculated output intensity as a function of the coating length with different reflection coefficient µ related to the weight ratio. (d) Linear responses of the four probes with different coating lengths under varied dose rates of X-ray; inset is the photographs of the four probes. (e) Performance comparison of the four probes.
Fiber-optic detectors are fabricated with an exposed-core length of ∼8 mm, and then functionalized by scintillating-composite coatings with varied weight ratios (10-70%) and coating thicknesses (32-455 µm), as shown in Fig. 5(a). The thicknesses of the composite coating are calculated from the average diameters of the coated fiber cores. All the probes demonstrate good linear responses to the dose rate of X-ray in a range of 31-1575 µGyair/s. Figure 5(b) presents the radio-luminescence of the optimal fiber probe (70%, ∼329 µm) under X-ray illumination and its linear sensitivity at a tube voltage of 35 kV. The dependence of the sensitivity upon the coating thickness of fiber-optic X-ray detectors are shown in Fig. 5(c). When the weight ratio (w) is 10%, the sensitivity increases proportionally with the coating thickness. When w gets larger (30% and 50%), the sensitivity finally inclines to stabilize, while it tends to decrease sharply at the weight ratio of 70%. Figure 5(c) can be replotted as a function of the GADOX content in the composite, which is calculated using the following equation:
where t is the coating thickness, r the core radius, l the coating length, ρ the mass density of the composite, and w the weight ratio. According to the product brochure of the optical fiber, the value of r is 490 µm. As displayed in Figs. 5(d)–5(e), the sensitivity and the LoD show strong dependance on the content of GADOX, except the divergence of the sensitivity appearing at a larger GADOX content.Fig. 5. (a) Optical micrographs of the exposed fiber core coated with scintillating composites, labeled with weight ratios of GADOX and coating thicknesses (scale bar 500 µm). (b) Responses of the optimal detector (70%, 329 µm) to different dose rates of X-ray; insets are photographs of the detector probe with and without X-ray (35 keV) illumination to demonstrate the radio-luminescence of the scintillating composite. (c) Impact of the coating thickness on the detection sensitivity. (d) Sensitivity and (e) LoD of fiber-optic probes as a function of GADOX content in the composite coating.
It is evident that the intensity of the light propagating in the fiber core is co-determined by the intensity of radio-luminescence and the light scattering and reflection inside the composite coating. The radio-luminescence intensity is determined by the content of scintillator GADOX in the coating. The scattering loss of luminescence light increases with the increase of the GADOX weight ratio and the coating thickness. When the GADOX content is small and the scattering loss can be neglected, the increase of the sensitivity mainly results from the increased radio-luminescence, i.e., the increased GADOX content and the corresponding X-ray absorption. When GADOX content further increases, the radio-luminescence intensity increases and the scattering loss enhances as well. At a critical GADOX content or thickness, the scattering loss becomes prevailing and the sensitivity of the detector begins to drop. Theoretically speaking, the sensitivity at every weight ratio would first increase with the GADOX content, then reach stabilization, and finally decrease. The critical thickness should be smaller than the minimum required thickness for X-ray full-absorption. In this work, the optimal device is obtained with the weight ratio of 70% and the thickness of ∼329 µm, exhibiting the highest linear sensitivity of 29.8 s/µGyair and the lowest LoD (∼1.2 µGyair/s at an X-ray tube voltage of 35 kV).
To comprehensively understand the role of radio-luminescence generation and scattering loss, a mathematical model is constructed and displayed in Fig. 6(a). Take one differential element in the scintillator, the effective attenuation length h of X-ray reaching this region is
Fig. 6. Mathematical model of the fiber-optic X-ray detector. (a) Schematic of the differential parameters of the model. (b) Schematic illustration of sensitivity calculation as a function of the coating thickness; (c-d) calculated sensitivity as a function of the coating thickness and the GADOX content.
