Long-term stability of a TLD-based individual monitoring system

The thermoluminescence dosemeter (TLD) system at the Individual Monitoring Service (IMS) of the Nuclear Technology Institute (ITN) at the Radiological Protection and Nuclear Safety Department (DPRSN) comprises two 6600 Harshaw readers and the Harshaw 8814 TL card and the holder containing two LiF:Mg,Ti (TLD-100) dosemeters for the evaluation of H-p(10) and H-p(0.07). The readers are calibrated on a monthly basis and as part of the quality assurance programme implemented at the IMS a set of dosemeters is issued monthly to the DPRSN's Standard Dosimetry Laboratory for linearity measurements. The results obtained since November 2001 are presented. Fading and sensitivity change experiments are carried out every month covering 8 week periods so that enough time is given to simulate issuing, integrating and receiving times and respective delays. A set of 96 dosemeters organised in eight subsets of 12 are used. In each subset, four dosemeters are irradiated and stored at room temperature (RT), four are not irradiated at all and the last four are irradiated after storage. The 12 dosemeters of each subset are readout at the same time, one per week, covering the 8 week period. The results from the sets irradiated and stored at different periods allowed for the evaluation of fading and sensitivity changes experienced over the whole monitoring period and respective preparation time and readout delays. Time evolution charts of the reader calibration factors, of the linearity parameters and of the evolution of the integrated area in the region of dosimetric interest with storage at RT were obtained. This paper aims to quantify the long-term stability of the TLD system in use at the IMS.