Commissioning of the Canadian Primary Standard for Low Dose Rate Brachytherapy

Abstract

The purpose of this project is to investigate the performance of the NRC WAFAC and commission it as a primary standard for calibrating LDR sources. Significant enhancements were implemented to the WAFAC for improving signal to noise ratio, measurement reproducibility and eliminating electric field distortion. The validation of the WAFAC, comparing to the existing NRC primary standard, shows that the agreement between the two devices is within the combined measurement uncertainties of less than 0.5 %. The measurements made with a 125 I LDR seed gave results that were consistent at the 1 % level with the manufacturer's stated seed activity (traceable to NIST). The polarity effect and aperture size effect had been studied during the commissioning. The overall k = 2 standard uncertainty in the determination of air kerma strength using the NRC WAFAC is estimated to be 2.6 %, which reaches the target uncertainty for the standard. iii I would like to extend my appreciation to Dr. Dave Rogers for his encouragements, help and belief in me. I would also like to extend my appreciation to Dr. Dave Wilkins for providing me free I-125 seeds. I would also like to extend my appreciation to Dr. Ernesto Mainegra-Hing for his Monte Carlo simulations and discussions with me. I would additionally like to thank Mr. Stewart Walker, Mr. David Marchington, and Mr. Brad Downton for their assistance in electrical and mechanical matters. Finally I would like to extend my deepest gratitude to my wife Meilin and my son Yiwen. Without their love, support and understanding I could never have completed my graduate studies.