However, it is not practical to calculate Eff because it is a complicated function of the absorption coefficient, the reflection coefficient and the scattering coefficient [34]. Hence, we used the reflectance of scintillating composites in Fig. 3(d) instead of Eff, considering the similar optical process involving light scattering and reflection. The calculated results in Fig. 6(b) demonstrate similar trends between the coating thickness with the sensitivity as that of the experimental results in Fig. 5(c). The sensitivity would first increase then decline with the increase of coating thickness under the collaborative dominance of geometrical factor and optical factors, which are the decrease of the cone angle, and the simultaneous increases of luminescence generation and scattering loss, respectively. The calculated dependance of the sensitivity on the GADOX content in Fig. 6(c) diverges slightly from the experimental results in Fig. 5(d) at the larger GADOX content. This deviation is mainly due to the insufficient consideration of the scattering loss in the mathematical model, which is strongly related to the weight ratio. This mathematical model perfectly explains the dependance of the detector sensitivity on the GADOX content.
To investigate the dependance of the scattering loss on the weight ratio and coating thickness, w and t are tuned simultaneously while maintaining the content of GADOX powders and thus the total amount of luminescence in the composite coating is constant, as shown in Fig. 7(a) and Table 1. The coating surface of detectors changes from glossy to matte due to the increase of the surface roughness along with the weight ratio, as illustrated in Fig. 3(b). The four detectors all exhibit a linear sensitivity to different dose rates of X-ray and the response amplitude increases with the weight ratio of GADOX, as given in Fig. 7(b). The sensitivities and LoDs given in Fig. 7(c) show that a smaller coating thickness is better for X-ray detection in terms of higher sensitivity and lower LoD. Deduced from the Beer-Lambert law, the luminescence intensity of unit thickness decays exponentially with the penetration depth of X-ray. Hence the majority of the radio-luminescence is excited in the surface of the composite coating and suffers from a lower coupling efficiency resulting from a smaller cone angle at a larger thickness, as illustrated in Fig. 6(b) and Eq. (13). Besides, the dependence of the transmission and reflection properties of the GADOX composites upon the GADOX content illustrated in Figs. 3(e) and 3(f) also strongly indicate that the coupling coefficient is only a function of the cone angle (geometric factor) when the GADOX content is fixed. Therefore, with the same level of radio-luminescence, a smaller thickness leads to a better detector performance. This provides an effective guidance and a feasible approach to the performance improvement of fiber-optic radiation detectors.
Fig. 7. Performances of fiber-optic detectors with composite coatings containing equal content of GADOX powders. (a) Photograph and optical micrographs of probes with different coatings. (b) Linear responses of detectors to the dose rate of X-ray. (c) Comparison of the sensitivity and the LoD of different detectors.
Table 1. GADOX contents in the composite coatings with different weight ratios
The intriguing advantages of the performance-improved fiber-optic X-ray detector prompted us to find its potential applications as a flexible and small-sized in-vivo real-time dosimeter, for the localized dose monitoring during radiation treatment, where the conventional bulky ionizing chamber is not suitable. As demonstrated in Fig. 8(a), the fiber detector is placed on the bed under a CT scanner, mimicking a localized radiation therapy using high-energy X-rays, to evaluate the performance of the fiber detector. The GADOX weight ratio in the composite is chosen as 70% to maximize the output signal, and the integration time of the optical spectrometer is reduced to 1 s to increase the instantaneity of the detecting system. The radiation sensitivity of three detectors with different coating thicknesses (∼100 µm, ∼300 µm and ∼500 µm) is investigated under different dose rates (adjusting the tube current) with a fixed tube voltage of 80 kV. It can be seen that all the detectors demonstrate good linearity to the dose rate and a larger coating thickness is favored for the detection of high-energy X-ray in terms of the higher output intensity and sensitivity, as displayed in Fig. 8(b). The impact of the tube voltage (X-ray energy) on the detector performance is further studied using the detector with the largest coating thickness of ∼500 µm. The detector demonstrates linear responses to the dose rate under all tube voltages, as shown in Fig. 8(c). The dependence of the sensitivity and LoD upon the tube voltage is summarized in Fig. 8(d). With the increase of the tube voltage (X-ray energy), the sensitivity increases and the LoD decreases correspondingly, which can be attributed to the larger penetration depth and the corresponding higher RL collection efficiency discussed in the above. The best LoD of ∼0.26 µGyair/s is achieved at a tube voltage of 120 kV, much lower than the current requirement for X-ray diagnostics (5.5 µGyair/s) [35], which indicates the prominent potentials of this fiber-optic detector for medical applications.
