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Fading properties of thermoluminescent materials

A recent survey of the literature on thermoluminescent dosimetry characteristics, with particular respect to fading properties, has shown a wide range of formats in which this information is presented. Thus Horowitz (1981) in his detailed review of TLD literature quotes published work on different materials giving such fading rates as ‘29% in one day’, ‘30% in the first month’ and ‘ 5 % after 50 days’. Again in McKinlay (1981) a variety of formats is found, e.g. ‘35% in 24 h’ and ‘6% in 6 months’. It is indisputable that the properties of individual materials used for thermoluminescent dosimetry vary widely, let alone the characteristics of the different materials, and fade rates may be quoted for different peaks or for different conditions. However, the comparison of different materials or of different treatments would be made much simpler if a standard approach could be adopted for the description of fading. The user will have two primary concerns in regard to fading: ( i ) the absolute magnitude of fading; and (ii) the effective lifetime of the dosemeter before its accuracy is too severely compromised. The level of accuracy demanded from thermoluminescent dosemeters in many applications suggests that a suitable form for quoting fade rate could be the time for fading of 5 % to occur. The various examples quoted above would then have, using the linear assumption, 5% fade times of 4 h, 5 days, 50 days, 3 h and 5 months. Comparison is immediately simplified and the relative merits for different applications are more easily assessed.