The routine monitoring of the occupationally exposed individuals constitutes an integral part of radiation protection program implemented at the Vinca Institute of Nuclear Sciences (INNV). The Individual monitoring service (IMS) at the INNV, accredited according to quality standard EN ISO/IEC 17025, is using Harshaw 6600 Plus automatic reader and thermoluminescent dosimeters (TLD) from two manufacturers (Harshaw, RadPro international GmbH). Working procedures are implemented according to Technical Recommendations for Monitoring Individuals Occupationally Exposed to External Radiation, RP 160 (European Commission, 2009). Among general consideration, RP 160 is suggesting that routine calibration of dosimetry system should be performed taking into consideration secondary charged particle equilibrium. As TLD system in use at IMS INNV comprises of TLD reader and of TLD cards, this consideration should be applied in both cases: when the reader and the TLD cards are being calibrated. During reader calibration, secondary charged particle equilibrium can be ensured using build-up PMMA plate of certain thickness placed in front of dosimeters. In this paper, influence of the build-up plate on reader calibration is being analyzed. This influence is observed through changes in reader calibration factor (RCF). RCF values obtained during reader calibration procedure with and without 2 mm thick build-up plate are presented. Exposures of whole body TLDs placed on slab phantom were done with Cs-137 radiation source located at Secondary Standard Dosimetry Laboratory (SSDL) at INNV. TLDs from both manufacturers had 2 detectors for measuring the dose equivalent at the depths of 10 mm and 0.07 mm. TLD reader was calibrated to measure effective dose in terms of Hp(10) and local skin dose in terms of Hp(0.07). RCF values for detectors measuring Hp(10), from both manufacturers, when irradiated without the plate were around 5% higher than RCF values obtained when irradiations were done with the plate. Values of RCFs for detectors measuring Hp(0.07) were around 1% higher when the plate had not been used. As RCF value is inversely proportional to measured dose, reader calibration without build-up plate, in terms of Hp(10), would lead to underestimated dose values. The observed RCF deviation of Hp(0.07) detector is regarded as consequence of exposure conditions uncertainties. In conclusion, if a constant geometry and constant operational conditions of TLDs are achieved, the only part of the TLD system that is not stable for a long time period is the TLD reader. To avoid inaccurate results, RCF should be orderly checked and reader calibration should be done according to widely accepted standards, currently in effect.3rd International Conference on Radiation and Applications in Various Fields of Research (RAD), Jun 08-12, 2015, Budva, Montenegr