Fig. 8. Demonstration of the fiber-optic detector as a dosimeter for radiation treatment. (a) Schematic of the experimental setup. (b) Output signal intensity of fiber-optic detectors with different coating thicknesses under radiation of CT X-rays with different dose rates (tube voltage: 80 kV). (c) Linear response of the fiber detector with a coating thickness of ∼500 µm to different X-ray dose rates under varied tube voltages. (d) Dependence of the sensitivity and LoD upon the tube voltage.
4. Conclusion
In summary, we fabricated high-performance fiber-optic X-ray detectors consisting of scintillating-composite coatings on the exposed fiber cores. The effects of composites coating length, the coating thickness, and the GADOX weight ratio on detector performances were systematically investigated. The fast-curing capability of UV glue is the key to fabricate the high-quality coatings with the high GADOX weight ratio, the uniformly distributed GADOX powders, and the uniform thickness. Besides the high-performance scintillator, the light coupling efficiency including the scattering loss and the geometric factor plays a critical role in the detector performance. A higher sensitivity can be achieved by increasing the weight ratio of GADOX powders and decreasing the thickness, resulting from the reduced scattering loss. With the topmost weight ratio of 70% and a coating thickness of ∼329 µm, the corresponding detector exhibits a good linear response to the dose rate of X-ray in a range of 31-1575 µGyair/s, and an extraordinary LoD of ∼1.2 µGyair/s at a tube voltage of 35 kV, which is lower than the reported fiber-optic X-ray detectors to our knowledge. The best LoD of ∼0.26 µGyair/s is achieved at a tube voltage of 120 kV under a CT scanner, much lower than the current requirement for X-ray diagnostics (5.5 µGyair/s), indicating the huge potential for applications in remote and real-time X-ray detection where conventional devices are limited such as localized in-vivo dose monitoring during radiotherapy. This systematic investigation provides a thorough understanding of the determining factors of fiber-optic radiation detector and the radio-luminescence generation and the scattering process inside the composite for the further performance improvement.
Funding
National Key Research and Development Program of China (2018YFF01010000); Department of Science and Technology of Sichuan Province (2021JDTD0021).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. F. Nishikido, T. Moritake, S. Kishimoto, and T. Yamaya, “X-ray detector made of plastic scintillators and WLS fiber for real-time dose distribution monitoring in interventional radiology,” in 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC). (IEEE, 2012), pp. 1272–1274. DOI: https://doi.org/10.1109/NSSMIC.2012.6551311.
2. M. A. Suarez, T. Lim, L. Robillot, V. Maillot, T. Lihoreau, P. Bontemps, L. Pazart, and T. Grosjean, “Miniaturized fiber dosimeter of medical ionizing radiations on a narrow optical fiber,” Opt. Express 27(24), 35588–35599 (2019). [CrossRef]
3. W. J. Yoo, S. H. Shin, D. Jeon, S. Hong, S. G. Kim, H. I. Sim, K. W. Jang, S. Cho, and B. Lee, “Simultaneous measurements of pure scintillation and Cerenkov signals in an integrated fiber-optic dosimeter for electron beam therapy dosimetry,” Opt. Express 21(23), 27770–27779 (2013). [CrossRef]
4. L. D. Whittig and W. R. Allardice, “X-Ray Diffraction Techniques,” in Methods of Soil Analysis, A. Klute, Editor. (American Society of Agronomy, 1986), pp. 331–362. DOI: https://doi.org/10.2136/sssabookser5.1.2ed.c12.
