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Abstract
Scintillators play an important role in the field of nuclear radiation detection. However, the light output of the scintillators is often limited by total internal reflection due to the high refractive indices of the scintillators. Furthermore, the light emission from scintillators typically has an approximately Lambertian profile, which is detrimental to the collection of the light. In this paper, we demonstrate a promising method to achieve enhancement of the light output from scintillators through use of mixed-scale microstructures that are composed of a photonic crystal slab and a microlens array. Simulations and experimental results both show significant improvements in the scintillator light output. The X-ray imaging characteristics of scintillators are improved by the application of the mixed-scale microstructures. The results presented here suggest that the application of the proposed mixed-scale microstructures to scintillators will be beneficial in the nuclear radiation detection field.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Scintillators convert the energy of incident particles or energetic photons, including X-rays, γ-rays, neutrons, and α-particles, into a large number of photons with much lower energy in the visible or near-visible range, which can then be detected easily using photomultipliers, photodiodes, or avalanche photodiodes. Therefore, scintillators are used widely in high-energy physics experiments, medical imaging, security applications, measurements of the physics of the universe, and well logging [1–4]. In the scintillator-based radiation detection field, the efficient light output is mainly dependent on both the internal quantum efficiency and the light extraction efficiency. However, a large fraction of the light that is produced by the scintillator remains trapped inside the scintillator because of total internal reflection (TIR), which then leads to poor light extraction efficiency. Furthermore, the emitted light follows a Lambertian angular profile, which limits the number of photons that can be collected by the detectors. The two factors described above will seriously affect both the detection efficiency and the system performance.
To tackle these difficulties, use of photonic crystals (PhCs) has been proposed to improve the light extraction efficiency of scintillators via coupling of the evanescent field with the periodic structures of the PhCs on the wavelength scale [5]. A diverse range of preparation methods has been used to prepare the PhCs to be coated on the surfaces of scintillators, including nanoimprint lithography [6,7], soft-X-ray interference lithography [8,9], self-assembly [10–12], and hot embossing [13]. In addition, microlens arrays (MLAs), which are composed of multiple micron-scale microlenses, have been used as important components in numerous optical systems, including integral imaging systems [14], organic light-emitting diodes (OLEDs) [15–18], and digital displays [19,20]. A variety of MLA fabrication strategies have been developed to date, including self-assembly [21,22], screen printing [23], the photoresist reflow method [24,25], 3D-printing [26,27], and nano-imprinting [28] . In our recent research, we theoretically proposed and experimentally demonstrated the feasibility of use of MLAs to improve scintillator light extraction efficiency [29–32]. MLAs have demonstrated a prominent ability to control the directionality of the light output of the scintillators. However, it is found that the improvement of the light output of the scintillator by MLAs within the entire emergence angle range is relatively limited and that the MLAs realize an obvious enhancement of the light output at the emergence angle of 45°, but also partly reduce the light output in the normal direction. This is not conducive to maximizing the photon collection when using different detector layouts.
Both PhCs and MLAs can enhance the light extraction efficiency of scintillators, but the control of the enhanced light extraction and of directional emission for a unitary microstructure remains limited. To obtain more flexible control, we combine PhCs based on the wavelength scale and MLAs based on the micrometer scale to form mixed-scale microstructures (MSMs). Generally, MSMs are complex microstructures that are composed of two or more units of differing scales. In this paper, the MSMs are used with a multilayer structure that combines the characteristics of both diffractive optics and geometric optics to control the light extraction from the scintillator. We hope that these MSMs can achieve more effective control of the light extraction efficiency. The underlying physical mechanism responsible for the light extraction is revealed in detail using a combination of numerical simulations and experiments.
