Temporal Sensitivity Changes and Signal Fading of LiF: Tl, Mg Under Controlled Environmental Conditions

M.L. Rodrigues; S.-H. Hsu; K.J. Kearfott; J.E. Schlicht; M.T. Sami; and T.A. Lebeis (University of Michigan)

The fading of the lower temperature peaks in the common thermoluminescent dosimeter (TLD) material LiF: Tl, Mg is well known. In addition, changes in the sensitivity of this material between readout and irradiation have been reported. This work addresses in a systematic way both of these effects on the individual glow curve peak areas. A set of sixty TLDs, configured in cards each containing four chips, was used in both experiments. Signal readout was accomplished with a gas-jet reader using a heating protocol that allowed capture of the lower energy glow peaks. Irradiations were performed using a Cs 137 irradiator, with a minimum dose selected so that noise in the resulting glow curves would not produce significant variation in the resulting individual glow curve peak areas. All TLDs were irradiated for each experiment, and a specially designed phantom used to assure reproducible positioning. Glow curve deconvolution was achieved using curve-fitting routines provided with the reader. For the first experiment, irradiations were performed immediately following the standard annealing procedure. Times ranging from a few minutes to several days were allowed to elapse before the dosimeters were read out. For the second experiment, the TLDs were annealed, and then times ranging from a few minutes to several days waited prior to irradiation. The dosimeters were then quickly read out. During the waiting periods, the TLDs were held in a low background, temperature-controlled environment. Experiments were repeated using different storage temperatures. Polynomial curves were fit for the average individual peak areas as a function of either pre- or post- irradiation times. Attempts were made to develop a model of the observed behavior in terms of movement of trapped electrons among the various peaks using simple compartmental models.

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