Gamma Irradiation-Induced Discoloration and Annealing Characteristics of K9 Glass

Abstract

K9 glass is prone to developing color center defects under gamma irradiation, which exhibit strong absorption at specific laser wavelengths. However, most of these color centers exhibit an annealing phenomenon in natural environmental conditions, wherein their absorptive characteristics gradually diminish or even disappear. Hence, this study proposes employing a high-temperature accelerated annealing approach to address the color centers induced in K9 glass by gamma irradiation, aiming to attain stable absorption characteristics for specific wavelengths. Initially, experiments were conducted to generate color centers in K9 glass using gamma irradiation to investigate the influence of different irradiation doses on the optical absorption characteristics of K9 glass. Subsequently, the gamma-irradiated K9 glass was subjected to natural annealing at room temperature, wherein the unstable color centers exhibited a slow recovery process during annealing. Building upon this, high-temperature annealing was employed to expedite the recovery of unstable color centers in darkened K9 glass. Finally, a comprehensive analysis of the mechanisms behind gamma irradiation and high-temperature annealing in K9 glass was conducted using various material characterization techniques. The research findings hold significant importance for efficiently obtaining K9 glass with stable absorption at specific wavelengths, thereby further enhancing the optical performance of K9 glass in extreme environments.

1. Introduction

In high-energy radiation environments such as space optical systems [1], nuclear reactors [2,3], and intense laser systems [4], borosilicate glass (represented by K9 glass as a typical example) serves as a critical window material due to its excellent optical transmittance (>92% at 300–800 nm) and thermal-mechanical performance [5]. However, prolonged exposure to gamma irradiation induces glass network structural defects through ionization effects, forming color center defects with characteristic absorption bands [6,7,8]. This process results in a reduction in the transmittance of K9 glass and causes a performance drift in optical systems. Of more significant concern is that these irradiation-induced defects are predominantly in a metastable state, and even at room temperature, they gradually diminish through natural annealing processes [9]. This dynamic evolution poses a significant threat to the long-term reliability of optical components. In-depth research on the formation and evolution mechanisms of color centers induced in K9 glass by gamma irradiation, along with the development of controlled stabilization processes, has become a common challenge in the field of extreme environment optical materials.

Current research on the behavior of color centers induced by gamma irradiation in glass is primarily divided into the following three theoretical frameworks: defect structure analysis based on paramagnetic resonance [10]; kinetic models of alkali metal migration [11]; and evaluation of annealing effects under temperature fields [12]. Early scholars predominantly focused on borosilicate glass systems, where electron paramagnetic resonance (EPR) could clearly identify the presence of paramagnetic defects such as E’ centers (≡Si·, g = 2.002) and confirm their concentration exhibited a linear relationship with irradiation dose [10,13]. However, the simplified assumption of a single-component model is highly restricted when explaining the gamma irradiation response of complex compositions such as borosilicate glass. The Möncke team [14] proposed that the heterogeneity of the B-O network in borosilicate glass directly influences the path of irradiation energy deposition. When the B₂O₃ content exceeds 8%, BO₃ triangles preferentially absorb ionizing energy, forming non-bridging oxygen vacancies accompanied by the expulsion of Na⁺ ions. This theory is corroborated by the EPR and experimental results of the Boizot team [15] in France, who found that electron beam irradiation reduced the surface Na⁺ concentration in sodium–calcium–silicate glass by 36% while significantly increasing the OH^- group content, confirming alkali metal migration as a key factor driving defect recombination. Through molecular dynamics simulations, Yang et al. [16] further revealed that the fluctuation magnitude of Na-O bonds in glass under irradiation was 180% higher than that of Si-O bonds, indicating that Na⁺ networks are more prone to radiation-induced structural relaxation [17,18]. However, despite these findings, much of the aforementioned research has focused on characterizing the defect generation stage. A systematic exploration of the annealing kinetic parameters required for defect stabilization (such as temperature thresholds and time effects) is still lacking.

