Full-angle high-reflection ultrathin composite film realizing high spatial resolution of scintillation crystal array

Nuclear medicine imaging is an important means of disease diagnosis in modern medicine,¹ which can directly monitor the concentration distribution of radionuclides in organs and tissues and its change over time and study metabolic functions at the molecular level, such as tumors and nervous system diseases.^2,3 X-ray detectors play a decisive role in nuclear medicine imaging systems. Detectors with high spatial resolution can further improve the imaging quality by allowing accurate location of the lesion. Additionally, shortening the imaging time without changing the original radiation dose reduces the damage to the human body.⁴ For this reason, nuclear medicine imaging such as Positron Emission Computed Tomography and Single Photon Emission Computed Tomography have an urgent demand for high spatial resolution scintillation detectors.^5–8 

The essence of the performance optimization of high spatial resolution scintillator detectors lies in the rational design and optimization of the scintillation photon transport process in the crystal to obtain a higher overall detector performance.^9–14 The surface treatment of scintillation crystals is one of the effective means to optimize the transmission and collection of scintillation photons. It has been shown that changing the surface treatment of a scintillation crystal can improve the light collection efficiency by about 25% and the time resolution by about 15%, respectively,¹⁵ which is particularly important for suppressing the time jitter introduced by the scintillation light transmission process to improve the spatial and time resolution of the detector and facilitate the design of time of flight detectors.¹⁶ Also, the thickness of the crystal surface material has a significant impact on the fill factor and sensitivity of submillimeter scintillation detectors.¹⁷ For instance, the use of a common Enhanced Specular Reflector or Teflon reflective material with a thickness in the range of 0.07–0.1 mm results in a low fill factor of only 64% of effective detection volume for a crystal array consisting of a pixel size of 0.5 × 0.5 mm².¹⁸ However, the use of a reflection film with a thickness of only 30 μm can increase the fill efficiency by 38%.¹⁸ 

Cerium doped lutetium yttrium oxyorthosilicate (LYSO) scintillator crystals are widely used in the radiation detection and imaging field.¹⁹ The urgent needs for application in these fields are the improvement of detection sensitivity and spatial resolution, which has increased the light output of the scintillation crystal array. Multilayer dielectric coatings have high reflectance and, thus, are often used in LYSO detectors to improve their light output. Inorganic materials such as magnesium fluoride (MgF₂, n = 1.38),²⁰ silicon dioxide (SiO₂, n = 1.46),²¹ niobium oxide (Nb₂O₅, n = 2.32),²² titanium dioxide (TiO₂, n = 2.1),²³ and aluminum oxide (Al₂O₃, n = 1.77)²⁴ can be used in the design of dielectric films. Sputtering, electron-beam evaporation, metal-organic chemical vapor deposition, vacuum evaporation technologies, molecular beam epitaxy, atomic layer deposition, and other techniques can be used for high-reflection (HR) coatings.^25,26 However, these high-reflection films are generally designed according to a 0° angle of incidence. Although photons incident at 0° angle can obtain high reflectance, the scintillation photons are distributed at all spatial angles. Thus, the high-reflection film cannot collect the photons at full-angle incidence, which has a negative impact on the light output of the crystal array.²⁷ Li et al. achieved a high reflectance by coating the surface of LSO (lutetium silicate) crystals with MgF₂/CeO₂/dielectric film/layer of the metallic aluminum film. However, they neglected to consider the average reflectance within the emission spectral band of LSO crystals.²⁸ Sun et al. used electron-beam evaporation to plate a 48-layer film consisting of HfO₂/SiO₂/TiO₂, which provided a distributed Bragg reflector partially similar to the work in this paper, but did not analyze and simulate the angle of incidence of the LYSO crystals in a comprehensive and systematic way as in the present work.²⁹ Xie et al. designed and optimized an 11-layer antireflective film, which acted as a reflection-reducing film, and the angle was not analyzed.³⁰ 

The goal of this study was to develop a membrane system that could achieve 99.3% reflectance over a wide range of incidence angles (from 0° to the critical angle of total reflection) based on the optical simulation results. To ensure high reflectance, a 60-layer dielectric coating was designed by SiO₂/TiO₂/Ta₂O₅. A light barrier layer was added to further block light crosstalk between two pixels. Metal Al film and milky-white diffuse reflective adhesive were chosen to coat the designed dielectric film labeled as Routes I and II, respectively. This work compared the optical barrier of the detector head on the ideal inner surface and the practical outer surface of the scintillation crystal pixel and obtained the mechanism of action of the different optical barrier layers based on theoretical and experimental results. We found that the optical barrier of Al film achieves the full-angle high reflectance and both routes maintain high reflectance within 0°–90° and, thus, improve the light output of LYSO arrays. This paper can provide theoretical guidance and versatile solutions for high spatial resolution scintillation crystal arrays.

