Introduction
In recent years, several types of thin-film polymer materials such as Gafchromic [1], clear Perspex [2], cellulose triacetate [3], and Sunna [4] have been listed as conventional radiation dosimeters according to the ISO/ASTM 51261(2002) standard. Compared with liquid or other types of dosimeters, organic-film dosimeters feature a low economic cost, stable fluorescence signal response, wide adaptability in shape and size, high detection dose limit, and wide dose detection range [5, 6]. They demonstrate significant potential in medical treatments, radiation processing control (such as food preservation and industrial material modification), nuclear track detection and visualization, dose monitoring at radiation sites, and nuclear decommissioning facilities [5, 7-9].
However, the aforementioned film dosimeters present some disadvantages in the actual dose-detection process, including low environmental adaptability, complex production processes, easy dose saturation, and short shelf life. Currently, studies pertaining to organic-film dosimeters focus primarily on two aspects. The first is investigation into new polymer films with favorable dose responses as alternative materials for dose detection, such as the development of new organic–inorganic composite materials. However, the processing of composite materials involves the issues of low material compatibility, complex manufacturing processes, unsatisfactory dose-response linearity, low durability, low long-term signal stability, rapid aging, low environmental adaptability, and high cost [10, 11]. The second the further analysis of the dosimetry characteristics of commercial or potential dosimeter materials, investigation into their applicable dosimetry scenarios, and proposal of schemes for structure optimization and performance improvement [12, 13]. Polycarbonate (PC) is a thermoplastic engineering plastic known for its excellent light transmission, impact resistance, ultraviolet-radiation resistance, high mechanical strength, and ease of processing [14]. They are widely utilized in industrial manufacturing, space science, electronic instruments, and medical devices [15]. Further investigations into the radiation effect of PC revealed that the aromatic ring structure in PC exhibited greater susceptibility to energy absorption and deposition compared with the aliphatic structure [16]. Irradiation can induce disorder in the internal structure of PC, thus resulting in intermolecular crosslinking (at 30 kGy), chain scission (at 200 kGy), and free-radical generation [16-18]. Subsequent studies on the radiation modification of PC revealed its radiophotoluminescence properties [19, 20] and suggested PC as a promising material for radiation-dose detection [21, 22]. During the initial period, relevant investigations on the radiation-dose detection of PC films focused primarily on analyzing the measurable dose range, photoluminescence (PL) intensity, optimal excitation wavelength, fluorescence lifetime, optical quantum yield, light absorption, and transmission. However, few studies have investigated the dosimetry characteristics of PC films; therefore, PC films were primarily used for fluorescence-tracking detection in the early stages instead of dosimeters.
The dosimetric characteristics of PC films must be investigated before they can be used for practical dose detection [23]. These include considerations such as thickness applicability, preheating time, temperature and humidity dependence, dose-rate response, dose linearity, in-batch uniformity, readout reproducibility, annealing, self-decay, and energy response. Soliman et al. investigated the PL spectra of different types of Makrofol PC materials (BL 95/8/2 6-2, DE 1-1, E, DE 1-4, KL 3-1005/1 6-2, DE 6-2, LT 6-4, and DE 7-2) irradiated with 60Co-gamma rays (300 kGy) [24]. The findings revealed the distinct PL characteristics of each film, thereby enabling the application of different types of PC films in diverse scenarios based on their unique properties. Abdul-Kader et al. investigated a Makrofol LT 6-4 PC film irradiated with 60Co-gamma rays and observed a decrease in the PL intensity with increasing irradiation dose within the range of 150-950 kGy and a linear correlation coefficient of 0.96 [25]. Posavec et al. reported that the PL intensity of irradiated PC films decreased with increasing temperature and that the ratio of PL peak intensity at 77-293 K (I77 KI193 K) was 1.93, thus demonstrating clear temperature dependence during testing [26]. Galante et al. discussed commercial PC detection films employed for monitoring