2. Structural design and numerical simulation
A hybrid simulation method that combined the rigorous coupled wave analysis (RCWA) method with the Monte Carlo ray tracing method was used to obtain the light output efficiency and the spatial distribution of the microstructure-coated scintillator [33]. This was achieved using the bidirectional scattering distribution function (BSDF). The BSDF was used to characterize the scattering properties from the surface of an arbitrary structure and described the radiance of the scattered light as a function of the incident angle, the wavelength, and the polarization. Both the bidirectional transmittance distribution function (BTDF) and the bidirectional reflectance distribution function (BRDF) were included in the BSDF. The hybrid simulation approach is summarized as Fig. 1. First, in the RCWA-based simulation, a simulation model of the PhCs and scintillator was established. Second, the parameters of the PhCs and scintillator were set, including the refractive index, the wavelength of the scintillator, the diameter of the PhCs. Third, the simulation region set as the period boundary condition, which was due to the obvious periodic structure of the PhCs. Fourth, the simulated convergence test was performed to determine the accuracy of the simulation. Then, the optical properties of the surface PhCs could be obtained. The complete scattering information calculated from the PhCs was saved as a BSDF. Next, in the simulation based on Monte Carlo ray tracing, a simulation model of MLAs and scintillators was established. The parameters of the MLAs and scintillator were set, including the size and refractive index of the scintillator, the diameter of the MLAs. The calculated BSDF was then used as a surface property of the scintillator. In addition, the simulated conditions were determined. The total number of rays in scintillator was set to be 1 million and the power threshold of an arbitrary light ray was set to be 1%, suggesting that when the power dropped less than 1% of its original energy of the light, this ray will be abandoned. Finally, the spatial distribution of the light output of the MSM-coated scintillators was obtained. The scintillator surface covered with microstructures served as the light-emitting surface and the other surfaces were called non-emitting surfaces.
Figure 2(a) shows a schematic illustration of a Bi4Ge3O12 (BGO) scintillator (refractive index nsc=2.15, dimension 20×10×1 mm3) coated with MSMs on its surface and the definitions of the related parameters, where the microstructures are used to enhance the light output and the directional emission of the scintillator. The MSMs are composed of a PhC layer and an MLA layer. The PhC layer is attached directly to the scintillator surface, and the MLA layer then covers the PhC layer. The PhC layer is composed of a monolayer periodic array of polystyrene spheres (refractive index np=1.59, diameter Dp=500 nm) and a 30-nm-thick conformal TiO2 layer covers the surfaces of the spheres. The MLA layer is composed of a monolayer hexagonal array of hemispherical polystyrene microlenses (refractive index nm=1.59, diameter Dm=4.5 µm). The emergence angle and the azimuth angle of the sample are labeled θem and φ, respectively.
Fig. 2. (a) Schematic illustration of MSMs coated on the surface of a scintillator and definitions of the related parameters. (b) Simulated light outputs from a PhC-coated scintillator (DP=500 nm, np=1.59, d=30 nm), an MLA-coated scintillator (Dm=4.5 µm, nm=1.59), an MSM-coated scintillator, and a reference scintillator as a function of the emergence angle (θem) and the azimuth angle (φ). (c) Simulated angular profiles of the light outputs from the reference scintillator, the PhC-coated scintillator, the MLA-coated scintillator, and the MSM-coated scintillator. (d) Angle-integrated enhancements of the scintillators from the emergent angle ranges of 0°–60°(AI) and 0°–35°(PI).