Two distinct research viewpoints exist in the glass annealing process optimization field: temperature dominance and time synergy. As early as 2010, Gusarov et al. [19] conducted studies on the effect of gamma irradiation on multi-component glasses at various temperatures. They observed that gamma irradiation increases the optical absorption of the glass. The optical absorption induced by a 200 °C thermal annealing for 24 h was completely suppressed. Furthermore, introducing cerium elements can enhance the glass’ light absorption in the ultraviolet range. Wang et al. [9] at Shanghai University investigated the influence of thermal annealing on defect concentration in silica glass. Their study revealed that when the annealing temperature exceeded 700 °C, there was a significant decrease in defect concentration in the glass, leading to improvements in transmittance and radiation resistance properties. Morgan et al. [20] at the University of Michigan reported on the differences in optical properties of fused silica glass resulting from gamma irradiation and thermal annealing treatments. Following gamma irradiation, simultaneous annealing at 800 °C did not restore the material to its pre-irradiated state. Fused silica with low hydroxyl content retained multiple spectral absorption bands in the 220 nm to 900 nm range, with multiple peaks attributed to the generation of E’ centers. Thus, it can be seen that following gamma irradiation, further high-temperature heat treatment of silica glass is a highly effective method for the restoration of its color center concentration. However, further investigation is required to explore the rate and mechanism of concentration recovery.

Therefore, current research on the formation threshold and saturation behavior of color centers in K9 glass under gamma irradiation, annealing process design, and environmental stability is significantly limited. These shortcomings severely constrain the rational design and engineering applications of radiation-resistant optical glass. Therefore, this study aims to elucidate the dose saturation pattern of color center formation in K9 glass under gamma irradiation, establish the synergistic relationship between annealing temperature and time, and explore residual defects’ chemical structural failure and their environmental response characteristics. Ultimately, the goal is to develop a gamma irradiation-high temperature annealing synergistic process that combines specific wavelength high transmittance with long-term stability. This process aims to provide optical material protection for extreme optical systems.

2. Materials and Methods

Standard K9 glass samples were purchased from CDGM GLASS Co., Ltd. (Chendu, China) for experimental specimens, with 10 mm × 10 mm × 5 mm dimensions. Each K9 glass sample was individually suspended and packaged in polytetrafluoroethylene sample boxes. Before the experiment, each experimental sample and packaging box underwent a cleaning process involving ultrasonic cleaning, high-pressure water rinsing, and light baking to ensure cleanliness. The adequately cleaned K9 glass experimental samples were treated using a 30 kGy Co-60 gamma irradiation device (BFT, Mianyang, China). As shown in Figure 1a, the schematic diagram of the setup mainly consists of a source frame, an array of arranged Co-60 sources, and a sample placement bracket. The source frame is used to secure and support the Co-60 sources, ensuring their stability during operation. The Co-60 sources are arranged in an array to achieve uniform irradiation of the samples. The sample placement bracket provides an accurate irradiation position for the K9 glass samples. At a distance of 15 cm from the center of the cobalt source, the dose rate reaches a maximum of 6.86 Gy/s. Due to its ability to generate high-energy gammas with intense penetration, no residues, and no contamination, this device is widely utilized in fundamental research on gamma radiation. All experimental procedures were conducted in a Class 10,000 cleanroom, with an ambient temperature between 22 °C and 24 °C and a relative humidity between 45% and 55%. Figure 1b displays photographs of K9 glass after gamma irradiation at various doses (0–300 kGy). It can be observed from the figure that as the gamma irradiation dose increases gradually, the color of the K9 glass deepens progressively, indicating a pronounced darkening phenomenon. All single-factor experiments were repeated three times to ensure the reliability and reproducibility of the experimental results. Data with significant deviations were subjected to retesting and evaluation.