radiation fields inside cylindrical container products. The correlation coefficient between the absorbed dose and response value was 0.99, thus indicating a favorable linear dose response [27]. Resta et al. irradiated Makrofol-KG PC films with 28Si+ ions at 0.5, 1, and 2 MeV and discovered that the PL intensity of PC varied by two orders of magnitude at the same radiation dose of 1 and 2 MeV 28Si+ ions, thus highlighting a significant difference in the energy response [28]. However, reports regarding the effects of PC thickness adaptability, preheating time before PL testing, and ambient humidity on dose detection, all of which can affect detection accuracy, are scarce. These factors have been considered in studies involving other types of dosimeters. Kattan et al. observed that polyvinyl-chloride films of varying thicknesses irradiated with 0-125 kGy gamma rays exhibited different degrees of radiation sensitivity [29]. Bhata et al. investigated a 250 μm- diethyl terephthalate film (also known as Garfilm-EM) irradiated with equal doses of 60Co-gamma rays under various relative humidity (RH) conditions and discovered that the net absorbance of the film remained stable after 14 days, except for a rapid decline on the first day, thus indicating that the dose response of this film was affected by humidity during irradiation [30]. Mejri et al. discussed a commercial inorganic glass dose tablet exposed to different RH environments after γ-irradiation; the net absorbance of the dose tablet showed a distinct trend of reverse proportional decline within 22 days, thereby indicating a significant humidity response during storage after irradiation [31]. In summary, only a few dosimetric characteristics have been briefly mentioned in existing studies pertaining to the detection performance of various types of PC films, and an overall discussion on a specific PC film has not been provided.
In this study, PL spectroscopy was employed to investigate the dosimetric characteristics of engineered PC films after electron beam (EB) irradiation (0-600 kGy). The effects of the PC-film thickness, PC preheating time after irradiation, temperature during PL spectrum test, ambient humidity during irradiation, and ambient humidity of the PC storage environment after irradiation on the PL spectral intensity were investigated. Additionally, the in-batch uniformity, readout reproducibility, dose linearity, PL-intensity attenuation characteristics of the PC after irradiation, and PC response to different EB energies were investigated. This study aims to provide more detailed data indicators and analysis results for each dosimetric characteristic point, comprehensively evaluate the possibility of using PC films as a dosimeter in practical radiation detection, and provide references for dosimetry studies involving other types of PC materials.
Materials and Methods
Sample preparation
The PC film used in this study was fabricated by Covestro Polymer Co., Ltd., Shanghai, China. It is a nontoxic, odorless, colorless, and transparent glassy amorphous organic polymer [32]. The film exhibited a density of 1.25g/cm3, a light transmittance of 90% ± 1%, a refraction index of 1.585 ± 0.001, a linear expansion rate of 3.8×10-5 cm/°C, and a thermal deformation temperature of 140 °C. PC films of various thicknesses, i.e., 0.2, 0.3, 0.5, 0.8, 1.0, 1.5, and 2.0 mm, were prepared. The surface of the PC film was flat and the thickness was uniform. The films were cut into 20 mm×20 mm squares, and the protective layers were removed from both sides for further use.
EB irradiation
Electron accelerators with different energies (3.5, 4, 4.5, 10, and 20 MeV) were used for PC irradiation in this study. A DD-type electron accelerator (Jinwo Technology Co., Ltd., CGN, China) operating in an energy range (EB) of 3.5–5 MeV and a beam power range of 30-200 kW was used. Additionally, an IS1020 backwave electron linear accelerator (Xianghua Huada Biotechnology Co., Ltd., Hunan, China) operating at an energy (EB) of 10 MeV and a rated beam power of 20 kW was used. The IS1020 high-energy electron accelerator (Huada Biotechnology Co., Ltd., Guangzhou, China) was also operated at an energy (EB) of 20 MeV and a maximum beam power of 20 kW. The PC film was placed on a conveyor belt and passed through the central area of the irradiation window multiple times to receive the required dose, with a single dose of 10 kGy per pass. Irradiation was performed at room temperature, and the radiation dose was traced using dichromate. The sample was uniformly irradiated using an EB with a dose deviation of 1%.