A composite film consisting of the dielectric film, metal film, and diffuse reflection film was employed to achieve high reflectance performance. This approach possesses full-angle reflectance and light-blocking effects. Figure 1(a) reveals the 60 layers of Ta₂O₅/SiO₂/TiO₂ dielectric layer configuration and 1 layer of Al film (Al film reflector) and reflection adhesives glue (highly adhesive film reflector).

Among the dielectric materials, TiO₂, Ta₂O₅, and SiO₂ exhibit excellent properties such as low extinction coefficient, good adhesion, physical and chemical durability, and high light transmittance in the visible spectrum (360–650 nm), which matches the LYSO emission spectral band.³¹ In Fig. 1(a), the 60 layers of dielectric layer which comprises Ta₂O₅/SiO₂/TiO₂ are employed as the ultrathin HR film. The dielectric layer consists of three quarter-wavelength reflection films with central wavelengths of 420, 500, and 575 nm. The outermost layer is the photo blocking layer, which has two routes, as detailed in Sec. II B.

The structural foundation of the above distributed Bragg reflectors (multilayer dielectric film) was represented by the formula S{(LH₁)¹⁰(LH₁)⁹(LH₂)¹¹}A, where S denoted LYSO substrates, A denoted air medium, L denoted SiO₂, H₁ denoted TiO₂, and H₂ denoted Ta₂O₅. In the wavelength range of 420–500 nm, the refractive indices of SiO₂, TiO₂, and Ta₂O₅ were 1.46, 2.41, and 2.10, respectively.³¹ 

where N represents the number of periods, n_S denotes the refractive index of the substrate, n_L represents the refractive coefficient of low refractive index materials, n_H represents the refractive coefficient of high refractive index materials, and n_A represents the refractive index of the air medium.

Table I displays simulation parameters. The simulation results are elaborated in Sec. III A. The simulation model was established and improved by comparing the reflectance spectra of the film samples with tfcalc simulations.

Al film is the only material with high reflectance from the ultraviolet to the visible light range and its surface can produce a thin oxide layer in the atmosphere. Therefore, as shown in Fig. 1(a), the Al film was used as a light-blocking layer in the Al film reflector of this study. The reflectance and transmittance spectra were measured using a UV-Vis spectrophotometer (Lambda 950, PerkinElmer, USA) equipped with a variable angle attachment with the aid of a UV-Vis spectrophotometer (U-3900H, HITACHI, Japan). The absorbance loss of the Al film was calculated by subtracting the reflectance and transmittance spectra. tfcalc simulations are used to examine the effect of aluminum layer thickness on dielectric film reflectance.

Diffuse reflection films result in higher uniformity in the horizontal and vertical distribution of photons, making the reflectance of the material less affected by the angle of incidence. In this study, a reflection adhesive with high reflectance in a wide spectral range (200–1000 nm), especially in the UV band, was prepared as a light-blocking layer for the highly adhesive film reflector, as shown in Fig. 1(a). The thickness of the reflection adhesive has a greater effect on its reflectance than its adhesive force. The thickness tests were done on a UV-Vis spectrophotometer (U-3900H, HITACHI, Japan). The reflectance spectra were measured with the same UV-Vis spectrophotometer (Lambda 950, PerkinElmer, USA) equipped with a variable angle attachment with the aid of a UV-Vis spectrophotometer (U-3900H, HITACHI, Japan).

Before the application of the coating, all LYSO crystal samples are chemically and manually polished to ensure a surface roughness of less than 10 nm.³³ Chemo-mechanical polishing was carried out using an automatic precision lapping and polishing machine (UNIPOL-802, Cogent, China). The optical surface profile roughness tester (ZYGO, USA) was used to obtain 3D images of the postpolishing substrate, as shown in Fig. 1(b). The structure of the films was examined using a high-resolution field emission scanning electron microscope (SEM) (Verios G4, FEI, USA). Thin films were deposited using a vacuum coater (UNIVEX, Leybold, Germany). The Al film was provided by 3M company.

