Extremity TLD Monitoring at OPG: Calibration, Quality Assurance, and Type Testing

Abstract

At Ontario Power Generation (OPG), extremity doses are measured using Thermoluminescent Dosimeter (TLD) chips. These chips are lithium fluoride crystals doped with magnesium and titanium (LiF:Mg,Ti). In order to accurately measure the dose received by a worker, it is essential to understand and quantify all potential influencing factors relating to the extremity dosimetry system. These factors include the way the system is calibrated, the response of the TLD chips to different types and energies of radiation, the effects of the different chip holders used for chip irradiations, and environmental factors. A special investigation was undertaken to identify and quantify the source of a long-standing bias in quality assurance (QA) test results between the Whitby Health Physics Laboratory (HPL) and the National Research Council - Canada (NRCC). The four major factors contributing to the bias were determined to be: 1) The response of LiF:Mg,Ti to the different irradiation sources used at the Whitby HPL (137Cs) and at the NRCC (60Co). 2) A difference in sensitivity between the field chips and the chips used to calibrate the readers at the Whitby HPL. 3) Differences in exposure rate standards used at the NRCC and at the Whitby HPL. 4) Different chip holders, used to hold the chips during an irradiation, at the NRCC and at the Whitby HPL. Experimental tests and Monte Carlo simulations have been performed to determine the relative response between various chip holders used at the Whitby HPL and at the NRCC. The experimental results were found to be in close agreement with the results generated by Monte Carlo N-Particle (MCNP). Monte Carlo simulations have also been used to determine the effects of adding different thicknesses of material in front, behind, and around a single TLD chip. The properties of a new bulk chip holder are also examined with the intention of replacing the current bulk chip holder. After making an individual sensitivity correction, it was determined that the response across the new holder was uniform. This implies that two chips placed anywhere in the holder will receive the same dose. The remaining part of the project focused on the identification and quantification of several factors which can influence TLD results. These tests included a measurement of the repeatability of the TLD process, the minimum time required between the annealing and the irradiation, and a 7-day fade test. For the first type test, sets of measurements were repeated under identical conditions to examine how repeatable a given measurement may be. The average relative standard deviation of the TLD readings from the chips was measured to be 1.7%. A second type test was conducted to identify the minimum amount of time required between the annealing period and the irradiation. This minimum time required for an accurate measurement was determined to be about 24 hours, but only about 6 hours are required for a relative measurement. A 7-day fade test was conducted to determine the amount of signal lost in the first week. Using the equation of the trend-line, a fade rate of 1.5% per week was calculated for the first week.