PL spectrum
An FLS1000 steady-state/transient fluorescence spectrometer (including conventional and variable temperature types) manufactured by the Edinburgh company, UK, was used to analyze the PL spectra of the irradiated PCs. The conventional type was operated under the following parameters: excitation-spectrum scanning wavelength range, 250–450 nm; emission- spectrum scanning wavelength range, 340–800 nm; scanning rate, 5°/min; step size, 1 nm; and integration time, 0.2 s. A xenon lamp was used as its excitation source, and the test was performed at ambient temperature. The variable-temperature type featured a built-in variable-temperature component (the Oxford Optistat thermostat) with an optional temperature range of 273-373 k and an insulation accuracy of ± 1 k. The emission-spectrum scanning wavelength range was 395-800 nm, and the other parameters were set based on the conventional type. The irradiated PC film was placed at the same position on the PL spectrum test platform to ensure the consistency of the PL measurement path.
Analysis of dosimetric characteristics
Thickness applicability
A typical feature of thin-film dosimeters is their availability at different thicknesses [29]. To evaluate the response of PC films with different thicknesses to EB irradiation, seven groups of PC films with different thicknesses (0.2, 0.3, 0.5, 0.8, 1.0, 1.5, and 2.0 mm) were prepared. Each group of films was irradiated with different doses (0, 100, 300, and 600 kGy) under a 10 MeV EB, and irradiation treatment was performed at ambient temperature in an air atmosphere. Subsequently, the PL intensity was analyzed, and the relationship between the PL peak intensity and irradiation dose of the PC films with different thicknesses was obtained through PL spectral testing at the same ambient temperature.
Preheating time
Generally, the non-luminous center effect occurs in irradiated polymer films. When the irradiated PC film is preheated prior to the PL test, its PL intensity changes accordingly. Therefore, the optimal preheating conditions for irradiated PC films must be determined to achieve higher PL intensity values. In this study, eight groups (three parallel samples in each group) of 0.3 mm PC films were irradiated at 100 kGy by a10 MeV EB at ambient temperature in an air atmosphere and then preheated at 60 °C for 10, 30, 40, 60, 90, 120, 180, and 300 min, separately. Finally, the PL peak intensities of the PC films preheated for different durations were measured.
Temperature dependence
Temperature dependence refers to the effect of ambient temperature on the PL intensity of the PC film during PL-spectrum testing [33]. To investigate the relationship between them, 0.3 mm PC films were irradiated at 100 kGy using a 10 MeV EB, and three parallel samples were prepared. PL tests were conducted at eight temperature points, i.e., 273, 278, 283, 293, 303, 323, 343, and 373 k. The variation in the PL intensity with the ambient temperature during the PL test was analyzed.
Humidity dependence
The humidity of the environment affects the dose response of polymer films. Therefore, the effect of RH in air (both during irradiation and post-irradiation storage) on the PL intensity of the PC film was evaluated in this study [30, 31]. To establish different RH gradients in the range of 12.4–97.2%, various saturated salt solutions were prepared using the techniques reported by Wexler and Hasegawa (1954) and Levine (1979). Table 1 lists the types of saturated salt solutions selected and their corresponding RH values under sealed conditions. The corresponding saturated salt solution (25 ml) was added to a borosilicate glass bottle to establish a different humidity gradient.
| Salt solution type | Relative humidity (%) |
|---|---|
| LiCl·H2O | 12.4 |
| MgCl2·6H2O | 33.6 |
| Mg(NO3)2·6H2O | 54.9 |
| NaCl | 75.5 |
| K2SO4 | 97.2 |
In the study of humidity dependence during irradiation, the 0.3 mm PC film was suspended in sealed glass bottles in different humidity environments (12.4, 33.6, 54.9, 75.5, and 97.2%) and stored at room temperature for seven days to balance the ambient humidity of the PC film. Subsequently, the glass bottles were irradiated at 100 kGy using a 10 MeV EB, after which the five groups of PC films were subjected to PL tests. In the study of humidity dependence during storage after irradiation, each group of PC films was first irradiated using a 10 MeV EB (100 kGy) and then encapsulated in bottles under different humidity conditions for seven days under environmental regulation. Finally, PL tests were performed and the effect of humidity on the PL intensity of the PC films was analyzed.
