The Quasi-Equilibrium Problem in Thermoluminescence

Abstract

The chapter begins with defining the quasi-equilibrium (QE) in thermoluminescence (TL). It is shown that it is the values of the system parameters namely the concentration N of the traps and the recombination centers (RC) and their cross-sections σ which solely determine whether or not the QE condition would be satisfied. This dependence may be used to answer the question whether QE is satisfied in the TL of real materials. Greater are the values of these parameters better it is for the QE condition. A function of N and σ is derived which defines the lower limit of these parameters for satisfying the QE condition. Some earlier workers had concluded that the QE may be satisfied only when recombination rate dominates over the retrapping rate during the glow curve recording. In this chapter this condition is shown to be unimportant. The importance of the recombination lifetime τ _rec is highlighted. Smaller is the value of τ _rec better it is for the QE to be achieved. It is found that the condition τ _rec/T _R  < 10⁻³ sets the upper limit of τ _rec in any glow peak, where T _R is the time span of the glow peak being measured. It is found that QE depends also on the heating rate β. When β is increased above a certain limit, the glow peak may begin to widen. At this point the glow peak enters from QE into non-QE domain. It is suggested that this property may be used to test whether a glow peak is being recorded under the QE condition. The limiting value of β above which this effect may be seen depends on the parameters of system under investigation. As an illustration, this test is applied to the glow peak No. 5 of LiF-TLD phosphor.