Figure 2(a) displays the SEM images of the distributed Bragg reflector. The film thickness is 3.84 μm and clearly displays the constitution of 60 layers. Figure 2(b) showcases the composition change of the dielectric film within the thickness range of 3.84 μm. Clearly, the first 38 layers (∼2.4 μm) are created by alternating periodic structure of SiO₂ and TiO₂ and the final 22 layers (∼1.4 μm) consist of SiO₂ and Ta₂O₅ cycling.

The simulation results of various dielectric film layer architectures at 420 nm, including superlattices of SiO₂/TiO₂, superlattices of SiO₂/Ta₂O₅, and the combination of both (the distributed Bragg reflectors) are depicted in Figs. 3(a) and 3(b). Figure 3(a) shows that the superlattices of 38 layers of SiO₂/TiO₂ exhibit high reflectance only at an incidence angle above 30°, while the superlattices of SiO₂/Ta₂O₅ have less than 97% reflectance below 20°. From the results, it was seen that the superlattices of SiO₂/TiO₂ can achieve high reflectance at high angles and the superlattices of SiO₂/Ta₂O₅ have high reflectance at low angles. Therefore, the combination of both can realize full-angle high reflectance [Fig. 3(b)]. The simulation of wavelength-resolved reflectance in Fig. 3(b) revealed that the superlattices of SiO₂/TiO₂ can generate a wide high reflectance band in the range of 430–650 nm, which did not coincide with the central wavelength of LYSO emission peak (420 nm). The (LH₂)¹¹ exhibited a narrow high reflectance band at 360–450 nm. The combination of the two can achieve high reflectivity in the 360–650 nm band. Conclusively, the distributed Bragg reflector dielectric film had high potential for achieving robust reflectance spanning from 0°–90° and covering the wavelength range of 400–650 nm. This noteworthy finding can guide the light collection layer design of scintillation crystal arrays with high spatial resolution imaging applications.

The practical results of incidence angle and wavelength-resolved reflectance are presented in Figs. 3(c) and 3(d). The average reflectance values were found to be 96.9%. Notably, distinct optical properties were observed at non-zero incidence angles, with reflectance decreasing as the angle of incidence increases. The average decline ranged from 2.2% to 2.5%. Specifically, the reflectance of dielectric reflective film first moderately decreases and then accelerates between 8° and 70°. Figure 3(d) illustrates the reflectance spectra of the high-reflection thin-film coating for different angles of incidence (8°, 30°, 38°, 58°, and 70°) across the wavelength range of 300–700 nm. The results in Fig. 3(d) clearly demonstrate significant variations in the reflectance spectral profile depending on the incidence angle, with the highest reflectance of 97.0% observed at 8° and the lowest reflectance of 94.8% at 70°. However, these findings fell short of the targeted reflectance of 99.3% by 2.3%, indicating the need for further optimization efforts.

Only the dielectric film is transparent and transmits light. This work designs a light-blocking layer in the outer layer of the dielectric film to investigate the difference in reflectance between the two paths.

The absorbance spectra of Al film and dielectric film are compared in Fig. 4(a) by measuring four different angles (10°, 35°, 60°, and 75°). The absorbance spectra of both films can be obtained from Fig. S1,³⁵ which shows two comparative plots of reflectance plus transmittance of Al films and dielectric films. It is observed that the Al film absorbs about 10% of the energy compared to the dielectric film. Figure 4(b) shows the reflectance curves of the dielectric film plus different thicknesses of Al film. Since the attenuation length of the Al film is less than 50 nm, we selected the thicknesses of 10, 20, 30, and 40 nm, respectively, and their shapes basically overlapped within the emission bands of the LYSO (400–650 nm). Therefore, changing the thickness of the Al film on the dielectric film only causes a very slight deviation in the reflectance. Figure 4(c) compares the average reflectance of a single Al film, a dielectric film, a simulated dielectric film, and a dielectric film with an additional Al film in the wavelength range from 300 to 700 nm. The test results show that the side wave bands of the dielectric film are flatter than the simulated results due to the precision of film thickness control during preparation. In the wavelength range from 400 to 650 nm, the average reflectance of the dielectric and Al combined film was 99.1%, while the average reflectance of the pure dielectric film was 97.1%. Thus, Al film can promote the reflectance of pure dielectric film to 2.0%.