In-batch uniformity
In-batch uniformity refers to the PL-intensity uniformity of PC films produced in the same batch under the same irradiation dose. It is a key index for describing the dosimetric characteristics of RPL materials [34]. Three groups of 0.3 mm PC films (15 parallel samples per group) were irradiated (10 MeV EB) at 100, 300, and 600 kGy, separately. Irradiation treatment was performed at ambient temperature in an air atmosphere. The PL peak intensity of each sample was measured. The relative mean deviation (RAD), as expressed in Eq. (1) was used to evaluate the degree of uniformity in the PL intensity.
Readout reproducibility
To investigate whether multiple PL tests affect the dose information stored in the PC film, three groups of 0.3 mm PC films were irradiated (10 MeV EB) at 100, 300, and 600 kGy, separately. Irradiation treatment was performed at ambient temperature in an air atmosphere. Each film was excited repeatedly and measured 10 times to obtain the PL peak intensity. To assess the dispersion of PL intensity, the coefficient of variation was estimated using Eq. (2).
Dose linearity
The dose linearity, which refers to the linear relationship between the measured signal intensity and the radiation dose, is a crucial parameter in dose detection [35]. In this study, the 0.3 mm PC films were irradiated with different doses (0, 20, 50, 100, 200, 400, and 600 kGy) using a 10 MeV EB (irradiation treatment was performed at ambient temperature in an air atmosphere). The relationship between the irradiation dose and PL intensity of the PC films after irradiation was analyzed via PL-spectrum tests, and the dose capture range of the PC film was determined. The correlation coefficient (R2) was used as a statistical indicator to reflect the degree of linear correlation.
Self-decay
Self-decay refers to a process in which the signal intensity of an irradiated sample declines with time [36]. To analyze the change in the PL peak intensity of the irradiated PC film after a certain duration, the 0.3 mm PC film was irradiated (10 MeV EB) at 300 kGy (irradiation treatment was performed at ambient temperature in an air atmosphere) and then stored in a dark and dust-free area. The PL intensities of the PC films were measured on days 0, 1, 3, 5, 10, 20, 30, and 60 after irradiation. The decay characteristic curve was obtained by performing an appropriate function fitting.
Electron energy response
Electron energy response refers to the difference in the PL peak intensity when the PC films are irradiated with the same dose under different EB energies and is typically depicted as the relationship between the PL intensity and energy [37]. In this study, five EB energies (3.5, 4, 4.5, 10, and 20 MeV) were selected to irradiate 0.3 mm PC films at a dose of 100 kGy. Irradiation was performed in an air atmosphere at room temperature. The PL peak intensity of the PC film irradiated with a 10 MeV EB was used as a normalized basis to calculate the deviation in the PL peak intensity of PC films irradiated with EBs of other energy levels. (Positive and negative deviations were used, where values higher and lower than the reference value were positive and negative values, respectively.)
Results and Discussion
The color of the PC film transformed to yellow and darkened gradually as the irradiation dose increased. This yellowing is associated with the formation of color centers during irradiation, such as phenoxyl and phenyl radicals. Radiation induces an increase in the internal structural disorder of PC, thereby creating conditions conducive to the formation of color centers [38]. However, the fading of the irradiated samples left for a certain duration is attributable to the oxygen reaction of free radicals within the PC film [39]. Based on excitation spectra tests, the optimal excitation wavelength of the PL spectrum for the PC film was determined to be 320 nm.
