Comparison of time-dependent effects, decision limit and residual signal in Harshaw LiF (Mg, Ti) and LiF (Mg, Cu, P)

Full text: The personal dosimetry service of the UK Health Protection Agency - formerly the National Radiological Protection Board (NRPB) - is currently commissioning a body TLD system based on the use of HarshawTM 8800 readers and two-element cards. As part of the process, studies have been carried out into the long-term time dependence of response, the limit of detection, and the magnitude of the signal remaining after recommended processing. TLD cards containing both conventional lithium fluoride (LiF:Mg, Ti) and the high-sensitivity material (LiF: Mg, Cu, P) were available, thus providing a comparison between the two types of material. Two time-dependent effects were examined to 180 days after anneal: the deterioration of TLD sensitivity with time (ageing), and the deterioration of latent signal with time (fading). In both cases the LiF: Mg, Cu, P performed better, showing effects of less than 5 %, compared with up to 15 % for LiF: Mg, Ti. Control dosemeters were used in the experiments, providing an opportunity to additionally assess the increase in decision limit Lc- defined as two standard deviations above background - as a function of time. For issue periods of up to two months, Lc for conventional LiF remained just under 20μSv, with the high-sensitivity LiF: Mg, Cu, P achieving half of this figure; for longer periods, the value of Lc increased for both materials, as a result of increasing natural background. Published performance data on LiF: Mg, Cu, P has highlighted residual signal - the signal remaining after routine read-out - as a potential problem. Figures are presented which show the residual signal at routine dose levels to be no more than 1.5 % of the primary signal, with the present card configuration and processing. (author)