Abstract
Sources of error in a thermoluminescent dosimetry (TLD) system based on LiF/PTFE dosemeters are identified, and means described for reducing them by appropriate design of instrumentation and dosemeters, and suitable calibration and other operational techniques. A theory of thermoluminescent dose measurement has been developed, with special reference to the use of semi-transparent dosemeters for the measurement of penetrating and nonpenetrating radiations. Studies of the thermoluminescence of lithium fluoride prepared with various activators are described, and the thermoluminescence mechanism in LiF:Mg,Ti is discussed. The major component in the system is a new readout instrument which has been designed to accept a wide variety of dosemeter configurations, and which employs several novel techniques to minimise errors during the readout of dosemeters. The light measurement system utilises variable negative feedback both to set the sensitivity to a desired level and to automatically maintain that level against system drifts. A heating cycle control system has been developed which employs a unique thermocouple cold junction compensation circuit to ensure reproducible readout cycle temperature. Different dosemeter forms, criteria for comparing them, and the particular characteristics of phosphor/PTFE dosemeters are described. Sources of sensitivity and background variations in LiF/PTFE dosemeters have been investigated, enabling the design of the dosemeters and operational techniques for precise measurements with them at both radiological protection and radiotherapy dose levels to be optimised. The dependence of the sensitivity of LiF/PTFE discs on thickness and optical transmission is explained in terms of the theory previously developed for semi-transparent dosemeters. An 80°C pre-irradiation anneal has been shown to be essential for precise measurements with LiF/PTFE dosemeters, due to the characteristics of the glow curve structure of the fine LiF powder used in their manufacture. The limiting factor in low dose measurements with LiF/PTFE discs is a background signal believed to be due to oxygen adsorbed too deeply in the PTFE surface to be removed by nitrogen flow. A number of recommendations for future work are given.