To illustrate the effect of the MSMs on the scintillator’s light output intuitively, the simulated light outputs of a PhC-coated scintillator, an MLA-coated scintillator, an MSM-coated scintillator, and a reference scintillator are shown as functions of θem and φ in Fig. 2(b). For the plain scintillator without any added structures (the reference), the light output approximately follows the Lambertian type pattern and there is no obvious azimuthal dependence. However, for the PhC-coated scintillator, the result indicates that the pattern has a sixfold symmetry characteristic of a triangular lattice, which is attributed to the periods of the PhCs. Additionally, for the MLA-coated scintillator, the directional control of the light output was particularly obvious and was mainly concentrated around the high angle direction of 45°. We have proved that the control of the light output directionality is caused by the regulation of the light reflected from the bottom surface of the scintillator by the MLAs [29]. From the simulation results, the light output also shows no obvious azimuthal dependence. In contrast, for the MSM-coated scintillator, we were fortunate to find that the scintillator’s light output still has the azimuthal symmetry of the triangular lattice. The light output directivity was not obvious, but there has been significant angular broadening. To provide further understanding of the light output enhancement, simulated angular profiles of the light output for the reference scintillator, the PhC-coated scintillator, the MLA-coated scintillator, and the MSM-coated scintillator are all shown in Fig. 2(c). Significant enhancement of the light output for all samples within the emergence angle range of 0°–60° can be achieved, but the directional emission control behavior is quite different. When compared with the reference scintillator, the enhancement ratios are 1.96-fold at the emergence angle of 0° and 2.80-fold at the emergence angle of 45° for the PhC-coated scintillators and the MLA-coated scintillators, respectively. However, the corresponding enhancement ratios are 2.67-fold and 2.48-fold at emergence angles of 0° and 35°, respectively, for MSM-coated scintillator. As shown in Fig. 2(d), the light outputs of the scintillators are integrated over the emergence angle ranges of 0°–60° (all integration, AI) and 0°–35° (partial integration, PI). When compared with the reference scintillator, the MSMs can achieve a 2.39-fold enhancement over the 0°–60° range and a 2.41-fold enhancement in the 0°–35° range. However, the PhCs and MLAs show only 1.51-fold and 1.67-fold enhancements in the 0°−35° range and 1.40-fold and 2.04-fold enhancements in the 0°−60° range, respectively. These results suggest that MSMs can improve the light output of the scintillator significantly beyond that achieved using the PhCs and the MLAs, and show that the light output enhancement occurs over a wide angular range.
To provide a further understanding of the improved scintillator light output mechanism produced by the MSMs, the simulated transmittance characteristics of the PhC-coated scintillator, the MLA-coated scintillator, the MSM-coated scintillator, and the reference scintillator are shown as a function of incidence angle (θin) in Fig. 3(a). For the non-emitting surfaces of the scintillators, both air and absorption boundary conditions are considered. The former conditions consider multiple reflections, including Fresnel reflections and total internal reflection at the non-emitting surfaces, which means that the reflected light at the emitting face can be reflected by the non-emitting faces of the scintillator and subsequently re-extracted using the microstructures when it reaches the emitting face again. The latter conditions do not consider the multiple internal reflections that occur at the non-emitting faces, which means that when the reflected light reaches the non-emitting faces, it will then be absorbed by the non-emitting faces and thus cannot contribute to the ultimate extraction efficiency. Therefore, the transmittance is related to either multiple extractions (ME) for the air boundary conditions or a single extraction (SE) for the absorption boundary conditions.
Fig. 3. (a) Simulated transmittance for single extraction (SE, dashed line) and multiple extractions (ME, solid line) as a function of the angle of incidence (θin) for different structures on the scintillators. (b) Schematic diagram of the five divided regions used for the simulations. (c) Simulated angular profiles for each region of the MSM-coated scintillator.
According to Snell’s law, at a scintillator-air interface, we can define the critical angle as θc1 = arcsin(nair/nsc). For the reference scintillator, the transmittance drops to zero beyond θc1. The transmission spectra of the reference scintillator for the SE and the ME are relatively similar, which implies that the multiple extractions cannot contribute to the ultimate extraction efficiency. For the MLA-coated scintillator, it is obvious that the transmittance for both the SE and the ME increases significantly when the incident angle is beyond θc1. Additionally, the value of the ME is higher than that of the SE, indicating that the multiple extractions make a significant contribution to the ultimate extraction efficiency. For the PhC-coated scintillator, there are two obvious peaks in the transmission spectra for the SE when the incident angle is greater than θc1, which means that the light can be extracted, these two peaks are mainly derived from the diffraction properties of the periodic array of PhCs and the whispering gallery modes of the nanospheres [5]. It is also found that multiple extractions broaden the transmission peak significantly, thus indicating that more light can be extracted. However, for the MSM-coated scintillator, when the incident angle is less than θc1, the transmittance for the SE shows an obvious drop when compared with that of the PhC-coated scintillator, but there are still two transmission peaks after the incident angle exceeds θc1. This implies that part of the light extracted via the periodic diffraction of the PhCs will be reflected into the scintillator by the MLA layer. However, the increase in transmittance for the ME indicates that the reflected light will be multiply extracted through the non-emitting surfaces.