5. A. A. Bunaciu, E. g. Udriştioiu, and H. Y. Aboul-Enein, “X-Ray Diffraction: Instrumentation and Applications,” Crit. Rev. Anal. Chem. 45(4), 289–299 (2015). [CrossRef]
6. J. A. Griffith and T. S. Bayer, “A bio-inspired chemical sensor of gamma and neutron radiation,” Sens. Actuators, B 190, 818–821 (2014). [CrossRef]
7. A. Kulkarni, C. K. Vyas, H. Kim, P. C. Kalsi, T. Kim, and V. Manchanda, “Online optical monitor of alpha radiations using a polymeric solid state nuclear track detector CR-39,” Sens. Actuators, B 161(1), 697–701 (2012). [CrossRef]
8. E. A. Miller, T. A. White, B. S. McDonald, and A. Seifert, “Phase Contrast X-Ray Imaging Signatures for Security Applications,” IEEE Trans. Nucl. Sci. 60(1), 416–422 (2013). [CrossRef]
9. G. Zentai, “X-ray imaging for homeland security,” Int. J. Signal Imaging Syst. Eng. 3(1), 13 (2010). [CrossRef]
10. D. Sporea, A. Sporea, S. O’Keeffe, D. McCarthy, and E. Lewis, “Optical Fibers and Optical Fiber Sensors Used in Radiation Monitoring,” in Selected Topics on Optical Fiber Technology, M. Yasin, S. W. Harun, and H. Arof, eds. (University of Limerick Institutional Repository, 2012). DOI: https://doi.org/10.5772/29119.
11. P. Mazzoldi and A. Miotello, “Radiation effects in glasses,” Radiat. Eff. 98(1-4), 39–54 (1986). [CrossRef]
12. G. OriglioProperties and radiation response of optical fibers: role of dopants. 2009. Available from: http://portale.unipa.it/export/sites/www/dipartimenti/fisica/home/attachments/tesi_dottXXI/Origlio_PhD.pdf.
13. S. Girard, C. Marcandella, G. Origlio, Y. Ouerdane, A. Boukenter, and J.-P. Meunier, “Radiation-induced defects in fluorine-doped silica-based optical fibers: Influence of a pre-loading with H2,” J. Non-Cryst. Solids 355(18-21), 1089–1091 (2009). [CrossRef]
14. D. Sporea and R. Sporea, “Setup for the in situ monitoring of the irradiation-induced effects in optical fibers in the ultraviolet-visible optical range,” Rev. Sci. Instrum. 76(11), 113110 (2005). [CrossRef]
15. E. Friebele, K. Long, and M. Gingerich, “Radiation damage in single-mode optical-fiber waveguides,” Appl. Opt. 22(11), 1754–1757 (1983). [CrossRef]
16. E. Friebele, “Optical fiber waveguides in radiation environments,” Opt. Eng. 18(6), 186552 (1979). [CrossRef]
17. D. McCarthy, S. O. Keeffe, E. Lewis, D. G. Sporea, A. Sporea, I. Tiseanu, P. Woulfe, and J. Cronin, “Radiation Dosimeter Using an Extrinsic Fiber Optic Sensor,” IEEE Sens. J. 14(3), 673–685 (2014). [CrossRef]
18. Ó. Esteban and A. I. de Andrés, “Dynamic gamma-ray monitoring at radioprotection levels with extrinsic polymer optical fiber sensors,” Sens. Actuators, A 271, 83–87 (2018). [CrossRef]
19. W. J. Yoo, D. Jeon, J. K. Seo, S. H. Shin, K.-T. Han, W. S. Youn, S. Cho, and B. Lee, “Development of a scintillating fiber-optic dosimeter for measuring the entrance surface dose in diagnostic radiology,” Radiat. Meas. 48, 29–34 (2013). [CrossRef]
20. K. W. Jang, D. H. Cho, W. J. Yoo, J. K. Seo, J. Y. Heo, J. Y. Park, and B. Lee, “Fiber-optic radiation sensor for detection of tritium,” Nucl. Instrum. Methods. Phys. Res. A 652(1), 928–931 (2011). [CrossRef]
21. W. J. Yoo, S. Shin, D. Lee, K. Jang, S. Cho, and B. Lee, “Development of a Small-Sized, Flexible, and Insertable Fiber-Optic Radiation Sensor for Gamma-Ray Spectroscopy,” Sensors 15(9), 21265–21279 (2015). [CrossRef]
22. K. W. Jang, B. S. Lee, and J. H. Moon, “Development and characterization of the integrated fiber-optic radiation sensor for the simultaneous detection of neutrons and gamma rays,” Appl. Radiat. Isot. 69(4), 711–715 (2011). [CrossRef]
23. B.S. Lee, Y.M. Hwang, H.S. Cho, S. Kim, and S. Cho, “Fabrication of fiber-optic radiation sensor tips with inorganic scintillator for remote sensing of X or γ-ray,” in IEEE Symposium Conference Record Nuclear Science 2004. (IEEE, 2004), pp. 865–868 DOI: https://doi.org/10.1109/NSSMIC.2004.1462344.