Thickness applicability
The variation in the mean value of the PL peak intensity detected by PC films of different thicknesses with the irradiation dose is shown in Fig. 1. The results show that the PL intensity of PC films with a thickness greater than or equal to 0.3 mm decreased as the dose increased, whereas the PL intensity of PC films with a thickness of 0.2 mm showed the opposite trend as the dose increased. The color of the PC film with the same thickness darkened as the irradiation dose increased. At the same irradiation dose, the color of the PC films darkened as the film thickness increased. The darker the color of the PC film, the greater was the internal radiation damage. When the thickness of the PC film exceeded 0.5 mm, the PL peak intensity of the PC film treated with the same irradiation dose decreased as the thickness increased. The thicker the PC film, the higher was the deposited energy under the same irradiation dose, thus facilitating the formation of a two-layer structure composed of a top carbonization layer and a bottom cross-linked layer [40]. The formed carbonization layer affected the transmission of excitation and emission light in the PL spectral test.
As shown in the fitting curve, only the PL intensities of the 0.3 mm and 0.2 mm PC films were linearly related to the irradiation dose (0–600 kGy), and their linear correlation coefficients were 0.962 and 0.834, respectively. However, the 0.2 mm PC film exhibited low mechanical strength (which implies its susceptibility to mechanical damage and deformation), low dose linear response values (i.e., it may not be able to absorb sufficient radiation energy, thus resulting in inaccurate measurement results), and dose-saturation tendencies. Therefore, a 0.3-mm-thick PC film was selected as the analysis sample in this study, thus aligning with the thickness of existing organic- film dose-detection materials such as low-density polyethylene films (0.3 mm) [41] and Indian Garfilm-EM films (0.25 mm) [30].
Preheating time
Figure 2 shows the PL emission spectrum of the PC film irradiated at (SI100kGy) after being preheated at 60 °C for various durations. In the preheating-time range of 0-300 min, the PL peak intensity of the PC film initially increased and then decreased as the preheating time increased, with the maximum reached at approximately 180 min. As shown in the upper-right corner of Fig. 2, the PL peak intensity at 10, 30, 40, 60, 90, 120, and 300 min represent 68.9%, 76.4%, 78.4%, 79.2%, 83.7%, 88.4%, and 67.2% of the PL peak intensity at 180 min, respectively. The initial increase in the PL peak intensity is attributable to secondary electrons inside the PC film after irradiation being captured by non-luminescent centers. These electrons escape after absorbing heat energy and are recaptured by the luminous center. However, when the preheating time exceeds 180min, the fluorescence signal of the PC film is quenched by heat more easily, thus resulting in a decrease in the fluorescence intensity. These results suggest that the PL intensity of the PC film can reach saturation after preheating for a certain period and that the optimal preheating time at 60 °C is 180 min. Based on existing literature, such as the optimal preheating condition for silver-doped inorganic glass RPL dosimeters being 40min at 90 °C [42], future studies may investigate the possibility of reducing the preheating time by appropriately increasing the preheating temperature.
Temperature dependence
After irradiation, the PL spectra of the PC films were recorded at various ambient temperatures. The variations in the PL spectra of the irradiated (100 kGy) PC films at different ambient temperatures are shown in Fig. 3. The emission spectrum peak of the PC film was located at 470 nm. Additionally, a weak peak at 410 nm emerged and disappeared upon excitation with 350 nm UV light, thus indicating that it was the scattering peak of the sample. Within the temperature range of 273–373 k, the PL intensity decreased as the ambient temperature increased. The PL peak intensity at 373 k decreased to 13.3% of its initial intensity at 273 k, and this decreasing trend is consistent with the effect of temperature on the PL intensity of fluorescent materials [26]. This may result from the inhibition of non-radiative recombination processes at low temperatures [43], with fluorescence thermal quenching occurring as the temperature increases, thus resulting in a reduced PL intensity [44]. The relationship between PL peak intensity and temperature is shown in the upper-right illustration in Fig. 3. They exhibit a relationship expressed by y = - 0.2609 x + 99.2735 (where y is the PL peak intensity and x is the ambient temperature), with a correlation coefficient of 0.964. These results indicate that the PC film maintained a favorable temperature-dependent linear relationship within the range of 273–373 k, thus suggesting its utility as a temperature sensor in future applications.
