Nucl. Tracks Radiat. Meas., Vol. 21, No. I, pp. 163-167, 1993 0735-245X/93 $6.00 + .00 Printed in Great Britain Pergamon Press Ltd STUDY OF AGEING EFFECTS IN LiF:Mg, Ti BY ANALYSIS OF THERMOLUMINESCENCE GLOW CURVES A. J. J. Bos and T. M. PITERS Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL 2629 JB Delft, The Netherlands Abstract--The effects of ageing on the thermoluminescence of LiF(TLD-100) crystals have been investigated by computerized glow curve analysis. After an annealing of 1 h at 400°C and quick cooling the crystals were aged at 80°C for 0, 6, 20, 60, 200, 800 and 1200min and readout at 6'C s ~ or 0.24"C s -~. The intensities of peaks 2, 3 and 4 decrease with increasing ageing time. Peak 5 initially increases (up to 20%) due to clustering of dipoles and decreases at longer ageing times due to precipitation. Dramatic changes in the values of the activation energies of peaks 2 (from 1.35 to 0.72 eV), 3 (from 1.44 to 1.17eV) and 4 (from 1.55 to 2.32 eV), have been observed for ageing times longer than 20 min and a readout heating rate of 6-C s- ~. These changes are smoothed out at when the readout heating rate is slow. The strong changes of E values indicate a trap modification process which may be caused by precipitation of defect clusters. The relevance for practical dosimetry is discussed. 1. INTRODUCTION FOR MORE than two decades LiF has been used as a practical radiation dosimeter. However, despite much research the understanding of its thermolu- minescence mechanism is poor. The most widely used and also extensively studied material is LiF(TLD- 100) marketed by Harshaw Chemical Company which consists of LiF doped with approximately 170ppm Mg 2+ ions and approximately 7 ppm Ti4+ ions. For dosimetric applications a thermal treatment (annealing procedure) is a necessity. An almost uni- versally accepted standard pre-irradiation annealing procedure (Horowitz, 1990) consists of a high tem- perature anneal (400'C for 1 h followed by a quench to room temperature) and a low temperature anneal (ageing at 80°C for 24 h). The high temperature anneal will disperse the Mg 2+ impurity ions, which are believed to be involved in the trapping of charge carriers. The low temperature anneal reduces the contribution of the low temperatures glow peaks (peaks 2 and 3; see Fig. 1 for the peak annotation) which are more sensitive to fading and therefore undesirable in practical radiation dosimetry. In a previous investigation (Bos et al., 1992) we studied the effects of the cooling rate and readout heating rate on trapping parameters of samples treated with only a high temperature annealing pro- cedure. The results showed that the activation ener- gies for all glow peaks vary strikingly indicating that the defect structure, which constitutes the traps for the charge carriers, is changing. In this study we investigate the influence of the widely applied low temperature annealing at 80°C on the glow curve. More specifically, we are interested in the decrease and/or growth of the glow peaks and in the trapping parameters as a function of the ageing time. In earlier studies on ageing effects (Zimmerman et al., 1966; Grant and Cameron, 1966; Harris and Jackson, 1968; Booth et al., 1972; Taylor and Lilley, 1982a,b,c) no glow curve analysis (with resolving the separate glow peaks) was applied. Small but important changes in the shape of the glow curve may therefore not be observed. With glow curve analysis, including the determination of the trapping parameters, it is hoped to gain new information which may contribute to a better understanding of the processes taking place during the low temperature anneal and the TL properties which are important in practical dosimetry. 2. MATERIALS AND METHODS The samples used in this study were LiF crystals TLD-100 polycrystalline chips (3.2 x 3.2 × 0.38 mm) obtained from Harshaw Chemical Company, Ohio. Thin crystals were used to minimize temperature gradients during readout. These samples originate from Batch No. 6 of TLD-100 samples described previously (Bos et al., 1990). The ageing experiments were performed with a specially designed micropro- cessor controlled annealing oven. In this oven the chips are enclosed in an anodized aluminium tray (8.3 × 8.3 × 0.4 cm, 83 g) which is heated by four i.r. heating lamps and cooled by pressured air from two venturi cooling pipes positioned above and below the tray. The temperature is measured with a thermo- couple inside the middle of the tray and controlled by a PID controller MICRO-96-TPI0 from Newtronic allowing very reproducible annealing cycles. The oven has a very small thermal mass providing rela- tively fast cooling rates. All samples were annealed at 400_+ I~C for I h, cooled down quickly in a 163