3.3.7. Thermal Fading

Thermal fading is defined as the loss of stored thermoluminescence TL intensity during the storage after irradiation. Typically, the thermal stability of the dosimetry material is based on two categories: annealing, storage temperature, readout, radiation types, and time, which are discussed in the previous terms. Second, the thermal stability is strongly dependent on the nature of the material; in this context, the activation *E* energy of the depth trap should be (*E* > *kT*), to liberate an electron from a trapping center.

The desirable dosimeters possess a glow curve that starts upward after 140 ◦C and reaches a maximum peak between 200–250 ◦C. However, commercial TLD materials possess fading. The fading may occur through defects or be due to their recombination in trapped holes [81–83]. The thermal fading property of the new host TL material in this work was examined. In this case, 0.5 mol% Ag-doped ZnO was exposed to 1 Gy and 4 Gy. The irradiated TL materials were divided into two groups, where each group had 35 samples and was stored in an opaque box (dark environment) at room temperature. The readout of TL started directly after 1 h and continued for 45 days.

Figure 15 displays the fading curve behavior of the samples. The signal loss was recorded at 8% for 1 Gy and 20% for 4 Gy in the first hour. After 45 days of irradiation, the signal loss was recorded at 32% and 40% for the cases of 1 Gy and 4 Gy, respectively. The residual signal noticeably decreased with the increase in the dose value of the X-ray radiation.

**Figure 15.** Thermal fading characteristics of Ag-doped ZnO thin films exposed to X-ray.
