Sandra Hopkins Final Report.pdf - University of Surrey

This formula provides the idealised case for a single trapping level. At low T the curve risesexponentially and after reaching a maximum ‘the glow peak’ it falls to zero. In reality there will bemore than one trapping level each of which will give rise to a glow peak maximum. The area andpeak height of each glow peak depends on the number of lattice defects and the amount of impurityatoms present.The TLDs used are known as TLD-100 (LiF : Mg : Ti). This is Lithium Fluoride doped withmagnesium and titanium. This is the most understood and commonly used phosphor.Although the exact process is unclear, it is suggested that the magnesium ions form electron traps incombination with certain defect centre in the lattice. The effect of titanium is thought to be in theformation of luminescence recombination centres.The reader used is a Harshaw model 5500 automatic TLD reader. Figure 10 shows an example ofthis TLD reader.The reader uses nitrogen gas during the read process mainly to suppress the chemiluminescencesignals from the dosimeter and the reader. In addition it purges the Photomultiplier assembly,keeping moisture out of this area. The dosemeters are lifted into the read chamber by a vacuumpick. The TLD is heated as a function of time according to a time temperature profile (TTP). Thisprofile is divided into three parts; the preheat, acquisition and anneal. The preheat ensures that alldosemeters in a group have a common starting point and can also be used to eliminate the fasterfadinglow temperature peaks. The dosimetric data is collected and the glow curve generated duringthe acquisition stage. The light detected by the photomultiplier tube at this stage will generate acurrent proportional to the amount of light detected. The anneal stage is used to hold thetemperature at a sufficiently high temperature to ensure that all the signal is removed. A calibrationis required to be carried out to produce a reader calibration factor which converts the raw data innano coulombs into the desired dose units, in this case mGy.13