The detailed process of gamma irradiating the K9 glass samples involves placing the K9 glass on the transmission system of the irradiation device and then initiating the irradiation apparatus to expose the sample to the gamma radiation emitted by the Co-60 source. During the irradiation process, thermoluminescent dosimeters were used to monitor the dose received by the samples, ensuring the accuracy of the irradiation dose. For K9 glass irradiated with varying gamma doses, the optical transmittance in the range of 300–1200 nm was determined using an ultraviolet-visible (UV-Vis) spectrophotometer (UV-1900, Shanghai Meixi Instrument Co., Ltd., Shanghai, China). To ensure the accuracy of all spectral measurement data for K9 glass, we conducted blank measurements on the spectrophotometer before each measurement. Subsequent measurements were initiated only after achieving a transmittance of 100.0%. Measurements were taken at different time points post-irradiation to observe the color center formation and disappearance process. We employed infrared spectroscopy (FTIR-650, Tianjin Gangdong Technology Co., Ltd., Tianjin, China) and X-ray photoelectron spectroscopy (AXIS SUPRA, Kratos Analytical Ltd., Manchester, UK) to investigate further the changes in all chemical bonds and elemental composition of K9 glass induced by gamma irradiation. Further observation of the effects of gamma irradiation on internal defects in K9 glass material was conducted using an electron paramagnetic resonance spectrometer (EPR200-Plus, Guoyi Quantum (Hefei) Technology Co., Ltd., Hefei, China). During the laser irradiation process of K9 glass, a Q-switched ultraviolet (351 nm) nanosecond pulse laser was utilized. The laser system featured an 8 ns pulse width, a Gaussian beam profile, and an 8 mm beam diameter. The laser was vertically irradiated onto the surface of the K9 glass at a power density of 0.5 J/cm², with each point receiving 10 individual irradiation pulses. The nano-pulsed laser scanned the entire surface of the K9 glass in a serpentine pattern, with a 2 mm overlap between adjacent points, both horizontally and vertically. The K9 glass irradiated with a nanosecond pulsed laser was stored and measured alongside other K9 glass samples in the same environment.

3. Results

3.1. Color Center Formation in K9 Glass Induced by Gamma Irradiation

K9 glass develops color centers that absorb specific wavelengths of light after gamma irradiation. Analyzing the spectral information of K9 glass is beneficial for investigating the impact of gamma irradiation on color centers. Figure 2a illustrates the transmittance spectra of K9 glass in the 300–1200 nm range after gamma irradiation at different doses. It can be observed from the figure that, under conditions without gamma irradiation, K9 glass exhibits high transmittance for light in the 350–1200 nm range, reaching approximately 90%. Only the transmittance of light in the ultraviolet range experiences a slight decrease, dropping to around 67%. At different gamma irradiation doses, the transmittance of K9 glass in the 900–1200 nm range shows no significant changes, indicating that gamma irradiation does not affect this range of K9 glass. However, with increasing gamma irradiation doses, the transmittance of K9 glass in the 300–900 nm range gradually decreases. The closer the spectrum is to 300 nm, the greater the reduction in transmittance of K9 glass. When the gamma irradiation dose exceeds 5 kGy, the transmittance of K9 glass at 300 nm reaches 0%.

From Figure 2a, it can be observed that the spectral transmittance of K9 glass undergoes significant changes between 300 nm and 700 nm after gamma irradiation, suggesting that gamma irradiation may lead to the formation of color centers in K9 glass that strongly absorbs within this wavelength range. Therefore, we selected 5 characteristic wavelengths at 351 nm, 450 nm, 550 nm, 650 nm, and 700 nm between 300 nm and 1200 nm to compare and analyze the variation in transmittance of K9 glass with gamma irradiation doses, as shown in Figure 2b.