Figure 4(d) shows the reflectance spectra of Al film reflector coating at different wavelengths (410, 420, 430, 440, and 450 nm) at various incident angles, namely, 8°, 30°, 38°, 58°, and 68°. The reflectance results are in the range of 96.0%– 99.9% and the reflectance remains consistent at all angles of incidence, which means that the Al coating not only improves the reflectance but also allows full-angle reflectance. It is worth noting that the highest reflectivity of 99.6% is observed at 450 nm, which has reached the experimental target. Figure 4(e) shows the variation of reflectance with wavelength for the Al film reflector. The very high overlap of the five curves indicates that a high degree of overlap in reflectance at different angles can be achieved by plating a full-angle reflection metal film on the outside of the dielectric film. Also, the average reflectance of the aluminum film in combination with the dielectric film is approximately 2.0% higher compared to the multilayer dielectric film. This is consistent with the results in Fig. 4(c). In addition, the reflectivity variation curves for five different test angles lie in the same plane, which is consistent with the results in Fig. 4(e).

In order to obtain the optimum thickness of reflective adhesive, we tested the reflectivity of reflective adhesives with different thicknesses (4, 8, 12, 25, 80, and 100 μm) at 420 nm wavelength as shown in Fig. 5(a). The results show that the reflectivity is relatively highest at 12 μm. Further, Fig. 5(b) shows the variation of reflectance with wavelength for the above mentioned reflective adhesives with different thicknesses. The result in Fig. 5(b) shows that the thinner the thickness, the relatively lower the reflectivity. In addition, Fig. 5(c) shows the reflectance of five thicknesses of reflective adhesives (4, 8, 12, 20, and 25 μm) on dielectric films. Figure 5(d) shows the reflectance versus wavelength curves for five thicknesses (4, 8, 12, 20, and 25 μm). The results show that the 12 μm thickness of the reflective gel performs relatively best with an average reflectance of 96.1%. Based on these findings, 12 μm adhesive was selected as the optimum thickness in this study.

Figure 5(e) shows the reflectivity-resolved results for the reflection adhesive, the dielectric film, and the dielectric film combined with the 12 μm reflection adhesive. The results show that the reflectivity of the adhesive layer alone is very low, at 30%– 40%. The reflectance of the dielectric film alone was measured at 94.4%. In contrast, the average reflectance of the dielectric film combined with the reflection adhesive was significantly higher at 96.1% in this wavelength range. However, in the passband region between 400 and 650 nm, the average reflectance of the dielectric film alone was 97.1%, while the average reflectance of the dielectric film combined with a reflection adhesive was slightly higher at 97.6% (Al film reflector was 99.1%). Figure 5(f) is a three-dimensional stereogram illustrating the incidence angle-wavelength-reflectance characteristics of the highly adhesive film reflector, focusing on the incidence angle-reflectance variation at five different wavelengths (400, 410, 420, 430, 440, and 450 nm). The inner results show reflectance values between 94.6% and 96.2% at different wavelengths and angles, with the highest reflectance observed at 450 nm and the lowest reflectance observed at 400 nm. Notably, the smooth trend indicates that the reflection gel can provide consistent reflectance performance at different incidence angles. The results for the outer layer show that from 400 to 650 nm, the average reflectance of the dielectric film combined with the reflection gel is comparable to that of the dielectric film alone. Furthermore, in the wavelength range of 400–450 nm, the reflectance of the dielectric film plus the reflection adhesive is only slightly higher than that of the dielectric film by about 1.0%. Thus, compared to the Al film reflector, an adhesive with low reflectivity can achieve high reflectivity at full angles, but with relatively low enhancement of the dielectric film.

The comprehensive trajectory of scintillation photons, spanning from generation to absorption, is intricately influenced by the reflectance properties of the surface of the scintillation crystal.³⁴ This phenomenon is visually depicted in Fig. 6(a), which shows a schematic diagram of the actual light collection within the LYSO scintillation crystal. Specifically, the scintillating photons decay and subsequently collect light from the interior of the LYSO crystal. However, the experimental data can only be measured from the outer surface, as shown in Fig. 6(b). Therefore, in this study, the reflectance of the inner and outer surfaces was simulated to find the actual mechanism of action of the different film layers. Simulation-based optical properties (R, n) are used for optical output simulations and compared with experimental data for a comprehensive evaluation of reflectance properties.