Humidity dependence
The effects of the ambient humidity on the PL intensity of the PC film during and after irradiation are shown in Figs. 4(a) and (b), respectively. A comparison of the results shown in the two figures above shows that the PL intensity of the PC film decreased as the RH increased (during irradiation and post-irradiation storage), whereas the peak position remained unchanged. However, when the ambient humidity increased from 12.4% to 97.2% during irradiation, the PL peak intensity of the PC film decreased to 74.3% of the initial value, whereas when the ambient humidity increased from 12.4% to 97.2% after irradiation, the PL peak intensity of PC film decreased to 53.3% of the initial value. Thus, one can conclude that the effect of the storage-environment humidity on the PL intensity of the PC film after irradiation is more significant compared with the case during irradiation, and that the PL response value of the PC film decreases with an increase in the ambient humidity. This phenomenon is attributable to the radiation, which generated the corresponding free radical sites on the PC main chain. Moreover, the interaction between the radiation and water molecules generates free radicals and active particles, which can interact with the PC film. Therefore, the increase in environmental humidity during irradiation generates more free radicals, which further interact with the PC film. The increase in humidity in the storage environment after irradiation accelerates the diffusion of oxygen into the PC matrix and further oxidizes the PC, thus reducing the PL intensity [30]. The humidity-dependence results of the PC films are similar to those of FWT, Mylar, Melinex, and PET film dosimeters [45-49]. Further discussion is warranted regarding the declining trend of PL intensity over time under different ambient humidities.
In-batch uniformity
The PL peak intensities detected for the different PC films produced in the same batch after the irradiation under the same dose are shown in Fig. 5. At radiation doses of 100, 300, and 600 kGy, the variation ranges of the PL peak intensity counts were 5.1×105 - 5.76×105, 3.18×105 - 4.2×105, and 1.76×105 - 2.92×105, respectively. As the irradiation dose increased from 100 to 600 kGy, the fluctuation range of the PL peak intensity increased. The relative average deviation corresponding to the three doses were 2.73%, 8.4%, and 12.8%, respectively. These results show that the in-batch uniformity of the PC film under doses less than 100 kGy is ideal and that the signal uniformity is similar to that of a GD-300 dosimeter (the deviation values of GD-300 at doses of 0.2, 20, and 200 mGy are ± 1.7%, ± 1.3% and ± 1.1%, respectively) [24, 42]. However, when the dose exceeded 100 kGy, the relative average deviation of the in-batch uniformity of the PC film increased, and its effect on the dose detection accuracy should be considered [50].
Readout reproducibility
Under three different radiation doses, the PC film was repeatedly excited and measured 10 times; the corresponding PL peak intensity values are shown in Fig. 6. The results show that the variation ranges of the PL peak intensity counts at irradiation doses of 100, 300, and 600 kGy were 5.04×105 - 5.56×105, 3.54×105 - 3.7×105, and 2.16×105 - 2.81×105, respectively, while the mean values of the PL peak intensity counts were 5.277×105, 3.616×105, and 2.547×105, respectively. The dose response is consistent with the data shown in Fig. 5. Additionally, the standard deviations of the three data groups were 0.158, 0.044, and 0.23, respectively, and the coefficients of variation of the PL intensity values were calculated to be between 1.2% and 8.6%. Compared with the GD-351 dosimeter (whose coefficient of variation for readout reproducibility in the dose range of 0.2-200 mGy is 15%-19%) [42], the the PC film exhibited a smaller standard deviation and coefficient of variation. This indicates the favorable readout reproducibility of the irradiated PC film.