To analyze the mechanism of the directivity of light output, at the interface between the scintillator and the PhC layer, we label the angle θc2 = arcsin (np/nsc). The scintillation light source can be divided into five regions, designated region 1 (0–θc1), region 2 (θc1–θc2), region 3 (θc2–(180°-θc2)), region 4 ((180°-θc2)–(180°-θc1)), and region 5 ((180°-θc1)–180°), as shown in Fig. 3(b). The non-emitting surfaces are set to have the air boundary condition, which means that the total light output is given by the summation of the light outputs for each region, but the contribution from each region is different. Figure 3(c) shows the directional emissions contributed by each of the five regions for the MSM-coated scintillator. The incident light in region 1 is extracted via the diffraction of the PhCs and is then concentrated at a high angle through the MLAs. For region 2 and region 4, only a small proportion of the light can be extracted directly via the PhCs to then enter the MLAs and be emitted into the air; this is shown as a weak directional emission. Most of the light will be diffracted by the PhCs back into the scintillator, and can then be re-extracted after multiple reflections at the non-emitting surfaces. This fraction of the emitted light is mainly concentrated in the normal direction. However, for region 3 and region 5, the light exits almost entirely through the lateral sides and the bottom of the scintillator and thus does not contribute to the directional emission from the light-emitting surface. As a result, with the aid of the control properties of the MSMs, broadened directional emission can be obtained.
To analyze the effects of the different parameters of the MSMs on the light extraction efficiency, Fig. 4(a) shows a simulated angular distribution of nanospheres with different diameters and the corresponding distribution of nanospheres when coated with a conformal layer. For the scintillators that were covered with nanospheres only, the 1.22-fold angle-integrated enhancement ratio in the 0°–60° range has been obtained for nanospheres with a diameter of 500 nm, which demonstrates that when the period of the photonic crystal becomes closer to the scintillator emission wavelength, the light extraction efficiency then becomes more significant. For the scintillators that are covered with the 500-nm-diameter nanospheres, the maximum light output intensity occurs at 0°. However, the maximum light output intensity is at 6° or 40° for the nanospheres with diameters of 400 or 600 nm, respectively. These results show that the directional emissions of the scintillators are mainly affected by the period of the structure. A conformal layer on the surface of the prefabricated PhCs represents an effective way to increase the number of optical modes, thus leading to further light extraction enhancement. However, this conformal layer does not affect the original directionality. Figure 4(b) shows the simulated light output of the MSM-coated scintillator as functions of both θem and Dm. The angle distribution profiles are nearly identical for the MSM-coated scintillators with different values of Dm. This is because the duty cycle of the large-sized microlens does not vary with respect to Dm under the conditions for the close packing arrangement, which is consistent with the results obtained from the MLA-coated scintillator [29]. In addition, the angle-integrated enhancement ratios at various values of nm (1.1–2.4) for the MLA-coated and MSM-coated scintillators are shown in Fig. 4(c). Regardless of these structures, the optimal condition for light extraction efficiency is obtained when nm is slightly larger than nsc. The results described above provide the fundamental understanding required for design of the MSMs to allow them to control the light output of the scintillators.