The experimental results from Figure 2b indicate that the transmittance of K9 glass at the five characteristic spectral positions decreases gradually with increasing gamma irradiation doses. The rate of transmittance variation sharply declines within the dose range of 20 kGy, followed by a gradual slowdown in the rate of transmittance decrease as the dose increases. When the gamma irradiation dose exceeds 20 kGy, the transmittance of K9 glass at each characteristic spectral position remains essentially unchanged. Therefore, it can be inferred that when the gamma irradiation dose reaches 20 kGy, the color centers formed in K9 glass are essentially saturated, with their concentration almost no longer increasing with further gamma irradiation doses. Furthermore, it is observed that the color centers formed in K9 glass after gamma irradiation exhibit higher absorption toward ultraviolet laser light.

3.2. Natural Annealing of Darkened K9 Glass

Due to the inherent instability of color centers induced in optical materials by gamma irradiation, we placed K9 glass irradiated with various doses of gammas in a Class 100 cleanroom environment at room temperature (25 °C) for 1 day, 10 days, 20 days, 30 days, and 90 days, respectively. We then conducted transmittance measurements at the 351 nm position of the spectrum to simulate the natural annealing phenomenon of darkening in K9 glass after gamma irradiation, as shown in Figure 3a. The graph illustrates that gamma-irradiated K9 glass gradually increases transmittance at 351 nm with extended storage time, demonstrating a pronounced annealing effect whereby the concentration of color centers within the K9 glass decreases over time. Moreover, the increase in transmittance values for K9 glass with different color center concentrations under natural environmental storage is essentially uniform. This indicates that the annealing phenomenon of color centers in K9 glass is universal and unrelated to color center concentration.

In order to facilitate the analysis of the annealing behavior of darkened K9 glass, Figure 3b presents the relationship between the normalized recovery rate of transmittance (ΔT/T₀) at the 351 nm position of K9 glass and the ambient storage time, using the data from one-day annealing as a baseline. The graph shows that the transmittance of K9 glass at 351 nm, irradiated with different doses of gammas, increases with prolonged ambient storage time. The recovery rate of transmittance increases with the dose of gamma irradiation, yet as the ambient storage time further increases, the slope of the transmittance recovery rate variation gradually diminishes. With an increase in gamma irradiation doses, the recovery rate of transmittance for K9 glass increases during ambient storage, indicating that higher concentrations of color centers exhibit a more rapid decrease in concentration during ambient storage at room temperature, highlighting a more pronounced annealing process.

3.3. High-Temperature Annealing of Darkened K9 Glass

Based on the research in Section 3.2, it is evident that the color centers in K9 glass material, following gamma irradiation under ambient temperature conditions, are unstable and exhibit a slow recovery process. In order to expedite the recovery of unstable color centers in K9 glass material, we are considering utilizing high-temperature conditions to accelerate the recovery of these unstable color centers, with the aim of achieving the solidification of color centers within K9 glass. To facilitate the study of the effects of solidification conditions on darkened K9 glass, all experimental samples of K9 glass were initially subjected to darkening treatment using gamma irradiation at a dose of 20 kGy. Subsequently, the impact of solidification conditions on the recovery of color centers in darkened K9 glass was investigated.

Figure 4a illustrates the relationship between the transmittance of darkened K9 glass in the 300–800 nm range and the annealing time under an annealing temperature of 100 °C. High-temperature annealing treatment significantly increases the transmittance of K9 glass in the 300–800 nm range. As the annealing time increases, the transmittance of K9 glass gradually rises, albeit with a very limited extent of increase. In order to provide a more intuitive representation of the relationship between the transmittance of K9 glass and annealing time, Figure 4b illustrates the variation in transmittance of K9 glass at typical wavelengths of 351 nm, 450 nm, 550 nm, 650 nm, and 700 nm following annealing at 100 °C. Compared to the just completed darkened state of K9 glass, the transmittance of K9 glass significantly increases across all wavelengths after a 2 h annealing process. Particularly noteworthy is the nearly twofold increase in transmittance of K9 glass at 351 nm following a 2 h high-temperature annealing treatment. With further increases in the duration of high-temperature annealing, the transmittance at various wavelengths of K9 glass continues to rise slowly. An examination of the increasing trend in the slope of the curves in Figure 4b reveals that after high-temperature annealing for more than 8 h, the rate of transmittance increase in K9 glass markedly decreases, with no significant variations in transmittance. This suggests that a considerable portion of the unstable color centers in K9 glass have already recovered.