24. Q. Zhuang, H. Yaosheng, M. Yu, Z. Wenhui, S. Weimin, Z. Daxin, C. Ziyin, and L. Elfed, “Embedded structure fiber-optic radiation dosimeter for radiotherapy applications,” Opt. Express 24(5), 5172–5185 (2016). [CrossRef]
25. K. W. Jang, D. H. Cho, W. J. Yoo, S. H. Shin, H. S. Kim, S. C. Chung, B. Lee, H. Cho, and S. Kim, “Development of two-dimensional fiber-optic radiation sensor for high energy photon beam therapy dosimetry” Journal of Nuclear Science and Technology, 4 (2008). DOI: https://doi.org/10.1080/00223131.2008.10875891.
26. A. De Andrés, Ó. Esteban, and M. Embid, “Improved extrinsic polymer optical fiber sensors for gamma-ray monitoring in radioprotection applications,” Opt. Laser Technol. 93, 201–207 (2017). [CrossRef]
27. S. Sen, M. Tyagi, K. Sharma, P. S. Sarkar, S. Sarkar, C. B. Basak, S. Pitale, M. Ghosh, and S. C. Gadkari, “Organic–Inorganic Composite Films Based on Gd3Ga3Al2O12:Ce Scintillator Nanoparticles for X-ray Imaging Applications,” ACS Appl. Mater. Interfaces 9(42), 37310–37320 (2017). [CrossRef]
28. A. I. Andrés, S. O’Keeffe, L. Chen, and Ó. Esteban, “Highly Sensitive Extrinsic X-Ray Polymer Optical Fiber Sensors Based on Fiber Tip Modification,” IEEE Sens. J. 17(16), 5112–5117 (2017). [CrossRef]
29. R. K. Mishra, J. Cherusseri, E. Allahyari, S. Thomas, and N. Kalarikkal, “Chapter 10 - Small-Angle Light and X-ray Scattering in Nanosciences and Nanotechnology,” in Thermal and Rheological Measurement Techniques for Nanomaterials Characterization, S. Thomas, R. Thomas, A. K. Zachariah, and R. K. Mishra, eds. (Elsevier, 2017), pp. 233–269. DOI: https://doi.org/10.1016/B978-0-323-46139-9.00010-4.
30. F. Wang, D. Liu, B. Yang, and Y. Dai, “Characteristics and Synthesis Mechanism of Gd2O2S:Tb Phosphors Prepared by Vacuum Firing Method,” Vacuum 87, 55–59 (2013). [CrossRef]
31. R. Bujila, A. Omar, and G. Poludniowski, “A validation of SpekPy: A software toolkit for modelling X-ray tube spectra,” Physica Medica 75, 44–54 (2020). [CrossRef]
32. J. H. Hubbell and S. M. Seltzer, X-Ray Mass Attenuation Coefficients, in X-Ray Mass Attenuation Coefficients, E. P. J. Linstrom and W. G. Mallard, eds. 2004, National Institute of Standards and Technology: Gaithersburg MD, 20899. DOI: https://doi.org/10.18434/T4D01F.
33. Mitsubishi Chemical, Eska Polymer Optical Fiber. https://www.pofeska.com/pofeskae/product/01/index.html (accessed 2021 April, 20);
34. I. Kandarakis, D. Cavouras, G. Panayiotakis, and C. Nomicos, “Evaluating x-ray detectors for radiographic applications: A comparison of ZnSCdS: Ag with and screens,” Phys. Med. Biol. 42(7), 1351–1373 (1997). [CrossRef]
35. Q. Chen, J. Wu, X. Ou, B. Huang, J. Almutlaq, A. A. Zhumekenov, X. Guan, S. Han, L. Liang, Z. Yi, J. Li, X. Xie, Y. Wang, Y. Li, D. Fan, D. B. L. Teh, A. H. All, O. F. Mohammed, O. M. Bakr, T. Wu, M. Bettinelli, H. Yang, W. Huang, and X. Liu, “All-inorganic perovskite nanocrystal scintillators,” Nature 561(7721), 88–93 (2018). [CrossRef]