Dose linearity
The PL emission spectra of the PC films irradiated at different doses (0–600 kGy) are presented in Fig. 7. A broad emission band was observed within the range of 400–600 nm. The PL spectral peak position of the irradiated PC films was redshifted compared with that of the unirradiated PC films (pristine). This is attributed to the formation of defects after irradiation or the partial release of hydrogen molecules, which resulted in the generation of carbon-rich clusters and a subsequent reduction in the optical bandgap energy [25]. Additionally, the PL intensity decreased as the irradiation dose increased. The PL peak intensity of the PC irradiated with 600 kGy decreased by 63.2% compared with that of the pristine samples (0 kGy). During the PL process, energy is transferred to the chromophore sites via UV excitation, and radiative recombination occurred in the thermalized electron–hole pairs o, thus resulting in fluorescence [51]. The decrease in the PL intensity may be associated with the formation of internal defects caused by radiation [52]. Radiation-induced disturbance in the internal structure of the PC resulted in the emergence of defect states, including chain scission and intermolecular crosslinking [53]. The formation of defects created a new radiative-recombination level for electrons and holes in the PC [54].
Moreover, the fluorescence peak positions of all samples were concentrated at 470 nm. The relationship between the PL peak intensity value (y) and the irradiation dose (x) within the dose range of 20–600 kGy is shown in the upper right of Fig. 7. Linear-regression fitting was performed, and the linear-regression equation y = - 0.00695 x + 6.32303 was obtained, with a correlation coefficient of 0.965 (the standard error of this correlation coefficient is 1.72%). This correlation coefficient closely matches that of the Makrofol LT 6-4 PC film in the range of 150–950 kGy [25], thus demonstrating that the PC film maintained a favorable linear relationship in the dose capture range of 20–600 kGy.
Self-decay
The attenuation characteristics of the PL peak intensity in the irradiated PC film (300 kGy) within 60 days are shown in Fig. 8. Clearly, the PL peak intensity decreased continually with time. This may be due to the ambient temperature or other factors, thus resulting in a reduction in fluorescence centers inside the PC-irradiated film. The results indicate a significant decrease in the PL peak intensity within the first 20 days after irradiation, with the peak intensity decreasing to 64% of its initial value. Subsequently, the declining trend became gentler over the following 40 days, with the peak intensity decreasing to 60% of the initial intensity within 60 days. Upon fitting with the ExpGro1 function, the following decay equation was obtained:
Electron energy response
Figure 9 shows the PL peak intensity of the PC film irradiated by EBs of different energy levels (3.5, 4, 4.5, 10, and 20 MeV) under the same dose (100 kGy). The results show that the PL peak response values of the energy points were 3.499×105, 3.422×105, 3.393×105, 3.352×105, and 3.236×105, respectively. Using the PL peak intensity value at 10 MeV as the normalized basis, the deviations of the PL peak intensity value at 3.5, 4, 4.5, and 20 MeV were +4.38%, +2.09%, +1.22%, and -3.46%, respectively. Based on a comparison with the energy-response characteristics of GD-351M RPL and LiF: Mg, Ti dosimeters [56], the signal intensity of the PC film exhibited a similar trend, although the amplitude of the change was smaller. This is because of the low density and small thickness of the PC film as well as the marginal difference in the electron energy response under irradiation with a high-energy EB (≥ 3.5 MeV).
Conclusion
In this study, the thickness applicability, preheating time, temperature and humidity dependence, in-batch uniformity, readout reproducibility, dose linearity, self-decay, and electron energy response of engineered PC films after EB irradiation were discussed based on PL-spectrum analysis. Upon excitation with 320 nm UV light, the fluorescence peak of the PC film appeared at an emission wavelength of 470 nm. The results show that the optimal thickness for dose detection using the PC film was 0.3 mm and that the optimal fluorescence value can be obtained by preheating at 60 °C for 180 min. However, environmental factors such as temperature (during PL spectral testing) and humidity (both during irradiation and post-irradiation storage), can affect the PL intensity. At a low irradiation dose (100 kGy), the dose-response uniformity of the PC film was ideal. In the dose-capture range of 20–600 kGy, the PL spectral peak intensity of PC films showed favorable dose linearity (R2 = 0.965) and readout reproducibility (Cv ≤8.6%). Additionally, when a PC film is used for radiation-dose measurement, a PL-spectrum test should be performed to prevent PL-signal decline after irradiation. Based on a comprehensive analysis of the aforementioned dosimetric parameters, this PC film can be proposed as a promising RPL material and demonstrates potential application in high-radiation-dose detection.
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