Using the transmittance at 351 nm as an example, this analysis examines the impact of annealing at 100 °C compared to room temperature annealing on the recovery rate of transmittance in darkened K9 glass, as shown in Figure 5a. The transmittance recovery rate of K9 glass after 2 h of annealing at 100 °C following gamma irradiation is comparable to that after 30 days of annealing at room temperature. This indicates that high-temperature annealing at 100 °C enhances the transmittance recovery rate of K9 glass by approximately 100–300 times. This significant increase in efficiency is of paramount importance for the decomposition of unstable color centers in K9 glass and the attainment of stability in its performance.

To further investigate the influence of annealing temperatures on color center recovery in K9 glass, Figure 5b presents the transmittance spectra in the 300–800 nm range of K9 glass after gamma irradiation under various annealing conditions, including no annealing, annealing at 50 °C, 100 °C, 150 °C, and 300 °C. In all cases, the gamma irradiation dose is 30 kGy, and the annealing duration is 8 h. With the gradual increase in annealing temperature, the transmittance of K9 glass increases progressively across the entire 300–800 nm wavelength range, and the magnitude of this increase shows a nearly linear relationship with the annealing temperature. This suggests that compared to annealing time, annealing temperature has a greater impact on the transmittance of darkened K9 glass, indicating a more significant influence on the recovery rate of color centers.

Figure 6a,b, respectively, present the relationship between the transmittance of K9 glass at wavelengths of 351 nm and 550 nm and the ambient temperature exposure time after 24 h of high-temperature annealing at 100 °C and 300 °C. Following high-temperature annealing, K9 glass exhibits good stability in transmittance at both 351 nm and 550 nm wavelengths, with the transmittance variation rate of K9 glass after 300 °C annealing being less than 5% after an ambient temperature exposure of over 400 days, as depicted in Figure 6b. Compared to annealing at 100 °C, K9 glass annealed at 300 °C demonstrates superior color center stability.

4. Discussion

In the above research, we used spectral information to characterize the effects of gamma irradiation and different annealing conditions on K9 glass based on the optical properties of color centers. To further investigate the mechanisms underlying color center generation induced by gamma irradiation and the high-temperature annealing process in K9 glass, we conducted a comprehensive analysis of the chemical bonds, elemental composition, and defects in K9 glass [10,21], as illustrated in Figure 7a,b, as well as Figure 8. Figure 7a displays the infrared spectra of K9 glass after exposure to varying doses of gamma irradiation. The peak around 1380 cm⁻¹ in the infrared spectrum corresponds to the stretching vibration peak of B-O bonds in BO₃ units [22]. This is attributed to a small amount of boron (B) element in K9 glass, forming B-O chemical bonds. Before gamma-ray irradiation, this peak can be detected on the surface of K9 glass. Subsequent gamma irradiation results in the breaking of the B-O bonds. It can be observed that with increasing irradiation doses, there is no significant change in the infrared spectral information of K9 glass. This indicates that gamma irradiation does not alter the chemical bond structure of K9 glass.