Fig. 4. (a) Simulated light output of the PhC-coated scintillator for various Dp. (b) Simulated light output from the scintillator as a function of the emergence angle (θem) and the diameter of the individual microlens (Dm). (c) Simulated enhancement ratios with respect to the reference scintillator as a function of the refractive index of the microlens (nm).
3. Sample preparation and performance characterization
Figure 5 shows a schematic of the workflow for MSM fabrication on the surface of a BGO scintillator. First, a mixture of polystyrene nanospheres (with Dp = 400, 500, and 600 nm) in alcohol and deionized water were blended with specific proportions. The mixture was then dropped onto a silicon wafer, which had previously been treated with a 10% mass fraction of a sodium dodecyl sulfate solution. The polystyrene nanospheres were dispersed uniformly on the silicon wafer surface and formed a monolayer arrangement via self-assembly. Second, the silicon wafer that was covered with the polystyrene nanospheres was immersed slowly into deionized water, thus allowing the nanospheres to float on the water’s surface and form a monolayer hexagonal close-packed array by mutual force. Third, these nanosphere arrays were transferred onto the surfaces of the BGO scintillators, which were cut and polished to dimensions of 20×10×1 mm3. Fourth, the samples were coated with a TiO2 shell using an atomic layer deposition system (SUNALE R-200, PICOSUN). Fifth, the samples were re-covered with a polystyrene microsphere monolayer formed on the PhC layer surface via self-assembly. Sixth, the samples were heat-treated in an oven at 115°C for 40 min. The MSM-coated BGO samples were then finally prepared.
The self-assembly method offers advantages over traditional lithography in terms of the ease of preparation over a large area with low cost, and atomic layer deposition provides a unique technique for deposition of conformal and homogeneous thin films with precise thickness control at the angstrom level [34], which is also highly desirable for large-area sample preparation.
The surface topography was measured using a Hitachi S-4800 scanning electron microscope (SEM). An ultraviolet light-emitting diode (365 nm) was fixed on the rotatable testing platform and excited the samples to emit light. The full frame transfer charge-coupled device (FFT-CCD) detector (PG-2000-Pro, Ideaoptics) was rotated horizontally to receive the light from the scintillator at different emergence angles and optical signal was then converted into an electrical signal by a fiber spectrometer. Eventually, the angle-resolved emission spectra were obtained. The samples are excited on the bottom surface of the scintillator, and the front surface with the microstructures is the light-emitting surface, as shown in Fig. 6. X-ray excited luminescence spectra were acquired by using an X-ray source (tungsten target, 80 kV, 40 keV) to excite the samples and using a monochromator (SBP300, Zolix) with a photomultiplier (PMTH-S1-CR131, Hamamatsu) to record the signals. In the X-ray imaging characteristics, the samples were excited by an X-ray source (Moxtek, TUB000140, 40 kV, 200 µA) filtered by an aluminum plate. The scintillation light was recorded by an intensified charge coupled device (ICCD) camera (Andor, DH34-18F-63). The imaged object was placed in front of the samples and the signals were collected and processed by a computer.
4. Results and discussion
Figure 7 shows the SEM image of the morphology of the MSM-coated BGO scintillator. The image clearly shows that the upper layer is a hexagonal arrangement of closely-packed MLAs (Dm = 4.5 µm) and the lower layer is a conformal PhC layer (Dp = 500 nm and d = 30 nm). TiO2 was deposited uniformly on all exposed surfaces to form a spherical shell. This structure is basically consistent with the theoretical design shown in Fig. 2(a).
Fig. 7. (a) Top view SEM image of the MSMs. (b) Top view SEM image of the nanospheres (DP=500 nm). (c) Side view SEM image of the MSMs. (d) Partially enlarged SEM image of the MSMs.
