3.1. Glass Characterization
The X-ray diffraction (XRD) pattern was used to confirm the formation of the synthesized compound. and the result are shown in Fig. (1) that proved that all of the samples are in a glassy state.
The study of thermal scanning of the glass is important because it provides information about the thermal properties of this glass in addition to some structure or phased transformations (8). DTA was used to check the characteristic temperatures, the glass transition temperature (Tg), crystallization (Tp) and melting temperatures when the glass heated, and its heat capacity. Other properties also change in a narrow temperature range, called the extent of transformation or the glass transformation where the structural molecules of the glass network acquire movement, which makes it possible to change. The first endothermic peak corresponds to the glass transition temperature (Tg). The exothermic peak follows the glass transition temperature that indicates the phase of crystallization temperature (Tp). The exothermic effect usually follows either one or several endothermic effects and is known as the melting temperature (Tm). Figure (2) shows the results of the DTA obtained, and the mixed alkali and alkaline elements effect added to the glass matrix shows a clear deviation. It turns out that the value of Tg varies from one sample to another.
Figure (3) shows the glass transition temperature values, Tg, of the glass samples, which increase slowly as the alkali and alkali earth elements oxide mixture content increases, i.e. the structural connectivity increases. In addition, Tg relies on the BO3 and BO4 groups, adding to the compactness of the structure and an improvement in the temperature of Tg temperature.
The thermal stability of the glasses is the result of a glass structure, which has a closely supported structure. Conversely, the thermally unstable glasses frame has a loose packaging.
The stability of the glass showed a difference and decreases with a decreasing the silicon ratio and a change in the proportions of the mixture of alkali and alkali earth elements in the composition of the bioglass, which indicates that the homogeneity of the glass changes with the composition G3 (23.92% CaO, 51.5% SiO2, 2.90% P2O5, 21.64%Na2O), and this is in line with the infrared spectra and density.
3.2. Thermoluminescence study
The radiation dosimetric achievement of a TL material strongly relies on the structure of its glow curve, such as maximum TL- intensity, and position of glow peaks . So, after exposure to the test dose (50 Gy from gamma-ray), the TL glow curves of G1, G2, G3, and G4 were studied, and the findings are shown in Fig. (4). It can be seen in the figure that G1, G2, G3, and G4, have the same shape of glow curves which indicated that these glasses contain the same types of traps. Hole traps are formed by non-bridging oxygen defects and fused silica, while electron traps are formed by empty Si and Na orbitals (9; 10).
while the glow curves for G1, G2, G3, and G4 are identical in form, the TL-intensities and peak position are dissimilar, as seen in Fig. (5). this figure clearly shows that G2 has the highest TL-intensity, followed by G1, G3, and G4, in that order. G1 and G2 have the same chemical composition except G2 contain 6% from P2O5 which may be responsible for the production of more electron traps in G2 than G1. The difference of TL- intensity between different types of glass may be due to the presence of different amounts from the chemical composition of the glass (SiO2, Na2O, CaO, P2O5, TiO2).
Also, the peak positions for G1, G2, G3, and G4, respectively, are different. The peak position for G1 at 454 K, G2 and G4 at 460 K, and G3 at 470 K. According to previous studies, the ideal glow curve for glass should have a single peak with maximum temperature between 453, and 523 K. (11; 12). Figure (6): Shows the glow curves of G1, G2, G3, and G4 respectively after irradiated with different gamma doses from 0.25 to 1000 Gy. For all doses, there are slight differences in the peak temperatures for all the bioglass matrices. However, with rising radiation dose, the TL-Intensity in all bioglass forms increases. This means that as the radiation exposure raises, the number of active traps increases, corresponding to a rise in the number of recombination traps, resulting in an increase in TL-Intensity when reading the bioglass samples. (13; 14).
3.3. Repeatability of TL measurements
One of the most important characteristics to be met in any dosimeter is the accuracy of this dosimeter with reuse. The coefficient of variation (CV) of TL-response for a particular dosimeter that undergoes the same treatment should not exceed ± 7.5% (15; 16). Therefore, all bioglass samples were subjected to cycles of irradiation at test dose (50 Gy), readout, and annealing to test its repeatability and (CV) was obtained by using Eq. (1):
Where, the average of the readouts is m, and the standard deviation is SD.
The CV values for G1, G2, G3, and G4 were found to be 6.7%, 5.3%, 6.3%, and 2.2% respectively, which are lower than the recommended value (7.5%) in all bioglass types.
3.4. Dose response
The linearity of the relation between irradiation dose and TL- intensity is one of the most critical features about any dosimeter, since it defines the range of radiation dose for a dosimeter. the supralinearity index F(D), is a tool to estimate a material's linearity, which first introduced by Horowitz (1981) and Mische and McKeever (1989)(4; 17–19)using the Eq. (2):
Where, f(D) is the TL- intensity at a low dose ‘D’, and f(D1) is the TL intensity at a high dose ‘D1’, the supralinearity index F(D) is equal to one within linear region, F(D) is higher than one within supralinear region and F(D) is lower than one within sublinear region. It was found that in the case of G1, G3, G4, the dose-response curves were sub-linear from 1 up to 25 Gy and become linear from 100 up to 1 kGy. However, in the case of G2, it was sub- linear from 1 up to 10 Gy and become linear from 25 up to 1 kGy as shown in Table (1). The dose-response for G1, G2, G3, and G4, respectively are shown in Fig. (7). Table (2) shows the linear dose range for the present bioglass matrixes types and those of previous works(20–23).
3.5. Kinetic analysis
Deconvolution functions for general orders of kinetics derived by kities, et al., (24) have been used to determine kinetic parameters such as activation energy and frequency factor, and the result of deconvolution for G1, G2, G3, and G4, respectively are seen in Fig. (8). Table (3) shows the activation energies and frequency factories for G1, G2, G3, and G4 respectively.
The kinetic parameters also have been determined by the peak shape according to Chen method (9; 15) and the result is shown in Table (4).
From these results, it can be observed that there is an agreement between activation energies and frequency factories calculated by two methods.