Figure 7b presents the XPS spectral information of the surface of K9 glass after exposure to different doses of gamma irradiation. The XPS spectrum of the unirradiated K9 glass surface shows a distinct Na 1s electron peak, indicating the presence of sodium elements on the surface of the K9 glass sample. However, after gamma irradiation, the Na 1s electron peak on the surface of all samples disappears. This observation is consistent with the conclusions reported by Boizot et al. [15]. After electron irradiation, the concentration of oxygen elements on the surface of alkali metal-containing (Na, K, etc.) fused silica glass increases, leading to the formation of O₂ molecules. This phenomenon is attributed to the migration of alkali metal ions in the glass or the transformation of defects. Without gamma ray irradiation, two peaks for Na in K9 glass: Na₂O-SiO₂ and Na 1s were observed. Sodium in K9 glass primarily exists in Na₂O, connected to the silicate network, forming a stable structure. Under gamma ray irradiation at a dose of 100 kGy, the Na 1s peak on the surface of K9 glass completely disappears, leaving only the Na₂O-SiO₂ peak. This indicates that gamma ray irradiation disrupts the chemical bonds formed by sodium, leading to a change in the chemical environment of sodium ions and resulting in the disappearance of the Na 1s signal. By analyzing the changes in the distribution of Na elements in K9 glass after gamma irradiation, it can be inferred that the surface Na elements migrate towards the material’s interior to form Na-containing defects (color centers) after irradiation. This process increases light absorption at certain wavelengths, thereby causing darkening of the K9 glass surface.

To further validate the conclusion that gamma irradiation induces color center defects in K9 glass, we employed an electron paramagnetic resonance spectrometer (EPR) to test the paramagnetic defects within the material. This is illustrated in Figure 8. The fundamental principle of EPR testing for internal defects in materials involves resonant absorption of microwaves by paramagnetic unpaired electrons. Figure 8 presents the room temperature EPR measurement results of untreated K9 glass, darkened K9 glass, high-temperature annealed K9 glass, and laser-irradiated K9 glass after annealing. From the analysis of defects in K9 glass materials, it is evident that paramagnetic defects (g = 2.003) appear in K9 glass after gamma irradiation, which may be attributed to E’ centers (−Si≡·) [23]. These defects essentially disappear after annealing but reappear following laser irradiation.

Based on the aforementioned research findings, it is observed that the absorbance at 351 nm significantly increases in high-temperature annealed K9 glass compared to untreated K9 glass. However, the EPR spectra of annealed K9 glass do not exhibit a notable increase compared to untreated K9 glass. Therefore, it can be inferred that this defect may not be the primary cause of the enhanced light absorption at 351 nm in annealed K9 glass materials. In conclusion, the color centers responsible for the enhanced light absorption at 351 nm in K9 glass materials are likely due to the formation of numerous charged defects within SiO₂ after gamma irradiation. These charged defects capture the metal ions doped in the glass, thereby giving rise to non-paramagnetic defects (color centers) containing metal ions. The specific mechanism behind the darkening of K9 glass upon gamma radiation exposure involves the formation of color centers in the glass due to gamma irradiation, as shown in Figure 9. These color centers enhance light absorption, leading to the discoloration and decrease in transmittance of K9 glass. The concentration of color centers increases with the dosage of gamma radiation, resulting in a gradual decrease in transmittance as the gamma radiation dosage increases.

5. Conclusions

In conclusion, this study experimentally demonstrates the induction of color centers in K9 glass and their annealing effects through gamma irradiation. The results indicate that gamma irradiation can introduce color centers in K9 glass, enhancing its absorption of light in the 300–900 nm wavelength range. The mechanism behind the induction of color centers in K9 glass by gamma irradiation is based on charged defects within SiO₂ capturing doped metal ions post-irradiation, thereby forming non-paramagnetic color centers containing metal ions. However, the optical absorption characteristics of K9 glass post-gamma irradiation are unstable, displaying significant annealing effects with increased ambient rest time at room temperature. Building upon this, we propose that high-temperature annealing can expedite the annealing of unstable color centers in darkened K9 glass, increasing efficiency by 100–300 times and maintaining the optical stability of K9 glass at room temperature. Therefore, based on the results of this study, it is established that employing a combination of gamma irradiation-induced darkening coupled with high-temperature annealing can enhance the stable light absorption of K9 glass at the 351 nm wavelength. This method is simple to operate, non-destructive to the glass crystal structure, and economically viable.