The angle-dependent emission spectra obtained under UV excitation for the reference sample and the MSM-coated sample are shown in Figs. 8(a) and 8(b), respectively. The reference sample shows an approximately Lambertian profile without a specific direction. When compared with the reference sample, the enhancement and directional control of the light output produced by the MSMs are particularly obvious. The light output shows 2.15-fold and 2.50-fold enhancements at 0° and 35°, respectively. However, the light outputs from the PhC-coated sample and the MLA-coated sample are enhanced by approximately 1.80-fold and 2.62-fold at 0° and 45°, respectively, as shown in Figs. 8(c) and 8(d).
Fig. 8. (a) Angle-dependent emission spectra under UV excitation for the reference sample, (b) the MSM-coated sample, (c) the PhC-coated sample, and (d) the MLA-coated sample.
The emission spectra of the reference scintillator, the PhC-coated scintillator, the MLA-coated scintillator, and the MSM-coated scintillator in the normal direction are plotted in Fig. 9(a). The spectra indicate that the MLA-coated scintillator shows no obvious wavelength dependence for its light output enhancement ratio. However, the enhancement ratios were dependent on the wavelength for both the PhC- and MSM-coated scintillators, which are attributed to the effect of the period of the PhCs. Furthermore, Fig. 9(b) presents the enhancement spectra of the samples acquired under UV and X-ray excitation in the normal direction. The enhancement ratios for the UV and X-ray excitations are nearly consistent, which suggests that the enhancement of the light extraction efficiency is independent of the excitation method. The simulated and measured enhancement ratios of the samples are shown as a function of θem in Fig. 9(c). The experimental and simulated results show fundamental agreement in spite of slightly large differences in some emission angles, thus verifying the effectiveness of the theoretical designs. Additionally, the differences in consistency are mainly attributed to some partial defects that occurred during sample preparation.
Fig. 9. (a) Measured emission spectra under UV excitation for the samples in the normal direction (0°). (b) Enhancement spectra of the coated samples with respect to the reference sample in the normal direction under UV and X-ray excitation. (c) Simulated and measured enhancement ratios for the light outputs of the samples as a function of θem.
The effects of the PhCs, the MLAs, and the MSMs on the X-ray imaging properties based on the BGO scintillation converter were also studied. To evaluate the suitability of the samples for use in X-ray imaging, we concealed a metallic spring in an opaque pill and imaged the closed pill using X-rays, as illustrated in Fig. 10(a). The MSM-coated sample exhibits better imaging brightness, we can infer that the microstructure can improve the brightness of the image by increasing the light output of the scintillators. It was interesting to find that the PhC-coated sample and the MLA-coated sample both showed misty images, whereas the MSM-coated sample presented a much clearer image. The spring is equivalent to a simple resolution card, and the images show that the MSM-coated sample maintains similar spatial resolution to that of the reference sample with improved imaging brightness. Figure 10(b) presents the vertical grayscale profiles along the vertical yellow dashed lines for the samples, which were extracted from Fig. 10(a). When compared with the reference sample, the average grayscale value from the bright part of the images for the MSM-coated sample shows an enhancement ratio of 2.58, which is slightly higher than the ratio of 2.15 measured in the normal direction in Fig. 9(a).
Fig. 10. (a) X-ray imaging characterizations of the different samples. (b) Grayscale profiles along the vertical yellow dashed lines for the different samples.
5. Conclusions
In conclusion, we have demonstrated that use of MSMs on the surface of a scintillator makes a major contribution to the wide-angle enhancement of the scintillator’s light output. Hybrid simulation methods enable effective realization of the structural design and the parameter optimization of these MSMs. We have explored the effects of the different parameters of the MSMs on the scintillator’s light output and have analyzed the principle of the wide-angle light output. In addition, the X-ray imaging characteristics of the scintillator is improved by addition of the MSMs, which would be beneficial in boosting the performance of practical detectors.
Funding
National Natural Science Foundation of China (11804252, 11975168); State Key Laboratory of Intense Pulsed Radiation Simulation and Effect (SKLIPR2022).
Disclosures
The authors declare no conflicts 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.
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