Sandra Hopkins Final Report.pdf - University of Surrey

lattice defects which are an important part of the thermoluminescence process. When the crystalabsorbs ionising radiation free electrons are produced. These may become trapped in the latticedefects. Additionally, the holes that are produced in conjunction with the free electron, may alsobecome trapped. Many hole centres are thermally unstable and may decay quickly at roomtemperature. The electrons will remain trapped provided they do not gain sufficient energy withwhich to escape. The energy required depends on the depth of the trap and the temperature of thematerial. Released electrons may recombine with holes at luminescence centres with the excessenergy being radiated as visible or ultraviolet photons. This electron capture and delayedrecombination with a hole at a luminescence centre is what makes up the process ofthermoluminescence. The complete process is displayed in figure 9Figure 9 : A simple energy band model for thermoluminescenceA glow curve is a plot of thermoluminescence intensity against temperature and can be derivedfrom the electron release formula. The equation for the glow curve intensity from electrons at asingle trapping level E is given as;I = n o Cexp – [ 1/R .s.exp(-E/kT)dT].s.exp(-E/kT)37Where E is the trapping level, R is the heating rate, n o is the number of electrons present at time t oand temperature T o , C is a constant related to luminescence efficiency, k is the Boltzmann constant,T is the temperature and s is a frequency factor associated with the particular lattice defect.12