3.3.8. Optical Fading

Optical fading is another significant parameter for TLD materials; however, the optical fading dramatically depends on the light intensity, wavelength, and time exposure [83]. In the current study, optical fading was measured to estimate the sensitivity of the Ag-doped ZnO thin films to sunlight and room light. The samples were initially kept in a dark container during all the time measurements. Two groups of 30 samples were irradiated by 4 Gy of X-ray radiation and numbered, where each data point identified the outcomes of five samples. For six hours, the first group of samples was exposed to sunlight, while the second group was exposed to room light (by fluorescent lamp).

Figure 16 shows the behavior of the optical fading of Ag-doped ZnO thin films. The stored signals for the first group of samples yielded a loss of 53% after 1 h and 70% after 6 h of direct sunlight exposure in ambient conditions. As for the second group of samples, the stored signals yielded a loss of 30% after 1 h and 46% after 6 h. The trapped electrons or holes can be released optically at low temperatures, which suggests that further recombination could occur between the opposite charge carriers with an increasing absorbed temperature. Thus, a decrease in the TL intensity is expected when the proposed dosimeters are directly exposed to sunlight or room light [84]. This behavior has been observed in our previous works with multilayer thin films and nanopowder (pellets). The results revealed that the samples exposed to sunlight lost the signal more than the samples exposed to room light; this shrinking in the stored signals is attributed to the UV in sunlight. In light of this, the study concluded that the TL dosimeters should be stored in opaque containers when utilized [25].

**Figure 16.** The optical fading of Ag-doped ZnO thin films exposed to sunlight and fluorescent light.

3.3.9. Minimum Detectable Dose (MDD)

For this study, the minimum detectable dose (MDD), also known as the lowest level detection, was calculated using Equation (6):

$$D\_0 = (B^\* + 2\sigma B)F\tag{6}$$

where *σB* is the standard deviation of the background; *B\** is the average background TL (zero dose reading); and F is the calibration factor expressed in Gy nC−1.

In the current study, the five samples were read to measure the background before irradiation and exposed to 1 Gy of X-ray radiation. The average background and standard deviation for Ag-doped ZnO thin films were 0.414 and 0.099 nC, respectively. Additionally, the calibration factor recorded 0.0168 Gy nC−1, substituting Equation (7). Consequently, the low detectable dose of the Ag-doped ZnO thin film was found to be 10.31 mGy. Table 2 shows the MMD for the Ag-doped ZnO thin film. The type of model of TLD reader is a significant factor in determining MMD [85].

$$F = Dose\ (Gy)/TL\ (n\text{C})\tag{7}$$

**Table 2.** The minimum detectable doses (MMD) for the Ag-doped ZnO thin film.


3.3.10. Percentage Depth Doses (PDD)

The percentage depth dose (PDD) is vital to determine the dose delivery, especially for cases that involve applying a narrow beam and a small field size [86]. PDD is given by the division of the absorbed dose at any depth, D, to the absorbed dose at a specific reference dose, which is along the central axis of the beam, as shown in Equation (8):

$$\%PDD = D\_d / D\_0 \tag{8}$$

In this study, X-ray radiation (80 kVp, 100 Ams) was applied to determine the depth dose distribution and compare the use of a PTW Markus parallel plate chamber, TLD rods, and TLD 100 chips, a host material of Ag-doped ZnO thin films. As previously described in Figure 1, with a slight difference, the procedure was set up with a source-to-surface distance (SSD) of 80 cm and a field size of 20 × 20 cm2. Furthermore, the phantom consisting of Perspex slabs included a slab with a thickness of 1 cm, where 10 slabs were used to ge<sup>t</sup> a maximum depth of 10 cm. The thin film set up in a slab with a thickness of 2 mm was placed in square holes, and the dose delivery was 3 Gy. The ionizing chamber, TLD rods, TLD 100 chips, and Ag-doped ZnO thin films were placed at different depths (from 0 to 10 cm).

Figure 17 shows the performance of the measurements at different depths, from the surface *D*0 up to *D* = 10 cm. The depth doses (each data point representing five samples' outcomes) were normalized to *D*0 = *Dmax*. Ag-doped ZnO and all references recorded *Dmax* at a depth of 0 cm. The *PDD* appeared to gradually decrease from the surface until the depth of 6 cm. The *PDD* values of Ag-doped ZnO at a depth of 5 cm were higher than the *PPD* values of the TLD 100 chips, ionization chamber (IC), and TLD rods, which were found to be 28.33%, 24.33%, 27.00%, and 25.00%, respectively. The variation in the values of depth dose is subjected to many factors, such as the location of effective point samples, field size (may be due to the scattering of radiation), measurement of detectors, presence of air gap between detectors and layers of Perspex phantom, and the <sup>Z</sup>*eff* of the materials.

**Figure 17.** Percentage depth dose curve of the Perspex phantoms in 30 cm × 30 cm field size at 60 kVp of the X-ray energy with delivery dose 3 Gy.