Figure 7 shows the simulation results of the above model at 420 nm, which shows an uneven curve. The reflectance spectrum of the inner surface light collection of the Al film reflector with respect to the incident angle is shown in Fig. 7(a). At nonlarge angles, the inner surface light collection of the Al film reflector shows a significant decrease in reflectance, by about 9.8% compared to the outer surface (99.7% for the outer surface light collection and 89.9% for the inner surface light collection). The inner surface collector has less reflectance than the outer surface collector. In addition, the outer surface light collector shows low reflectance at certain unmeasured angles (e.g., 51° and 72°). As shown in Fig. S2(a),³⁵ the dips in Fig. 7(a) come from dielectric film interference cancellation. The curves in Fig. 7(a) are the result of the superposition of two types of films, i.e., the dielectric film and the Al film. The two bumps just correspond to the dips of dielectric film at two incident angles. In Fig. 7(b), the reflectance profiles of the inner and outer surface light collection of the highly adhesive film reflector were simulated over a range of incidence angles from 0° to 90°. The results show that the reflectance performance of the inner surface light collection of highly adhesive film reflector exhibits significant fluctuations below 40°, with a clear minimum below 55% occurring below 60°, which is much worse compared to the reflectance of the outer surface light collection. As can be seen from Fig. S2,³⁵ the dips of the outer surface of Fig. 7(b) also correspond to the two dips of the dielectric film; while the multiple dips of the reflectance of the inner surface of Fig. 7(b) below 40° are also due to interference elimination of the dielectric film, interference elimination still exists after the addition of reflection adhesive, destroying the critical incidence angle of total reflection, and high reflection of the large angle of transmittance light due to diffuse reflection.

Figure 7(c) shows the inner surface light collection reflectance of the Al film reflector compared with the dielectric film. The designed dielectric film has a critical total reflection angle range from 40° to 60°. When the incident angle is beyond 60°, the reflectance reaches 100% due to the total reflection effect. When the incident angle is between 40° and 60°, the reflectance decreases sharply (i.e., transparent band) because of light transmission and then increases significantly with the extensive offset of incident angle. The high reflectance below 40° can be attributed to the light coherence emphasis. With the addition of an Al film, the light transmission effect can be suppressed completely in the incident angle range of 40°–60°. Thus, the composite film consisting of dielectric film and Al film can realize full-angle high reflection.

Figure 7(d) shows the simulation results for the inner surface light collection of the highly adhesive film reflector, comparing the reflection gel used in this study with and without the reflection gel. The analysis of the reflection gel is challenging due to the inhomogeneous layer and surface roughness caused by the presence of micrometer-sized particles. The three reflectance curves in the figure clearly show that the gel enhances and increases the reflectance of the dielectric film at incidence angles greater than 40°. As a result, the region where the reflectivity of the experimental dielectric film drops sharply (40°–60°) is flattened. However, the reflectivity did not improve until the incident angle reached 40°, and the reflectivity of the experimental dielectric film decreased sharply from an initial reflectivity of about 99%, showing a large fluctuation of about 80%.

In this study, we designed an ultrathin dielectric multilayer film suitable for LYSO. We also investigated the mechanism of how Al film and diffuse reflection film affect the dielectric film. The results of the experiment led to the following conclusions: (1) The dielectric coating, formed by alternating TiO₂ and SiO₂ for LYSO compared with materials such as Ta₂O₅, i.e., could form highly reflection films with wider passband. (2) The Al film enhances the reflectance of the dielectric film more than the reflection adhesive. (3) Both the Al film and the reflection adhesive could achieve full-angle reflectance and increased light output.

The findings demonstrated that dielectric layers with alternating high and low refractive indices are excellent choices for coating the surfaces of LYSO crystals. It is feasible to achieve high reflectance at a full incidence angle from crosstalk in LYSO arrays using Al films or reflection adhesive. The magnetron sputtering method for coating Al films is more efficient, while the coated reflection adhesive is more economical. This study provided a comprehensive analysis that established a strong foundation for future research aimed at enhancing the spatial resolution of x-ray detectors.

This work was supported by the National Key R&D Program of China (No. 2022YFB3503900) and the National Natural Science Foundation of China (NNSFC) (No. 12275347).

The authors have no conflicts to disclose.

Jing Yang: Data curation (equal); Formal analysis (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal). Linwei Wang: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Supervision (equal). Zhang Chen: Data curation (equal); Formal analysis (equal); Methodology (equal); Software (equal); Supervision (equal). Zhongjun Xue: Data curation (equal); Formal analysis (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – review & editing (supporting). Shuwen Zhao: Supervision (equal); Validation (equal); Visualization (equal); Writing – review & editing (lead). Dongzhou Ding: Funding acquisition (equal); Project administration (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.