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2.1 Basic concept for thermoluminescent
Basically, thermoluminescence is where the absorbed radiation can be released from the crystalline materials in form of luminescence when being heated at its maximum temperature. The processes of the thermoluminescence have two stage. Alawiah (2015) used one trap and one center (OTOR) as a model. To describe the TL properties, the theory of the energy band is used (Alawiah, 2015 and Piters, 1993).

2.1.1 Mechanism of thermoluminescence
The first stages in thermoluminescence process are by absorption of energy from ionizing radiation which causes the change of the system from equilibrium to a metastable state. This stage is known as energy storage. To stabilize the absorbed energy, electronic excitation, and the displacement damage is controlled. The defect creation can happen if the electronic excitation and the displacement damage is not stabilized correctly. Some of the localized electronic states are occupied by a non-equilibrium concentration of electrons cause radiation-induced defects and less amount of electrons being trapped. The electron-hole pair production and excitation creation work as energy storage for electronic excitation (Shodhganga.inflibnet.ac.in. 2018).

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Mobile holes and electrons in the crystal structure are will be formed the electron-hole pair production in the material after being irradiated. Preexisting impurities or radiation-induced defect have defected which causes the presence of a mid-gap state and form between the two energy bands. There are two types of energy bands; valence band (placed at the outermost energy level and consists of electron-hole pairs) and conduction band (the higher energy level where the electron is free to move and produce electric current).

By referring to Attix (2004), there are two types of crystal-lattice imperfections such as electron traps and hole traps that might be located the luminescence centers where when the electron and hole are permitted to recombine causes the light to be emitted. This traps might be located either in the conduction band or in the valence band.

Figure 2.1 Energy-level diagram of the energy storage stage
Note. From Shodhganga.inflibnet.ac.in. 2018
When radiation is applied, the electron passing through to conduction band from valence band and the hole charged area in the valance band became positively charged. The electron excited into the conduction band and recombine with either electron trap and holes trap. Hole traps and electron traps are known as a luminescence center.

Figure 2.2 Energy-level diagram of the energy release stage
Note. From Shodhganga.inflibnet.ac.in. 2018

As energy is released in the form of light when being heated known as relaxation stage where relaxation of the system back to the equilibrium. When the temperature reaches its maximum limits, the energy released the traps in conduction band results in the release of the storage energy by electron de-excitation event and cause the change in material from the metastable state to ground state. The electron is then free to be trapped again or recombine with the hole in the hole trap that emits energy in the form of light. The hole traps are known as recombination station for this mechanism.

2.1.2 Randall-Wilkins theory
The trapped charge carries escaped can be determine using first-order kinetic energy at a temperature (T) as described by Randall and Wilkins 1945 (Attix, 2004). The equation used is:
p=1/?=???exp?^(-E/kT)
Where p is the probability of escape per unit time (-s)
? is the mean life time in the trap
? is the frequency factor
E is the energy depth of the trap (eV)
k is the Boltzman’s constant (1.381 × 10-23 J K-1 = 8.62 × 10-5 eV K-1)
The values of ?, E, and k are assumed as constant. With increasing in T causes the values of p to increase and ? to decrease. When T has scanned upward linearity against time, the rate of escape of trapped electron increase with starting temperature at room temperature. The maximum temperature, Tm is followed by decreases in the electron trapped supply as the trapped electron is gradually exhausted.